Curtis Peebles

With Contributions

by Richard P. Hallion
Library of Congress Cataloging-in-Publication Data

Peebles, Curtis.

 Probing the sky : selected NACA/NASA research airplanes and their

contributions to flight / by Curtis Peebles.

 pages cm

 Includes bibliographical references and index.

1. Research aircraft--United States--History. 2. United

States.--National Aeronautics and Space Administration--Research. I. Title.

 TL567.R47P4349 2015

 629.133’3--dc23

 2014039331

Copyright © 2014 by the National Aeronautics and Space Administration.

The opinions expressed in this volume are those of the authors and do not 

necessarily reflect the official positions of the United States Government or 

of the National Aeronautics and Space Administration.

This publication is available as a free download at

http://www.nasa.gov/ebooks.

National Aeronautics and Space Administration

Washington, DC
iii

Table of Contents

Introduction: Toward—and Into—the Unknown v

Chapter 1: Confronting the “Sound Barrier”: The Bell XS-1 ............................... 1

Chapter 2: Flying Test Tube: The Douglas D-558-1 Skystreak.......................... 45

Chapter 3: Proving the Swept Wing: The Douglas D-558-2 Skyrocket............. 85

Chapter 4: Unfulfilled Promise, Serendipitous Success: 

The Douglas X-3 Stiletto................................................................ 135

Chapter 5: Versatile Minimalist: The Northrop X-4 Bantam............................ 167

Chapter 6: Transformative Pioneer: The Bell X-5.............................................. 207

Chapter 7: Progenitor of the Delta: The Convair XF-92A .................................. 243

Round One: A Reflection 269 

Appendix: Technical Specifications for the Round One Aircraft 280

Acknowledgments 282 

Selected Bibliography 283

Index 307
The scale of what’s involved in undertaking research flights is made clear in this iconic image 

of the D-558-2, the B-29 launch plane, and the ground support personnel and equipment. The 

photo was taken in front of the NACA hangar at South Base shortly before the move to the present facilities. (NASA Photo)
v

Introduction

Toward—and Into—the Unknown

In the decades since the Wright brothers’ first flights, a body of knowledge 

and tools, created in an evolutionary process of small steps, had been built 

up to guide engineers and researchers in developing new aircraft. The early 

wood-and-fabric biplanes had given way to all-metal monoplanes. Aircraft 

size, range, and payload had also grown, until the oceans could be spanned in 

a fraction of the time a ship would take. Speed became the critical factor in 

both commercial and military operations. 

But by the early 1940s, speed itself had become the problem. With aircraft 

flying near the speed of sound, the old rules of subsonic aerodynamics no longer 

applied and the old tool of aeronautical research—the wind tunnel—no longer 

worked. Consequently, engineers lacked the means to determine if their designs 

would withstand actual flight conditions. 

The technology, tools, and procedures needed in this new realm of flight 

completely transformed the fields and practices of aerodynamics, propulsion, 

structures, design theory, materials, flight control systems, life-support systems, 

escape systems, safety procedures, wind tunnels, and data collection systems 

and methodologies. The body of knowledge for the supersonic era was effectively recast and made anew. 

Thomas Kuhn’s term “paradigm shift” is now much overused both as an 

expression and as an intellectual concept, but, in the case of the midcentury 

high-speed revolution, it is certainly appropriate. What aeronautical science 

had accrued for subsonic flight following the Wrights’ first flight in 1903 

was rendered almost entirely irrelevant by the late 1940s and the onset of the 

supersonic era. 

The engine of transformation was the turbojet. It spelled the end of one era 

in aviation and the dawn of another, pushing people and machines across the 

sonic divide faster than they had gone before, forcing transonic and supersonic 

researchers to address the mysteries that were affecting airplanes entering this 

new and unknown realm. 

Ironically, while it was this new form of propulsion that drove the supersonic breakthrough, the turbojet itself could not yet take the researchers and 

pilots far enough into the realm to conduct the vital research data they sought. 
Probing the Sky

vi

The turbojet of the 1940s was a new and still immature technology. The 

temperatures in its combustion chambers (burner cans) and which the spinning turbine blades endured approached the limits of contemporary metallurgical science, making them unsuited then for supersonic flight. In order to 

explore the supersonic regime, researchers turned to rocket planes, relegating 

turbojets to slower speeds. Some of these turbojets became famous, such as 

the XS-1 (later designated simply the X-1, the first of the postwar “X-series” 

research airplanes). Others are little remembered today—but each played its 

own distinctive part in taking aviation from the subsonic to the supersonic era. 

Collectively, these first research aircraft are known as “Round One,” the transonic and supersonic probers of the 1940s and 1950s that preceded “Round 

Two,” the hypersonic North American X-15 of the 1960s.

This story is not simply one of strange-looking airplanes and their courageous test pilots. It offers, as well, insights into the history of aerospace technology and science and of shifting technology and (yes) paradigms within that 

field. The turbojet and the rocket changed the rules. Together, they led to the 

X-1 and the other research airplanes. These airplanes were also why a small 

group of engineers came to a vast dry lakebed in the Mojave Desert, establishing there, arguably, the world’s premier center for aeronautical research and 

development, the Air Force Test Center–NASA Armstrong Flight Research 

Center complex (and the associated airspace) at Edwards Air Force Base, CA. 

Creating the new era, with its own rules and tools, took place against the 

dynamic background of the Cold War, forcing and feeding the need for everhigher performance. But at the onset of that era, lacking reliable tools to determine what would work and what would not, engineers had few means to decide 

what paths they should follow. In a real sense, they faced the same situation 

as the Wright brothers, when they were building their first kites and gliders in 

1899–1902, with little reliable information, much of which was contradictory 

and misleading. They discovered that little of the available information was reliable. They discovered that they had to reject treasured assumptions and awake 

to new realities. They had to rethink their concepts of stability and control. 

For the test pilots, there was yet something more: since the potential of failure 

and death was very real, when they went aloft, their skill had to be matched 

by even greater courage. Knowledge, dedication, expertise, and courage: of the 

mix of such was the crafting of the transonic and supersonic revolution, made 

manifest in the skies over the Mojave over a half century ago.

viii

A semispan airplane model on the wing of a P-51 in January 1946. In an adaptation of the 

wing-flow technique, a half-scale model of an airplane was attached to the upper surface of 

a wing. As the airplane made a dive, airflow over the wing accelerated to high transonic or 

supersonic velocity. (NASA)
CHAPTER 1 

Confronting the “Sound Barrier”: 

The Bell XS-1

The resistance of a wing shoots up like a barrier as we approach the speed of sound.

—W.F. Hilton1

In 1935, when British aerodynamicist W.F. Hilton inadvertently coined the 

phrase “sound barrier,” the piston engine aircraft was nearing its performance 

peak, producing approximately one horsepower per pound of engine weight. 

But aeronautical engineers were already outrunning the knowledge they had 

acquired in the three decades since the Wright brothers’ first flight at Kitty 

Hawk, NC, in December 1903. Up to this time, designers had always treated 

the airflow over wings as if it were an “inviscid” or incompressible fluid. This 

simplified their calculations, and the errors induced by this convenient (if false) 

assumption were too small to be of significance. 

With aircraft now exceeding 400 miles per hour (mph), the compressibility 

of air could no longer be ignored. But engineers and scientists lacked the insight 

on how the required revolutionary changes could be accomplished. The wind 

tunnel, the aerodynamicist’s standard tool since before the Wright brothers, was 

of little help. As the airflow neared the speed of sound, shock waves formed on 

the models and supports and reflected onto the tunnel walls, rendering the data 

questionable from Mach 0.8, just below the speed of sound, to approximately 

Mach 1.2, just beyond the speed of sound. While tunnels could hint at some 

of the flow changes induced by high-speed flight—for example, a dramatic 

loss of lift and simultaneous sharp increase in drag—they could not furnish 

precise quantified data that would permit accurate analysis.

Thus, very suddenly, a new realm of flight loomed—mysterious, unknown, 

dangerous, and destructive. Pilots making high-speed dives found that control 

surfaces would not move or did so to little effect. In some cases, their airplanes 

broke up or dove uncontrollably into the ground. Others, more fortunate, 

found that the controls would respond normally once the airplanes reached 

lower altitudes. The phrase “sound barrier” encapsulated the mystique of this 

realm. Very quickly, a mythology developed regarding supersonic flight. Myths 

unsupported by scientific fact gained traction, some of which bordered on the 

1
Probing the Sky

2

bizarre: Some held that a pilot’s voice would become caught in the throat as the 

plane “broke” the sound barrier, while others alleged that time would reverse 

and the pilot would become younger after going supersonic.2

The Onset of Transonic Research

The first indications of these problems appeared in 1918. Aircraft of that time 

had top speeds of about 100 mph. However, the tips of their propellers were 

rotating at speeds as high as 300 to 650 mph. It was Frank W. Caldwell and 

Elisha N. Fales, of the Army Air Service Engineering Division, who made the 

first realistic measurements of propeller aerodynamics. Using a wind tunnel 

able to reach speeds of 450 mph, they discovered that “[w]hen the stress reaches 

a certain value, two adjoining layers of air begin to slide past each other and 

the character of the flow is changed.”3

They had observed flow separation, which occurs as airflow passing over an 

airfoil accelerates. As a consequence, the airfoil’s drag increases, reducing its 

efficiency. Because a propeller’s airfoil and that of a wing are similarly affected, 

the propeller tests gave the first indications of what would occur as aircraft 

reached the same speeds.4

Lyman J. Biggs and Dr. Hugh L. Dryden of the Bureau of Standards 

expanded upon this initial work over the next 

decade. Between 1924 and 1931, Biggs and 

Dryden undertook four separate studies. The 

first study used the same six airfoils Caldwell 

and Fales had tested, but at speeds of 375 mph 

to 682 mph, confirming their earlier results.5

Biggs and Dryden’s second set of tests also 

involved the six airfoils, but at speeds of 383 

mph (Mach 0.5) to 830 mph (Mach 1.08). This 

reconfirmed the earlier results, determined that 

flow separation was the cause of the increased 

drag, and expanded the speed range.6

Hugh L. Dryden undertook 

some of the first research into 

aerodynamics at transonic speeds 

during his research on propeller 

airfoils. The aerodynamic effects 

the propeller airfoils experienced 

gave indications as to what would 

occur as aircraft flew close to the 

speed of sound. (NASA)

This was followed by a third, much more 

expansive study of 24 propeller airfoil shapes, 

tested at speeds of Mach 0.5, 0.65, 0.8, 0.95, 

and 1.08. The study’s results showed a general 

rule: thin airfoils had lower drag at higher speeds, 

making them more effective. In contrast, thick 

airfoils showed greatly increased drag at higher 

speeds. Another general observation was that 
 Confronting the “Sound Barrier”: The Bell XS-1

3

when flow separation occurred, all the airfoils’ efficiencies were reduced “nearly 

to the same level irrespective of their efficiency when the flow is smooth.”7 As 

a result, “[t]he efficient sections therefore suffer most.”8 Because of the different Mach numbers that were used for each test, Biggs and Dryden were able 

to show that the rate of the increase in drag rose abruptly at speeds well below 

Mach 1, while the flow separation occurred above Mach 0.8 in most cases. 

Biggs and Dryden’s fourth and final research study was on the advantages 

of “circular arc” airfoils. One of the airfoils used in the third study was of this 

type. A circular-arc airfoil has a flat lower surface, while the upper surface is a 

segment of a circle. Despite its being a thick airfoil, Biggs and Dryden found 

that the circular-arc shape was more efficient than conventional airfoil shapes 

at high speeds. They wanted to see if this applied as a general rule. The tests 

involved eight circular-arc airfoils at Mach 0.5, 0.65, 0.8, 0.95, and 1.08. By 

using the same speeds as the earlier tests, they could make a systematic comparison. The results were surprising—one circular-arc airfoil was “extremely 

inefficient” at low speeds but had only 80 percent of the drag of a conventional 

airfoil at speeds between Mach 0.85 and 1.08. Biggs and Dryden realized that 

this result offered an elegant solution to the loss of propeller efficiency. They 

wrote, “These results indicate that it would be beneficial to use circular-arc sections for the outer part of a propeller blade intended for use at high tip speeds, 

retaining sections of the conventional type nearer the hub where the thickness 

ratio is large and the speed low. It seems not unreasonable to expect that a 

circular-arc section can be used profitably over the outer third of the blade.”9

Although these studies were conducted by the Army Air Corps and the 

Bureau of Standards, their results were all published by the National Advisory 

Committee for Aeronautics (NACA), which was established in 1915 “to supervise and direct the scientific study of the problems of flight with a view to their 

practical solution.”10 The NACA was created in response to the advances in 

aeronautical research and aircraft design being made in Europe. Over time, 

the NACA increasingly focused its efforts on finding “practical solutions” that 

could advance aeronautics, establishing a pattern of inquiry that constitutes 

one of the foundational underpinnings of the more recent NASA, which was 

created in 1958 to address the challenges of the emergent post-Sputnik space 

age. The propeller-airfoil research represented milestones in the understanding of compressibility effects at transonic and supersonic speeds. Although it 

was not done to make supersonic flight possible, but rather for the seemingly 

mundane goal of improving propeller efficiency, it nevertheless had profound 

significance for the future assault on the “sound barrier.”11

This focus on practical applications and incremental improvements would 

serve the NACA well in the 1920s and the Depression years of the 1930s. 

It reflected the evolutionary nature of aviation technology in this period. 
Probing the Sky

4

Wood-and-fabric biplanes were giving way to all-metal monoplanes, aircraft 

ranges and speeds were steadily increasing, and passenger comfort and safety 

were improving.12 The wooden Fokker trimotors of 1929 had, a decade later, 

been replaced by all-metal DC-3s. 

In all of these endeavors, the NACA played a significant role. Its work in airfoil 

development transformed aircraft design practice and established a global standard. While agency researchers did miss—as many others in America and elsewhere did as well—the potential significance of the gas turbine engine, the agency 

did support studies on reaction propulsion systems, and its many investigations 

of conventional aircraft propulsion and propulsion systems design resulted in 

much more efficient engine installations on aircraft, typified by nacelle and 

cowling location, and the nearly ubiquitous NACA cowling of Fred Weick. As 

well, the NACA’s detailed studies on flight structures, bolstered by its persistent 

analysis of foreign research, hastened the transformation of the airplane from 

a wooden, braced biplane to a metal, cantilever, and monocoque monoplane.

The Turbojet Revolution

Serious interest within the NACA in reaction propulsion began at the same 

time as the propeller-efficiency studies and likewise involved piston-engines, 

not pure turbojets. In both America and abroad, early concepts for a “jet” 

engine involved using a piston engine to power a reciprocating air compressor. 

The resulting high-pressure air was then fed into a combustion chamber, where 

fuel was added and ignited. The hot gas efflux exited via a convergent-divergent 

nozzle to produce thrust.13

Edgar Buckingham of the Bureau of Standards undertook a study of jet 

propulsion for the Army Air Service using this conceptual engine. (No actual 

hardware or subscale engine existed.) His conclusions were published in 1923 

by the NACA.14 Buckingham found that the hybrid piston engine/jet concept 

had numerous flaws, which included high fuel consumption, a much heavier 

weight than a piston engine, an increased number of moving parts, and the 

resulting lower reliability and higher maintenance requirements. Buckingham 

concluded, “There does not appear to be, at present, any prospects whatever 

that jet propulsion of the sort here considered will ever be of practical value, 

even for military purposes.”15 Buckingham’s conclusions were incorrect, both 

for this hybrid concept and, more significantly, for the higher-performance 

(and more technologically demanding) turbojet. It should be noted, however, 

that the Bureau of Standards and the NACA were far from alone in such judgments, which, generally, were accepted more broadly by the global engineering 

and scientific community as a whole.
 Confronting the “Sound Barrier”: The Bell XS-1

5

Subsequent NACA studies in the 1920s and early 1930s used the same 

flawed, conceptual engine and reached the same flawed conclusions. In part, 

this was due to the inefficient hybrid design, incorrect assumptions about 

the large size and weight of the compressor, and the research culture at the 

NACA. The Langley Memorial Aeronautical Laboratory, which was the only 

NACA research facility until 1941, had only a small engine research division, 

and its members were largely focused on the piston engine. Thus, it was three 

European countries—Nazi Germany, Great Britain, and Fascist Italy—and 

not the United States, that made the most significant jet-related technology 

breakthroughs of this era. (It should be noted, however, that the same complacency afflicting American aeropropulsion experts was likewise generally found 

overseas. Only after inventors outside the aeropropulsion mainstream—Hans 

von Ohain in Germany and Frank Whittle in Great Britain—demonstrated 

the value of the gas turbine was it effectively “seized” by the aeropropulsion 

community and subsequently made a normative element of aircraft design.)16

Neither von Ohain nor Whittle was aware of the other’s work until after the 

outbreak of the Second World War. Hans von Ohain, a young physicist, began 

his work on jet engines in 1933. Von Ohain’s engine was a more advanced axialflow turbojet engine design than Whittle’s concept, but, even so, he finished 

first, thanks in large measure to the support of Ernst Heinkel, a leading aircraft 

manufacturer in Germany. Von Ohain’s turbojet engine powered the He 178, 

which made the world’s first flight by a pure jet aircraft on August 24, 1939.17

Frank Whittle, von Ohain’s equivalent in Great Britain, began working on a 

turbojet engine design earlier, in 1928. He persevered in the face of official 

skepticism, metallurgical challenges, and a lack of serious financial support, 

and finally he successfully ground-tested a centrifugal-flow jet engine in 1937, 

following this with a flying example four years later, the Gloster E28/39, which 

flew on May 15, 1941, as the first British jet aircraft. At first glance, Whittle’s 

engine resembled a Moss or Rateau-type turbosupercharger in design, but it 

had very great differences, notably, of course, in its direct connection, via a 

single drive shaft, of the “hot” turbine and the “cold” compressor.18

Despite growing interest in gas turbine propulsion, the NACA remained 

focused on the original ducted fan design, assisted by downstream burning 

with the jet efflux passing through a convergent-divergent nozzle. In early 

1940, Albert E. Sherman undertook a second look at Buckingham’s report on 

jet propulsion. Sherman examined a jet’s feasibility in terms of flight at 500 

mph. His conclusion was that jet propulsion offered the possibility of high 

speeds without the heavy machinery Buckingham had thought necessary 17 

years previously. Sherman’s report was discussed at an April 11, 1940, meeting 

in the office of Henry Reid, the head of the Langley Aeronautical Laboratory. 
Probing the Sky

6

The senior staff in attendance accepted the report and concluded that Langley 

should undertake jet research. 

Dr. George W. Lewis, the NACA Director of Aeronautical Research, sent 

a letter to Langley on April 22, 1940, approving the start of work on a combustion test rig for what was now called the “Jeep” engine, but which was 

still, at heart, simply a refined version of the conceptual design Buckingham 

had examined in 1923. This was a piston-engine/combustion-chamber design, 

rather than a true turbojet. It was built using sheet iron and a salvage engine, 

but the low priority and demands of other work meant 2 years passed before 

Jeep test runs would begin. 

Europe was now engulfed by war, and, while the United States was still 

officially neutral, a close watch was kept on Germany’s aircraft developments.19

General H.H. “Hap” Arnold, chief of the U.S. Army Air Corps (shortly to be 

reorganized as the U.S. Army Air Forces [AAF]), sent a letter on February 25, 

1941, to Dr. Vannevar Bush, the chairman of both the NACA and the National 

Defense Research Committee. Arnold warned that the Germans had made 

considerable progress on rocket propulsion, which threatened to make existing fighter aircraft obsolete. He continued, “Further investigation by a large 

group of able scientists is immediately needed.”20 Bush formally established the 

NACA Special Committee on Jet Propulsion, with retired professor William F. 

Durand as chairman. Durand had been a member of the NACA since it was 

organized and was still active at the age of 82. Bush told him in a March 18, 

1941, letter about the Jeep engine and commented, “It seems to have great 

possibilities and I cannot find any flaw in their arguments.”21

Eastman N. Jacobs, an engineer in Langley’s airflow research branch who 

had been studying reaction propulsion and thrust-augmented systems since the 

late 1920s, wanted to do more than ground-test the Jeep engine. He wanted 

to use the Jeep to power a specially designed high-speed research aircraft. His 

concept, which emerged from the drawing tables in July 1942, had a cylindrical 

fuselage, a nose-inlet, a shoulder-mounted straight wing, and a V-tail, and it 

sat on tricycle landing gear. The pilot’s cockpit was just behind the inlet. The 

research plane used a wing with a 15-percent thickness-chord ratio (i.e., the 

wing’s depth from top to bottom was 15 percent of the distance from the wing’s 

leading edge to its trailing edge) and an NACA high-speed airfoil. The power 

came from an 825-horsepower Pratt & Whitney R-1535 radial piston engine, 

to compress the incoming air, and the downstream fuel-injection and burning 

Jeep propulsion system. The drag due to compressibility effects above 550 mph 

made the airplane’s top speed uncertain. However, the team felt confident that 

the airplane could reach transonic speeds. 

Ironically, the driving force behind the proposal was not propulsion 

experimentation itself, but the loss of aircraft in high-speed dives. Lockheed 
 Confronting the “Sound Barrier”: The Bell XS-1

7

P-38 Lightnings, Republic P-47 

Thunderbolts, and Bell P-39 

Airacobras had all suffered fatal tail 

failures. The Navy’s Curtiss SB2C 

Helldiver had similar problems 

(and some foreign aircraft did as 

well). Tests at both Langley and 

the new Ames Research Center 

(the second NACA center, created in 1941 to support the West 

Coast aircraft industry) showed 

that the P-47 and P-39 needed to 

have their tail structures strengthened to withstand the higher-thanpredicted air loads at high speeds. 

With the SB2C, compressibility at 

high speeds was distorting the wing 

surface, causing flow separation 

and buffeting turbulence, which in 

turn caused the horizontal stabilizers to flutter, exceeding their structural strength and breaking them 

off. To cure the problem, the wing 

structure was stiffened, preventing 

the distortions. Additionally, the 

tail had to be redesigned to withstand the 13-g loads experienced 

in dives.22

This design, proposed by Eastman Jacobs 

in 1941, constituted the first NACA research 

aircraft. The airplane was powered by a Jeep 

engine, which combined a piston engine to 

compress the air and a combustion chamber 

to produce thrust. The Jeep engine had already 

been made obsolete by British and German 

turbojet engines. (NASA)

Despite the need, Jacobs’s proposed research aircraft and the Jeep engine 

would never be built. In April 1941, Hap Arnold had visited Britain and been 

informed of British work with the Whittle engine. He immediately arranged 

for the importation of Whittle engine technology to the United States. Out 

of this came the first American “black” aircraft program: to develop a highly 

secret jet engine test bed. Bell Aircraft Corporation was awarded a contract to 

develop the plane, and General Electric another contract to develop its engines. 

The plane itself was assigned an abandoned propeller-driven fighter designation 

(P-59) used for another program to mask its own development. 

On October 1, 1942, at Muroc Army Air Field, California (now Edwards 

Air Force Base), the Bell XP-59A jet made its first flight. The aircraft was powered by a pair of General Electric I-A Whittle turbojet engines. Durand was on 

hand for the airplane’s first flight at Muroc and had known about the airplane 
Probing the Sky

8

from its beginning. Due to the secrecy of the project, however, he could not 

discuss it with the other committee members. With an advanced turbojet 

already in production, the research aircraft and the Jeep engine project were 

obsolete. The Jeep was formally canceled in March 1943, marking the end of 

formal American interest in ducted fan approaches. From now on, America 

would be firmly fixed on the turbojet—and the rocket.

During the summer of 1943, Jacobs went to England and visited the Royal 

Aircraft Establishment, where he learned about the state of British turbojet 

engine technology. When Jacobs returned to Langley, he drafted a letter strongly 

protesting the military services’ refusal to fund a research aircraft. In the letter, 

he stated that the United States was making the same mistake that the British 

had, namely, applying a revolutionary engine to a conventional airframe. But it 

was his final comment that carried the most weight. In his warning, Jacobs demonstrated uncharacteristic perception for someone from an agency increasingly 

being seen as having failed to meet its primary mission of advancing American 

aviation technology. “The development of the jet power units themselves,” he 

warned any and all, “had progressed beyond the development of suitable airplanes to employ them.”23

Jacobs realized that the unknowns of compressibility were no longer the 

single issue facing designers. The revolutionary turbojet engine had to be 

matched with an equally revolutionary airframe design. He argued that a unified effort involving the military services, aircraft manufacturers, and the NACA 

was needed to solve the multiple unknowns of high-speed flight. This effort 

would be done “with a view to producing quickly extreme-performance aircraft 

of several types to be developed around existing units and suitable to exploit to 

the full the capabilities of those existing jet-propulsion power plants.”24

Two Roads Toward the Transonic

The Jacobs research aircraft was not the only such proposal being discussed. 

John Stack, the head of the Compressibility Research Division, met with Lewis 

in the spring of 1942 to discuss the development of such an aircraft. Lewis 

did not like the idea of a research aircraft, but he did respect Stack’s ability, 

self-assurance, ambitions, and skills as a researcher. Lewis did not approve 

development work on a research airplane due to wartime pressures on the 

NACA. Stack came away from the meeting with a sense that Lewis did not 

object to a low-priority effort to identify the design features that a transonic 

research aircraft should have. 

As Jacobs had done, Stack also assembled a team to define the research aircraft. The members included engineer Milton Davidson and junior engineering 
 Confronting the “Sound Barrier”: The Bell XS-1

9

aide Harold Turner, Jr. (Turner had 

worked on Jacobs’s aircraft design.) 

Stack’s design had a top speed of 

Mach 0.9, as he was not interested 

in actual supersonic flight. The limited performance of early turbojet 

engines made a successful Mach 

0.9 flight marginal in any event. By 

early summer of 1944, the team had 

completed a preliminary design of 

a small turbojet-powered research 

aircraft capable of Mach 0.8 to 

Mach 1 speeds.25

John Stack, the NACA researcher who masterminded the design and construction of the D-558 

series. He was determined to get a jet-powered/

transonic research airplane and was opposed 

to the rocket-powered/air-launched/supersonic 

XS-1. In this 1946 photo, a D-558-1 wind tunnel 

model is in the background and Stack is holding 

a swept-wing D-558 model. This eventually led 

to the D-558-2 Skyrocket. (NASA)

Stack informed selected friends 

within the AAF and Navy aeronautical communities of the study; the 

only military officer who expressed 

strong support for his research aircraft was Captain Walter Stuart 

Diehl of the U.S. Navy, a noted 

aeronautical engineer, author of an influential aerodynamics textbook, expert 

on seaplane design, and senior naval representative to the NACA. During 

late 1943, Diehl met several times with the chief of the Navy’s Bureau of 

Aeronautics’ structural branch. He argued that a research airplane was the only 

means to convince people that the “sound barrier” was “only a steep hill.”26

Support for a research airplane also came from an industrial source. On 

December 18, 1943, Durand held a special meeting at the NACA Headquarters 

on the subject of jet propulsion. At the meeting, he asked a basic question: 

What should the United States do with the turbojet propulsion technology 

being developed?

Among those in attendance was Robert W. Wolf of Bell Aircraft, who was 

the designer of the XP-59A’s airframe, cockpit, and propulsion system. Wolf 

had also traveled to Britain in 1943 to review its progress in turbojet technology. He knew more powerful engines were under development that would 

allow aircraft to reach transonic speed in level flight. 

During the discussion, Wolf suggested that a high-speed, jet-powered 

research aircraft be designed. These data were urgently needed, as new jets 

would soon be facing the same structural and control problems in level flight 

that had befallen the P-38, P-39, P-47, and SB2C during dive tests. After the 

meeting concluded, Wolf put his ideas into a letter sent to Lewis on December 

29, 1943. He wrote:
Probing the Sky

10

It appears quite possible to construct a single engine aircraft based 

on available gas turbine jet power plants which will fly at speeds in 

level flight exceeding the critical Mach numbers of currently used 

types of wings. If this aircraft were designed with enough inherent 

versatility to changes in control surfaces, wings, etc., it should be 

possible to develop usable control surfaces such as ailerons, dive 

control flap, tail surfaces, etc., which would work satisfactorily at 

or above the critical speeds of the wings. Furthermore, this could 

be done in level flight and would not be subject to the dangers 

and difficulties associated with the high accelerations encountered 

in current dive programs.27

Lewis’s response was more positive than that he had given Stack in 1942. He 

wrote Wolf that the NACA was highly interested in learning more regarding 

compressibility and aircraft stability and control at transonic speeds by means 

of a turbojet-powered research aircraft. Lewis continued that the NACA was 

giving this project “our very serious concern.”28

A 1944 photo of John Stack when he 

was the head of Langley’s Compressibility 

Research Division. With wind tunnels incapable of providing reliable transonic-speed 

data, attention began to focus on building 

a research aircraft to provide the needed 

data. This resulted in a conflict between 

the NACA and the Army Air Forces, as 

well as within the NACA over the aircraft’s 

design—subsonic and jet powered versus 

supersonic and rocket powered. (NASA)

The AAF had some intelligence 

reports on Germany’s rocket and jetpowered aircraft, though these often 

offered sketchy and in some cases contradictory information. It was also clearly 

interested in transonic flight research. 

In mid-January 1944, the Development 

Engineering Branch of the Materiel 

Division at AAF Headquarters authorized a study of a research airplane for 

investigating aerodynamic phenomena 

between 600 and 650 mph. At the same 

time, Ezra Kotcher requested that the 

Design Branch of the Aircraft Laboratory 

at Wright Field conduct a comparative 

study of two different research airplane 

concepts. The first would be powered 

by a General Electric TG-180 axial flow 

turbojet engine with a thrust of 4,000 

pounds. The other aircraft design would 

use a proposed 6,000-pound-thrust 

Aerojet rocket engine.29

While this study was underway, two 

joint military-NACA conferences took 
 Confronting the “Sound Barrier”: The Bell XS-1

11

place at Langley on March 15 and 16, 1944, to deal with the issue of a 

transonic research aircraft. One meeting was chaired by Navy Captain Diehl 

and the other by Colonel Carl Greene of the Materiel Command. Greene 

was Diehl’s AAF equivalent, functioning as the permanent AAF liaison officer 

at Langley. While the NACA leadership had now accepted the need for a 

specialized research aircraft, the military services—with numerous responsibilities for waging a global war against ruthless foes—were still divided over 

the need, design, and objectives: was, for example, the aircraft to be purely 

experimental? Or might it have operational features or even be the basis of an 

operational design?30

The AAF’s Design Branch completed its studies for the supersonic research 

airplanes in April 1944. Kotcher’s design was dubbed the “Mach 0.999 study,” 

a reference to the popular belief in an impenetrable sound barrier. A rocketpowered aircraft was seen as offering the best approach to collect supersonic 

flight data. The thrust of the rocket engine meant higher speeds could be 

reached at higher altitudes, eliminating the need to make dive tests, as well as 

the associated risks for both pilot and plane. 

This question of using a turbojet versus a rocket motor for the research 

airplane sharply divided research airplane proponents in the AAF and the 

NACA. Kotcher had presented the preliminary results of the Mach 0.999 study 

at a May 15–16 meeting with the NACA. Stack was critical of the design in 

large part because of the rocket engine. NACA researchers believed this form 

of propulsion would be unsafe. If the rocket failed during the takeoff from a 

runway, they argued, the heavily loaded airplane would crash and explode. 

A rocket-powered aircraft, Stack believed, could not meet the research 

requirements. Its flight time would be brief compared to that of a turbojetpowered aircraft (he did not envision the air-launching that would make them 

highly productive). This meant a rocket-powered aircraft would not be able 

to collect the volume and quality of data the researchers needed. While the 

AAF vehicle was designed to fly above Mach 1, the NACA’s research airplane 

was limited to flying at transonic speed. Finally, the NACA objected that the 

performance of a rocket-powered vehicle would not be representative of an 

operational aircraft’s. In contrast, the experience of flying a turbojet research 

aircraft would be applicable. Kotcher countered that the XP-59A had shown 

that existing turbojet engines lacked the thrust needed to fly at transonic speeds, 

while the Mach 0.999 study showed that rockets could reach such speeds. 

The AAF gave its reply in a final round of meetings at Langley on December 

13–14, 1944. Not surprisingly, the AAF rejected the NACA jet-powered aircraft proposal, as the subsonic concept was too conservative. The AAF wanted 

the airplane to reach Mach 1.2 (approximately 800 mph, depending on the 

altitude), and, as NACA researcher John V. Becker later observed, “The Army 
Probing the Sky

12

was putting up the money and they decided to do it their way.”31 In late 

December 1944, 1 week after the meeting, the AAF began negotiations with 

Bell Aircraft Corporation to build the XS-1 (Experimental Sonic-1) rocketpowered research aircraft, the result of a fortuitous chance meeting at Wright 

Field between Kotcher and Bell chief engineer Robert J. Woods.32

Stack was committed to acquiring a turbojet-powered research aircraft, for if 

it did not have the fullest performance of the rocket-powered one, it would nevertheless have greater endurance and might well, in his view, be safer and more 

reliable as well. Thus, after Kotcher (whom Stack highly respected) made it clear 

that the AAF was determined to build a rocket-powered research aircraft based 

on his Mach 0.999 study, Stack turned to the Navy’s Bureau of Aeronautics. 

His message was dire: he told Navy personnel that the AAF’s rocket-powered 

aircraft was unlikely to survive enough flights to provide significant data. His 

efforts had the desired results. 

Abraham Hyatt, a Marine Corps officer and aeronautical engineer, proposed 

a research aircraft design in September 1944 that matched the NACA concept. 

It would have a top speed of 650 mph at sea level, use a turbojet engine, have 

thin 10-percent thickness-chord wings, take off and land under its own power, 

have room for flight instrumentation, and have excellent low-speed handling. 

These requirements were not surprising, as the Bureau of Aeronautics stated 

that the Navy “was only interested in obtaining an airplane which met with 

the full approval of the NACA.”33

In late December 1944, Diehl, Hyatt, and Captain William “Bill” Sweeney, 

of the Bureau of Aeronautics, showed L. Eugene “Gene” Root, of the Douglas 

Aircraft Corporation, the preliminary specifications for the jet-powered 

research aircraft. Douglas was a major contractor for Navy aircraft, and Gene 

Root took the Navy specification to Douglas’s El Segundo plant and met with 

its chief engineer, Edward Henry “Ed” Heinemann. Heinemann formed a 

small design team and, by February 1945, had completed a preliminary design 

for the Douglas Model 558 High-Speed Test Aircraft.

This aircraft would be the first of a three-part effort. The D-558 phase 1 

would have a top speed of about Mach 0.89. In the phase 2 effort, two of the 

aircraft would be modified to have both a Westinghouse 24C turbojet engine 

and a Reaction Motors Incorporated rocket. This would allow them to fly at 

speeds between Mach 0.89 and about Mach 1 in level flight. The data from 

the phase 1 and 2 aircraft would be used by Douglas to produce engineering 

drawings and a mockup of a phase 3 operational Navy combat aircraft. 

The Bureau of Aeronautics approved the new design of the D-558 phase 1 and 

2 aircraft. To differentiate the two aircraft designs, the phase 1 aircraft were given 

the Douglas model number D-558-1, while the phase 2 aircraft were called the 
 Confronting the “Sound Barrier”: The Bell XS-1

13

D-558-2. The Bureau of Aeronautics also outlined a development program that 

gave the NACA a major role in the management of the flight research program. 

The scale of what is involved in undertaking research flights is made clear in this iconic image 

of the D-558-2, the B-29 launch plane, and the ground support personnel and equipment. The 

photo was taken in front of the NACA hangar at South Base shortly before the move to the present facilities. (NASA)

The NACA, the Navy, and Douglas would each have access to one of the 

D-558-1 aircraft. The NACA research pilots would use their aircraft to collect data on air loads, flutter, engine performance, and stability and control 

at transonic speeds. Douglas test pilots would fly the company’s D-558-1 for 

information useful for the D-558-2 operational aircraft. In early March 1945, 

the NACA sent a letter to the Navy giving its observations: “The Committee 

is certain that the procurement of these two models of high-speed research 

airplanes will permit making of a large advance in aerodynamic knowledge 

in the transonic region of flight and every attempt should be made to make 

these aircraft available to the NACA for flight research as soon as possible.” 

After several inspections of the D-558-1 mockup and revisions of the design to 
Probing the Sky

14

correct what NACA representatives saw as shortcomings, the D-558-1 design 

was approved.34

While Navy-NACA-Douglas relations were smooth, the same was not 

completely true of the AAF–NACA–Bell XS-1 effort. The initial disagreement was within the Langley staff and concerned the best wing to use on the 

aircraft. Both groups knew that a thin wing was preferable to a thick wing for 

high-speed flight. The reason was that thick wings have a lower “critical Mach 

number,” the velocity at which a shock wave forms, leading to an immediate 

large increase in drag, causing the so-called “shock-stall.” 

Stack and fellow wind tunnel researchers wanted the XS-1 to have a wing 

with a 12-percent thickness-chord-ratio wing. The group took this position 

because the thicker 12-percent wing would encounter flow changes at speeds 

of Mach 0.75 to 0.9. The aerodynamicists wanted data in this speed range to 

correlate with their wind tunnel data. As a result, they recommended that Bell 

fit this thicker wing to the XS-1 to deliberately cause it to fly within the risky 

shock-stall flight regime. 

The opposite viewpoint was expressed by Robert R. Gilruth, a researcher 

at Langley, who argued for a very thin wing, with a thickness as low as 5 percent. He believed the XS-1 and its pilot 

would need every advantage they could 

get to survive the unknowns of transonic 

flight. If a thin wing was used, the airplane would be much safer to fly. 

Robert R. Gilruth invented the wing-flow 

technique to collect transonic data from 

an airplane in a dive, and he supported 

the rocket-powered XS-1. Stack effectively designed the jet-powered D-558-1. 

Their activities following the establishment 

of NASA reflected their outlooks. Gilruth 

worked on piloted space flight activities, 

and Stack retired when the NACA ceased 

to exist. (NASA)

The thick/thin wing debate had a 

larger importance. If the 12-percentthickness wing was selected, the XS-1 

would only be able to reach Mach 0.9; 

it could not go supersonic. In contrast, 

Gilruth was arguing that to increase 

safety, a very thin wing should be used to 

lessen the inherent unknowns and dangers of transonic flight. Ironically, this 

safe approach also meant that the XS-1 

could exceed Mach 1. 

Floyd Thompson reviewed the arguments and evidence and decided that 

Gilruth’s thin-wing concept was preferable. Stack responded by suggesting that 

the XS-1 be fitted with two different sets 

of wings. The first would be a 10-percentthickness wing, less than what Stack had 
 Confronting the “Sound Barrier”: The Bell XS-1

15

wanted but more than what Gilruth recommended. The second wing would 

have an 8-percent thickness-chord ratio. This was slightly thicker than Gilruth 

had argued for, but much less than what Stack wanted. The logic was clear to 

Thompson and other Langley managers, and, in March 1945, they decided to 

have Bell build the two sets of wings.35

A far more serious set of objections arose as Bell Aircraft began construction of the XS-1. The original design had cylindrical internal fuel tanks for 

the liquid oxygen and alcohol/water fuel positioned fore and aft of the wing 

carry-through structure. A turbopump fed the oxidizer and fuel into the rocket 

engine. Difficulties in building this turbopump led to the cylindrical tanks’ 

being replaced by two spherical tanks. High-pressure nitrogen gas would now 

feed the fuel into the engine. The nitrogen gas was carried in multiple spherical 

tanks. To withstand the pressure of the nitrogen gas, both the storage tanks and 

the oxygen and alcohol/water fuel tanks had to be made of thick metal. The 

result was that the XS-1’s landing weight was increased by a ton and the fuel 

supply dropped due to the lesser volumetric efficiency of spherical tanks. As 

a result, a ground takeoff would reduce the XS-1’s maximum speed to below 

Mach 1.

Larry Bell had decided at an early stage that the XS-1 should be air-launched 

from a four-engine mother ship such as a Douglas C-54 Skymaster transport 

or Boeing B-29 Superfortress. Air-launching avoided the risk of the heavily 

loaded XS-1’s suffering an engine failure on takeoff, undoubtedly resulting 

in the loss of both plane and pilot. With the war still underway, however, no 

aircraft were available. Langley researchers, in contrast, expected the airplane to 

take off from a runway, fly the speed run, and make a normal runway landing. 

Due to the lack of a mother ship, the launch issue was still open. 

With the redesign of the fuel system, the only option to reach the AAF’s 

planned Mach 1.2 top speed was an air launch from a mother ship. This would 

carry the XS-1 to an altitude of 30,000 feet, at which point the XS-1 would be 

released. Although an air-launch profile had earlier been considered as a safety 

measure, it was now mandatory to meet the AAF’s speed goals. An additional 

problem was that the smaller fuel supply meant that the rocket’s burn time had 

dropped from 5.4 minutes to around 2.6 minutes. These changes reopened the 

disagreements between the NACA and the AAF.36

Upon hearing of the changes, Stack sent a memo to Langley’s chief of 

research. The XS-1 “may prove to be unsuitable.”37 Stack listed five basic 

requirements the research airplane had to meet before he would be satisfied:

• Speed greatly in excess of the critical Mach number.

• Duration at full power for complete observations in level flight at 

steady conditions.

• Takeoff, flight, and landing with self-contained power units.
Probing the Sky

16

• Flexibility to permit changes of all principal components such as 

wings, tail surfaces, and canopies.

• Space for adequate instrumentation.38

Stack then noted that the XS-1 fell short of these basic requirements. In 

particular, the aircraft lacked the needed duration at full power as well as the 

ability to take off, fly, and land with self-contained engines. It also probably 

could not now meet the requirement for adequate instrumentation. 

Because the XS-1 did not meet the required duration at full power, Stack 

continued, it also now failed to meet the requirement for speed in excess of 

the critical Mach number. The fuel supply was so limited that the XS-1 would 

burn its entire fuel supply to reach 35,000 feet and then be unable to make 

the level speed run. While speed runs could be made at lower altitudes, their 

research value was minimal, as it was only at altitudes of around 35,000 feet 

that speeds in excess of the critical Mach number could be reached. In closing, 

he urged that a major effort be made to develop the turbopump needed for the 

original fuel system design. 

Another factor that disturbed engineers at Langley, perhaps Stack most of 

all, was control of the program. The AAF began looking for possible test sites 

other than Langley. Langley was a busy airport located in a heavily populated 

area, and any emergency ran the risk of killing and injuring people on the 

ground. A possible move meant the Langley researchers would not have the 

direct control over the flight research program that they wanted and were 

accustomed to.39

The initial test sites considered for the XS-1 other than Langley were Muroc 

Army Air Field in California and Wendover Field in Utah. Both were in isolated areas ideally suited for safety and security. The move to a remote site 

prompted objections from both NACA Headquarters and Langley personnel. 

Stack dashed off an angry memo to John Crowley, chief of research at Langley, 

on June 14, 1945, which said, “This airplane originated here as did the P-80 

program. If we are to do research of this kind we must have the airplane here. I 

do not believe we should again be treated as a service as was in the case with the 

P-80. If the shifting of this aircraft to a western station materializes I propose 

that we transfer all work beginning right now so we can free our people to do 

research with our present equipment.”40

By late summer of 1945, the issue of the test site was still unresolved. The 

candidate sites being checked by Bell test pilot Jack Woolams included Muroc 

Army Air Field; Wendover Field, UT; Salina Field, KS; Daytona Beach, FL; 

Marietta Field, OH; and several airfields in Texas. Stan Smith of Bell thought 

Muroc, deep in the Mojave desert, 100 miles from Los Angeles and over 2,100 

miles from Buffalo (Bell Aircraft headquarters), was too far away and recommended Daytona Beach.41
 Confronting the “Sound Barrier”: The Bell XS-1

17

Why Not Swept Wings?

By this time, a much larger issue had arisen that damaged the NACA’s reputation even more than its earlier neglect of jet-engine technology or any disagreements with the AAF over rocket planes and tests sites. Ironically, it also 

represented a major breakthrough in the design of supersonic aircraft. It was 

the result of theoretical calculations by Robert T. Jones at Langley.

Jones’s accomplishment began with a study of a dart-shaped missile in 

late 1944. The missile’s slim delta shape required a new approach to calculate 

its lift. Jones saw that he could apply the mathematical studies of the airflow 

around airships that Max Munk had made in 1924 to analyze the flow around 

the missile, buttressed by studies by Hsue-Shen Tsien, a von Kármán associate 

from the Guggenheim Aeronautical Laboratory at the California Institute of 

Technology, for supersonic flow around projectiles and slender bodies. 

Jones began studying the mathematics of “potential flows” at supersonic 

speeds in early 1945. This term refers to fluid motion where there is no 

rotation of the fluid element. Jones realized he was deriving the same equations as he had using his earlier lifting theory. These new equations now also 

included compressible flow. Jones recalled that Tsien had said that some 

slender projectiles exhibited no effects of compressibility when rotated. Jones 

took his earlier calculations out of the desk drawer and incorporated the 

compressible flow equations. Jones found that very slender wings lacked 

compressibility effects.

Trying to find a physical explanation, Jones undertook a series of elaborate 

calculations and eventually found that this lack of compressibility effects was 

the result of sweepback on the lift generated by large-span wings. Again, it was 

a paper by Munk that gave Jones an understanding of the physical process. 

Munk’s paper had dealt with the effects on aircraft stability due to wing dihedral and sweepback at low subsonic speeds. Munk noted that for an airplane 

in level flight, only the component of velocity at a “normal angle” (i.e., at a 

perpendicular angle) to the wing’s leading edge affected the production of lift. 

Although Munk’s paper only applied to subsonic aerodynamics, Jones believed 

it could apply to slender wings in supersonic flows. 

When Jones finished his calculations, he had a unified theory that encompassed swept wings with angles ranging from 0 degrees to 90 degrees and 

covered all possible wing configurations rather than solely slender wings. 

By late April of 1945, Jones had completed a report on his calculations. 

Jones noted, “The analysis indicates that for aerodynamic efficiency, wings 

designed for supersonic speed should be swept back at an angle greater than 

the Mach angle and the angle of sweepback should be such that the component of velocity normal to the leading edge is less than the critical speed of 
Probing the Sky

18

the airfoil sections. This principle may also be applied to wings designed for 

subsonic speeds near the speed of sound, for which the induced velocities 

resulting from the thickness might otherwise be sufficiently great to cause 

shock waves.”42

The physical explanation of these results was complex, but the results were 

astonishing. With the Mach line ahead of the wing, the “streamlines,” a smooth, 

nonturbulent flow over the upper surface of a wing following a set pattern, 

would curve and follow paths consistent with a flow at subsonic speeds, even 

though the velocity was actually supersonic. 

This meant the effective Mach number was reduced compared to the actual

Mach number of the airplane. Jones had suspected that this would occur, but 

he was surprised by the size of the reductions his calculations indicated, which 

showed that the effective Mach number would be three to five times smaller 

than that on a straight wing. This limited the amount of wave drag, which was 

due to changes in velocity through a shock wave. Also reduced were the effects 

of compressibility shock, which involved significant changes in the patterns of 

airflow pressures, densities, and temperatures. 

The advantages of swept wings were not limited to supersonic flight, Jones 

found. He noted the sudden increase in drag when the airflow “may be avoided 

by increasing the angle of sweepback so that the normal component of the 

velocity not only is subsonic but is also less than the critical speed of the airfoil 

sections. This principle may also be applied to wings designed for subsonic 

speeds near the speed of sound.”43

Jones wrote a memo to Crowley telling him he had “made a theoretical 

analysis which indicated that a V-shaped wing would be less affected by compression than other planforms. In fact, if the angle of the V was kept small 

relative to the Mach angle, the lift and center of pressure remain the same at 

speeds both above and below the speed of sound.”44

The next step was to test the theory in actual flight. Jones concluded the 

memo by asking Crowley to approve wing-flow and drop-body testing of 

different wing shapes “designed to minimize compressibility effects.”45 These 

alternative techniques had been developed in response to the inaccurate data 

from wind tunnels at transonic speeds. Gilruth had developed the wing-flow 

technique. A model connected a balance mechanism located in the starboard 

gun bay of a P-51 Mustang protruded above the upper surface of the wing, and 

an NACA pilot then dove the plane to over Mach 0.80. While the aircraft itself 

was still subsonic, the airflow over the model was supersonic, to as high as about 

Mach 1.4. While this produced useful results, the short duration of the test 

methodology—and the obvious dangers attending diving into the dense lower 

atmosphere at compressibility-inducing Mach numbers—limited its appeal. 
 Confronting the “Sound Barrier”: The Bell XS-1

19

A semispan airplane model on the wing of a P-51 in January 1946. In an adaptation of the 

wing-flow technique, a half-scale model of an airplane was attached to the upper surface of a 

wing. As the airplane made a dive, airflow over the wing accelerated to high transonic or supersonic velocity. (NASA)

A second approach was drop-body tests. These were streamlined shapes with 

either rectangular or swept wings, dropped from 40,000 feet by a B-29. During 

their fall, the shapes reached Mach 0.9 to Mach 1.27. The forces acting on the 

wings were measured using onboard instruments, and the measurements were 

transmitted to ground stations. Their trajectories were followed by radar and 

optical tracking. Crowley gave his approval, and Gilruth’s flight research section 

began testing to verify or refute the advantages of swept wings. 

While these tests were still underway, Jones’s draft paper met with strong 

opposition from Dr. Theodore Theodorsen, head of the Physical Research 

Division, a leading theoretical physicist, and chairman of the Langley publication committee. He had had a contentious dispute with Eastman Jacobs in the 

1930s over the use of wind tunnels versus theoretical calculations. Theodorsen 

was similarly critical of Jones’s results. 

Theodorsen did not agree with Jones’s arguments or his conclusions, calling them “hocus-pocus” and demanding that they be clarified with some “real 

mathematics.” Worse, he dismissed Jones’s results as “a snare and a delusion.” At 

the end of the publication committee meeting, Theodorsen demanded that the 

material on Jones’s swept-wing theory be removed from the paper. Considering 

the weight he carried because of his stature and term of service, Langley management agreed with Theodorsen’s objections and decided publishing the paper 

without experimental proof could not be justified.46
Probing the Sky

20

The debate was settled by the end of 

May 1945, and Jones’s reputation was 

redeemed with the completion of the 

wing flow and drop-body tests. They 

showed that Jones’s predictions were 

valid. The data showed a reduction of 

wing drag by a factor of almost four 

using a swept-wing configuration. The 

results were also confirmed by Macon 

C. Ellis and Clinton Brown, using a 

section of wire at a large angle of sweep 

in Langley’s supersonic wind tunnel.47

A 35° swept-wing model mounted on the wing 

of a P-51 Mustang. The “wing flow” technique 

provided data on airflow at transonic speeds 

that were otherwise unobtainable through wind 

tunnel tests. (NASA)

Langley’s engineer-in-charge, 

Henry Reid, sent Jones’s completed 

paper to NACA Headquarters in early 

June 1945. Reid noted that Theodorsen still did not agree with Jones’s arguments and conclusions and had refused to participate in the paper’s editing. The 

paper, titled “Wing Plan Forms For High-Speed Flight,” was issued on June 

21, 1945, as a Confidential Memorandum Report. A second edition was issued 

3 weeks later, as an Advanced Confidential Report, with a wider distribution 

to both the military services and specific personnel in the aviation industry.48

At this same time, Allied intelligence teams were searching for documents 

and other data on advanced technology developed by Germany. They soon 

discovered that extensive work on high-speed airfoils and sweptback wings 

had been done. In contrast, the NACA had done virtually no research in these 

fields during the 1930s and 1940s. The failure to anticipate the advantages of 

swept wings was seen by the military and aircraft contractors as still another 

NACA failure to keep pace with European aviation advances, and it caused 

an angry response. 

Brigadier General Alden R. Crawford, chief of the AAF’s Production 

Division, asked NACA Chairman Jerome Hunsaker in October 1945 why 

no mention of Jones’s swept-wing theory had been made during earlier XS-1 

design reviews. Had the AAF known about this, General Crawford claimed, the 

XS-1 design could have been changed to incorporate swept wings. The general 

was incorrect—the AAF had been told about Jones’s paper before the first XS-1 

mockup review. The AAF had specified straight wings for the XS-1 because its 

planned jets used straight wings. For its part, the NACA had rejected using a 

swept planform because, in the minds of Langley researchers (as they informed 

the AAF), logically, it made little sense to further burden the research airplane 

with yet another unproven design concept. Even so, the NACA’s reputation 
 Confronting the “Sound Barrier”: The Bell XS-1

21

was again damaged, and, for the record, Jones himself had hoped that the 

XS-1 might incorporate a swept-wing planform.49

Flight Tests and Unresolved Issues

As 1945 drew to a close, the XS-1 project neared the flight-test phase. Colonel 

George F. Smith, chief of the Engineering Division’s Experimental Aircraft 

Project Division at Wright Field, who oversaw the XS-1 project, sent a letter 

to Lewis on November 23 to outline the AAF’s position on several issues. The 

initial Bell glide flights would be made at Pinecastle Army Air Field in Florida. 

This base had a 10,000-foot runway; was being equipped with an SCR-584 

radar and optical tracking equipment; had predictable weather; and was nearer 

to the Bell, AAF, and NACA facilities than Muroc. Pinecastle’s remote location 

also provided security. Colonel Smith’s letter also made clear that the inability 

to develop a turbopump would not be allowed to delay the project and that 

the XS-1 would be flown using the high-pressure fuel system. Not surprisingly, 

Stack had very different ideas about the NACA’s role and was not willing to 

change them.50

Senior officials with the XS-1 research program 

included, from left, Joseph Vensel (head of 

operations), Gerald Truszynski (head of instrumentation), Capt. Charles Yeager (XS-1 pilot), 

Walter C. Williams (head of NACA Muroc Flight 

Test Unit), Maj. Jackie Ridley (XS-1 project engineer), and De E. Beeler (chief engineer). (NASA)

Stack wrote a three-page memo on December 28, 1945, to research director Crowley regarding the NACA’s views on XS-1 flight research program 

management. Stack wrote that the Pinecastle flights were “to determine the 

feasibility of operating from Langley Field,” a notion he and other NACA 

engineers still harbored. Stack 

added that after the Bell contractor 

flights were completed, Langley 

would take over pilot responsibilities for the tests. His earlier 

objections to the XS-1 design still 

remained. He wrote that the NACA 

should not take possession of the 

XS-1 project until the aircraft was 

flown with the turbopump system 

and fuel for a powered landing 

and was capable of a ground takeoff. Finally, he emphasized in the 

memo to Crowley that the NACA 

had to insist that these conditions 

be met.51

On January 8, 1946, NACA 

Headquarters sent a reply to Colonel 
Probing the Sky

22

Smith’s letter. Although the NACA letter supposedly agreed with the AAF position on the XS-1 program, in fact, it was “accepting” Stack’s rejection of the 

Army position. The NACA letter stated the following:

• The final Army acceptance would occur after the rocket motor was 

demonstrated and without the need for the B-29 launch aircraft.

• The Pinecastle flights were to determine if Langley Field operations 

were feasible.

• Bell Aircraft would supply the pilot until the NACA decided to take 

over the program.52

By the time the NACA’s letter was sent, the Bell glide flights were underway at Pinecastle Field. Bell test pilot Jack Woolams made 2 captive flights 

and 10 free flights between January 25 and March 6, 1946, flying the first 

Bell XS-1 (AAF serial 46-062), equipped with a 10-percent thickness-chord 

wing and 8-percent thickness-chord tail (changed subsequently to an 8-percent 

thickness-chord wing and 6-percent thickness-chord tail before it began its 

powered flights). The Pinecastle glide flights provided the answers to several 

issues. The XS-1 had good low-speed flight characteristics but poor visibility 

through the flush windshield, and the air-launched concept was practical. 

More important, the Pinecastle flights eliminated the use of either Langley 

Field or Niagara Airport for subsequent XS-1 powered test flights. A much 

wider landing area was required for safe operations. Of the 10 glide flights, 

the first landed short of the runway. On the fourth flight, the left main gear 

retracted after touchdown and the XS-1 went off the runway and suffered 

minor wing and fuselage damage. On the fifth landing, the nose gear retracted, 

causing minor damage. Even a 10,000-foot runway lacked the qualities needed 

for a consistently acceptable unpowered touchdown. Additionally, bad weather 

caused numerous canceled flights. The XS-1 required a site with better and 

more predicable weather conditions.53

Despite the hopes of both Langley’s Stack and Bell Aircraft’s Smith, the only 

site that met the requirement was Rogers Dry Lake at Muroc Army Air Field 

in California. With a surface area of 44 square miles, Rogers was the largest 

dry lake in the world. Its surface was dried silt, able to support 250 pounds 

per square inch. The lakebed’s north-south distance was 11 miles. It enjoyed 

clear skies some 350 days a year. The lakebed was located in the middle of the 

Mojave Desert, far from prying eyes, yet was less than a hundred miles from 

Los Angeles and its aircraft contractors. The XP-59A had first flown there, and 

the advantages of the site for flight testing were clear to project personnel.54

The next step was the powered flights, using the Reaction Motors, Inc., fourchamber XLR-11 6,000-pound-thrust rocket engine.55 As part of its contract 

with the AAF, Bell Aircraft had to demonstrate that the XS-1 was controllable to a speed of Mach 0.8 and could withstand an 8-g pullout. Although 
 Confronting the “Sound Barrier”: The Bell XS-1

23

this concluded Bell’s obligations under the contract, company management 

assumed they would also make the first Mach 1 flight. The longstanding practice was that contractor test pilots made the envelope expansion flights, up to 

the airplane’s maximum speed.

The powered XS-1 flights would not be made by Woolams, who had been 

killed while practicing for the 1946 Thompson Trophy air race. His replacement 

was Chalmers H. “Slick” Goodlin. He was only 23 years old when assigned 

to be the XS-1 test pilot, but he had amassed extensive flight experience. He 

had soloed before turning 17; he had served in the Royal Canadian Air Force 

as a flight instructor, in the Royal Air Force as a Spitfire pilot, and in the U.S. 

Navy as a test and ferry pilot—all before joining Bell in December 1943 as a 

production test pilot.56

Group photo of the groundbreaking for the new main building at the High-Speed Flight Research 

Station. From left to right are Gerald Truszynski (head of instrumentation), Joseph Vensel (head 

of operations), Walter C. Williams (director of the High-Speed Flight Research Station), Marion 

Kent (administration officer), and California state official Arthur Samet. (NASA)

The NACA was also preparing for the Muroc tests. In September, Hartley 

Soulé, chief of the Stability Research Branch at Langley, selected Walter C. 

Williams to oversee a small team, including engineers, instrument technicians, 

and technical personnel, who were to support the Bell XS-1 powered flights. 

They were called the NACA Muroc Flight Test Unit. Williams and four other 

NACA engineers arrived at Muroc in September 1946. Six more arrived in 
Probing the Sky

24

October, and the two women “computers,” who reduced the data from the 

onboard instrumentation into charts and graphs, arrived in December. The 13 

personnel were initially considered to be a unit on a temporary assignment to 

the desert base. The NACA Muroc Flight Test Unit gained permanent status 

on September 7, 1947. It became the High-Speed Flight Research Station on 

November 14, 1949, and was later renamed simply the NACA High-Speed 

Flight Station.

They found themselves in a place very different from Virginia. Facilities 

at Muroc were meager, with the initial NACA facilities consisting of a single 

hangar shared with the AAF, plus two rooms. Darkroom facilities were lacking. Support facilities were the loading pit for the XS-1, the liquid-oxygen and 

liquid-nitrogen tanks, and a fuel trailer to mix the alcohol/water fuel. 

Housing was also marginal. Williams found an apartment in Palmdale, 40 

miles from Muroc. He was the exception. Single engineers and technicians 

lived in “Kerosene Flats,” the decaying wartime housing in the town of Muroc 

near the edge of the lakebed. In late 1946, the Marine air station in Mojave 

was closed, and married NACA personnel moved into the base housing. In 

both cases, the housing was low-quality wartime construction, cold at night, 

and in dilapidated condition. 

The biggest change from Langley was the landscape. Muroc was in the 

wide-open spaces of the southwestern desert, with hot, dry weather during the 

summer, while winter saw freezing temperatures and occasional snow. It was so 

bleak that some Langley engineers and their families outright refused to leave 

the familiar shores of Tidewater, VA. Of those who came west, some adapted 

and thrived, while others soon returned to the Chesapeake.57

The second XS-1 (AAF 46-063) arrived at Muroc on October 7, 1946, under 

the belly of the B-29 launch aircraft. The aircraft had the thicker 10-percent 

thickness-chord wings and 8-percent thickness-chord horizontal tail flown 

on the first airplane at Pinecastle 9 months earlier. Goodlin initially made a 

series of glide flights to become familiar with the XS-1’s handling and flight 

characteristics. The first glide-flight attempt was made 2 days later but was 

aborted before launch. It was rescheduled and successfully made on October 

11. A total of four glide flights were made by December 2, 1946. The way 

was now clear for the initial powered flights. The first attempt, on December 

9, was aborted before launch. The next try, on December 20, was successful. 

Goodlin made a total of 12 powered flights in the second XS-1 over the next 

3 months, with a final flight on February 21, 1947. Two more attempts were 

aborted before launch. 
 Confronting the “Sound Barrier”: The Bell XS-1

25

Settling Differences, Building a Program

While Bell’s powered flight tests were going smoothly, the internal differences 

continued. The Bell and NACA personnel had different concepts of the role 

of the contractor XS-1 glide and powered flights. For Bell, these were to prove 

the basic airworthiness of the XS-1, within the contract specifications. This 

was to be a quick program, which Richard Frost of Bell Aircraft noted did not 

“envision any lengthy series of scientific tests to investigate all the byroads of 

stability in its various forms.”58

In contrast, Williams told Frost and Robert Stanley (also with Bell) during 

a September 1946 meeting at Muroc that the NACA wanted complete stability 

and control data from the upcoming powered flights, as well as aerodynamic 

and structural load data. The data needed, he continued, included longitudinal 

stability characteristics, control in both steady and accelerated flight conditions, and the buffet boundary.59 Frost responded that no special flights would 

be made to collect such data. Bell would accept its test pilot’s assessment of 

stability and control issues to meet the contract requirements. There would 

be no delays in the flight to accommodate the NACA’s research requirements. 

On April 11, 1946, Colonel Smith wrote to Lewis to outline the AAF’s 

position regarding its understanding and plans for the XS-1 project. Once the 

Bell test flights were completed, the NACA would undertake the high-speed 

flights and supply a pilot and data collection system and personnel. If the 

NACA concluded the aircraft was unsafe, the XS-1 could be returned to AAF 

control. The letter also stated that the AAF position was for the air launch to 

continue, as this was the most practical and safest method of flight testing for 

the XS-1. Colonel Smith made it clear that the AAF wanted a “firm understanding” between the AAF and the NACA on these issues before the end of 

the Bell powered flights.

Williams wrote a memo to Melvin N. Gough, chief of the Flight Research 

Division at Langley, in response to the AAF letter. Williams stated that the 

NACA should not accept the XS-1 until the turbopump had been successfully 

tested. He also rejected the use of the B-29 launch aircraft. Williams also noted 

that Bell was planning to make the initial powered flight at Muroc, having 

rejected operating from Langley. 

Gough had his own requirements, which included the capability to take 

off and climb to 35,000 feet under its own power. While he suggested that 

the NACA might waive this requirement once the B-29 operations gained 

experience, Gough insisted that the NACA not accept the XS-1 without 

the turbopump and reserve fuel capacity for emergency landings. Langley’s 

engineer-in-charge, Henry Reid, in a letter to NACA Headquarters written 

on April 29, 1946, noted that unless these requirements were met, the NACA 
Probing the Sky

26

“will not undertake to supply a pilot and 

operate the airplane.” Reid asked NACA 

Headquarters to press the AAF to begin 

the construction of XS-1 fueling and 

handling facilities at Langley. Doing so 

would allow the transonic research flights 

to begin soon after the acceptance testing 

was completed. Despite the results of the 

Pinecastle glide flights, Langley personnel still intended to fly their XS-1 from 

their facility. 

As for the XS-1 test plan itself, 

Williams outlined a two-phase approach 

on June 7, 1946, to Gough. The first phase 

would determine stability and control at 

high Mach numbers. Flight would be 

made in increasing Mach number increments, with eight flights made for each 

speed increment. A total of 48 “successful 

flights” would be made in the first phase 

to reach the XS-1’s “operational limits.” 

If a change in wing thickness was needed, 

the entire sequence would have to be repeated, requiring another 48 flights. A 

“successful flight” was defined as one in which all systems functioned correctly, 

the pilot flew the mission exactly as planned, and all the instrumentation operated properly. The odds of this were about 50/50, indicating that as many as 

100 flights might actually be needed.

Melvin N. Gough, chief test pilot at Langley, 

was among the group of staff members 

who wanted the XS-1 to take off and land 

on the Langley runway and who opposed 

the air-launch technique, supporting development of the D-558-1. (NASA)

The second phase would be to measure aerodynamic loads on the wings 

and tail and collect additional drag and performance data along with stability and control measurements. This phase would be much shorter, with only 

16 flights being planned for each wing thickness. Reid forwarded the plan to 

NACA Headquarters on June 24, 1946. What was not in the plan was just 

as important. The XS-1 would be flown to its “operational limits.” Williams 

never mentioned making supersonic flights. His plan was also a long, deliberate, step-by-step effort, requiring a year or more to complete.60

The differing viewpoints between Bell, the NACA, and the AAF were such 

that insignificant matters gave rise to anger. As 1946 ended and the first powered flights were made, both the AAF and Bell began to publicize the XS-1 

project (even though the XS-1 was a classified project and was being flown at 

a restricted base). Stories were carried in the New York Times and later in Time

magazine, which focused on Bell, the AAF, and Goodlin. 
 Confronting the “Sound Barrier”: The Bell XS-1

27

On December 12, 1946, the day after the New York Times article was published, Williams sent a letter to Soulé at Langley. Williams complained that 

the Bell-AAF publicity was about an airplane that, he claimed, did not really 

exist. They “should admit that the airplane was not designed as a supersonic 

aircraft but rather a high transonic airplane.”61 He continued that the supersonic capability was only because they were “forced” to use the B-29 launch 

plane to achieve the altitude and flight time. The issue of straight wings versus 

swept wings also raised Williams’s ire. He dismissed comments that straight 

wings were on the XS-1 because it was not known if swept wings could be used 

“for the birds.” The real reason was that no one knew much about swept wings 

when the aircraft was being designed.62

Both Williams and James Voyles, the civilian AAF representative for the 

XS-1 project, believed Stanley might go as far as to authorize Goodlin to “accidentally” exceed Mach 1. While the suspicions were unjustified, they reflected 

the sharp disagreements emerging among all players over the program’s goals 

and future direction.

Even the test site still seemed subject to debate. In January 1947, a group of 

Ames researchers came to Muroc and were shown around by Gough. They were 

impressed by the XS-1 support facilities, including the loading pit. Lawrence 

Clousing of Ames commented about it, and Gough replied that a similar 

pit would be built at Langley. Clousing was surprised, but Gough told him 

Goodlin had landed the XS-1 on the paved Muroc runway. Operating the 

rocket plane from Langley, Gough said, would not be a problem.63

The NACA’s plans for the XS-1 had also undergone a change from those 

in Williams’s June 7, 1946, letter. NACA management expected to acquire 

one of the XS-1s in July 1947, which would mark the start of phase 1 of the 

three-phase research effort. The first phase was built around the XS-1 and the 

D-558-1 aircraft. Reflecting Stack’s preferences, phase 1 involved only transonic flights. Phase 2, using the as yet unbuilt Bell XS-2 and Douglas D-558-2 

swept-wing research aircraft, would reach speeds up to supersonic. Not until 

phase 3 would flights above Mach 1 be made by the NACA. The third phase 

would use the XS-3, a jet-powered aircraft with low-aspect straight wings, and 

a proposed third derivative of the D-558-1 (which was never built).64

Both Bell Aircraft and the AAF felt that the NACA approach was too slow 

and cautious, and indeed it was. Robert Stanley told Larry Bell that the NACA 

was only marking time until the D-558 was ready. Stanley also feared that the 

NACA would follow the earlier pattern of testing done with the P-80 and 

P-84 aircraft. Stanley believed the NACA planned to fly the XS-1 at altitudes 

of 20,000 to 30,000 feet until the pilot encountered trim changes or buffeting, 

at which point the NACA flights would stop. Stanley pointedly added, “At 

these low altitudes, they could do the same with a P-84 since it reaches Mach 
Probing the Sky

28

trouble at part[ial] throttle.” Little wonder, then, that Stanley saw no point in 

testing the XS-1 in this manner.65

The AAF had its own issues with both Bell and the NACA. From the beginning, reaching Mach 1 was the Army’s primary goal for the XS-1 project. This 

was to be accomplished in the shortest time possible. The NACA seemed, 

at best, reluctant to take on the challenge. Williams’s initial flight plan and 

the later three-phase effort were both going to be slow, long-term efforts. In 

contrast, the AAF’s focus was on the near term. The swept-wing XP-86, then 

under development, would be approaching supersonic speeds as early as late 

1947, and the AAF had to have data on the possible risks it faced and needed 

the data sooner, not in a year or more.

The AAF also had management and monetary issues regarding Bell’s efforts. 

The contractor’s proposal included no specifics on the length of its project or 

any guarantees of results. Colonel Smith also objected to Goodlin’s “bonus 

money.” The practice itself was not unusual, and contractor test pilots had long 

been paid extra money for risky flights. Bell management and Goodlin had 

reached a “handshake agreement” that he would receive $150,000 for making 

the first supersonic flight in the XS-1, a not-unreasonable sum given the risks 

involved. But the AAF was suffering from the postwar funding cutbacks, and 

money for the XS-1 was running short. The AAF was unable to meet the proposed payment, and Smith would not agree to so large a pilot bonus. 

Instead, he offered Bell a fixed-price contract, which included specific 

requirements Bell had to meet during the tests. At the same time, the NACA’s 

perceived foot-dragging and continued objections to AAF decisions convinced 

Smith and other AAF personnel that they had to pursue a new approach to 

meet the AAF’s research needs.66

The new AAF plan was a two-part complementary program. The AAF 

would reach Mach 1 in a minimum number of flights, using a service test pilot. 

The NACA would conduct a detailed, incremental flight research effort using 

its pilots. Bell’s involvement would end once the company’s original contractual 

obligations were completed. 

By the spring of 1947, as the AAF was debating the future of the XS-1 

flight program, the Flight Test Division personnel were confident that AAF test 

pilots were ready for the challenges that the XS-1 posed. Colonel Osmond J. 

Ritland recommended to Colonel Smith that the Flight Test Division be given 

responsibility for the supersonic test flights.67 Based on Ritland’s assessment, 

Smith asked Colonel Albert Boyd, chief of the Flight Test Division at Wright 

Field, if his pilots could undertake the accelerated flight test plan for the XS-1. 

Boyd’s reply was, “You bet.” 

Bell turned down the proposed fixed-price contract, and on May 1, 1947, 

Smith notified AAF Headquarters of the company’s decision, adding, “As a 
 Confronting the “Sound Barrier”: The Bell XS-1

29

result of this notification, discussion is now underway with [the] view of having 

this program taken over by [the AAF Air Mobility Command] Flight Test 

Division.”68 Larry Bell appealed to General Carl Spaatz, the AAF commander, 

to reverse the decision (ultimately to no avail).

Following this interchange, the Flight Test Division began planning for 

the AAF program. The instrumentation on the XS-1 would be the minimum 

required for measuring the speeds and altitudes reached during its flights. As 

many as five glide and powered flights were planned just to reach Mach 0.8. 

While the AAF flights would be done in parallel with the NACA’s effort, the 

Army would focus on achieving a Mach 1.1 flight in the shortest time possible. 

The first XS-1, fitted with the thinner 8-percent thickness-chord wings and 

6-percent thickness-chord horizontal stabilizers, had a higher critical Mach 

number and speed capability than the thick-wing second XS-1 and was thus 

better suited for the AAF’s accelerated assault on Mach 1. A key decision made 

by the AAF flight planners was to make the supersonic flights at high altitudes, 

thereby reducing the dynamic pressure the XS-1 would experience and minimizing the loads encountered should it experience abrupt transonic pitching 

and buffeting. Boyd and Lieutenant Colonel Fred Ascani now reviewed the 

records of the 125 pilots at the Flight Test Division and compiled a list of 

candidates to fly the plane.69

They selected Captain Charles E. “Chuck” Yeager as the primary XS-1 pilot. 

Yeager was then 24 years old, married, and a P-51 ace from World War II. While 

Yeager lacked both a college degree and formal engineering training, Boyd 

considered him the best instinctive pilot he had ever known. Yeager was the 

engineers’ choice because he had an uncanny ability to return from a test flight 

and tell the engineers exactly what the airplane had done in response to what 

control input at what stage of the flight, in language the engineers immediately 

understood, and all after a flight was over, no matter what else had transpired 

during the flight. Yeager was clearly a standout, even among the very select 

group of test pilots composing the AAF’s Flight Test Division at Wright Field.

For Yeager’s backup, Boyd selected First Lieutenant Robert A. “Bob” 

Hoover, another outstanding intuitive fighter test pilot. The AAF’s engineerin-charge/project manager was test pilot Captain Jackie L. “Jack” Ridley, 

holder of a master’s degree in aeronautical engineering from the California 

Institute of Technology, who could furnish the engineering support the two 

pilots required.70

On June 24, 1947, General Spaatz concurred with Smith’s accelerated plan 

for XS-1 testing, and the following day, Yeager, Hoover, and Ridley joined 

personnel from the Flight Test Division and the Aircraft Projects Section to 

develop the accelerated test plan, preparing as well for a meeting with NACA 

personnel at the end of the month to finalize the two-phase test plan. 
Probing the Sky

30

Langley’s personnel proved less than enthusiastic in the change of program 

direction. In a June 13, 1947, letter to NACA Headquarters, Henry Reid indicated that Langley researchers had never liked the Bell Aircraft Corporation’s 

XS-1 flight program and had only agreed to it in order to establish a good working relationship with the company, thereby ensuring they got the modifications 

that agency engineers wanted before finally accepting the aircraft. The Bell 

Aircraft Corporation’s test program would also have shown that the XS-1 was 

safe—as far as the NACA was concerned—before the NACA flew the airplane. 

Neither of these goals really mattered now that Bell was out of the project. 

The result, Reid drily noted, “is considered unfortunate by the Laboratory.”71

Reid was also skeptical about the AAF’s plans. While promising NACA 

Headquarters that Langley would cooperate with the joint effort and that the 

agency had no objection to the AAF’s taking over the project, he did want 

the AAF to acknowledge that Langley disagreed with the expedited flight 

plan and still preferred its own plan, as it would provide the largest amount 

of research data.72

It was this last point that turned out to be the redeeming element for the 

NACA in all this, although no one knew it at the time. In the rush to Mach 

1, the AAF made it clear that the details were of secondary concern, while to 

the NACA those details were the only concern, and this had been the source 

of so much friction between the two groups. In time, knowing exactly what 

was happening all along the way turned out to be essential, at which point 

the NACA’s methodical, ever-so-slow approach to increased speeds was its 

trump card. But in mid-1947, the logic of more rapidly accelerating the drive 

through Mach 1 was unassailable, particularly given the urgency of new high 

subsonic and transonic aircraft programs such as the XF-86.

AAF and NACA representatives met at Wright Field on June 30 and July 

1, 1947, to discuss the coordination of their respective XS-1 plans. Colonel 

P.B. Klein opened the meeting by stating that the AAF planned to undertake 

an accelerated flight plan with the XS-1 to achieve speeds of Mach 1.1 at altitudes of 50,000 to 60,000 feet in the shortest possible time. AAF personnel 

were aware of Reid’s comments and stressed that the XS-1 was a joint effort. 

Colonel Boyd emphasized that the Flight Test Division would “appreciate all 

of the assistance the NACA personnel could give them in conducting this 

program.”73 AAF personnel also asked Bell Aircraft for the help of Richard 

Frost, who had been part of the Bell XS-1 team. He had provided technical 

advice during the Bell flights and urged that all the AAF tests parties cooperate with him. 

Hartley Soulé presented a summation of what had been learned thus far, 

the possible technical problems that lay ahead, and the NACA-recommended 

procedures that should be used. He noted that no significant compressibility 
 Confronting the “Sound Barrier”: The Bell XS-1

31

effects on the XS-1 had been identified up to Mach 0.82. He added, “It is 

apparent from the flight and wind tunnel data that above M-0.85 large 

changes in stability and control and vibrational changes are to be expected. 

These have been anticipated in the Langley flights, which will be made at an 

altitude of 30,000 feet, and plans have been made to increase speed cautiously 

in small increments and to explore conditions at each increment thoroughly 

before proceeding to a higher speed.”74

NACA researchers had concerns about the AAF’s plan to make the tests at 

high altitudes, and Soulé focused on this in his remarks. He noted that the 

Mach 0.8 flight had been made at 30,000 feet, which resulted in a dynamic 

pressure on the XS-1 of about 250 pounds per square foot. “At 60,000 feet 

for the same Mach number of 0.8, the dynamic pressure would be about 65 

pounds per square foot. It appears doubtful, therefore, that any inadvertent 

attitude to which the airplane might go as a result of stability and control 

changes could result in any structural failures at 60,000 feet.”75

Reflecting how little was then known of transonic flying, Soulé cautioned, 

“The pilot should avoid prolonged glides to lower altitudes where the density 

is higher because conditions may change critically between the acceleration 

and deceleration phase of the flight, and consequently such glides may be 

extremely dangerous.”76 He also recommended that AAF personnel read 

several NACA flight-data reports, the results of wind tunnel testing, and 

other testing results. 

He also provided AAF personnel with a copy of the NACA flight plan 

and asked for a copy of theirs in return. The Flight Test Division representatives told them no flight plan had been developed listing specific Mach 

numbers and altitudes. Decisions on such matters would not be made until 

the flights were underway and would be based on data analysis and Yeager’s 

recommendations after each flight. 

Aware of the rising tension in the room, Boyd assured Soulé and the rest 

of the NACA contingent that the Flight Test Division flight planning was 

to be guided by common sense, sound engineering experience, and a focus 

on safety. He stressed, however, that the flight plan would be progressive and 

brief; the goal remained to fly supersonic in the shortest time, and they would 

not be diverted from this objective. The official start of the AAF accelerated 

plan was July 10, 1947. The time required to reach Mach 1 was estimated to 

be 60 to 90 days. As events would prove, the high end of this estimate was 

right on the mark.77
Probing the Sky

32

Toward—and Beyond—the Unknown

Following briefings at Bell Aircraft, Yeager, Hoover, and Ridley traveled to 

Muroc, arriving there on July 27. Frost began a 4-day XS-1 “familiarization 

schooling” for the new arrivals. Frost later summed up the unknowns of 

supersonic flight in the summer of 1947. Regarding an impregnable sound 

barrier, “At best, I was ambivalent about it. In hindsight, people may well say 

that the so-called ‘sound barrier’ really didn’t prove to be a barrier at all...but 

let me assure you, the conditions Chuck was facing going into those flights 

were very much a barrier in our minds at that time.... It wasn’t within our 

power to give Chuck any real assurances about what might happen in any 

one of a multitude of different circumstances.”78

As with Woolams and Goodlin, Yeager began by making glide flights 

in the first XS-1. Three of these were made between August 6 and August 

9, 1947. Yeager commented that the little saffron speedster was “graceful, 

responsive, and beautiful to handle.”79

Due to a shortage of B-29 parts, it was 3 weeks before the powered flights 

could begin. Yeager made his first powered flight on August 29 and reached 

Mach 0.85. This was followed by four more powered flights in September. Yeager 

increased the XS-1’s speed in small increments, going from Mach 0.89 to 0.91, 

and then to Mach 0.92. In the process, Yeager was assessing the XS-1’s stability 

and control, elevator and stabilizer effectiveness, and buffet. This continued with 

another two flights, on October 5 and 8. While there had been problems, the 

accelerated effort made good progress. The October 8 flight reached Mach 0.925.

The next flight was made on October 10, 1947, reaching an indicated 

airspeed of Mach 0.94 at 45,000 feet. Yeager then pulled back on the control 

column and was surprised when the aircraft failed to respond. The shock 

wave on the horizontal stabilizers had moved aft until it was on the elevator 

hinge line. The elevators had lost all effectiveness, and, as a result, Yeager had 

no pitch control of the XS-1. He wrote in his flight report, “[T]he control 

column could be moved to the limits of travel each way with little force and 

very slow response in airplane attitude.”80

In fact, things were far less bleak than this report suggests. Aerodynamicists 

had predicted that the XS-1 would pitch up or down at Mach 1 and that 

without the elevator’s pitch control, the aircraft might be lost. That had been, 

after all, the reason that the XS-1 had an adjustable horizontal tail installed 

in the first place. Significantly, NACA testing had already indicated that the 

XS-1 had to use the adjustable stabilizers in order to fly through the speed 

of sound. At the request of the AAF, the NACA had tested a 1/16-scale Bell 

model of the XS-1 in the Langley 8-foot high-speed tunnel at speeds of up to 

Mach 0.945 and Reynolds numbers of 1.18 × 106

. 
 Confronting the “Sound Barrier”: The Bell XS-1

33

The tests clearly revealed the steady decrease in elevator control effectiveness above Mach 0.87, Langley researcher Axel T. Mattson noted in a report 

issued in May 1947 (5 months before Yeager’s flight): “At a Mach number of 

0.9, however, the airplane, because of an indicated diving tendency with loss and 

reversal in elevator control, will require the use of the stabilizer as a trim control. 

Control by the use of the stabilizer is effective at least up to a Mach number 

of 0.93, the limit for these tests.”81 (Emphasis added.) So available data clearly 

indicated both the necessity for using stabilizer trim in the transonic region 

and the likelihood that it would resolve any pitch control problems. In any 

case, Yeager’s associate Jack Ridley reassured the airman that by moving the 

stabilizers in very small increments of 1⁄4 to 1⁄3 of a degree, Yeager could retain 

pitch control without the elevators. 

A genuine surprise came from John Mayer while he was reducing the data 

from the flight. After correcting for errors in the XS-1’s airspeed system, he 

found that the aircraft had reached a true airspeed of Mach 0.957. After working through the weekend, he again revised his analysis, concluding the XS-1’s 

true airspeed was Mach 0.997 at 37,000 feet. Only the narrowest of margins 

still remained. 

On the morning of October 14, 1947, the B-29 with the XS-1 attached 

underneath took off from the Muroc runway. Due to the loss of pitch control 

on the previous flight, NACA engineers told Yeager to limit his speed to Mach 

0.96 unless he was certain he could safely fly faster. Yeager himself was not in 

the best condition as he climbed into the XS-1 cockpit. The previous weekend, 

he had fallen from a horse at aviatrix Florence “Pancho” Barnes’s notorious 

Happy Bottom Riding Club and had broken two ribs. In pain, Yeager went 

to a civilian doctor in Rosamond to have them taped up, rather than go to an 

Air Force flight surgeon and risk being grounded. Ridley, by now a firm Yeager 

friend, knew about the mishap and cut a 10-inch length of broom handle to 

help Yeager lock the hatch from inside the rocket plane, despite his broken ribs. 

The drop was made at 10:26 a.m., at an altitude of 20,000 feet and an 

airspeed of 250 mph. This was slower than expected, and Yeager had to lower 

the XS-1’s nose to avoid stalling. He fired the four rocket chambers in rapid 

succession and began to accelerate and climb. He shut down two of the rocket 

chambers and began using the movable stabilizer for pitch control, finding it 

“very effective.” 

Yeager leveled out at approximately 42,000 feet and fired a third rocket 

cylinder. The XS-1 accelerated rapidly to an indicated airspeed of Mach 0.98. 

The Machmeter needle fluctuated and jumped off scale. There was no violent 

buffeting. The aircraft did not pitch up or down. There were no indications 

that something unusual had happened. Yeager held the speed for 20 seconds 

before shutting off the engine. Yeager radioed, “Ridley! Make another note. 
Probing the Sky

34

There’s something wrong with this Machmeter. It’s gone screwy!” Ridley 

replied, “If it is, we’ll fix it. Personally, I think you’re seeing things.”82 NACA 

personnel analyzed the flight data and determined that the XS-1 had reached 

a speed of Mach 1.06. The task had taken 96 days since the AAF took over 

the program on July 10, 1947. Within hours, the achievement was classified, 

and no public announcement was made.83 Not quite three months later, at 

the express direction of Air Force Secretary Stuart Symington, the Army Air 

Forces held a secret conference at Wright Field to present the results of the 

program to a select audience of the Nation’s leading designers, engineers, and 

aeronautical researchers.84 NACA’s Muroc Flight Test Unit harvested the technical results of the flight in a series of analytical reports issued over the next 

several years, buttressing the quick look afforded by the Air Force’s accelerated 

flight test program.85

Beyond Yeager’s landmark achievement awaited a new series of unknowns. 

There were many design ideas about how a supersonic airplane should be configured. However, there was little solid information and, with wind tunnels still 

unable to provide reliable data, few means to find out more. There remained 

an interrelated series of problems in aerodynamics, propulsion, and aircraft 

configurations to bedevil and bother the aeronautical community in the years 

ahead. Nevertheless, if there were many challenges and unknowns ahead, there 

was one major bedrock accomplishment: the myth of an impenetrable barrier 

had been forever vanquished.
 Confronting the “Sound Barrier”: The Bell XS-1

35

Endnotes

1. Opening quote from Louis Rotundo, Into the Unknown: The X-1 

Story (Washington, DC: Smithsonian Institution Press, 1994), p. 6.

2. James R. Hansen, Engineer in Charge: A History of the Langley 

Aeronautical Laboratory, 1917–1958 (Washington, DC: NASA 

SP-4305, 1987), p. 253; Richard P. Hallion, Supersonic Flight: 

Breaking the Sound Barrier and Beyond—The Story of the Bell X-1 

and Douglas D-558 (London: Brassey’s, 1997), p. x. When asked 

about these stories, John McTigue, a former X-15 engineer, recalled 

reading such claims in newspaper articles and letters to the editor circa 1944–45. The stuck-voice story was false, as the air in the 

cockpit was not moving with respect to the pilot. The time-reversal 

story represents a misunderstanding of the theory of relativity. As an 

object nears the speed of light, time slows down. The crew of a starship traveling at nearly the speed of light would experience a flight 

as lasting only a few months. Simultaneously, back on Earth, years, 

decades, or even centuries would pass. In no case, however, can time 

actually be “reversed.”

3. F.W. Caldwell and E.N. Fales, “Wind Tunnel Studies in 

Aerodynamic Phenomena at High Speeds,” Report No. 83 (NACA, 

1920), p. 59.

4. Ibid., pp. 60, 77, 86, 89.

5. L.J. Briggs, G.F. Hull, and H.L. Dryden, “Aerodynamic 

Characteristics of Airfoils at High Speeds,” Report No. 207 (NACA, 

1924), pp. 465, 478.

6. L.J. Briggs and H.L. Dryden, “Pressure Distribution Over Air Foils 

at High Speeds,” Report No. 255 (NACA, 1926), p. 555.

7. L.J. Briggs and H.L. Dryden, “Aerodynamic Characteristics of 

Twenty-Four Airfoils at High Speeds,” Report No. 319 (NACA, 

1929), pp. 327–328, 345–346.

8. Ibid.

9. L.J. Briggs and H.L. Dryden, “Aerodynamic Characteristics of 

Circular-Arc Airfoils at High Speeds,” Report No. 365 (NACA, 

1931), pp. 67, 69.

10. NASA, “The National Advisory Committee for Aeronautics: Tracing 

NASA’s 95-Year-Old Roots,” http://www.nasa.gov/centers/ames/news/

features/2010/95_anniversary_prt.htm, updated March 3, 2010.

11. Hansen, Engineer in Charge, pp. xxviii, 2, 126–133.

12. Eric Schatzberg, Wings of Wood, Wings of Metal (Princeton: Princeton 

University Press, 1998). This book looks at the social factors, as 
Probing the Sky

36

opposed to engineering requirements, behind the switch from 

wooden construction toward all-metal aircraft. These include a perception that metal was “modern,” while wood was a “preindustrial” 

material. Another factor was the influence of the U.S. military on 

aircraft design.

13. For a discussion of these, see G. Geoffrey Smith, Gas Turbines and 

Jet Propulsion for Aircraft (New York: Aircraft Books Inc., 1946), pp. 

34–50. For the jet engine more generally, see Edward W. Constant 

II, The Origins of the Turbojet Revolution (Baltimore: Johns Hopkins 

University Press, 1980), pp. 63, 64, 70–72, 77, 89, 93, 95; and 

Robert Schlaifer and S.D. Heron’s Development of Aircraft Engines 

and Fuels (Boston: Harvard Business School, 1950).The bestknown of these hybrid concepts was the “Campini engine,” after 

the Italian engineer Secondo Campini, who successfully flew one in 

1940–41, but many other concepts for such engines existed as well. 

In England, the term “motorjet” was also used. For Campini’s work, 

see Gregory Alegre, Campini Caproni, no. 5 of the Ali d’Italia series 

(Turin: La Bancarella Aeronautica, n.d.).

14. For American work, see James St. Peter, The History of Gas Turbine 

Development in the United States...A Tradition of Excellence (Atlanta, 

GA: International Gas Turbine Institute of the American Society of 

Mechanical Engineers, 1999).

15. Edgar Buckingham, “Jet Propulsion for Airplanes,” Report No. 159 

(NACA, 1923), p. 85.

16. For a particularly useful introduction to the history and development of the jet engine, see Walter J. Boyne and Donald S. 

Lopez, eds. The Jet Age: Forty Years of Jet Aviation. Washington, 

DC: National Air and Space Museum in association with the 

Smithsonian Institution Press, 1979.

17. Margaret Connor, Hans von Ohain: Elegance in Flight (Reston, VA: 

American Institute of Aeronautics and Astronautics, 2001).

18. For Whittle’s view of his work, see his autobiographical Jet: The 

Story of a Pioneer (New York: Philosophical Library, 1954) and John 

Golley’s (in association with Whittle and with technical assistance 

from Bill Gunston) semiautobiographical Whittle: The True Story 

(Washington, DC: Smithsonian Institution Press, 1987). See also 

John Grierson’s memoir-history Jet Flight (London: Samson Low, 

Marston & Co., Ltd., 1944); and Andrew Nahum’s thoughtprovoking Frank Whittle: Invention of the Jet (Duxford, U.K.: Icon 

Books, 2004).
 Confronting the “Sound Barrier”: The Bell XS-1

37

19. For the impact (in both directions) of turbojet development and 

combat requirements, see Sterling Michael Pavelec, The Jet Race and 

the Second World War (Annapolis, MD: Naval Institute Press, 2007).

20. Hansen, Engineer in Charge, pp. 230–231.

21. Hansen, Engineer in Charge, pp. 231–232. Dr. Vannevar Bush’s 

interest in the Jeep engine concept also reflected his own technological conservatism and academic mindset. Despite his accomplishments as a scientist, he endorsed the same jet concept the NACA 

was studying. For the record, Bush consistently displayed a pattern 

of being too quick to dismiss the possibilities of technological breakthroughs. In late 1945, for example, he told a congressional committee that a long-range rocket armed with a nuclear weapon was 

impractical and that the Congress and the American people should 

leave it out of their thinking. In 1949, he predicted it would be 

many years before the USSR would build an A-bomb. The Soviets 

tested their first A-bomb that year.

22. Ibid., p. 260; Hallion, Supersonic Flight, pp. 19–20.

23. Hansen, Engineer in Charge, p. 244.

24. Ibid., pp. 236–245; Hallion, Supersonic Flight, pp. 18–21. The 

XP-59A itself was an example of what Jacobs was describing. It was 

initially intended to evolve into an operational combat aircraft rather 

than remain a technology demonstrator. The goal was to have a combat aircraft ready sooner than first building an experimental aircraft, 

followed by an operational fighter. (Much like the Durand Committee 

had proposed for Jacobs’s research aircraft.) While the turbojets were 

revolutionary, the XP-59A’s airframe was a late-1930s high-drag 

design. This limited it to a top speed of 389 mph. As a result, the few 

P-59s built were used as jet trainers to introduce pilots into the higherperformance Lockheed P-80A that succeeded it.

25. John V. Becker, The High-Speed Frontier: Case Studies of Four NACA 

Programs, 1920–1950 (Washington, DC: NASA SP-445, 1980), p. 90.

26. Hansen, Engineer in Charge, p. 260.

27. Hallion, Supersonic Flight, p. 22.

28. Ibid., pp. 20–23.

29. Ibid., pp. 24–25.

30. Hansen, Engineer in Charge, pp. 260–261.

31. Becker, The High-Speed Frontier, pp. 91–92.

32. Becker, The High-Speed Frontier, p. 91; Hansen, Engineer in Charge, 

pp. 271–273. “XS-1” continued to be used into 1948, when the Air 

Force revised its designation system. The “S” was dropped for all of 
Probing the Sky

38

the research aircraft (XS-1, XS-2, XS-3, and XS-4), and they became 

the X-1, X-2, X-3, and X-4.

33. Hallion, Supersonic Flight, pp. 31–32, 61, quotes the Hyatt memo 

at length, which was USN Bureau of Aeronautics AER-E-225-AH, 

“Proposed High-Speed Research Airplane” (September 22, 1944). 

See also Rotundo, Into the Unknown, p. 21; and Hansen, Engineer in 

Charge, pp. 273–274.

34. Ibid., pp. 65–67, 69, 70; Hansen, Engineer in Charge, pp. 290–291. 

Unlike the AAF or the Air Force, the U.S. Navy never had a specific designation for a purely experimental aircraft. Until September 

1962, the Navy and Air Force had separate aircraft designation 

systems, even for the same aircraft.

35. Hansen, Engineer in Charge, pp. 275–279; Hallion, Supersonic 

Flight, pp. 71–72. Stack and Gilruth also jointly recommended that 

the thickness-chord ratio of the XS-1’s horizontal stabilizer be less 

than that of the wing. This was to prevent them from both experiencing shock wave formation at the same time, with attendant 

changes such as turbulence, loss of control effectiveness, etc. They 

also recommended attaching the horizontal stabilizer midway up 

the vertical fin to avoid a loss of effectiveness due to the wing wake 

impinging upon it. A final joint recommendation was to have the 

horizontal stabilizer be adjustable, effectively making the combined 

stabilizer and elevator surface almost an “all-moving tail.” As an 

airplane’s speed increased, the shock wave moved aft on the horizontal stabilizer. When it reached the hinge line of the movable 

elevator, the control surface would be blanked out and the pilot 

would lose pitch control to such a degree that, in many cases, he 

could freely move the elevator without his control inputs having any 

effect upon flight path control. To avoid this, the stabilizer’s angle 

could be changed by the pilot, giving a measure of control. The 

same design approach was also applied to the D-558-1 and the Air 

Force’s F-86E Sabre jet fighter. Later supersonic aircraft such as the 

American F-100A and Soviet MiG-19 had a one-all-moving horizontal stabilizer.

36. Hallion, Supersonic Flight, pp. 52–53.

37. Hansen, Engineer in Charge, p. 288.

38. Ibid., p. 288.

39. Ibid., pp. 288–289.

40. Rotundo, Into the Unknown, p. 31.

41. Ibid., pp. 35–36. Stan Smith and Bell Aircraft management had 

a private reason for rejecting Muroc Army Air Field in California. 
 Confronting the “Sound Barrier”: The Bell XS-1

39

The West Coast was the home, by the mid-1940s, of many U.S. 

aviation contractors. Had the XS-1 gone to Muroc, many of the 

engineers might have decided to forego the Buffalo winters for the 

California sunshine.

42. Robert T. Jones, “Wing Plan Form for High-Speed Flight,” in 

Collected Works of Robert T. Jones, TM X-3334 (Washington, DC: 

NASA, 1976), p. 379.

43. Ibid., p. 379; Richard P. Hallion, “Lippisch, Gluhareff, and 

Jones: The Emergence of the Delta Planform and the Origins 

of the Sweptwing in the United States,” Aerospace Historian 26, 

no. 1 (spring 1979), passim; Hansen, Engineer in Charge, pp. 

282–283; MSN Encarta, “Sound Barrier,” http://uk.encarta.msn.

com/text_781533139__3/Sound-Barrier.html, accessed November 

30, 2009 (cached page). The development of Robert T. Jones’s wing 

planform paper shows the complex process by which technological 

and scientific breakthroughs are made. It started with Jones’s efforts 

to develop a lift theory for a dart-shaped missile. This, in turn, led 

Jones to a pair of very different studies. The first, by Max Munk, was 

of low-speed airflow around dirigibles, while the other, by Hsue-Shen 

Tsien, was on supersonic airflow around spinning projectiles, which 

indicated a lack of compressibility effects on these bodies. When Jones 

factored Tsien’s equations into his calculations, it appeared that very 

slender wings would also show no compressibility. Next, Jones recalled 

another paper by Munk, which dealt with the effects on aircraft stability due to dihedral and wing sweep. This was the first paper to describe 

the basic effects of sweepback on a wing. But because it only dealt 

with low subsonic speeds, the implications for transonic and supersonic flight went unrealized. Once Jones incorporated Munk’s second 

paper into the calculations, he was able to create a general theory that 

was valid for all wing-sweep angles. 

What Jones did not know was also important. Adolf Busemann, a 

German aerodynamicist, was among those giving papers at the Volta 

Congress on High-Speed Aeronautics in 1935. Part of his highly 

theoretical paper dealt with “arrow wings” as a means of reducing 

wave drag. Busemann’s theory did not include the idea of positioning the wing behind the Mach line, however. As a result, the cross 

flow over the wing was still supersonic. Three American scientists 

attended the Volta Congress—Eastman Jacobs, Hugh Dryden, and 

Theodore von Kármán. None of them viewed Busemann’s idea as 

important. Not until after Jones had written an initial draft of his 

paper was a 1942 British translation of Busemann’s paper found in 
Probing the Sky

40

the Langley library. Jones incorporated a citation listing it in the 

revised text. See Richard P. Hallion, “Sweep and Swing: Reshaping 

the Wing for the Jet and Rocket Age,” in Richard P. Hallion, ed., 

NASA’s Contributions to Aeronautics, vol. 1: Aerodynamics, Structures, 

Propulsion, Controls, SP-2010-570-Vol 1 (Washington, DC: National 

Aeronautics and Space Administration, 2010), pp. 5–17. 

Of the three papers by Munk and Tsien, written in the 1920s and 

1930s, only one was related to supersonic flight. Despite this, they 

played a role in Jones’s development of this swept-wing paper. This 

influence was due to Jones’s ability to realize their relevance to the 

problems he was investigating, despite their subject matter. The use 

of swept wings on the German Me 262 jet fighter and the Me 163 

rocket fighter also had no influence on Jones’s paper, as these aircraft 

were subsonic and the swept wings were used primarily to maintain 

their center of gravity. Indeed, the Me 163 had profoundly poor—

even dangerous—high-speed stability and control characteristics 

induced by its tailless swept-wing planform, as discussed subsequently in the chapter on the Northrop X-4 Bantam.

44. Hansen, Engineer in Charge, p. 284.

45. Ibid., p. 284.

46. Ibid., pp. 105–107, 284–285.

47. Ibid., pp. 281–284; Hallion, Supersonic Flight, p. 45.

48. Hansen, Engineer in Charge, pp. 284–285.

49. Ibid., pp. 289–290; Hallion, Supersonic Flight, pp. 46–47.

50. Rotundo, Into the Unknown, pp. 43–44.

51. Ibid., p. 50.

52. Ibid., pp. 51–59, 62. Langley’s location near the ocean and nearby 

rivers, as well as its humid climate, all ruled out operating the XS-1 

from the site. Pinecastle had better weather than Langley, but this 

still interfered with the glide flights. Given the poor visibility from 

the XS-1 cockpit and the landing accidents during the Pinecastle 

glide flights, it seems the loss of the airplanes would have been nearly 

certain had the flights operated from Langley’s runway.

53. Hallion, Supersonic Flight, p. 91.

54. John Ball, Jr., Edwards: Flight Test Center of the USAF (New York: 

Duell, Sloan and Pearce, 1962), pp. 38–41.

55. For the story of this remarkable engine, see Frank H. Winter, 

“‘Black Betsy’: The 6000C4 Rocket Engine, 1945–1989,” a paper 

presented at the 23rd Symposium on the History of Astronautics, 

40th International Astronautical Congress of the International 

Astronautical Federation, Malaga, Spain, October 1989.
 Confronting the “Sound Barrier”: The Bell XS-1

41

56. Hallion, Supersonic Flight, pp. 89–91.

57. Ibid., pp. 91–92, 98–102; Richard P. Hallion and Michael H. Gorn, 

On the Frontier: Experimental Flight at NASA Dryden (Washington, 

DC: Smithsonian Books, 2003), pp. 21–24.

58. Rotundo, Into the Unknown, pp. 102–103.

59. Ibid., pp. 118–119.

60. Ibid., pp. 104–109, 120, 125–126; James O. Young, Meeting the 

Challenge of Supersonic Flight (Edwards Air Force Base, CA: Air 

Force Flight Test Center History Office, 1997), p. 35.

61. Rotundo, Into the Unknown, p. 151.

62. Ibid., p. 152.

63. Ibid., p. 162. The reality was that the airplane Williams described 

was the one that did not exist. The turbopump would not be ready 

for several more years, no provisions for a reserve fuel supply were 

ever included in the aircraft, and a ground takeoff capability was 

impractical and unsafe.

64. Ibid., pp. 39, 181.

65. Ibid., pp. 191, 193–194; Young, Meeting the Challenge of Supersonic 

Flight, p. 36.

66. Young, Meeting the Challenge of Supersonic Flight, pp. 34–37.

67. Ibid., pp. 38–39.

68. Ibid., p. 40.

69. Young, Meeting the Challenge of Supersonic Flight, pp. 39–41.

70. Ibid., pp. 41–42.

71. Rotundo, pp. 230–231.

72. Ibid., pp. 230–231.

73. Young, Meeting the Challenge of Supersonic Flight, p. 45.

74. Ibid.

75. Ibid.

76. Ibid.

77. Ibid., pp. 45–46.

78. Ibid., p. 47.

79. Ibid., p. 48.

80. James O. Young, Supersonic Symposium: The Men of Mach 1, Air 

Force Special Code (AFSC) Special Study, September 1999, p. 219.

81. Axel T. Mattson, “Force and Longitudinal Control Characteristics 

of a 1/16-scale Model of the Bell XS-1 Transonic Research Airplane 

at High Mach Numbers,” NACA RM L7A03 (May 21, 1947), 

p. 9–10. All NACA Research Memorandums in this book can be 

found on the NASA Technical Reports Server (http://ntrs.nasa.gov/

search.jsp) or within Dryden Flight Research Center’s archives.
Probing the Sky

42

82. From flight transcript, reprinted in Young, Meeting the Challenge of 

Supersonic Flight, p. 73.

83. Hallion, Supersonic Flight, pp. 113–119, 124–125; Rotundo, Into the 

Unknown, pp. 274–279; Young, Meeting the Challenge of Supersonic 

Flight, pp. 52–59. The secret held until December 22, 1947, when 

the Los Angeles Times carried a front-page story. The headline read, 

“U.S. Mystery Plane Tops Speed of Sound.” The story had a number 

of significant errors: Captain Charles “Yaeger” had broken the sound 

barrier at 70,000 feet, NACA research pilots Herbert Hoover (true) 

and Howard Lilly (false) had also flown supersonic, and Goodlin 

had made the first 30 flights (false). Not until June 15, 1948, was 

the flight officially confirmed in a joint press conference by General 

Hoyt S. Vandenberg, Air Force Chief of Staff, and Dr. Hugh L. 

Dryden, NACA Director of Research. On December 17, 1948, 

the Robert J. Collier Trophy, aviation’s most prestigious award, 

was given to the XS-1 project. Its citation read: “To Robert Stack, 

Research Scientist, NACA, for pioneering research to determine the 

physical laws affecting supersonic flight, and for his conception of 

transonic research airplanes; to Lawrence D. Bell, President, Bell 

Aircraft Corporation, for design and construction of the special 

research aircraft X-1; and to Captain Charles E. Yeager, U.S. Air 

Force, who, with that airplane, on October 14, 1947, first achieved 

human flight faster than sound.” 

It is ironic that Stack, who preferred the D-558 over the X-1 and 

who objected to its propulsion system, its launch method, and its 

test site, should win the Collier Trophy for its achievements. In contrast, Ezra Kotcher, whose “Mach 0.999 study” and whose support 

of a rocket engine over a turbojet led directly to the X-1, was most 

unfortunately ignored, at least in the Collier citation.

84. The symposium papers were subsequently published by the USAF 

as Air Force Supersonic Research Airplane XS-1 Report No. 1 (Wright 

Field, Dayton, OH: USAF Air Materiel Command, January 9, 

1948). This historic report was subsequently reprinted and reissued 

in October 1997 by Richard P. Hallion, then the Air Force Historian 

at the Air Force History and Museums Program, Headquarters Air 

Force, Washington, DC, in commemoration of the 50th anniversary 

of the first piloted supersonic flight.

85. These reports included the following: Ellwyn E. Angle and Euclid C. 

Holleman, “Determination of Longitudinal Stability of the Bell X-1 

Airplane from Transient Responses at Mach Numbers Up to 1.12 at 

Lift Coefficients of 0.3 and 0.6,” NACA RM L50I06a (November 7, 
 Confronting the “Sound Barrier”: The Bell XS-1

43

1950); De E. Beeler and John P. Mayer, “Measurements of the 

Wing and Tail Loads During the Acceptance Tests of Bell XS-1 

Research Airplane,” NACA RM L7L12 (April 13, 1948); L. Robert 

Carman and John R. Carden, “Lift and Drag Coefficients for the 

Bell X-1 Airplane (8-Percent-Thick Wing) in Power-Off Transonic 

Flight,” NACA RM L51E08 (June 25, 1951); Hubert M. Drake 

and John R. Carden, “Elevator-Stabilizer Effectiveness and Trim of 

the X-1 Airplane to a Mach Number of 1.06,” NACA RM L50G20 

(November 1, 1950); Hubert M. Drake, Harold R. Goodman, 

and Herbert H. Hoover, “Preliminary Results of NACA Transonic 

Flights of the XS-1 Airplane with 10-Percent-Thick Wing and 

8-Percent-Thick Horizontal Tail,” NACA RM L8I29 (October 13, 

1948); Hubert M. Drake, Milton D. McLaughlin, and Harold R. 

Goodman, “Results Obtained During Accelerated Transonic Tests 

of the Bell XS-1 Airplane in Flights to a Mach Number of 0.92,” 

NACA RM L8A05a (April 19, 1948); Harold R. Goodman and 

Hubert M. Drake, “Results Obtained During Extension of U.S. 

Air Force Transonic-Flight Tests of XS-1 Airplane,” NACA RM 

L8I28 (November 16, 1948); Harold R. Goodman and Roxanah B. 

Yancey, “The Static-Pressure Error of Wing and Fuselage Airspeed 

Installations of the X-1 Airplanes in Transonic Flight,” NACA RM 

L9G22 (August 12, 1949); Walter C. Williams and De E. Beeler, 

“Results of Preliminary Flight Tests of the XS-1 Airplane (8-Percent 

Wing) to a Mach Number of 1.25,” NACA RM L8A23a (April 6, 

1948); and Walter C. Williams, Charles M. Forsyth, and Beverly 

P. Brown, “General Handling-Qualities Results Obtained During 

Acceptance Flight Tests of the Bell X-1 Airplane,” NACA RM 

L8A09 (April 19, 1948).
44

The third D-558-1 parked on the South Base ramp with three ground personnel. The Skystreak 

was one of two concepts for how best to gain data on transonic flight. It represented a traditional design, had straight wings, and was jet-powered, with a top speed just below Mach 1. It 

was funded in part by the Navy and built by Douglas to NACA specifications. In contrast, the 

Army Air Forces XS-1 was rocket-powered, with a top speed well above Mach 1. (NASA)
CHAPTER 2

Flying Test Tube: 

The Douglas D-558-1 Skystreak

The Model D-558 gives the impression of being an outstandingly excellent job of 

design and engineering, and a very sound airplane for research purposes....

—Captain Frederick M. Trapnell, U.S. Navy1

If the D-558-1 could have been promoted in the early forties, it would have been 

timely. But coming into the flight picture as it did in 1947, it was unnecessary.

—John V. Becker, NACA

The Douglas D-558-1 Skystreak always flew in the shadow of the Bell XS-1.2

If radical by the standards of conventional propeller-driven airplanes, then by 

the standards of the rocket-powered XS-1 the Skystreak was a very conservative 

design, reflecting the NACA’s desire for a turbojet-powered research airplane 

that could effectively “loiter” in the transonic regime, nibbling at the sonic frontier. If overshadowed by its flashier rocket-powered contemporary, it admirably 

fulfilled the expectations of the NACA, playing a significant role in the early years 

of high-speed research with its capability to undertake a wide range of research 

activities over a 6-year period, effectively “freeing up” its higher-performance 

rocket-powered rivals to explore the frontiers of the supersonic regime while it 

generated detailed information on the high-subsonic and transonic. 

In performance, appearance, mode of operation, and systems, the D-558-1 

appeared little different in design from the first generation of U.S. jets. Chief 

engineer Edward H. Heinemann, L. Eugene Root, Kermit Van Every, A.M.O. 

Smith, Robert C. Donovan, R.G. Smith, and the other members of the Douglas 

design team faced a number of challenges if the D-558-1 was to be capable 

of accomplishing the research goals that the NACA and the Navy’s Bureau of 

Aeronautics had set for it.3

The first of these tasks was to minimize the fuselage’s frontal area so as to 

maintain a high fineness ratio, the ratio of the fuselage’s length to its diameter. 

Drop-body tests showed that a high fineness ratio reduced drag at transonic 

speeds. At the same time, the fuselage shape had to prevent airflow from being 

accelerated to supersonic speeds, which would cause a flow-disrupting shock 

45
Probing the Sky

46

wave to form in the vicinity of the wing. To address these issues, the best solutions were also the simplest.

The team selected a cylindrical fuselage, one with a diameter just big enough 

to hold the TG-180 turbojet engine; indeed, afterwards, A.M.O. Smith (who 

determined the wing planform and airfoil section and did much of the work on 

the aerodynamics of the nose inlet) recalled the design process as “a case of wrapping the smallest airplane around the largest jet engine that was available.”4

 This 

shape also minimized airflow acceleration over the wing. A final design requirement was that of the fuselage/wing fillet, which needed a critical Mach number 

as high as that of the wing but which would not exhibit poor stall characteristics. 

Meeting this specification would require considerable NACA wind tunnel testing.

The wings used the NACA’s 65-110 airfoil (a symmetrical “65-series” wing 

section with a thickness-chord ratio of 10 percent), as it had a high critical 

Mach number, had good high-speed characteristics, and had been used previously by the Douglas engineers for their A-26 medium bomber, the highestperformance twin-engine medium bomber developed during the Second 

World War. As with the XS-1, the Douglas team employed a straight-wing 

planform. But in contrast to the relatively high aspect ratio of the XS-1’s wings, 

the D-558-1 had low-aspect-ratio wings (a short wingspan with a wide chord). 

Further, all the D-558-1s employed a 10-percent thickness-chord section, not 

the lower 8-percent thickness-chord section employed on the first of the XS-1s. 

The horizontal stabilizer was similar to that on the XS-1. It had a thinner airfoil 

than did the wing (6 percent for the stabilizer versus 10 percent on the wing) 

and was mounted higher up on the vertical tail, with the forward section made 

movable to retain control at high transonic speeds. The pilot had a switch on 

the control wheel for moving the stabilizer up and down.5

The D-558-1 structural design was described by Heinemann as being comparatively conventional and straightforward. It did have some unusual features, 

however, that were different from those of contemporary aircraft structures. 

In an era dominated by aluminum skinning, the Skystreak’s fuselage skin was 

a magnesium alloy slab 1/10-inch thick, which eliminated the need for formers or stringers, both used in conventional fuselage designs, thus ensuring a 

light yet rigid and strong fuselage tube, saving at least 60 pounds in structural 

weight over a conventional rib-stringer-skinning design. The only internal 

reinforcement used was at points of concentrated structural loads. As a result, 

the D-558-1’s internal volume was relatively large. 

The wings and the tail were of standard aluminum construction, using 

high-strength alloy in a conventional rib and spar matrix. To ensure that their 

external skins were as smooth as possible, sheet-metal fabricators and craftspeople attached the fuselage and wing skins to rigid contoured frames and then 

attached the internal framework directly to the skin. 
 Flying Test Tube: The Douglas D-558-1 Skystreak

47

The thin wings posed several design problems. First was fuel capacity, for 

the aircraft had to carry enough fuel for a research flight lasting about 30 minutes from takeoff through landing. The solution designers chose was to make 

the front half of the wing into an integral fuel tank, a so-called “wet wing.” 

Technicians sealed the wing interior using a synthetic rubber compound, which 

required 5 weeks to cure and dry. Once ready, the tank formed by this process 

held 230 gallons of kerosene fuel. To extend flight time and altitude, a pair of 

50-gallon tip tanks could also be carried, which when used added another half 

hour to flight duration (and which were, in fact, employed at various points 

during the test programs flown on the three Skystreak aircraft). 

The D-558-1 had tricycle landing gear, but, here, too, the thin wing created a challenge. While the nosewheel could retract into the forward fuselage 

behind the cockpit, the tightly packed fuselage—occupied by the engine, 

controls, instrumentation, and other systems—lacked any room for the twin 

main wheels. Accordingly, the only place for the main landing gear was inside 

the wings, but, given the thin airfoil, the wheels had to be much thinner and 

smaller than would have been typical for an aircraft of the D-558-1’s size. 

Douglas engineers approached the Goodrich Corporation to produce special 

20-by-4.4, 8-ply nylon tires. Originally, the tires were intended to operate at 

230 pounds per square inch (psi), though this was later reduced to a more 

forgiving 175 psi. As Heinemann later noted, “It was realized from the outset 

that this wheel and tire size was much smaller than desired for the load, but 

the selection was considered justified due to the serious effect larger wheels 

would have had upon the size of the airplane.”6

Additionally, Douglas engineers had the issue of pilot escape from the 

D-558-1 at high speeds to consider. Aeromedical experts were asked about 

escape options and indicated they doubted a human could withstand a normal 

bailout at high speeds. The air blast was considered too great for a pilot to survive, and the acceleration needed to propel both an ejection seat and the pilot 

clear of the vertical tail was in excess of human anatomical limits. 

The approach taken was to use a capsule escape system. The capsule’s nose 

was attached to the fuselage at four points. In an emergency, the pilot would 

pull a handle to release the four attachments, freeing the nose section. Once 

it fell away and slowed, the pilot pulled a second handle that dropped the 

seat back, then fell out of the nose section and opened his parachute. Similar 

capsules were later fitted to the D-558-2 and the Bell X-2 research aircraft. In 

both X-2 crashes, the capsules proved ineffective, and both pilots died.7

To aid optical tracking from the ground, the D-558-1 was painted insignia 

red (though this was subsequently changed to white, as the red actually made 

the aircraft far less visible against the dark blue Mojave sky). The aircraft carried 

standard U.S. “star-and-bar” markings, and the words “Douglas Skystreak” were 
Probing the Sky

48

painted on the nose in a stylized typeface suggesting speed and modernity. With 

its red paint finish, cigarette-like cylindrical fuselage, nose inlet, and rounded silver 

jet nozzle, the aircraft was not surprisingly nicknamed the “Crimson Test Tube.”8

“The arrangement of the airplane was quite conventional,” Heinemann subsequently recalled, “as it was believed the best way to obtain the largest amount 

of useful data in the shortest period of time was to employ only design features 

that were well known and did not involve uncertainty.”9

 He also noted, “It is 

considered necessary that all of the high speed aerodynamic data be obtained 

in level flight instead of vertical or near vertical dives as in the past.”10 These 

stipulations were a reflection of the NACA’s viewpoint, goals, and influence.

First Flights and Speed Records

In January of 1947, the D-558-1 #1 (Bureau of Aeronautics number [BuNo] 

37970) was completed and several months of ground testing was begun. Two 

trucks then transported the disassembled aircraft from the Douglas plant at 

El Segundo over the San Gabriel Mountains to the Mojave Desert and Muroc 

Army Air Field. After reaching Muroc on April 10, the aircraft was reassembled, 

the engine and other systems were tested, and preparations were made for the 

first flight. 

Veteran Douglas test pilot Eugene F. “Gene” May, who had flown for the 

company since 1941, was selected for the initial flights. The airplane was ready 

by April 15 for the first flight. May took off from the lakebed and immediately 

ran into trouble. The TG-180 engine suffered a partial power loss, and May 

landed straight ahead on the lakebed. When he applied the brakes, the left 

brake disintegrated and he had to hold the left rudder to keep rolling forward 

in a straight line. On April 21, similar problems plagued the second attempt, 

grounding the D-558-1 until the end of May 1947. 

Landing gear problems were encountered on the next six flights, with the 

gear either not retracting or locking in place. Even so, by the 12th flight, on July 

12, 1947, the aircraft had demonstrated satisfactory low-speed flight characteristics. Douglas engineers began modifications needed for transonic research 

flights. The low-speed, clear-bubble canopy was replaced with a V-shaped, 

reinforced high-speed hooded windscreen. The new canopy design was found 

by several pilots to be too small to allow them to wear a helmet, and they flew 

without one. With the work complete, the aircraft made an airspeed calibration check on its 13th flight. 

May now began a buildup to transonic speeds, beginning with the 14th 

flight made on July 17. During the course of the next six flights, May reached a 

speed of Mach 0.85 on August 5. During this same period, the second D-558-1 
 Flying Test Tube: The Douglas D-558-1 Skystreak

49

(BuNo 37971) was completed and delivered to Muroc Army Air Field, where 

it was to be used by NACA pilots. The next step, however, was an attempt at 

the world’s airspeed record.11

From left to right, Eugene May (Douglas Aircraft) and Howard Lilly (NACA research pilot) pose in 

front of the second D-558-1, which was destroyed in a crash on May 3, 1948. Lilly was killed in 

the accident, the first NACA research pilot lost in the line of duty. (NASA)

The Navy was interested in using the second D-558-1 to best the existing airspeed record of 615.778 mph, set by British Royal Air Force Group 

Captain E.M. Donaldson in a modified Gloster Meteor in September 1946. 

The rules for an official record recognized by the Fédération Aéronautique 

Internationale (FAI) required that an aircraft fly at an altitude below 250 feet 

(75 meters) and make four passes along a 1.864-mile (3-kilometer) course. 

While the discussions were underway, Donaldson’s record had fallen to the 

AAF. Colonel Albert Boyd broke the record on June 17, 1947, in a modified 

P-80R jet, reaching a speed of 623.738 mph, the last speed record set by the 

service before it transformed into the independent United States Air Force in 

September of that year. The higher speed increased the challenges both the 

airplane and pilots would have to overcome.

With the second D-558-1 delivered and test-flown, the way was clear for the 

record attempts. Navy Commander Turner F. Caldwell made the first record 

attempt on August 20, 1947, in the first D-558-1 and achieved an average 

speed in the four passes of 640.663 mph, breaking the standing record by 

16.924 mph. Marine Major Marion E. Carl’s turn came on August 25. He 

had an idea for a way to squeeze a little more speed from the second D-558-1. 
Probing the Sky

50

On Caldwell’s flight, the TG-180 engine had produced 100 percent rotations 

per minute (rpm) on the ground, but power readings dropped to 98 percent 

rpm once he took off. Carl convinced the Douglas ground crew to raise the 

engine’s rpm to 102 percent on the ground. As expected, the same rpm drop 

occurred after takeoff, leaving Carl with 100 percent rpm in flight. When his 

four passes were averaged, he had achieved 650.796 mph, raising the record 

by 10.133 mph, small but enough.12

With the world speed record now in Navy/Marine hands, attention shifted 

back to the test-and-research effort. The first D-558-1 resumed its contractor 

flights, piloted by May. Another 18 flights were flown before it was turned over 

to the NACA on October 23, 1947, a week after Chuck Yeager and the rival 

XS-1 had broken the sound barrier. 

The NACA D-558-1, the second of the three Skystreaks, had not been 

fitted with data instrumentation, so the first step in preparing the aircraft was 

to install a standard NACA recording package, overseen by the Muroc Flight 

Test Unit’s chief instrumentation and telemetry tracking engineer, Gerald M. 

“Gerry” Truszynski:13

Instrumentation Data Collected

Airspeed-altitude recorder Indicated airspeed and pressure altitude

Three-component accelerometer Normal, longitudinal, and transverse acceleration

Angular-velocity recorder Rolling velocity

Sideslip-angle recorder Sideslip measurement

Wheel-force recorder Aileron and elevator forces

Pedal-force recorder Rudder-pedal force

Control-surface position recorder Aileron, elevator, rudder, and stabilizer position

Consolidated oscilloscope Wing-bending moment/sheer load and horizontal 

tail sheer load

Common timing circuit Synchronize collected data

It was not until late November that the work had progressed far enough 

for a flight to be made. NACA research pilot Howard C. “Tick” Lilly had 
 Flying Test Tube: The Douglas D-558-1 Skystreak

51

been selected to fly the second D-558-1. The first NACA flight was made on 

November 25, 1947, for pilot familiarization, but it was cut short after instrumentation problems. Lilly made a second flight the following day, but it also 

had to be aborted due to both instrumentation problems and the failure of the 

landing gear to lock properly after retracting. With the onset of the winter rainy 

season, Rogers Dry Lake was flooded. Engine modifications also had to be made. 

Taken together, all these factors halted flight operations for several months.14

Project managers used the winter downtime to make another modification. 

As mentioned previously, on the ground and at low altitudes over the desert the 

Skystreak was eye-catching in its dark red finish, but optical and photographic 

trackers had a hard time spotting it when it flew at higher altitudes, against the 

dark blue Mojave sky. Walter C. Williams later wrote, 

It was found...that very little photographic contrast was being 

obtained between the red airplane and the relatively dark blue 

sky conditions prevalent in this area[,] with the result that photographs could not be obtained to ranges greater than the order of 

25 to 30 thousand yards. It was reasoned that the photographic 

contrast could be increased by using the lightest color possible 

against the darker sky. On this basis, the aircraft was test painted 

white and both visibility and photographability were found to be 

greatly increased. With proper filtering techniques, photographs 

of the airplanes are now taken to greater than 60,000 yards and 

are generally visible over their entire test flight range.15

Disaster: The NACA Loses Its First Pilot

The second D-558-1 had a history of landing gear problems. On November 

26, 1947, on the second NACA flight, the landing gear door would not lock. 

Between March 31 and April 7, 1948, the problem caused NACA flights 

4 through 7 to be aborted. After several successful research flights, the gear 

problem reoccurred on April 28 when the right landing gear did not retract, 

forcing an abort of the 16th NACA flight.16

The problem reoccurred on May 3, 1948. At noon, after takeoff on the 

18th NACA flight, the landing gear failed to lock in the full-up position. Lilly 

landed the aircraft, and the next several hours were spent troubleshooting the 

problem. When that was finished, the ground crew towed the D-558-1 to the 

west end of the runway; Lilly started the engine and took off heading east. The 

landing gear retracted normally, and the aircraft accelerated to a speed of about 
Probing the Sky

52

250 mph at an altitude of 100 to 150 feet above the runway. Several ground 

crewmembers continued watching the D-558-1 and saw “a large piece of white 

material” separate from the fuselage.17

Smoke and flames began coming from the fuselage as the aircraft maintained level flight for several seconds. The D-558-1 began a left yaw and right 

sideslip roll, which continued until the left wingtip, canopy, and vertical tail 

struck the lakebed. The impact point was about 1,800 feet beyond where the 

large piece of fuselage skin had landed. The aircraft bounced into the air and 

broke up, hitting the ground about 400 feet farther along the flightpath and 

scattering debris over a wide area. The fire was extinguished by Air Force personnel. Lilly was killed on impact, the first NACA research pilot killed in the 

line of duty since the agency’s founding in 1915. 

Crash investigators began by carefully searching for evidence along the takeoff 

path, locating and tagging debris and marking it on a grid chart to establish the 

sequence of events. The first pieces found were fragments of the engine compressor case and blades, bits of the fuselage skin from the top of the engine section, 

and paint chips, evidence of in-flight engine disintegration. These were located 

less than 2 miles from where takeoff had begun. About 0.2 miles farther down 

the flightpath was the 4-foot-square section of white fuselage skin spotted by 

witnesses. Examination showed no evidence of fire. Instead, it was determined 

that compressor case and blade debris had torn through the fuselage skin. 

These fragments struck a vulnerable spot on the top of the fuselage. The 

single set of rudder and elevator cables passed between this section of skin and 

the compressor case. As the fuselage skin separated, the control cables were 

“plucked,” causing a leftward movement of the rudder, a right sideslip, and 

a left roll. The right rudder cable and the up elevator cable were severed. In 

contrast, the left rudder cable, down elevator cable, and aileron cables survived 

the disintegration of the compressor case but broke on ground impact. 

Examination of the debris revealed that the fire had broken out after the 

compressor case had broken apart. The left side of the rudder was badly burned, 

but the right side showed no sign of fire, confirming that the rudder had moved 

to the left before impact. The fixed vertical tail, however, had heavy fire damage 

on both sides. The tail cone bore a burn pattern consistent with flames at an 

angle approximately that of a full left rudder.

The investigation board examined the TG-180 engine and determined that 

the rapidly spinning compressor rotor had suddenly stopped turning, causing 

the compressor shaft to fail. Many of the compressor blades were torn out, 

and the compressor casing had broken up with apparently explosive force. The 

board learned that three other TG-180 engines had suffered similar failures in 

earlier aircraft accidents, a pattern that pointed to fatigue failure of the blades as 

a potential primary cause. The board noted other possible causes: blades damaged 
 Flying Test Tube: The Douglas D-558-1 Skystreak

53

by objects being sucked into the engine, or a failure in a steel spacer ring between 

the 10th and 11th rotor stages. 

The board also learned that three TG-180 engines assigned for use in the 

D-558-1 aircraft had been rejected by NACA and Navy inspectors after numerous 

large nicks had been found on the edges of the engines’ turbine blades. Inspection 

of the compressor for such damage is practically impossible as a routine preflight 

check, but if present, it could lead to blade failure. Additionally, investigators 

learned that the steel spacer ring had been replaced in later-production TG-180 

engines with an aluminum ring to reduce the potential for failure. This change 

had not been made in the engines for the NACA aircraft.

The investigation board made a number of recommendations aimed at improving flight safety. The board wanted only newly manufactured TG-180 engines 

fitted in the two remaining D-558-1 research aircraft before flights were resumed. 

Board members also wanted improvements in preflight inspections, as well as in 

specifications and manufacturing and inspection procedures for turbine blades.

Further, the recommendation was made that controls, fuel lines and pumps, 

and electrical circuits located near the compressor section be protected and 

that control cables should be armored, shielded, or duplicated to protect them 

from damage. 

The board was also critical of the D-558-1 canopy and cockpit design. Lilly had 

not worn a crash helmet in the aircraft because the narrower high-speed canopy 

did not leave sufficient room for one. He had earlier removed the shoulder straps 

because he found them inconvenient. The board also discovered that the cockpit 

space was restrictive and quite dark. The pilot had to lower his head to see some of 

the instruments and could not see both outside and inside the cockpit at the same 

time. The board urged that a study and redesign of the cockpit be undertaken. 

Given that the idea of air-launching the XS-1 had been controversial among 

Langley researchers, and in light of their repeated demands that the aircraft 

be ground-launched, one of the board’s recommendations was ironic: “As a 

safeguard for personnel and valuable research equipment, the fairly well proven 

use of air launching should be given more consideration. The great difference 

in wing loading between take-off and landing, as well as the magnitude of 

the loading, and the desirability for maneuvering capability at the start of an 

uncertain flight emphasize the value of air launching.”18

First Research Results

As the investigation of Lilly’s crash was underway, Williams wrote up the first 

research results from the limited longitudinal-stability and control data taken 

during the second D-558-1’s short operational life. The data, taken during the 
Probing the Sky

54

Skystreak’s airspeed calibration flights, up to Mach 0.85, were published less 

than 2 months after the crash.19 The flights involved making level flights at a 

30,000-foot-pressure altitude and increasing the aircraft’s speed from Mach 

0.55 to Mach 0.85. During the flight, changes in elevator position and force 

required to trim the aircraft were recorded. These were used to determine the 

aircraft’s longitudinal stability and any trim changes caused by the effects of 

transonic speed, such as the formation and movement of shock waves on the 

wings and stabilizers. The flights were done at stabilizer incidence angle settings of 1.95° and 2.32°. 

Williams wrote, “The results of measurements of the elevator angle and 

force required for trim at Mach numbers up to 0.85 show that below a Mach 

number of 0.80 the D-558-1 airplane possesses positive static longitudinal 

stability. Above a Mach number of 0.82, there is a nose-down trim change.”20

Charting of the elevator force, specifically how many pounds of force the pilot 

had to use to counter any trim changes, made clear what had occurred. Between 

Mach 0.6 and 0.75, only minor pressure was needed to keep the D-558-1 level. 

Once the aircraft reached Mach 0.8, however, the amount of pressure required 

sharply increased. At Mach 0.85, the pilot had to pull back on the control yoke 

with nearly 10 pounds of force to keep the airplane level. 

As the NACA realized, the report constituted a “quick look,” rather than 

a detailed, incisive examination. The airspeed calibration data had not been 

completely evaluated, but the error margin was estimated to be 1 percent or 

less. This estimate was based on comparisons with the airspeed calibrations 

done with the XS-1, also at Mach 0.85. There were also instrumentation issues; 

no elevator-position data were obtained above Mach 0.8 for a stabilizer setting 

of 2.32°, as the recorder’s film had run out. Finally, the stabilizer incidence 

angles were close together, making it difficult to determine the relative elevator 

effectiveness over various Mach numbers. 

The results were considered valid, however, as the elevator force and angle 

data showed the aircraft had positive longitudinal stability up to a Mach 

number of about 0.80 with the control wheel both fixed and free. Above a speed 

of about Mach 0.82, the data showed that a trim change occurred, the first 

indication of compressibility effects in level flight. A similar trim change also 

appeared in the data derived with the second XS-1, which also had a 10-percent 

wing. NACA researchers subsequently issued further reports likewise based on 

the initial flights of the second Skystreak, following these reports with a more 

extensive study by Williams examining the aircraft’s stability characteristics 

during sideslips, dated April 18, 1949.21

The technique used by the pilot to measure the static directional stability 

of the D-558-1 was to slowly deflect the ailerons, creating gradually increasing 

sideslips. At the same time, he added enough rudder and elevator to maintain 
 Flying Test Tube: The Douglas D-558-1 Skystreak

55

level flight. The Skystreak was flying level at a constant altitude, but with its 

nose angled to one side. Two series of tests were flown. One was made at a 

10,000-foot-pressure altitude and at indicated airspeeds between Mach 0.50 

and 0.80. The second series was made at a 30,000-foot-pressure altitude, at 

Mach 0.50 and 0.84. (The indicated airspeed was above that where the nosedown trim change occurred.)

Once the data were collected, the rudder, aileron, and elevator positions 

and the forces and angles of bank were plotted on graphs as functions of 

sideslip angle. Bank angles were obtained from measurements of transverse 

acceleration. Variations of rudder position and force with the sideslip angle 

gave a measure of the aircraft’s static directional stability, with both fixed and 

free control. The dihedral effect, which is the rolling moment of an aircraft 

caused by the spanwise inclination of the wings, was measured with both fixed 

and free controls. This was illustrated in the variations of aileron position and 

force with the sideslip angle. The pitching moment due to the sideslip was 

indicated by the variation of the elevator position and force with sideslip angle. 

The variation of angle of bank with sideslip angle gave a measure of crosswind 

force characteristics. The report concluded:

• “The apparent directional stability of the D-558-1 was high 

throughout the speed range covered, but was greater at low altitudes 

than at high altitudes at any given Mach number. There was also an 

increase in directional stability with an increase in Mach number.”

• “The increase in directional stability at lower altitudes was probably 

due to a decrease in rudder efficiency, caused by distortions of the 

vertical tail and fuselage by higher dynamic pressure.” 

• “The dihedral effect was positive but low over the speed range tested.”

• “There was little to no change in pitching moment with sideslip and 

the cross-wind force was positive.”22

Several issues related to data collection had been neglected. No measurements of rudder forces were recorded at 10,000 feet. There were also discontinuities in the variation of both the aileron force and the position with 

sideslip angles near zero. Williams concluded that the problems with the force 

measurements were due to friction in the control system. He believed the 

position errors were caused by play in the linkage between the aileron and the 

point of measurement. Despite this, Williams believed the slope of the curves 

should provide a good measure of dihedral effect.23 These two reports contained 

preliminary data drawn from a few early flights. A more detailed report was 

issued on April 22, 1949, and contained information on the D-558-1’s highspeed characteristics, up to Mach 0.89, that was not included in Williams’s 

two early reports.24
Probing the Sky

56

The stability measurements used in the report were primarily derived 

from two of Lilly’s high-speed flights. The two flights, made at an altitude 

of 40,000 feet, reached Mach 0.89 and used stabilizer incidence angles of 

2.3° and 1.4°. The results of the different settings were significant. The 2.3° 

stabilizer-incidence angle resulted in the aircraft’s becoming increasingly 

nose-heavy as the Mach number increased above Mach 0.80. During the 

initial phase of the recovery maneuver, Lilly had to pull back hard on the 

control yoke to decrease the Mach number. As the Mach number decreased 

over a 10-second period, nose heaviness was also reduced, and Lilly had to 

reduce the pull force to avoid too great an acceleration.

The Skystreak’s behavior was very different at a stabilizer incidence angle 

of 1.4°. Above Mach 0.83, the Skystreak became increasingly tail-heavy. 

During the 24-second recovery phase, Lilly simply reduced the push force; 

the airplane’s speed dropped from Mach 0.88 to 0.834, and he completed a 

normal recovery. When the sideslip angle and the control forces and positions 

were plotted as a function of Mach number, the differences were glaringly 

apparent. Depending on the stabilizer setting, a pull- or push-force of 30 

pounds was required to correct trim. 

During both high-speed flights, Lilly reported that above Mach 0.84 the 

right wing became very heavy, and he had to make increased left aileron 

inputs to correct it. He added that the wing heaviness was not continuous, 

which made the aircraft’s lateral stability feel uncertain at its highest speed. 

Lilly found it difficult to determine the amount of lateral control needed 

for trim. His control movements resulted in lateral oscillations. The report’s 

author believed some of this problem to be due to aileron fraction. 

Some stability and control data were also collected during several incrementally increasing turns made at an altitude of 30,000 feet, at Mach numbers between 0.50 and 0.80. A single turn was made at 10,000 feet and 

Mach 0.71. The data from the turns showed positive longitudinal stability 

throughout the speed range. The lowest value was recorded at Mach 0.675, 

and beyond this point stability increased with increasing Mach number. 

These results were also consistent with those from the XS-1 flights. 

The single test at 10,000 feet indicated that the apparent stability was 

higher at this altitude than at 30,000 feet. The report noted, “Some of this 

difference can be accounted for by the effects of altitude but it is also possible that, because of the higher dynamic pressure at the lower altitudes, the 

apparent stability is altered by distortion effects.”25 This increase in apparent 

stability at lower altitude was also noted in Williams’s second report, also 

issued in April 1949.

Buffeting occurred on both of the high-speed runs, beginning at a speed 

of about Mach 0.85. The D-558-1’s buffet boundary was determined not 
 Flying Test Tube: The Douglas D-558-1 Skystreak

57

only during speed runs, but also in straight stalls and turns. The normal-force 

coefficients necessary to cause buffeting were plotted as functions of Mach 

number, to define the combination at which buffeting began. 

Data from the XS-1 flights confirmed the Skystreak results. The XS-1 was 

also flown with the same two stabilizer incidence angles as the D-558-1, and 

in both cases the aircraft became nose-heavy at higher speeds. The XS-1 also 

showed similar wing-heaviness behavior. This similarity was to be expected, 

as both had 10-percent wings and 65-110 wing sections.26

Modifications and Flights Resume 

With the death of Lilly and the destruction of the second D-558-1, the two 

remaining aircraft were grounded until the accident investigation was completed. During this period, the D-558-1 #1 (the Douglas aircraft) underwent 

modifications to fix the problems identified by the accident board and was 

returned to Muroc. 

Gene May resumed the Douglas flight-test program, which entered a dangerous phase during which stability and control data for speeds of up to the 

aircraft’s maximum Mach number were to be collected. Because of the D-558-

1’s limited performance capability, May had to make risky dive flights, similar 

to those done in the P-38s and other aircraft, rather than level speed runs, as 

were done with the X-1. 

The test program involved 10 dive tests, 5 with wingtip fuel tanks and 5 

without. A dive would begin at 40,000 feet, with a pullout following at around 

30,000 feet. On September 29, 1948, May exceeded Mach 1 in a 35° dive 

in D-558-1 #1, constituting the only time a Skystreak was to fly supersonic. 

Stability and control deteriorated badly at Mach 0.84, with the aircraft oscillating laterally. As speed increased, the left wing became heavy. Longitudinal 

stability decreased above Mach 0.94, with the aircraft “tucking under.” This 

was similar to what had been experienced with piston-powered aircraft during 

transonic dive flights. 

The dive tests with the tip tanks followed in early November 1948. With 

the stock tank configuration, drag was reduced and range increased. However, 

top speed was also reduced, and the takeoff roll was longer. The tanks were 

later fitted with endplates in a study of wing airflow. On November 4, 1948, 

Gene May made another dive test to Mach 0.945. The flight data showed the 

endplates had stopped spanwise flow separation over the wings, which disrupted lift. Despite this, the endplates were not used again on the D-558-1. 

These endplates, in some respects, anticipated the development of the vortexreducing winglet two decades later.27
Probing the Sky

58

The third D-558-1, which was assigned to the NACA, had been trucked 

to Muroc on November 4, 1947, but, with attention focused on the first and 

second aircraft, it had made only four flights by early 1948. Following Lilly’s 

crash, the third D-558-1 was disassembled and trucked back to the Douglas 

plant for modifications. The work involved adding duplicate control cables and 

1⁄4-inch stainless steel armor to protect the emergency fuel pump and fuel lines. 

Engineers also tested the vulnerability of the high-pressure fuel hoses to shrapnel from an exploding engine. This was done by firing .22-caliber rifle bullets 

into the standard fuel hose. The bullets easily punctured the standard hose. As a 

result, these were replaced by wire-wound fuel hoses, which were better able to 

withstand high-velocity impacts. 

The final change Douglas made was to repaint the third D-558-1 in an 

overall white finish. This posed a complication with the control surfaces. A 

letter from R.B. Cox, of Douglas, to Walter C. Williams, dated October 11, 

1948, noted: 

When the aircraft was sprayed with the white undercoat...the 

control surfaces were painted. I had seen a schedule of work, 

issued by the chief engineer for this division, which called for a 

white paint coat on the entire ship, except for the control surfaces. I called this to the attention of the aircraft project engineer 

and investigation proved the shop order was in conflict with the 

original order. Further investigation showed the reason for not 

painting the surfaces involved a problem of weight and balance. 

With the original red color the surfaces were just within the allowable margins and the addition of the white undercoat threw them 

over the limits. The solution was to remove the white, rubbing 

down into the red just slightly and then fog a light mist coat of 

red on the units.28

The modified and repainted third D-558-1 was returned to Muroc Army 

Air Field by Douglas in early November 1948. After the aircraft was reassembled, Douglas pilot Gene May completed a demonstration flight in the 

modified aircraft. He accomplished this on January 3, 1949, in a flight that 

included a 6.8-g pullout, left and right maximum sideslips at 580 mph, and a 

low-level pass at 605 mph indicated airspeed. 

The flight was apparently without mishap. However, an NACA safety representative making a postflight inspection discovered a damaged brass safety 

wire that had passed through the jet engine. Pulling the engine and conducting an inspection would have delayed turning the aircraft over to the NACA. 

Instead, Douglas simply installed a new TG-180 engine. The NACA took 
 Flying Test Tube: The Douglas D-558-1 Skystreak

59

formal delivery of the third D-558-1 on January 22, 1949. The ground crew 

then began preparing it for research flights, a process that lasted until April. 

As this was underway, the Douglas flight 

tests with the first Skystreak were concluded. A total of 101 flights had been 

completed. Douglas transferred the aircraft to the NACA, which put it in “dead 

storage” for use as spare parts. It never 

flew again. The third D-558-1 was the 

only one of the three Skystreaks still 

in use.29

Preparing the NACA Skystreak for 

research flights took much of 1949–50. 

Robert A. Champine was the first to 

fly the aircraft after it was turned over 

to the NACA. He had arrived at Muroc 

from Langley in October 1948. He made 

two D-558-1 pilot proficiency flights in 

April 1949. After Champine’s two flights, 

the aircraft was grounded for an engine 

change and remained grounded during 

the spring and summer of 1949. It would 

not be ready to fly until August. 

NACA research pilot Robert Champine 

climbs out of the D-558-1 after a flight. 

Champine became an NACA research

pilot in December 1947, and he retired in 

1979. On December 2, 1948, he became 

the sixth person to fly supersonic. (NASA)

NACA Research Flights Continue

Once the Skystreak was restored to flight status, the initial research focus was 

on its handling qualities. Piloting duties were split between Bob Champine 

and John H. Griffith, an NACA research pilot assigned to Muroc in August of 

1949. Between August and September of 1949, the two made seven flights.30

One issue explored during these flights was the effectiveness of the D-558-1’s 

ailerons. NACA flight 8 was made by Champine on August 31 and involved 

22 aileron rolls, of which 4 were at Mach 0.86. On NACA flight 9, made by 

Griffith on September 28, 16 aileron rolls were made, 4 of them above Mach 

0.875. The rolls were abrupt, with the rudder held in a fixed position, and 

made at speeds between Mach 0.6 and 0.89. 

The amount of aileron deflection was between one-eighth and one-half 

the total available deflection of ±15°. Most of the rolls were made at pressure 

altitudes of about 35,000 feet, although some were as low as 15,000 feet. The 

rolls were made in both directions. To make the research pilot’s task easier, a 
Probing the Sky

60

mechanical stop was placed in the cockpit to allow the pilot to hold a constant 

aileron deflection until a constant rolling velocity was established. 

As the Skystreak rolled, a yawing oscillation occurred, much like that of 

a wobbling top as it spins. This yawing was most apparent when the rolling 

velocity was increasing. Once the rolling velocity reached maximum value, 

however, the yawing damped out. Several of the roll maneuvers were studied 

to determine what effects the yawing had on maximum rolling velocity. No 

significant change was noted. 

Complicating the tests was the fact that the ailerons were slightly warped. 

With no load on them, this amounted to as much as a 1° difference between 

the aileron-cord line and the wing-cord line at different points along the span 

of each aileron. Ideally, there should have been a continuation of the airfoil. 

The results of the flights were described in a May 1950 NACA research 

memorandum. Altitude had no effect on the ailerons’ ability to control the 

aircraft. None of the test rolls were executed with more than half the available aileron deflection. The pilot believed the rolling velocities achieved with 

one-half deflection were enough to meet the maximum requirements for 

either test or military operations with the aircraft. Indeed, a full aileron deflection at Mach 0.85 at 35,000 feet would result in a complete revolution in 

0.95 seconds. 

The aileron forces required at a given deflection increased with indicated 

airspeed. The indicated value of force at maximum rolling velocity depended 

on the time required to reach maximum rolling velocity and the fraction in 

the aileron control system (estimated at ±5 pounds). When the force data were 

graphed, an approximately straight line resulted. This indicated that the total 

hinge-moment coefficient for a given aileron deflection was independent of 

the Mach number, at least for the speed range of the tests.

From the earlier stability and control research flights, as well as the aileron 

tests, a clearer understanding of the Skystreak’s lateral trim and handling 

characteristics at high Mach numbers was now available. The report noted 

the following: 

As the speed of the airplane is increased a right-wing heaviness 

becomes apparent to the pilot at about the same time as general 

buffeting of the airplane is encountered. As the speed is further 

increased the wing heaviness increases, a change in aileron trim 

force of about 7 pounds and a corresponding change in total 

aileron deflection of about one-half degree being required to 

trim...at a Mach number of about 0.88. The trim force and 

deflection for the wing-heaviness example quoted are typical, 

although in a few cases trim changes could not be detected on the 
 Flying Test Tube: The Douglas D-558-1 Skystreak

61

recording instruments and were not noticeable to the pilot. The 

lateral unsteadiness of the airplane at high speeds is evident on 

the time history from the rapid variations of force and deflection 

applied by the pilot in attempting to trim. It was also evident...

that the airplane has a short-period rolling-yawing oscillation of 

small amplitude. In addition to the wing heaviness, pilots have 

reported an intermittent “wing dropping” which occurs above a 

Mach number of about 0.86. This sudden rolling of the airplane 

occurs above a Mach number of about 0.86. This sudden rolling of the airplane occurs in either direction and appears to be 

associated with the general lateral unsteadiness of the airplane at 

high Mach numbers.31

As earlier reports had noted, the wing heaviness of the X-1 and D-558-1 

were similar, but more details were now apparent. On the X-1, the heaviness 

occurred at about Mach 0.85 and appeared to be related to an abrupt reduction 

in aileron effectiveness. The stability and control data on the Skystreak showed 

that aileron effectiveness did not decrease at speeds of up to Mach 0.89. Aileron 

effectiveness had not been investigated for small deflections, however. The 

report also noted, “A possible contributing cause of both the wing heaviness 

and wing dropping is probably asymmetric location and movement of shock 

waves on the wing resulting from construction asymmetry.”32

During September and October of 1949, two 60-cell manometers were 

installed to record right-wing surface-pressure-differential measurements. Six 

rows of orifices were cut into the upper and lower surfaces of the right wing, 

running along the chord from front to back. Row 1 was only 6 inches from 

the fuselage; row 6 was close to the tip. The pressure differential between the 

upper and lower wing surfaces was measured from rows 1, 2, 3, 4, and 6. Row 5 

measured individual surface pressures relative to the instrument compartment, 

and the instrument compartment pressure was measured relative to the static 

pressure, which was corrected to the free-stream static pressure by using radartracking data. This process calibrated the pressure measurements, eliminating 

errors and ensuring accuracy. 

The “plumbing” installed within the wing was extensive. The flush-type 

orifices in the wing skin were connected to the instrument compartment 

with 1⁄8-inch-inside-diameter aluminum tubing, which was connected to the 

manometer cells with 3/16-inch rubber tubing. The length of the aluminum 

tubing ranged from 6 feet at the wing-root stations to about 14 feet at row 6, 

near the wingtip. About 4 feet of rubber tubing was used on each line. 

Given the length of tubing, the maneuvers that were planned, and the need 

for precision data, the effects of instrument lag had to be considered. Ground 
Probing the Sky

62

testing indicated that any lag attributable to tubing length would be negligible. 

The lag in the airspeed recording system was calculated using established procedures and corrections made in the data. Considerable instrument lag in the 

airspeed recording system occurred during speed runs and the windup turns. 

Corrections were calculated and added to Mach-number and dynamic-pressure 

measurements. For the 1-g-stall measurements, lag was negligible because the 

pilot used a separate airspeed system.33

With the installation of the manometers now complete, the aircraft was 

returned to flight status on October 28, 1949, with Griffith making the first 

pressure-distribution flight. Two more pressure-distribution flights, one each 

by Griffith and Champine, were made by late November, before the year’s 

activities came to a close. 

More than 2 months passed before D-558-1 flights resumed. Technical 

problems continued, grounding the aircraft in February 1950 following an 

engine malfunction. Repairs took significant time, and it was not until April 

5, 1950, that pressure-distribution flights resumed. Griffith flew the third 

D-558-1 as Champine had returned in 1950 to Langley, where he continued as 

a research pilot. Following a flight on April 11, 1950, the aircraft experienced 

hydraulic problems after landing.34

Pressure-distribution research involved a large number of flights and a wide 

range of test maneuvers and procedures. Early research activities involved a 

1-g stall at a subcritical Mach number and 15,000 feet, a speed run to Mach 

0.90, and a windup turn at Mach 0.86. The 1-g stall was executed by gradually slowing the aircraft until it stalled. Other maneuvers were more complex. 

The speed run started with the Skystreak at 37,000 feet and Mach 0.70. 

The pilot dove to 33,000 feet and a Mach number of 0.90 and then began a 

gradual left turn, which he tightened until maximum allowable buffeting was 

reached. During the turn, airspeed dropped to around Mach 0.86. Once at a 

near-constant Mach number and an increasing normal-force coefficient, the 

pilot collected several data points. The ailerons were held near neutral during 

the maneuvers, and rolling velocities due to lateral oscillations were low.35

The third D-558-1 was also used to investigate a simple solution for reducing or delaying a range of effects resulting from compressibility, including buffeting, lateral instability, changes in trim, and reduction in control efficiency. 

Engineers attached “vortex generators” to the upper wing surfaces. Despite 

their impressive name, these were small airfoils with an NACA 0012 section 

and a chord of 0.5 inches, and they were positioned at 2-inch intervals. The 

generators were alternately tilted toward and away from the fuselage. They did 

not change the wings’ section profile and were added following Griffith’s April 

11, 1950, flight. The work took less than a month. 
 Flying Test Tube: The Douglas D-558-1 Skystreak

63

The first vortex generator research flight was made on May 5 as a continuation of pressure-distribution studies. The vortex generators on the first flight 

extended only from the wing’s mid-flap to the mid-aileron. The flight entailed 

takeoff and landing followed by a flight to altitude. The goal was to collect data 

on low-speed handling characteristics with the vortex generators installed. The 

initial configuration was modified for the second flight, with the vortex generators extending from the mid-flap section out to the wingtip. The final design, 

for the third flight, stretched the full width of the wing. 

The portions of the research flights made at altitude involved low-speed 

stalls, pull-ups in the buffet regions at speeds of up to Mach 0.89, and abrupt 

aileron rolls above Mach 0.7. Once the flights were completed, the vortex 

generators were removed and the flight conditions were repeated to collect 

baseline measurements. The sixth and final vortex generator flight was made 

on June 13, during which Griffith reached a speed of Mach 0.98. This marked 

the end of the pressure-distribution research flights.

NACA researchers analyzed the data and found that the vortex generators 

produced a number of effects. At speeds above Mach 0.85, the areas of flow 

separation were reduced. At Mach numbers greater than 0.85, the flow separation and the forward movement of the shock wave on the wing’s upper surface 

were reduced, though no change in the small-amplitude lateral oscillations 

could be identified. The buffet boundary and wing drop were both delayed by 

about Mach 0.05. The pilot reported that the intensity of the buffeting was 

“appreciably reduced” below the stall.36 No detrimental effects from the vortex 

generators were found on the Skystreak’s longitudinal and lateral control, at 

least for the conditions of the tests. The only negative result was an increase 

in drag. 

The NACA vortex generator test series had a major impact on aeronautics 

and aircraft design. Boeing was the first to use vortex generators on production 

airplanes. Rows of the little metal tabs soon appeared on the wings of B-47 and 

B-52 bombers, KC-135 tankers, and thousands of airliners.37 Other companies 

followed suit as well. 

The next research effort undertaken was the measurement of the Skystreak’s 

buffet boundary, which is the combination of speed and lift coefficients at 

which an aircraft experiences irregular shaking or oscillation due to turbulent flow or flow separation. The Skystreak was grounded for modification during the summer and early fall of 1950. A high-speed photographic 

manometer was added to measure wing-pressure distribution over a spanwise 

station, and a downwash vane for airflow measurements was added to determine the wing’s contribution to buffeting. A nose boom was also added for 

angle-of-attack measurements. 
Probing the Sky

64

While the aircraft was grounded, the engineers replaced its stabilizer with 

the first D-558-1’s instrumented stabilizer. The engineers had noticed a trim 

change during pressure-distribution flights that was due to a loss of elevator 

effectiveness. They were uncertain whether this was a result of changes in 

the actual pressure distribution or of physical distortion of the stabilizer and 

elevator. The solution was to replace the stabilizer. They completed the work in 

mid-October 1950, and D-558-1 flights resumed on October 26 with a check 

flight by Griffith, clearing the way for buffeting, tail-load, and longitudinalstability flights, which would be made over the course of the coming year.38

The check flight would be Griffith’s last in the D-558-1. He left the NACA in 

the fall of 1950 to become a senior test pilot on the troubled Chance Vought 

F7U Cutlass flight test program. 

His replacement was A. Scott Crossfield, a World War II Navy fighter 

pilot and gunnery instructor. Crossfield was a thorough-going aeronautical 

professional, both a consummate engineer and consummate pilot. He had 

attended the University of Washington, earning bachelor’s and master’s degrees 

in engineering during 1949 and 1950. Soon after, he joined the NACA as an 

aeronautical research pilot. His arrival reflected the new demands on test pilots 

wrought by transonic and supersonic flight. It was no longer enough to be a 

hot-shot fighter pilot with plenty of stick-and-rudder time. Both the new and 

the old test and research pilots were accomplished aviators, but for the new 

breed such as Crossfield, engineering knowledge was at least as important as 

flying skill.39

Crossfield made his first D-558-1 flight on November 29, 1950. It was 

both a pilot checkout flight and the beginning of the buffet, tail-load, and 

longitudinal-stability research project. He made a total of five flights before 

year’s end. The effort continued into the new year with Crossfield making 

another four research flights in January 1951. 

Another new pilot now joined the D-558-1 project. Walter P. Jones had 

arrived at the High-Speed Flight Research Station in September 1950 with 

both undergraduate and graduate degrees in aeronautical engineering from 

Purdue University. Jones made his first flight in the aircraft on February 13, 

1951—a pilot check flight that also included the collection of some buffet, 

tail-load, and longitudinal-stability data. Jones’s second Skystreak flight, on 

February 20, highlighted the risks of research flying. His oxygen regulator was 

faulty, and Jones began to suffer from anoxia. He nevertheless recognized his 

situation, aborted the flight, and landed safely. 

These were not the only dangers Crossfield and Jones faced. Engineers 

discovered that the elevator was twisting by as much as 2° during pull-ups at 

about Mach 0.80 and was also experiencing undesirable vibrations. Flight data 

indicated that these vibrations increased in amplitude in direct proportion to 
 Flying Test Tube: The Douglas D-558-1 Skystreak

65

the increase in Mach number and occurred in all high-lift conditions over the 

entire Mach number range, even if the airplane was in a stall. Elevator force 

and position indicators did not reflect the vibrations until they had reached an 

undesirable level. Though a precautionary x-ray examination of the elevators 

showed no sign of fatigue cracks, concerns persisted. 

NACA engineers contacted Douglas, as the latter had recorded similar problems during its flight program with the first Skystreak. During a Mach 0.94 

dive test, the first D-558-1 had experienced vibrations so severe that a “neverexceed” speed of Mach 0.92 had been established for the aircraft. Douglas 

engineers said they had attempted to reduce the vibrations by changing the 

position of the outboard elevators’ balance weights. The Douglas contractor 

flight program was ending, and the engineers admitted that they had not 

evaluated the change. More serious was the fact that the Douglas engineers 

had never informed NACA engineers of the speed limitation. 

The team at Douglas analyzed data from the NACA flights and discovered 

that the elevator vibrations had reached nearly 70 percent of the maximum 

stress limits of the design and could eventually cause a fatigue failure. Between 

the Douglas and NACA tests, more than a hundred flights had been made 

with the elevators from the first D-558-1, which had been fitted subsequently 

to the third D-558-1, and it was questionable how many more flights could 

be made without a failure. As a precaution, the first D-558-1’s elevators were 

removed and the third D-558-1’s elevators were reinstalled, since these had 

been used on only 22 flights and had a much longer service life. The switch 

took a significant amount of time, and it was not until late April 1951 that 

the work was completed.40

Jones made the first flight in the reequipped D-558-1 on May 2, 1951. 

Between May and late June, Crossfield and Jones made a total of six buffet, 

tail-load, and longitudinal-stability flights at speeds between Mach 0.835 and 

Mach 0.86. Once more, there was a change in personnel. During the remainder 

of the NACA Skystreak project, Crossfield made only three more flights, with 

Jones making a single additional D-558-1 flight. Subsequent research activities 

were undertaken once several new pilots joined the effort. 

Joseph A. Walker was the first of these. An Army Air Forces pilot in World 

War II, he had flown reconnaissance missions in P-38 Lightnings over Austria, 

the Black Sea, and southern France. At the war’s end, and already holding 

a bachelor’s degree in physics from Washington and Jefferson College, he 

joined the NACA’s Lewis Flight Propulsion Laboratory in 1945 as a physicist. He transferred to the High-Speed Flight Research Station in 1951 as a 

research pilot.41

Walker’s first pilot checkout flight came on June 28, 1951, during which 

he reached a speed of Mach 0.82. He made a second checkout flight on July 3, 
Probing the Sky

66

which also included tests of buffeting and tail loads. Walker made a total 

of 14 D-558-1 flights between June 28 and October 18, 1951. Although 

the D-558-1 flights represented a significant part of the High-Speed Flight 

Research Station’s activities, they were never routine.

The third D-558-1 on the lakebed, being prepared for a flight. A ground crewman is adjusting 

the pilot’s parachute straps, which the pilot cannot see. The aircraft is also positioned close 

to the lakebed shoreline to allow the maximum distance for a takeoff run. When the NACA 

contingent first arrived at Muroc Army Air Field in late 1946, they found the facilities of a wartime training base very different from those at Langley. Housing was subpar, the facilities were 

limited, and the climate was usually either too hot or too cold to work comfortably. Some NACA 

personnel left, but others adapted. (NASA)

On Walker’s third flight in the D-558-1, on July 17, he had to cut the 

mission short due to low fuel. Two flights later, on July 26, weather forced an 

inflight abort. Clouds prevented tests at altitudes above 15,000 feet. The next 

two flights were successful, but on August 10 the airplane suffered a fuel leak 

due to a malfunctioning vent valve, cutting short still another mission. After 

a successful flight on August 20, on which Walker reached a true airspeed of 

Mach 0.9, problems struck again. A hydraulic failure caused his August 22 

flight to be aborted. Walker’s final D-558-1 flight, on October 18, was for 

the collection of lateral-stability and landing data. Walker then moved on to 

other projects.42

Stanley P. Butchart took over research duties with the D-558-1. Like 

Crossfield and Walker, he was a World War II combat pilot, having flown TBM 

Avenger torpedo planes in the Pacific.43 Butchart entered the University of 

Washington after the war, earning bachelor’s degrees in aeronautical engineering 
 Flying Test Tube: The Douglas D-558-1 Skystreak

67

and mechanical engineering. While there, he and Crossfield served in the same 

Naval Reserve squadron before both joined the NACA in May 1951. Butchart 

made his first D-558-1 flight, for pilot checkout, on October 19, 1951. This 

was followed on November 9 by his first research flight, which was the last 

flight of the buffeted, tail-load, lateral-stability, and landing-study project. 

Skystreak flights made during this period resulted in the publication of 

several NACA research memorandums between early 1951 and early 1952. 

Additional tables of wing-pressure measurements were released in January 

1951. Like the data issued in December 1950, the new data were collected in 

windup turns. Unlike in the earlier tests, however, the new information was 

not limited to collection at a single speed of Mach 0.86. Rather, it covered 

Mach 0.67, 0.74, 0.78, and 0.82 at an altitude of 35,000 feet. Both reports 

consisted of tabulated measurements but lacked detailed analysis, in the interest 

of making flight-test data available to designers as quickly as possible.44

During the flights made with the second D-558-1, measurements had been 

made of the effects of different stabilizer incidents on the aircraft’s longitudinalstability and control characteristics. These preliminary results showed that 

minor changes in the stabilizer incidents caused major changes in longitudinal 

trim characteristics. Once flights resumed with the third Skystreak, the issue of 

the effects of the stabilizer incidents was revisited in a more thorough investigation. The tests were made using shallow dives, pullouts, and windup turns at 

altitudes ranging from 37,000 to 27,000 feet, at Mach numbers between 0.60 

and 0.89. The stabilizer incidences used during the research flights were 1.6°, 

2.2°, 2.6°, 2.7°, 2.9°, and 3.3°. These tests were considerably more complete 

than those made during initial efforts with the first D-558-1.

In newspaper accounts of test pilots’ experiences, the focus is more often on 

the drama and danger of the flights than on the test results. In NACA research 

memorandums, the text is dry and to the point. The results of the flights are 

depicted in charts and graphs and are often unclear to a lay reader. But sometimes, even dry text and charts make clear what has transpired. Regarding a dive 

to about Mach 0.89, “The data were obtained in a dive from about 37,000 feet 

with a stabilizer setting of 3.3°. At about 48 seconds, as the pilot attempted to 

pull out, the elevator angle and stick forces necessary to execute the maneuvers 

became excessive and the stabilizer had to be used to recover from the dive. 

The time history for this run was not extended beyond 48 seconds because the 

subsequent data were not satisfactory for analysis.... [I]t is evident from the 

figures that large changes in longitudinal trim occur at Mach numbers above 

about Mach 0.84.”45

The pull-stick force needed to recover from the dive with the 3.3° stabilizer 

setting was about 80 pounds. More important, the recovery was only possible by also using the adjustable stabilizer. The conventional fixed horizontal 
Probing the Sky

68

stabilizer and movable elevator, used since the early days of flight, were definitively shown to be obsolete for transonic and supersonic flight. The two-part 

design of the X-1 and D-558 was the origin of the all-moving tail fitted to the 

later model F-86 and subsequent high-performance aircraft and is still in use 

today. The report also noted, “The results indicate that large and rapid changes 

in elevator deflection and force were required for balance at Mach numbers 

above 0.84. At Mach numbers above about 0.84, a sharp decrease in the relative elevator-stabilizer effectiveness was shown and analysis indicated that a 

major part of the observed trim changes was explained by this decrease.... The 

increase in apparent stick-fixed stability parameter was attributed to a decrease 

of relative elevator effectiveness together with an increase of the stability of the 

airplane by a factor of 4 between Mach numbers of 0.75 and 0.89.”46

The third D-558-1 parked on the South Base ramp with three ground personnel. The Skystreak was 

one of two concepts for how best to gain data on transonic flight. It represented a traditional design, 

had straight wings, and was jet-powered, with a top speed just below Mach 1. It was funded in part 

by the Navy and built by Douglas to NACA specifications. In contrast, the Army Air Forces XS-1 was 

rocket-powered, with a top speed well above Mach 1. (NASA)

Though the bulk of research undertaken with the D-558-1 was similar to 

that done with conventional aircraft to understand stability and control issues, 

another study using the Skystreak looked to the future, toward the dawning 

computer revolution. By the early 1950s, engineers realized that automatically 

stabilizing dynamic systems were necessary for future supersonic aircraft. To 

design such a system, however, engineers needed aircraft frequency-response data 
 Flying Test Tube: The Douglas D-558-1 Skystreak

69

at high subsonic, transonic, and supersonic speeds. Acquiring these data involved 

the transfer functions of both the aircraft and the control systems. 

The procedure used to collect the data was to make several stick-fixed 2° 

elevator pulses, each lasting 0.5 to 1.0 second. The aircraft was in stabilized 1-g 

flight at a speed between Mach 0.52 and 0.90 and altitudes of 30,000 to 37,000 

feet. These pulses produced an initial aircraft oscillation of approximately ±1⁄2 

g to ±1 g and a pitching velocity of ±0.1 radians per second. As with other 

tests, precision was central to ensuring that the data were valid. A restricting 

device was attached to the elevator control that returned it to approximately 

the original position following the pulses and also maintained the fixed elevator 

condition as the oscillation subsided. The other controls—ailerons, rudder, and 

stabilizers—were fixed during the maneuvers. The stabilizer incident was fixed 

throughout the tests at a nose-down angle of –2°. Another factor affecting the 

data was the twist of the elevator, which amounted to about 0.4°. To produce 

an average value, the elevator position was recorded at four positions along the 

external span of the fuselage. The four control positions were averaged, and 

this average was used as the input function. 

When the flights were completed, Fourier transform (a mathematical process) was applied to the input and output functions, establishing the longitudinal frequency response of the aircraft as a function of Mach number. The report 

noted, “A comparison of the response data estimated from wind tunnel data 

with the experimental results showed good agreement. It was found that the 

maximum response amplitude was a minimum at a Mach number of 0.88. At 

lower Mach numbers (0.52 to 0.66) the effects of lift coefficient on frequency 

response are indicated.”47

1952–1953: The Skystreak’s Twilight Years

Much had changed at the High-Speed Flight Research Station since the D-558-1 

had been delivered in the spring of 1947. New X-planes had been built, resulting in a heavy workload for the limited number of engineers, technicians, and 

computers at the remote site. The Skystreak was also showing its age. Following 

Butchart’s second flight, it was grounded for maintenance work that involved 

repairing major fuel leaks and correcting engine-ignition problems. Work was 

completed in January 1952, but winter rains flooded Rogers Dry Lake, preventing flight operations. The research plan for the D-558-1 in early 1952 called for 

an investigation of vertical-tail loads. To support the tail-loads study, strain-gauge 

instrumentation was installed. Before this research activity could begin, however, 

or the strain gauges could even be calibrated, a study of lateral stability and aileronroll effectiveness was undertaken with the D-558-1. 
Probing the Sky

70

The D-558-1 resumed flight on June 25, 1952, with Crossfield as pilot, 

beginning the lateral-stability and aileron-effectiveness research flights. 

Crossfield made two flights before turning the project over to Butchart. The 

pilots made abrupt aileron rolls from Mach 0.4 to the Skystreak’s limiting 

Mach number at 10,000 feet, 25,000 feet, and 35,000 feet. Results showed 

that aileron effectiveness dropped rapidly above Mach 0.88. Butchart made a 

total of five flights between July 17 and August 12, 1952. He later recalled the 

cramped Skystreak’s cockpit. During takeoff on July 17, 1952, he reached for 

the landing gear handle but could not squeeze his hand between the control 

yoke column and the emergency oxygen bailout bottle strapped to his left leg. 

It took Butchart three tries to get the gear retracted.48

Horizontal stabilizer issues continued to plague the program. During a 

postflight inspection, the horizontal stabilizer mountings were found to be 

loose. High-Speed Flight Research Station engineers judged this to be due 

to wear over a long period of time, rather than to one-time damage resulting 

from a severe maneuver. The horizontal stabilizers were originally fitted to 

the first D-558-1, and about 100 flights already had been made with them. 

They were then sent to Langley to be instrumented. Once this was completed, 

the stabilizers were returned to the High-Speed Flight Research Station and 

reinstalled on the third Skystreak. By the time the loose mountings were discovered, 41 more flights had been made with the stabilizers. The risk posed by 

the loose mountings was considerable. The horizontal stabilizer was known to 

vibrate during buffeting, and during a Mach 0.90 dive, the vibration became 

quite pronounced. High-Speed Flight Research Station engineers decided 

to repair the vertical-load links and the pins that attached the links to the 

horizontal stabilizer. 

Repairing the horizontal stabilizers first required removing their skins. Once 

this was done, the strain gauges were inspected and found to need extensive repairs; they would have to undergo a complete recalibration. The strain 

gauges were critical for measurements of vertical-tail load as the stabilizers were 

mounted directly on the tail. 

The engineers estimated that repairs to the horizontal stabilizers and recalibrations of the tail-load gauges would take 3 months. This resulted in a management decision to omit the vertical-tail-load research from future D-558-1 

activities. Managers at NACA Headquarters learned of the decision and sent 

a letter to Hartley Soulé at Langley seeking his input on the advisability of 

abandoning the tail-loads study. A letter was also sent to the High-Speed Flight 

Research Station asking for information on the circumstances of the decision.49

Donald Bellman, an engineer at the High-Speed Flight Research Station, 

wrote the reply to NACA Headquarters. He noted that the 3-month delay 

needed to make the repairs and complete the recalibration “eliminated the 
 Flying Test Tube: The Douglas D-558-1 Skystreak

71

possibility of starting the vertical tail load program in the 1952 flying season. 

Prospective work of greater importance precluded the program from the 1953 

flying season, so the program was abandoned.”50

Research with the D-558-1 did not resume until January 29, 1953. 

Dynamic-stability measurements were the goals of the new flights. These would 

be accomplished by making elevator and rudder pulses at transonic speeds at 

25,000- and 35,000-foot altitudes. Although some dynamic-stability data that 

had been collected in earlier missions focused on lateral stability, about twothirds of the elevator pulse data and nearly all the rudder pulses had yet to be 

collected. Butchart made five dynamic-stability flights, the last on March 27. 

As before, a new pilot now joined the project. He was John B. “Jack” McKay, a 

Navy F6F Hellcat pilot during World War II. After the war, he attended Virginia 

Polytechnic Institute and graduated in 1950 with a bachelor’s degree in aeronautical engineering. McKay joined the NACA in January 1951, initially working at 

Langley for a brief period as an engineer before transferring to the High-Speed 

Flight Research Station. His assignment on the D-558-1 was to provide dynamicstability “fill-in” data. 

McKay made his pilot checkout on March 27, 1953. This was followed on 

April 1 and 2 by the two fill-in flights. At the time of his assignment, these 

flights were supposed to mark the end of research with the D-558-1. But 

High-Speed Flight Research Station managers had approved a new research 

project designed to investigate the effects of tip tanks on the Skystreak’s buffet 

characteristics, so McKay also piloted these flights. 

The first tip-tank flight, made on May 7, 1953, was aborted because a 

fuel-vent failure caused a leak in the left tank. The second flight, on May 12, 

was no more successful. The data recorder had not been turned on, so McKay 

came home with no data. Despite McKay’s shaky start on the project, the next 

four flights, on May 13, May 20, June 2, and June 3, 1953, were successful. 

Crossfield made the 78th and final NACA research flight in the third D-558-1, 

an investigation of low-speed stability and control in coordinated turns, on 

June 10, 1953. With his landing, the Skystreak passed into history.51

An Assessment

The Skystreak’s importance in the exploration of the supersonic frontier was, as 

the two opening quotations indicate, conflicted. This was due to many factors. 

The first was the aircraft’s limitations. John Stack did not want a supersonic 

airplane, but one that could fly at high-transonic speeds and provide the data 

that existing wind tunnels could not. The NACA’s large wind tunnels, then 

the best in the world, began to choke at the speeds at which compressibility 
Probing the Sky

72

problems were appearing with aircraft. Makeshift efforts designed to get around 

this stumbling block, such as wing flow and rocket-boosted models, had limitations, so building research aircraft remained the only option. 

The third D-558-1 Skystreak parked outside an NACA hangar at Edwards South Base in 1949. 

The aircraft was painted overall white to make photo tracking easier against the dark blue desert 

sky. The original dark red paint was intended to be highly visible but actually made the Skystreak 

difficult to spot. (NASA)

In a broader context, the D-558-1’s limitations reflected the NACA’s cautious approach to the sonic frontier. The agency’s approach in 1944–46 was 

characterized by a focus on research conducted through small steps rather than 

bold leaps and through incremental improvements in aircraft technology rather 

than wholesale revolutionary breakthroughs. Much of the NACA’s war work 

was in drag reduction for existing production aircraft and was focused on small 

increases in performance. The surprisingly ambivalent response to Robert T. 

Jones’s paper on swept wings (until confirmed by evidence of Nazi wartime 

work) was another reflection of this mentality; it continued with the NACA’s 

proposed X-planes research plan, which called for numerous flights with several 

different aircraft stretching over a long period, with supersonic flight achieved 

only at the very end—a program plan that led to the AAF ramming through 

its accelerated XS-1 assault. 

The Douglas engineers designing the D-558-1 were driven by the NACA’s 

recommendations for a simple first-generation transonic research aircraft, but 

also by the Navy’s (and their own) interest in possibly using the aircraft to at 
 Flying Test Tube: The Douglas D-558-1 Skystreak

73

least contribute to an operational naval fighter. The Skystreak represented a 

conservative design for a land-based jet aircraft, but a radical one for a naval 

fighter, in the age of straight-deck carriers more suited to straight-wing slowapproach-speed aircraft than to the “hotter” swept-wing jets. It is important to 

note that the advent of the swept wing did eventually result in pressures to build 

a swept-wing derivative of the D-558-1, which led to the Mach 2–breaking 

D-558-2 Skyrocket, first flown in February 1948. Compared to this pointed, 

streamlined (indeed elegant) design, the “original” D-558-1, with its cylindrical 

constant-diameter fuselage and straight wings and tail surfaces, looked archaic 

rather than futuristic. 

Despite all of this, the Skystreak was a remarkably productive aircraft. 

During the D-558-1 research flights, a wide range of activities was undertaken. 

The tests included high-Mach-number dives; research on the effects of tip tanks 

and on aileron effectiveness; directional-stability rudder kicks and side slips 

made to collect stability data; measurements of pressure distribution and lift 

pressure; and checks of vortex generators, buffeting and tail loads, longitudinal 

stability, and dynamic and lateral stability in the transonic range.52

The significance of the Skystreak’s accomplishments lies not in the tests 

themselves, many of which were the standard tests used on prototype and 

production aircraft, but rather in the database assembled across the range of 

speeds at which they were flown. This made the D-558-1 important. In this 

context, at least, it did not matter that the Skystreak lacked swept wings or 

rocket power. 

The capabilities and limitations of the D-558-1 and the XS-1 (renamed the 

X-1 in June 1948) resulted in a division of research methodology. Because the 

Skystreak could cruise for (relatively) prolonged periods at transonic speeds, 

it was used for such research, freeing the shorter-duration XS-1 for use in 

collecting data at supersonic speeds. The XS-1 was ill-suited to undertake sustained flights at transonic speeds because of the rocket engine’s ravenous fuel 

consumption, which left it with, at most, 21⁄2 minutes of powered flight time. 

Paradoxically, the D-558-1’s contributions were also more limited for the 

same reason. The X-1s and swept-wing D-558-2 continued to be successfully 

used for research activities into the late 1950s. The lower performance of the 

D-558-1 meant its useful lifespan was shorter; Soulé’s December 19, 1952, 

letter to NACA Headquarters regarding the cancellation of the vertical-tailloading studies with the Skystreak noted, “It should be understood that the 

abandonment of the proposed D-558-1 vertical-tail-load flight tests does not 

mean the abandonment of the study of vertical-tail loads at Edwards. Verticaltail load data are being obtained during flights of the X-5 and D-558-2 airplanes, and it is believed that the resulting information will be of more interest 

than such information on the D-558-1 airplane.”53 (Emphasis added.)
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74

The clear implication was that the D-558-1 was nearing the end of its usefulness and that the faster, rocket-powered Skyrocket had more to offer than 

the jet-powered Skystreak. 

Over time, as memories of the debates that attended the development of 

the XS-1 and D-558-1 faded, the myth that their separate roles had been 

intentional from the start developed in the research community. This mythology held that the two airplanes were deliberately planned to undertake the two 

disparate sets of research activities. While not true, and while later aircraft such 

as the F-86 were soon made available to the NACA for high-speed research, it 

is certainly true that the availability of the Skystreak as a complementary test 

system to the flashier XS-1 benefited postwar aeronautical research. As John 

Becker noted, “It was the D-558-1’s and not the advanced service aircraft that 

were used for extensive flight research at high subsonic speeds by [the] NACA, 

complementing coverage of the higher transonic speeds by the X-1s. It is quite 

understandable how some NACA managers by hindsight can see a logic in 

the way those two vehicles were used that did not really exist when they were 

promoted in 1944 and 1945.”54

For NACA researchers, pilots, and engineers, the Skystreak also provided 

initial experience with the new demands of research aircraft operations, but 

at a terrible price. The death of “Tick” Lilly (whose portrait hangs in Dryden 

Flight Research Center to this day) in the second D-558-1 crash highlighted 

design flaws in the aircraft, as well as in the approach taken to the project. 

These oversights included the fact that the engine lacked the latest upgrades and 

modifications, as well as the vulnerability of the control cables, fuel lines, and 

other components to damage. Acting on the recommendations of the accident 

board in the wake of Lilly’s crash, the Douglas X-3, Northrop X-4, and Bell 

X-5 research airplanes also received modifications, benefiting them greatly.55

There was a final accomplishment of the X-1 and D-558-1 programs that 

is often overlooked. The research aircraft were built in part because the existing wind tunnels choked between Mach 0.80 and low supersonic speeds. 

Researchers saw research aircraft as being a substitute for wind tunnel testing 

as drop bodies, rocket-boosted models, and the wing flow technique had 

been. From 1947 to the early 1950s, the D-558-1 and the X-1 represented 

the only tools available for collecting transonic data. The irony was that the 

construction of the research aircraft generated pressures forcing wind tunnel 

researchers to seek ways to fix the choking problem. 

Overcoming the wind tunnel limitations required several steps. The first 

was reducing the size of the model to just one-tenth of 1 percent of the tunnel 

throat area. Choking still occurred, but its onset was delayed from Mach 0.80 

to Mach 0.95. Langley researchers realized that in order to take advantage of 
 Flying Test Tube: The Douglas D-558-1 Skystreak

75

the small-model technique, the support-structures’ designs would have to be 

changed, as their surface area was now much larger than that of the models. 

The third D-558-1 in flight with scattered clouds in the distance and the desert below. The 

Skystreak did not have the impact that the XS-1 had on aviation technology, but it did contribute 

to the understanding of transonic flight. (NASA)

They developed the technique of attaching the model to a long rod, called 

a “sting,” which was placed farther downstream in the tunnel. A specially 

contoured insert on the tunnel’s wall was also added ahead of the sting. The 

two features both corrected the blockage of the tunnel and created a more 

uniform airflow. Langley researchers used an early version of the sting/liner 

beginning in the spring of 1946 for wind tunnel testing of the XS-1 and 

D-558-1 designs at speeds as high as Mach 0.92. 

Langley engineer Ray H. Wright made the next significant breakthrough. 

He proposed putting lengthwise slots in the throat of a wind tunnel test section, 

originally to eliminate the effects of wall interference. This technique evolved 

into using the slots to reduce the choking at high transonic speeds, a practice 

to which Italian aerodynamicist Antonio Ferri and the German inventor of 

the swept wing, Adolf Busemann, both expatriate theoreticians working at 

Langley in the postwar years, objected to at a September 1947 meeting with 

John Stack. They argued that though the slots would reduce the choking, at 

Mach 1 the data were unlikely to be valid. 
Probing the Sky

76

Stack was not overly concerned about the small remaining gap. His response 

to Ferri and Busemann’s objections was to say that if the slotted tunnel worked 

at Mach 0.995 and Mach 1.005, the gap in the middle was meaningless. It 

took several more years before the slotted tunnel was an operational reality. The 

initial modifications to Langley’s 8-Foot High Speed Tunnel (HST) exceeded 

Mach 1 in late 1948, but the airflow was “rough and uneven.” The slots had to 

be carefully shaped to achieve smooth transonic airflow. On October 6, 1950, 

transonic research operations in the 8-Foot HST began. The 16-Foot HST 

also began operation with a slotted throat 3 months later. The significance 

of the achievement was recognized in 1951 when Stack and his associates 

were awarded the Collier Trophy for the development of the slotted tunnel, 

indirectly another accomplishment attributable to the onset of the postwar 

X-series aircraft.56
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77

Endnotes

1. Opening quotations are from Hallion, Supersonic Flight, p. 141, and 

Becker, The High-Speed Frontier, p. 96.

2. During its development and service life, the Douglas Skystreak was 

variously referred to as the “D-558-I” and the “D-558-1.” The latter 

usage is more prevalent in Douglas, Navy, and NACA documentation, and, accordingly, this work uses D-558-1 for consistency.

3. See Edward H. Heinemann and Rosario Rausa, Ed Heinemann: 

Combat Aircraft Designer (Annapolis, MD: Naval Institute Press, 

1980), pp. 141–150.

4. Tuncer Cebeci, Legacy of a Gentle Genius: The Life of A.M.O. Smith

(Long Beach, CA: Horizons Publishing, Inc., 1999), p. 28.

5. Hallion, Supersonic Flight, pp. 69, 71–73.

6. Edward H. Heinemann, “The Development of the Navy-Douglas 

Model D-558 Research Project” (El Segundo, CA: Douglas Aircraft 

Company, November 17, 1947), pp. 3–7.

7. Heinemann, “The Development of the Navy-Douglas Model 

D-558 Research Project,” p. 7; Curtis Peebles, ed., The Spoken 

Word: Recollections of Dryden History, The Early Years, SP-2003-4530 

(Washington, DC: NASA, 2003), pp. 84, 110. Scott Crossfield, 

who flew the D-558-1 and -2, did not have a high opinion of their 

escape systems. He later noted in The Spoken Word, “This is the way to 

commit suicide to keep from getting killed. They never did have the 

development on them that they should have had, and they weren’t any 

good anyway. If you could make a capsule that was good enough to 

live through the emergency, you might as well fly it and throw away 

the airplane.” Stanley P. Butchart, who also flew the D-558-1 and -2, 

echoed Crossfield’s comments. Butchart added, also in The Spoken 

Word, “When you stop to think of it, [at] the higher speeds, and you 

drop the nose off, you’re going to get a very big negative g as you come 

out of there. So that restricts you as to how fast you can be going and 

still use that escape method.” Just such a problem killed Air Force 

Captain Milburn Apt on September 27, 1956, when he attempted to 

escape from the first Bell X-2 following his flight to Mach 3.2.

8. “Supersonic Douglas Skystreak To Race Speed of Sound for Navy” 

(Douglas press release), pp. 1–2; Hallion, Supersonic Flight, pp. 

74–75; Dryden Flight Research Center public movie Web page, 

http://www.dfrc.nasa.gov/Gallery/Movie/D-558-1/index.html, accessed 

February 11, 2010.
Probing the Sky

78

9. Heinemann, “The Development of the Navy-Douglas Model D-558 

Research Project” (El Segundo, CA: Douglas Aircraft Company, 

November 17, 1947), pp. 2–3.

10. Ibid.

11. Dryden Flight Research Center public movie Web page, http://www.

dfrc.nasa.gov/Gallery/Movie/D-558-1/index.html, accessed February 

11, 2010; Hallion, Supersonic Flight, pp. 140, 145.

12. Hallion, Supersonic Flight, pp. 140–142; Douglas Aircraft Company, 

“Skystreak World’s Speed Records,” September 19, 1947; Becker, 

The High-Speed Frontier, p. 96. The D-558-1 record stood until an 

F-86A broke the record on September 15, 1948, reaching a speed of 

671 mph. By then, F-86s were routinely exceeding Mach 1 in dives. 

The low-altitude speed run was originally designed for wood-andfabric biplanes, so pilots could not artificially increase their speed by 

making a dive from high altitude before beginning each run. By the 

late 1940s, with jets flying at speeds in excess of 600 mph in the dense 

lower atmosphere, this approach was very dangerous. A small control 

error would cause the aircraft to hit the ground. Ironically, while Carl’s 

average speed was higher, both he and Caldwell had matching Mach 

numbers of 0.828. This was because Mach number depended on 

altitude and air temperature. Caldwell’s flights were made at a temperature of a little over 75 °F. Carl’s flight, in contrast, was made at an 

air temperature of nearly 95 °F.

13. Walter C. Williams, “Limited Measurements of Static Longitudinal 

Stability in Flight of Douglas D-558-1 Airplane” (BuAero No. 

37971), NACA RM L8E14 (June 24, 1948), p. 2. The date on an 

NACA Research Memorandum was when it was issued.

14. Hallion, Supersonic Flight, pp. 142–144; Richard P. Hallion and 

Michael Gorn, On the Frontier: Experimental Flight at NASA Dryden

(Washington, DC: Smithsonian Books, 2003), p. 386.

15. Letter, Walter C. Williams to NACA, “Choice of Color for Research 

Aircraft at Edwards,” December 3, 1951, reprinted in J.D. Hunley, 

Toward Mach 2: The Douglas D-558 Program (Washington, DC: 

NASA, 1999), pp. 116–117. The word “photographability” is used 

in the text. The D-558-1s eventually had pronounced differences 

in their finishes. The first Douglas D-558-1 ended its career with a 

white fuselage and lower wings, and the canopy, wing tanks, rudder, vertical-tail tip, horizontal stabilizers, elevators, ailerons, speed 

brakes, and (apparently) upper wing surfaces and landing gear doors 

were red. The NACA’s third D-558-1 was all-white with the exception of the red rudder, elevators, and ailerons. The second D-558-1 
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79

had an NACA shield on its vertical tail. The third D-558-1 bore the 

NACA shield initially, then a yellow band with “NACA” in black 

and “NACA” on the upper right wing. Late in its service, a large 

black “X” was painted on both sides of its forward and aft fuselages 

for photo tracking.

16. Hallion, Supersonic Flight, pp. 224–225.

17. “NACA Aircraft Accident Investigation Report, Douglas D-558-1 

Airplane” (BuNo 37971), Muroc Air Force Base, Muroc, CA, May 

3, 1948, p. 2.

18. “NACA Aircraft Accident Investigation Report, Douglas D-558-1 

Airplane” (BuNo 37971), Muroc Air Force Base, Muroc, CA, May 

3, 1948. The D-558-1 was originally fitted with a bubble canopy 

for the Douglas flights. In videos of early flights, May, who was 

small in stature, can be seen wearing a silver crash helmet. When the 

V-shaped canopy replaced the original design, it was then too small 

to allow most pilots to wear a crash helmet. The accident report 

noted that the lack of a helmet and the shoulder straps would have 

made no difference in Lilly’s crash. During the speed-record flights, 

Caldwell had worn a helmet. Carl was taller than Caldwell, however, 

and he could not close the canopy while wearing a crash helmet. 

Carl had to wear a World War II–type cloth helmet.

19. Walter C. Williams, “Limited Measurements of Static Longitudinal 

Stability in Flight of Douglas D-558-1 Airplane, BuAero No. 

37971,” NACA RM L8E14, (June 24, 1948).

20. Ibid., p. 4.

21. Williams, “Limited Measurements of Static Longitudinal Stability in 

Flight of Douglas D-558-1 Airplane,” pp. 3–4, 10.

22. Walter C. Williams, “Flight Measurement of the Stability 

Characteristics of the D-558-1 Airplane (BuAero No. 37971) in 

Sideslips,” NACA RM L8E14a (April 18, 1949), pp. 1–4.

23. Ibid., p. 4.

24. William H. Barlow and Howard C. Lilly, “Stability Results 

Obtained with Douglas D-558-1 Airplane (BuAero No. 37971) in 

Flights up to a Mach Number of 0.89,” NACA RM L8K03 (April 

22, 1949).

25. Ibid., p. 4.

26. Ibid., pp. 3–5.

27. Scott Libis, Skystreak, Skyrocket, & Stiletto: Douglas High-Speed 

X-Planes (North Branch, MN: Specialty Press, 2005), pp. 26–28. 

Credit for the later development of the winglet goes primarily to 
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80

the remarkable NACA-NASA aeronautical scientist Richard T. 

Whitcomb of the Langley Research Center.

28. Hallion, Supersonic Flight, pp. 146, 260.

29. Ibid., pp. 146–147.

30. Hallion and Gorn, On the Frontier, p. 387.

31. Jim Rogers Thompson, William S. Roden, and John M. Eggleston, 

“Flight Investigation of the Aileron Characteristics of the Douglas 

D-558-1 Airplane (BuAero No. 37972) at Mach Numbers Between 

0.6 and 0.89,” NACA RM L50D20 (May 26, 1950), p. 5.

32. Ibid. pp. 1–6.

33. Earl R. Keener and Mary Pierce, “Tabulated Pressure Coefficients 

and Aerodynamic Characteristics in Flight on the Wing of the 

Douglas D-558-1 Airplane for a 1g Stall, a Speed Run to a Mach 

Number of 0.90, and a Wind-Up Turn at a Mach Number of 0.86,” 

NACA RM L50J10 (December 15, 1950).

34. Hallion, Supersonic Flight, pp. 147–148; Libis, Skystreak, Skyrocket, 

& Stiletto, p. 121.

35. Keener and Pierce, “Tabulated Pressure Coefficients and 

Aerodynamic Characteristics in Flight on the Wing of the Douglas 

D-558-1 Airplane for a 1g Stall, a Speed Run to a Mach Number of 

0.90, and a Wind-Up Turn at a Mach Number of 0.86,” p. 5.

36. De E. Beeler, Donald R. Bellman, and John H. Griffith, “Flight 

Determination of the Effects of Wing Vortex Generators on the 

Aerodynamic Characteristics of the Douglas D-558-1 Airplane,” 

NACA Research Memorandum RM L51A23, August 14, 1951, 

p. 8.

37. Ibid., pp. 4–7; Hallion and Gorn, On the Frontier, pp. 386–387. 

Vortex generators were also tested on the final flight of NACA 

X-1 #2 on October 23, 1951. The rocket cut out after two ignition attempts, and the aircraft glided to a landing. The aircraft was 

subsequently grounded due to possible fatigue failure of the nitrogen 

spheres.

38. Ibid., p. 149; Hallion and Gorn, On the Frontier, pp. 386–387.

39. Peebles, ed., Spoken Word, pp. 52–53, 77.

40. Hallion, Supersonic Flight, pp. 149–150.

41. Ibid., p. 135; Hallion and Gorn, On the Frontier, p. 388.

42. Ibid., p. 151; Peebles, ed., The Spoken Word: Recollections of Dryden 

History, The Early Years, pp. 103–104.

43. Butchart served with another outstanding young naval airman 

who became a close friend—then-Ensign George H.W. Bush, later 

President of the United States.
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81

44. Earl R. Keener, James R. Peel, and Julia B. Woodbridge, “Tabulated 

Pressure Coefficients and Aerodynamic Characteristics Measured in 

Flight on the Wing of the Douglas D-558-1 Airplane Throughout 

the Normal-Force Range at Mach Numbers of 0.67, 0.74, 0.78, and 

0.82,” NACA RM L50L12a (January 29, 1951), pp. 1, 4–5. One 

possible reason for the lack of analysis in the reports is the outbreak 

of the Korean War in June 1950. The straight-wing U.S. Air Force 

and Navy jet fighters were outclassed by the swept-wing Soviet MiG15, which entered combat in November 1950, forcing immediate 

deployment of the swept-wing USAF F-86A.

45. Melvin Sadoff, William S. Roden, and John M. Eggleston, 

“Flight Investigation of the Longitudinal Stability and Control 

Characteristics of the Douglas D-558-1 Airplane (BuAero No. 

37972) at Mach Numbers up to 0.89,” NACA RM L51D18 (June 

25, 1951), p. 4.

46. Ibid., pp. 1, 7, 16–18. The all-moving stabilizer on the F-86 gave 

U.S. pilots an advantage over the Soviet MiG-15 in air combat 

during the Korean War. An F-86 could dive away from an attacking MiG-15 and retain control despite reaching transonic and 

low supersonic speeds. This was because the MiG-15 still used 

fixed-stabilizer and movable elevators, making it unstable at transonic speeds and incapable of reaching Mach 1 under any flight 

conditions.

47. Ellwyn E. Angle and Euclid C. Holleman, “Longitudinal FrequencyResponse Characteristics of the D-558-1 Airplane as Determined 

from Experimental Transient-Response Histories to a Mach Number 

of 0.90,” NACA RM L51K28 (February 11, 1952), pp. 1, 4, 7–8.

48. Hallion, Supersonic Flight, pp. 150–151; Libis, Skystreak, Skyrocket, 

& Stiletto, p. 32.

49. Letter, Hartley A. Soulé to NACA Headquarters, “Request for 

Comments on Abandonment of D-558-1 Vertical-Tail Load 

Investigation,” December 19, 1952.

50. Letter, Donald R. Bellman to NACA, “Looseness of Stabilizer 

Mountings for the D-558-1 Airplane,” November 5, 1952; letter, 

Donald R. Bellman to NACA, “Reply to Request for Information 

on D-558-1 Vertical-Tail Load Investigation,” January 12, 1953.

51. Hallion, Supersonic Flight, pp. 152–153; Libis, Skystreak, Skyrocket, 

& Stiletto, p. 122; Hallion and Gorn, On the Frontier, p. 388. 

The fates of the different research aircraft reflected their perceived 

importance. The X-1 #1, in which Yeager made the first supersonic 

flight, was given to the Smithsonian Institution after its retirement 
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82

in May 1950. It is currently on display at the National Air and Space 

Museum in Washington, DC. It shares the main entrance with the 

Wright Flyer, the Spirit of St. Louis, the X-15 #1, and the Apollo 11 

Command Module. After Crossfield’s last flight in the third D-558-

1, it joined the first D-558-1 in dead storage at Edwards Air Force 

Base. Both aircraft were subsequently transferred. The first D-558-1 

was sent to California State Polytechnic College in San Luis Obispo, 

CA, in 1957. It was transferred to the National Museum of Naval 

Aviation in 1964, where it was restored and remains today. The 

third D-558-1 was sent for display at the Marine Corps Air-Ground 

Museum at Quantico, VA. The aircraft was displayed outdoors and 

rapidly deteriorated. Fortunately, it was transferred to the Carolina 

Aviation Museum, restored, and put on display in 2001.

52. Libis, Skystreak, Skyrocket, & Stiletto, pp. 116–122.

53. Soulé, “Request for Comments on Abandonment of D-558-1 

Vertical-Tail Load Investigation.”

54. Becker, The High-Speed Frontier, p. 96.

55. Hallion, Supersonic Flight, pp. 152–153.

56. Hanson, Engineer in Charge, pp. 312–327; Becker, The High-Speed 

Frontier, pp. 98–114.

84

The second D-558-2, NACA 144, parked on the ramp at South Base. Modifications made to 

allow air launch included the removal of the jet engine and fuel tanks. Between the air launch 

and the increased rocket burn time, the craft’s maximum speed was doubled. On November 20, 

1953, this aircraft made the first Mach 2 flight. (NASA)
85

CHAPTER 3

Proving the Swept Wing:

The Douglas D-558-2 Skyrocket

It was actual flight time that was the real education—five minutes in 

the air with the experimental ship was worth ten hours of study on the ground, 

and gradually I understood the magnitude of the horizon that lay out there, 

unknown, waiting to be probed in the rocket ship.

—William “Bill” Bridgeman1

Ed Heinemann and the Douglas design team had always envisioned that the 

D-558 project would entail multiple aircraft configurations, consistent with 

what the Navy’s Bureau of Aeronautics envisioned coming from the program. 

Originally, the Navy contract awarded to Douglas called for six D-558 phase 1 

aircraft, with different combinations of side and nose inlets and straight wings 

with three different airfoil sections. Phase 2 involved adding rocket boosters 

and replacing the TG-180 jet engine with a smaller Westinghouse 24C turbojet 

on three of the aircraft. Phase 3 originally called for a mockup of an operational 

combat aircraft.2

This plan underwent a radical change due both to the capture of research 

material from Germany, after the war, and to R.T. Jones’s paper on swept 

wings. In the early summer of 1945, John Stack suggested to Douglas that the 

D-558-1 incorporate a 35° swept wing. However, it was not until after analysis of German documents brought back to America by the Naval Technical 

Mission to Europe (“NavTechMisEu”), one of whose members was Douglas 

engineer A.M.O. Smith, that the NACA, the Bureau of Aeronautics, and 

Douglas agreed to examine a swept-wing derivation of the D-558 program 

effort. The formal request by the Navy and the NACA for the new configuration came in mid-August 1945. 

A practical consideration was the fact that the existing turbojet engines lacked 

the thrust capacity to reach high transonic speeds. As a result, Douglas, the Navy, 

and the NACA agreed from the program’s outset that a rocket-powered research 

aircraft would be needed to evaluate the full potential of swept wings at transonic and supersonic speeds. Fortunately, with the development of the Reaction 
Probing the Sky

86

Motors, Inc., (RMI) 6000C4 engine—which the AAF designated the XLR-11, 

and the Navy the XLR-8—just such an engine existed that could be applied to 

the D-558. The swept-wing D-558 would take off and climb to test altitude using 

a Westinghouse 24C jet engine. The pilot would then employ the RMI rocket 

engine to accelerate into the high transonic and low supersonic regime. Once 

this was completed, the rocket would shut down, and the pilot would restart 

the jet engine for the descent to a powered landing. Two separate engines and 

fuel systems would have to fit within the fuselage. Douglas engineers found it 

impossible to squeeze both a rocket and a jet engine into the D-558-1’s alreadynarrow fuselage, so they had to start from scratch in designing the D-558-2. The 

result was a much shapelier and elegant design, which became one of the iconic 

symbols of aeronautical progress in the postwar era.

NACA 144 at South Base. The “Turbine Ex” is the exhaust for the turbopump, which fed fuel 

to the rocket engine. The “LOX Prime” was part of the liquid-oxygen vent/jettison system. The 

X’s on the forward and aft fuselages were photo reference marks. South Base was built during 

World War II as a training field for B-24 and P-38 pilots. It was the home for the NACA contingent from 1946 to 1954, when it moved to the current facility. (NASA)

The D-558 Phase 2 Design Process

Kermit E. Van Every was assigned the task of designing the new aircraft, and he 

faced a number of design requirements. Among these was the Navy managers’ 

requirement that the swept-wing version should have the low-speed and relatively 

benign stall characteristics of the straight-wing D-558-1. Douglas engineers faced 
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87

a difficult task in meeting this requirement, as swept wings were known to have 

poor low-speed and stall behavior. At the same time, they had to ensure that the 

aircraft would be stable at high speeds. 

The D-558-1’s cylindrical fuselage and nose inlet were abandoned in the 

new design. Instead, the new aircraft had an elongated fuselage; a flush windshield similar to that of the XS-1 enhanced the plane’s sleek lines. Flush air 

inlets were located low on the sides of the forward fuselage. The fuselage diameter had to be increased, as compared with that of the D-558-1, for it contained 

the Westinghouse 24C jet engine and two tanks holding 250 gallons of jet 

fuel, the LR-11 rocket engine and tanks holding 195 gallons of water/alcohol 

fuel, 180 gallons of liquid oxygen (LOX), 11 gallons of 90-percent hydrogen 

peroxide to power the turbopump, and the helium used to pressurize the fuel 

system. The nose and main landing gear also retracted into the fuselage. Finally, 

the flight data instrumentation, totaling between 800 and 1,100 pounds, completed the payload. 

The rocket engine was mounted at the rear of the fuselage. The jet engine 

was positioned in the middle, with the air-inlet ducts passing around the liquidoxygen tank to reach the engine. The two tanks for the jet fuel were above 

the jet engine. The water/alcohol fuel tank was located above the jet engine’s 

exhaust pipe, which was angled slightly downward and exited under the aft 

fuselage. Saying that the airplane was oddly arranged—to say nothing of the 

mix of power plants—would be an understatement. In fact, the NACA never 

had another such amalgamation.

The D-558-1’s straight cylindrical fuselage cross section was retained for the 

D-558-2 at the wing-fuselage intersection, which gave it a measure of “area 

ruling” before the advent of the concept. Had the fuselage been an ogival body, 

like the X-1’s, local airflow velocity at the wing-fuselage juncture would have 

increased, causing early shockwave formation and, therefore, an increase in 

transonic drag. Its wing had a 35° sweep and a span of 25 feet, representing a 

compromise between low- and high-speed requirements. This also resulted in 

a lower aspect ratio than the D-558-1’s straight wings, further reducing drag. 

But it also complicated making the airplane safe to fly at low speed. Kermit Van 

Every blended a variety of elements to ensure that the D-558-2’s swept wing 

was suitable. He selected a modest 35° sweep angle since this was already, by 

the beginning of the postwar era, a much-studied planform, tested by German 

wartime researchers, employed on the postwar Bell L-39 swept-wing test bed (a 

Navy-funded research program), and intended for the AAF’s XP-86 (prototype 

for the F-86) and the planned Boeing XB-47 jet bomber. Van Every replaced 

the NACA 65 series airfoil used on the D-558-1 with a higher-lift NACA 63 

series airfoil. The latter had better low-speed/stall characteristics yet did not 

sacrifice much in high-speed performance. Next, he increased the wing’s area to 
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175 square feet compared to the D-558-1’s 150.7 square feet. He gave the wing 

a “reverse taper” (from a 10-percent thickness-chord ratio at the root to a more 

lift-friendly 12-percent thickness-chord ratio at the tip). Finally, he added flaps, 

wing fences, and automatic Handley Page wing-leading-edge slats to improve 

its low-speed behavior. The slats could either operate automatically or be locked 

open or closed. As with its predecessor, the D-558-2’s horizontal stabilizers were 

thinner than the wings, could be moved independently of the elevators for highspeed control, and were positioned high on the vertical tail to remain out of the 

wing wake. Van Every increased the stabilizer’s critical Mach number further by 

giving it a sharper 40° sweep, rather than the 35° sweep of the wings.3

First Flights

Douglas rolled out the first D-558-2 Skyrocket (BuNo 37973) on November 

10, 1947, less than a month after Yeager’s Mach 1 flight. The aircraft was 

not complete, however, as the LR-8 rocket engine had not yet been installed 

because the turbopump needed to supply it with propellants was not yet ready.4

In its place, a cone-shaped fairing was added for the initial flights. Once the ceremonies were finished, ground tests of the aircraft began. The turbojet engine 

underwent test runs on November 21. The aircraft had a flush canopy (like 

the XS-1) that added to its racy looks but afforded its pilot minimal forward 

visibility during approach and landing. Consequently, by December this had 

been replaced with a standard raised canopy with a V-shaped windshield. This 

provided much better visibility for the pilot even if it did spoil the aircraft’s 

sleek lines. Once these final details were complete, the D-558-2 was loaded 

on a flatbed truck and wrapped in tarps to conceal its shape. The convoy left 

for Muroc on December 10, 1947. 

After arriving at Muroc, the aircraft underwent further checkouts and the 

instrumentation was installed, which took the rest of December. The pilot 

selected for the early flights of the first D-558-2 was John F. Martin, a Douglas 

test pilot since 1940 who had a background as a United Airlines pilot. He had 

served as test pilot on the A-20, A-26, and C-54. 

Initial taxi tests were made on January 5 and February 2, 1948, clearing the 

way for the first flight on February 4. Problems were experienced on the first 

flight; the already-anemic Westinghouse J34 engine was sluggish during startup 

and acceleration. Things did not improve much when the takeoff roll across the 

lakebed took 15,000 feet, and, once aloft, Martin discovered the airplane suffered 

from a persistent “Dutch roll” lateral-directional (roll-yaw) oscillation, an early 

indication that Douglas needed to increase the height (and hence area) of the 

vertical fin, which it subsequently did, by 18 inches.5
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89

A D-558-2 jet and JATO rocket take off in 1949. Because the LR-8 rocket engine was not initially 

fitted, the program had to make do with this interim propulsion system to carry out research with 

the aircraft. The takeoff roll was long, hindering maximum performance. Modifications were subsequently made to allow the D-558-2 to be air-launched from a B-29. (NASA)

To correct the long takeoff roll, Douglas engineers prudently decided to 

attach two jet-assisted-takeoff (JATO) solid rocket boosters to the aircraft for 

an added boost. The first JATO-boosted flight was made on July 13, 1948. The 

rockets were fired midway through the roll and jettisoned after takeoff. This 

shortened the takeoff run to 8,210 feet, a little over half the distance required 

with the jet alone. As a result, the use of JATO rockets became standard for 

the jet-powered, ground-takeoff flights; they not only shortened the takeoff 

roll, but they also conserved fuel and improved safety. Various combinations of 

two, three, or four rockets were tested, with four rockets deemed best. Martin’s 

last D-558-2 flight was on August 25, 1948. Skystreak veteran Gene May 

now took over the Douglas test duties, making his first flight in the airplane 

on September 16, 1948. May began an extensive flight-test effort that lasted 

more than a year.6

By this time, the second D-558-2 (BuNo 37974), intended for the NACA, 

had been delivered to Muroc, and May made a pair of dive demonstration 

flights on November 2 and 7, 1948, after which the aircraft was formally turned 

over to the NACA on December 1, 1948. Like the first Skyrocket, the second 

D-558-2 also lacked the LR-8 rocket engine when it was delivered because the 

planned turbopump to feed it was still unavailable. In spite of this, the NACA 

accepted the aircraft since initial flights were to be made for general stability 

and control and air-load measurements at Mach 0.85, both of which could be 
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done with the jet engine alone. Once the LR-8 engine was available, the second 

Skyrocket would be returned to Douglas and the rocket installed. 

Engine problems and the installation and calibration of instrumentation 

lasted through the winter and spring of 1949, delaying the start of research 

operations. NACA research pilot Robert Champine made the first NACA 

flight on May 24 for pilot familiarization, instrumentation checkout, and 

general handling characteristics. Champine made two more research flights in 

June for data on stability, control, and wing bending, as well as for wing twist 

measurements. During the second flight, the cockpit camera caught fire, filling 

the cockpit with smoke. Two airspeed calibration flights were made in July, 

followed by a lateral-control-exploration flight in August.7

The dynamic-lateral-stability data collected in the first two NACA 

Skyrocket flights were analyzed. Test pilots made sudden control inputs, and 

the onboard instrumentation evaluated aircraft response. One test involved “a 

lateral oscillation of the airplane resulting from abrupt deflection and release of 

the rudder.” The research memorandum noted, “This maneuver was made at a 

Mach number of 0.63 and an altitude of 12,000 feet with the airplane in the 

clean condition. The data show the oscillation is slow to damp out especially 

at small amplitudes where the oscillation is practically of constant amplitude. 

The period of the oscillation is 1.6 seconds.”8

A second test maneuver made was a lateral oscillation while in the landing 

condition (gear and flaps down). The memorandum noted, “This oscillation 

was again induced by abrupt deflection and release of the rudder. In the landing condition, the airplane performs a constant-amplitude oscillation with a 

period of approximately 2.7 seconds.”9

A more complicated test maneuver measured the D-558-2’s behavior 

during part of the landing approach. The report described the events as follows: “During the first part of this time history, between 30 and 44 seconds, 

the pilot did not attempt to stop the oscillation by use of the ailerons or rudder 

and the airplane performed a constant-amplitude oscillation. From 44 seconds 

to 60 seconds the pilot used the ailerons and was able to damp the oscillation. 

Even though the pilot can damp the oscillation, the oscillation is objectionable 

particularly during landing approaches and landings because the controls must 

be moved almost constantly. The rough-air handling qualities of the airplane 

would probably be particularly objectionable.”10

The conclusion of the memorandum was that the D-558-2 showed differing dynamic-lateral-stability characteristics, depending on the situation. In a 

“clean” condition (flaps and landing gear up, slats locked), the airplane’s lateral 

oscillations were lightly damped, particularly when they were of small amplitudes. With the D-558-2 in a landing condition, however, the airplane had 

neutral oscillatory stability, which was indicated by the airplane undergoing a 
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91

constant-amplitude oscillation. The pilot could damp this oscillation, but doing 

so required vigorous control inputs, activity that was considered objectionable by 

the pilot, as it required almost continuous control-surface movements. These, in 

turn, rendered the D-558-2’s handling poor in less than ideal conditions, such 

as turbulence.

This was not the only handling problem the D-558-2 would experience. A 

bigger issue soon came to light. 

The NACA 144 undergoing wing-strain-gauge calibration in a hangar. Sandbags were piled on 

the upper wing to simulate aerodynamic forces. Strain-gauge readings were compared to calculated forces to determine any errors. These were used by the “computers” who reduced onboard 

data after a flight. The number of engineers required for the calibrations was impressive. (NASA)

Pitch-Up: The Insidious Threat

The seventh NACA flight, with Champine as the pilot, came on August 8, 

1949. The plan involved making a 4-g turn at Mach 0.6. Without warning, 

the Skyrocket’s nose pitched up with an acceleration of 6 g’s during the turn. 

Champine countered with full-down elevator, and the aircraft recovered. As 

a precaution, he immediately landed. What he had experienced was the first 
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proof that a swept-wing aircraft would pitch up during a hard turn. Tests 

made using models had hinted at the possibility that longitudinal instability 

would occur, resulting in the nose’s pitching up, but what the models had not 

indicated was the severity of the problem.11

The source of the pitch-up problem was a combination of several factors. 

A. Scott Crossfield, an NACA research pilot who made 62 D-558-2 flights, 

described the sequence: “The air we fly in doesn’t like high sweep angles. It 

doesn’t like severe taper ratios. And it doesn’t like low aspect ratios. And the 

D-558-2 had a little bit or a lot of every one of these. And it was classic in what 

it did as a swept wing.... The tips of the [D-558-2] wings tended to stall before 

the roots of the wings. And if that’s aft of the center of gravity, the airplane 

wants to pitch-up.”12

Pitch-up posed a number of risks, not least of which was loss of control. 

In a turning dogfight, a swept-wing fighter experiencing a pitch-up could be 

overstressed by g forces; the sudden loss of airspeed resulting from a pitchup could make an aircraft vulnerable to an enemy fighter’s attack or could 

cause it to enter a spin and be unable to recover. Pitch-up posed risks in 

day-to-day operations. A swept-wing aircraft maneuvering during a landing 

approach might pitch up, stall, and have insufficient altitude and airspeed to 

recover before crashing. In an extreme case, structural failure could occur. With 

swept-wing F-86s entering service and a new generation of swept-wing fighters 

and bombers in development, understanding pitch-up assumed critical and 

urgent importance. 

The NACA research effort with the second D-558-2 now focused on the 

pitch-up issue. Joining the project was another NACA research pilot, John 

Griffith. After Griffith’s checkout flight on September 12, 1949, he and 

Champine split flying duties. Griffith experienced an inadvertent pitch-up 

on his November 1, 1949, flight. The sequence was similar to one Champine 

had experienced on a flight made nearly 3 months before. Griffith made 

a 4-g turn at Mach 0.6, and the aircraft became longitudinally unstable, 

triggering a pitch-up. Griffith attempted to fly beyond the instability; the 

D-558-2’s angle of attack increased, and the aircraft yawed and rocked, then 

snap-rolled, turning completely upside down. Griffith recovered, confirmed 

that the Skyrocket had not been damaged, and continued the flight.13

Champine later described the experience of a D-558-2 pitch-up: “If 

you pulled up and got 4 or 5 gs, it would suddenly stall in such a manner 

that the lift distribution on the wing would cause it to pitch-up violently. 

It would go to extremely high angles of attack, between 45 and 60 degrees, 

and then it would start to roll violently, so the aircraft became completely 

and totally out of control—just spinning around in the sky. Once you fell 

into it you had no way of controlling it.”14
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93

Griffith’s pitch-up experience was analyzed in a 1951 NACA research 

memorandum as follows:

[T]he pilot attempted to fly the airplane in the normal-forcecoefficient range where the airplane was unstable (times between 

11 and 14 sec). After the airplane pitched up, the airplane first 

performed an unsteady rolling motion. The pilot used the rudder in attempting to control this rolling motion and caused the 

airplane to perform a 360° snaproll. The data indicate that very 

large angles of sideslip were reached during the snaproll. No 

sideslip-angle measurements were obtained during the maneuver, but integration of the yawing velocity indicates that sideslip 

angles on the order of 30° to 35° were approached. A maximum 

lateral acceleration of about 1.1g occurred during the snaproll. 

This lateral acceleration corresponds to a side force on the airplane 

of about 10,800 pounds. The distribution of side force on the 

airplane between the fuselage and vertical tail is not known, but 

it is likely that the load on the vertical tail, during the oscillatory 

yawed flight, approached the vertical tail design limit of 8,700 

pounds. The maximum rolling velocity which occurred during 

the snaproll is not known as the 2.6-radian-per-second range of 

the rolling-velocity recorder was exceeded, but it is likely that the 

maximum rolling velocity was on the order of 3.5 to 4.0 radians 

per second. In recovering from the snap-roll the airplane again 

reached a negative normal acceleration of 3.0g.

The pilot reported and the recording instruments showed that 

airplane buffeting occurred at normal accelerations slightly less 

than the acceleration at which the airplane became unstable. This 

buffeting served as a warning of the approach of instability. If the 

elevator is moved down when the airplane buffeting occurs, the 

response of the airplane is good and the instability can be avoided. 

In the pilot’s opinion the airplane is unflyable in accelerated flight 

in the lift-coefficient range in which it is unstable. If the pitch-up 

resulting from the instability is not checked by moving the elevator down as soon as it is noticed by the pilot, the angle of attack 

increases very rapidly and violent rolling and yawing motions 

occur when the high angles of attack are reached.15

Between May 1949 and January 1950, 21 NACA research flights had been 

made with the second D-558-2. The bulk of these were for longitudinal- and 

lateral-stability, lateral-control, and stall data. The results of these flights were 
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summarized in a series of NACA research memorandums.16 Beyond describing the pitch-up characteristics of the D-558-2, one of the reports also gave 

a preliminary assessment of the D-558-2’s longitudinal-stability and control 

characteristics based on these flights. It noted the following: 

With the slats locked and the flaps up, the airplane was longitudinally unstable at normal-force coefficients greater than approximately 0.8 in steady flight at low speeds and in maneuvering flight 

at Mach numbers up to at least 0.65. No data were obtained 

at high normal-force coefficients at Mach numbers greater than 

about 0.65 because of the power limitations of the airplane with 

only the jet engine installed. The instability proved objective to the 

pilots, particularly in accelerated flight because of the tendency 

for the airplane to pitch to high angles of attack very rapidly and 

because violent rolling and yawing motions sometimes occurred 

when the high angles of attack were reached. The instability probably resulted from a large increase in the rate of change of effective 

downwash at the tail with increase in angle of attack at moderate 

and high angles of attack. 

With the flaps down and the slats locked the longitudinal stability characteristics in steady flight at low speeds were very similar 

to the characteristics with the flaps up and the slats locked except 

that the instability occurred at a higher normal-force coefficient.

The degree of instability present with the slats unlocked and 

the flaps up or down was much less than with the slats locked and 

the pilots had only minor objections to the longitudinal characteristics of the airplane.

In steady flight in the Mach number range from 0.50 to 0.87 

the airplane is stable longitudinally and no abrupt trim changes 

occurred up to the highest Mach number reached, 0.87. The 

data indicate that only a slight reduction in the relative elevatorstabilizer effectiveness occurred in going from a Mach number of 

about 0.55 to 0.85.17

Into the Supersonic: The Skyrocket Turns to Air Launch

With the initial jet-powered NACA research flights completed, the second 

D-558-2 was returned to Douglas for modifications. The shortcomings of 

the ground takeoffs, even with JATO rockets, were now apparent. Bridgeman 

later described the experience of a JATO takeoff in the Skyrocket:
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

95

A mile of runway is eaten up and she hasn’t hit 100 [mph]. Up to 

80, now 90, there it is, 100. Hit the first two JATOs. A second, 

and a kick in the fanny, and another kick as the remaining JATOs 

fire off. She’s up to 180, roaring down the lakebed. This is the 

time; pull her nose up...this is no F-80! Without her rockets, 

climbing on jet alone, I can feel that she is far underpowered; she 

handles like a truck, heavy and large. I’ve got hold of something 

new all right.18

Clearly, something as simple as a blown tire from overheating during the 

long takeoff runs could spell disaster. Something had to be done, and the simplest solution was modifying the D-558-2s for air launching from a Boeing 

B-29 mother ship.

Though the first and second D-558-2s had been delivered without the LR-8 

rocket engine, the third (BuNo 37975) was equipped with the rocket power 

plant (as well as its J34 turbojet) from the start. Gene May made the first flights 

in the third D-558-2 beginning on January 8, 1949. On June 24, 1949, May 

flew this aircraft through the speed of sound, noting later that the “flight got 

glassy smooth, placid, quite the smoothest flying I had ever known.”19

But despite this, the D-558-2’s performance was still disappointing, a 

byproduct of having both the Westinghouse 24C turbojet and LR-8 rocket 

engine. Given the large amount of kerosene needed for the J34 turbojet for the 

climb to altitude and descent to landing, the D-558-2 had insufficient rocket 

propellant—the diluted alcohol/water fuel and liquid-oxygen oxidizer—to 

permit the Skyrocket to penetrate far into the supersonic regime. An NACA 

study showed that a jet-only ground takeoff was limited to a top speed of Mach 

0.9. Using the LR-8 for an added boost during ground takeoff resulted in a 

limiting top speed of Mach 0.95. NACA engineers concluded that the small 

increase in speed was not justified by the added risk of an LR-8 rocket ground 

takeoff. Following the fourth Douglas flight, on March 27, 1949, May had 

noted, “[The] ultimate performance of the aircraft will never be achieved with 

the limited supply of rocket fuel.”20

Air launch enabled getting the most out of the aircraft by eliminating the 

need for using the scarce rocket propellants to help kick the Skyrocket off the 

ground. Better still was turning the Skyrocket into an all-rocket boost-glider 

like the Bell X-1. Douglas engineers calculated that by removing the jet engine 

and adding more fuel for the rocket (using the space previously taken up by 

the jet engine and its inlets and exhaust system), an air launch would produce 

a top speed of Mach 1.46 to 1.6, dramatically increasing the Skyrocket’s value 

as a research tool. The NACA’s chief of research, Hugh L. Dryden, supported 

the Douglas proposal, suggesting the company modify the second aircraft to 
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an all-rocket/air-launch configuration at the same time the LR-8 rocket was 

installed. The third could be modified at the same time and used by Douglas 

in an air-launch demonstration before it was turned over to the NACA. 

Dryden sent a letter to the Navy on September 1, 1949, strongly recommending the modifications. Navy managers agreed, and, on November 25, 

1949, the Navy Bureau of Aeronautics issued a contract change ordering the 

second and third D-558-2 aircraft to be modified to an air-launch configuration 

and a B-29 modified as the mother ship, which, by the Navy’s nomenclature 

system, was designated a P2B-1S Superfortress. (The B-29 already modified 

for the XS-1 could not be used to drop the Skyrocket, as the sweptback wings 

and tail surfaces of the D-558-2 necessitated a completely different geometric 

set of modifications to the bomb bay.)

The third D-558-2 would retain the J34 jet engine and inlets, the LR-8 

rocket engine, and the existing fuel system. The only modifications would be 

the addition of retractable launch hooks. The aircraft was trucked back to the 

Douglas plant in El Segundo. Bridgeman made the first air launch of a D-558-2 

on September 8, 1950, using only the turbojet. Five more air launches followed, the last two using both jet and rocket power. Bridgeman described his 

experiences on his November 27, 1950, flight: 

Ten...nine...eight...seven.... My hand is on the data switch...

six...five. There. One hand on the wheel, one wrapped around 

the throttle. Four...three...two...one...drop!

From out of the dark, protective belly of the [B-]29 the world 

bursts over me in bright light. She’s free. I’m away from the mother 

ship clean and free. It works.... Rapidly I click on the four rocket 

tubes.... She heads out, still losing altitude but trimmed noseup.... I check the rocket pressures. In the green.... I pull it up 

and begin the climb.... Too much.... I drop the nose a fraction. 

Too far. Up again; it is now a matter of calculation and feel.... I 

push her nose over for the speed run. She accelerates rapidly in 

the thin sky-meadow of the higher altitude.

The buffet! Okay, that’s .91. But she doesn’t stop; she still 

buffets although we are through into .92, the other side, where 

she smooths out always. The buffeting continues... .93, .94.... I 

search the instrument panel and there’s the answer—the turbine 

out temperature is overboard by 500[°]. At once I jerk the throttle 

to idle, but it is not fast enough; the engine gives up. She flames 

out, loses thrust, and pitches over, throwing me in the harness 

and against the panel.... At once, in rapid succession, the rocket 

tubes fail. The altitude is too much for the jet engine. When she 
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97

pitched forward she unported the rocket fuel, starving off the 

tubes—all in a matter of ten seconds.21

With the completion of the company’s acceptance flights, the aircraft was 

turned over to the NACA on December 15, 1950, and given the designation NACA 145. Scott Crossfield became the project pilot. He made pilotfamiliarization and instrumentation check flights on December 22 and 27, 

1950. These were jet-only flights, as were the initial two research flights in 

March and April 1951. Crossfield made the first NACA jet-rocket flight on 

May 17, but he shut down the jet engine during the flight because of combustion instability. Joining the project at this point was NACA pilot Walter P. 

Jones, who made his first Skyrocket flight on July 20, 1951. Early activity in 

the series focused on lateral and longitudinal stability evaluated via aileron rolls 

and elevator pulses, as well as accelerated turns and pitch maneuvers. 

Longitudinal-stability characteristics were determined with the aircraft in a 

clean condition and in turning flight at Mach numbers between 0.5 and 0.96, 

at altitudes between 19,000 and 36,000 feet. The Skyrocket was stable up to 

moderate values of normal-force coefficients. At higher coefficients, however, 

a constant elevator deflection triggered a rapid pitching of the aircraft. At 

the start of the pitch-up, the stick force lightened and the pilot reversed the 

elevator control in an attempt to stop the pitch-up. This was unsuccessful, 

and both the angle of attack and normal-force coefficient increased until a 

recovery was made. 

In an effort to control the pitch-up, an outboard wing fence was added to 

both wings. Jones made the first flight on October 18, 1951, with the wing 

fences, at Mach 0.7. A second flight was made on November 9 at Mach 0.95. 

The pitch-up flights were halted until the following summer.22

The all-rocket second D-558-2 required more extensive changes before it 

returned to the air. It was sent back to the Douglas plant in El Segundo in early 

1950 and disassembled. The two jet fuel tanks were removed, and, in their 

place, a liquid-oxygen tank and an alcohol/water tank were added. This almost 

doubled the fuel supply to 345 gallons of liquid oxygen and 378 gallons of 

alcohol/water. The jet engine was removed, and its exhaust in the lower aft fuselage was faired over. Air inlets were also removed and flush panels added. The 

LR-8 rocket engine and its turbopump were mounted in the aft fuselage. The 

result was an aircraft optimized for high-speed/high-altitude research flights.23

On November 8, 1950, the second D-558-2 was loaded under the P2B-1S 

mother ship and flown back to what was now Edwards Air Force Base after 

Muroc had been renamed. As with the third D-558-2, Bridgeman would 

make the initial flights with the second one before it was turned over to the 

NACA for research work. This proved difficult. Several attempts were aborted 
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shortly before launch. At one point, a sign appeared on the hangar wall: “Old 

Skyrockets Never Die—They Just Jettison Away.” Bridgeman later said, “An 

experimental test pilot has to be right on the edge, at the peak of his performance; and the delay was sapping this capacity.”24 Finally, on January 26, 1951, 

the Douglas team’s luck seemed about to change. Then, as the countdown 

reached less than a minute to launch, Bridgeman saw that a rocket pressure 

gauge was dropping. Bridgeman radioed the P2B-1S’s pilot, George Jansen: 

“No drop. This is an abort,” and began shutting down the systems. Bridgeman 

was startled to hear Jansen begin the 10-second countdown. Bridgeman radioed: “Don’t drop me, George!” but Jansen had his thumb on the microphone 

key and could not hear Bridgeman’s calls. 

A jet-powered D-558-2 parked on the ramp at South Base in 1949. The jet intakes are located 

low on the forward fuselage, and the exhaust is on the underside of the rear fuselage. The 

aircraft has been fitted with wing fences, and the wing slats are extended. The original NACA 

shield appears on the vertical tail but was subsequently replaced by a yellow band with a winged 

NACA insignia. (NASA)

Bridgeman frantically pushed the circuit breakers back into place and 

centered the control stick, hoping he had not missed anything. When the 

launch count reached zero, the Skyrocket separated; once clear of the bomber, 

Bridgeman fired the four rockets in rapid sequence. They ignited despite the 

pressure readings, and the Skyrocket accelerated upward. 

By now, 35 seconds had passed since he had been dropped, and Bridgeman 

made his first radio call: “George, I told you not to drop me!” Colonel Frank 

“Pete” Everest, the Air Force chase pilot, replied, “You got keen friends, 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

99

Bridgeman.” Everest added, “He’s accelerating away from me in a climb. Looks 

like he’s doing all right...all four rockets appear to have lit off.” Because of 

the higher speed, the elevators were ineffective and Bridgeman had to use the 

movable stabilizers for pitch control. The peak speed reached was Mach 1.28 

in a slight dive.25

To prevent a repetition, a green light was rigged up in the P2B-1S cockpit to 

indicate that the D-558-2 was ready for launch, to be triggered by the research 

pilot in the test vehicle when he was ready in case there was another sticky 

microphone. The launch countdown was also shortened. It was not until April 

5, 1951, that the second Douglas flight was made, reaching a peak speed of 

Mach 1.36 at 46,500 feet. Bridgeman again experienced severe lateral oscillations, forcing him to shut off all four chambers of the engine before burnout. 

The fix for the oscillations was to add a rudder lock, which prevented it from 

moving at speeds above Mach 1. The all-rocket D-558-2 had been proven 

airworthy and had proven that an air launch was feasible. The Navy, Douglas, 

and Bridgeman now began a series of maximum-speed and -altitude flights. 

Bridgeman began the speed buildup on May 18, 1951. After launch, he 

climbed to about 55,000 feet and made a –0.8-g pushover to begin the speed 

run. He reached a maximum speed of Mach 1.72 at 62,000 feet before the 

engine shut down. This was followed on June 11 with a flight that reached 

Mach 1.79. Bridgeman was now the fastest man on Earth. Both these flights 

exceeded the most optimistic performance estimates made by Douglas for an 

all-rocket/air-launched Skyrocket. 

The Douglas engineers decided to try for even faster speeds. Doing this, 

however, required that a lower 0.25-g pushover be made. The flight was scheduled for June 23. Bridgeman was successfully launched; he ignited the rocket 

and began the climb. Reaching 60,000 feet, he made the 0.25-g pushover and 

accelerated in a shallow dive. When the aircraft reached Mach 1.5 it began 

rolling violently, throwing Bridgeman from side to side as the wings rocked 

back and forth as much as 70° in less than a second. Bridgeman’s attempts to 

bring the aircraft back under control actually aggravated the problem, and, as 

a last resort, he shut the engine down. 

But instead of calming, the rocking motion increased. Bridgeman pulled 

back on the control wheel and made a 4-g pullout. Now headed toward the 

safety of the lakebed and accompanied by the F-86 chase plane, he made a 

successful landing. NACA engineers determined that the aircraft had reached 

a top speed of Mach 1.85 at 63,000 feet. This represented a new speed record 

for a rocket-powered aircraft and was well beyond what both the NACA and 

the Navy had originally envisioned as the upper-end of D-558-2 performance.

But NACA managers were troubled by the continuation of the Douglas 

flight test project. The all-rocket aircraft had been proven airworthy, and NACA 
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researchers did not view record flights favorably. Speed and altitude records 

set during research flights were not significant. When setting records became 

the reason for a flight, airplanes crashed and pilots died. Research aircraft, their 

view held, were for use in collecting data for future aircraft development, and 

it was time—even beyond time—for the agency to have received the aircraft 

back for its own extensive research. Ultimately, the Navy agreed, but only after 

signing off on two more Douglas flights. 

An in-flight photo of a Skyrocket descending toward the lakebed with an F-86 chase plane 

following behind. Chase planes were key elements in undertaking research flights safely. The 

chase pilot provides an external set of eyes to warn of problems and calls out the research 

airplane’s altitude above the lakebed. (NASA)

The first of these came on August 7, 1951, as a maximum-speed flight. To 

avoid the instability experienced on previous flights, the initial pushover load 

factor was 0.8 g. Bridgeman then reduced this to 0.6 g. The aircraft became 

left-wing-heavy, however, and his attempt to correct this with the ailerons was 

unsuccessful, so he raised the loading back to 0.8 g, which restored lateral stability. Bridgeman then reduced it to 0.6 g until the rocket engine shut down. 

When the data reduction was complete, engineers found that the Skyrocket 

had reached Mach 1.87, with a possible error of ±0.05, at a pressure altitude 

of 67,300 feet. The maximum airspeed was calculated to be 1,243 mph ± 33 

mph. Bridgeman believed the aircraft capable of even higher speeds, stating, 

“[M]agic Mach 2 is attainable if lateral control can be maintained.”26
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

101

Bridgeman’s final Skyrocket flight was on August 15, a maximum-altitude 

flight. Charles Pettingall, chief aerodynamicist in the Douglas testing division, 

calculated the flightpath this time. It was designed to exceed the 72,395-foot 

altitude record set by the U.S. Army Air Corps–National Geographic Society 

Explorer II piloted balloon flight in 1935. After launch, Bridgeman ignited 

the rocket and reached a peak speed of Mach 1.35, slower than the maximum 

speeds attained in earlier launches. Passing through 63,000 feet, the D-558-2 

began rolling to the left. Even with full opposite aileron input, the airplane was 

slow to recover. The rocket engine shut down, and the aircraft coasted upward 

under its own momentum. Bridgeman described the view from an altitude 

no other human had reached: “Out of the tiny window slits there is the earth, 

whipped clean of civilization, a vast relief map with papier-mâché mountains 

and mirrored lakes and seas. The desert is not the same desert I have seen for 

two years. The coastline is sharply drawn with little vacant bays and inlets, a 

lacy edge to the big brown pieces of earth.”27

When calibration corrections were made to the air pressure data, results 

indicated that the Skyrocket had reached a pressure altitude of 77,500 feet ± 

500 feet. But the radar data had to be corrected for errors in the radar slant 

range and elevation angle, as well as for such factors as beacon delay, atmospheric refraction, and Earth’s curvature. Once done, the Skyrocket’s altitude 

above sea level was shown to have been 79,500 feet ± 65 feet. Bridgeman was 

now both the fastest and highest-flying man on Earth.28

Beyond setting the new speed and altitude records, the Douglas demonstration flights significantly increased knowledge of the D-558-2 flying qualities 

and handling. On one of the first supersonic flights, an uncontrollable lateral 

oscillation occurred, forcing the pilot to abandon the speed run to Mach 1.4. 

The magnitude of the oscillations was inversely related to the angle of attack: 

the lower the angle of attack, the more violent the oscillations. At higher 

speeds, such as on the Mach 1.85 flight, a different problem appeared. If the 

pilot made the pushover to begin the speed runs at too low a g-force, severe 

oscillations were triggered.29

The all-rocket second D-558-2 was turned over to the NACA on August 

31, 1951, and given the designation NACA 144. Research flights with both 

of the Skyrockets could now begin.30

The First NACA Supersonic Skyrocket Research Flights

There were two different Skyrocket configurations in NACA hands: the second 

D-558-2/NACA 144, with its all-rocket configuration, and the third D-558-2/

NACA 145, with a jet-rocket propulsion system. This dictated that each plane 
Probing the Sky

102

would be used for different types of research. Once the Douglas speed- and altitude-record flights had been completed, NACA engineers decided to explore 

the second Skyrocket’s operational limitations, with Crossfield as pilot. He had 

his first pilot check flight in the new aircraft on September 28, 1951, during 

which he experienced rough engine operation but still reached Mach 1.2. 

The D-558-2 was not only used for aerodynamic research, but also for the collection of operationally oriented data. The third Skyrocket, NACA 145, in which the jet/rocket propulsion system 

was retained, was fitted with pylons and simulated bombs to test their effects. (NASA)

He made three more flights in October and November. The first two flights 

provided data on longitudinal and lateral stability and control, loads, and aileron effectiveness at low supersonic speeds (Mach 1.28 and Mach 1.11). The 

final flight, on November 16, 1951, saw Crossfield reach a speed of Mach 1.65 

at 60,000 feet. The winter rains closed the lakebed until the spring of 1952. 

In contrast, the third Skyrocket, with its dual jet-rocket propulsion system, 

had a much more limited performance capability and thus was more suitable 

for transonic research, exploring such issues as swept-wing pitch-up. Once the 

lakebed dried out, these flights resumed on June 19, 1952, with Crossfield 

and Jones as project pilots.31 The initial flights were made to test aircraft 

response with an inboard and outboard fence on each wing. The first flight, 

by Crossfield, reached a speed of Mach 0.7. Subsequent flights were made at 

Mach 0.96. Four flights were made in this configuration, with one aborted 

due to a failed cockpit heater. The last flight in the series, on August 14, was 

made with the inboard wing fence removed.32
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

103

A 1951 photo showing one of a number of wing-fence configurations tested to counter the pitchup caused by swept wings. The inboard fence is mounted on top of the wing, and the outboard 

fence extends around the wing leading edge. (NASA)

 High-Speed Flight Research Station engineers Jack Fischel and Jack Nugent 

summarized the effectiveness of the wing fences in a research memorandum. 

The tests were made with the Skyrocket in a clean configuration in turning 

flight at speeds of Mach 0.5 to 0.96, at altitudes between 19,000 and 36,000 

feet. The flights were made both with the original wing configuration and with 

the addition of an outboard fence on each wing since wind tunnel testing had 

indicated that this fence would alleviate the pitch-up problem. 

Fischel and Nugent found the flight results less clear-cut than wind tunnel 

data had suggested they would be. They concluded: “The addition of wing 

fences appeared to provide only a slight improvement over the original configuration, inasmuch as the pitch-up occurred at only slightly higher values of 

normal-force coefficient for the modified airplane configuration.”33

Their analysis also included the pilots’ impressions, which were quite blunt:

In the pilots’ opinion, the airplane is uncontrollable for a range 

of normal acceleration of about 1 g and 11⁄2 g above the value at 

which the reported change in stability occurs; this behavior is 

very objectionable. At low speeds, if the pilot does not check the 

pitch-up by use of the elevator as soon as it is noticed, the angle 
Probing the Sky

104

of attack increases rapidly and violent rolling and yawing motions 

are experienced at large values of [angle of attack]. At high speeds 

the pitch-up appeared to be more severe and more abrupt. 

Throughout the speed range covered, the occurrence of a reduction in stick-free stability, almost simultaneously with the reduction 

in stick-fixed stability, tended to accentuate the pitch-up to the 

pilot. The pilot felt that even with improved control, as would 

result from an all-movable tail, flight above the stability boundary would not be sufficiently steady for gunnery or other precise 

maneuvering.”34

For the next series of flights, engineers tested several different wing configurations. The goal was to find a means of preventing instability and pitch-ups during 

accelerated longitudinal maneuvers at speeds of up to Mach 1 and altitudes of 

10,000 and 35,000 feet. The aircraft was flown with wing slats fully extended, 

both with and without inboard wing fences, and, finally, with the wing slats half 

extended and wing fences removed. Another change made, in order to improve 

the stick-force characteristics at moderate and large angles of attack, was to attach 

two bungee cords to the control column. 

Crossfield made the initial flight on October 8, 1952, at a speed of Mach 

0.97. During the flight, and while performing a turn at high speed, the 

Skyrocket pitched up 36° followed by the now-anticipated sharp roll-off and 

near loss of control. The final pitch-up research flight for the year was made 

on October 22, 1952, also by Crossfield. In this instance, the aircraft was 

configured without wing fences. During the flight, pitch-up occurred when 

the aircraft made turns.35

The results from the extensive series of tests were summed up in a 1954 

research memorandum written by Fischel: 

Opening the wing slats to the fully extended position improved 

the stability characteristics of the airplane by alleviating pitchup at Mach numbers of below approximately 0.8; however, at 

Mach numbers between 0.80 and 0.85 the severity of the pitch-up 

remained unaltered. At Mach numbers of about 0.98 and 1.00, 

maneuvers performed up to relatively high values of normalforce coefficient with slats fully extended exhibited no evidence 

of pitch-up; however, this effect has since been duplicated with 

the clean-wing configuration (no fences, slats retracted).

Removing the wing fences from the airplane configuration 

with slats fully extended caused the reduction in stick-fixed 

stability to become slightly more pronounced, and generally, 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

105

to occur at approximately the same or slightly lower values of 

normal-force coefficient.

With wing slats half extended and no wing fences, the airplane 

exhibited instability characteristics and pitch-up similar to that 

exhibited by the airplane with slats retracted and with wing fences.

With slats fully extended and wing fences removed, use of a 

bungee in the control system to alleviate or eliminate the stick-free 

instability caused the airplane to appear more controllable to the 

pilot and caused the decrease in stick-fixed stability to become 

less apparent and less objectionable.36

Fischel also described the pilots’ impressions of the aircraft’s behaviors:

In general, the pilots’ reports corroborated the data and conclusions reached for the maneuvers performed. With the slats fully 

extended at all Mach numbers below M ≈ 0.8, it is the pilot’s 

opinion that the airplane stability did not deteriorate appreciably 

after the initial decay, and as a consequence control was regained 

more rapidly than in the original slats-retracted configuration. At 

M ≈ 0.98 (wing fences on) and M ≈ 1.0 (wing fences removed), 

the airplane appeared controllable up to the maximum value of 

[angle of attack] attained. In both fence configurations with the 

slats fully extended the stability change most apparent to the pilot 

was lightening of stick forces at moderate angles of attack. The 

pilots reported a stick-fixed stability change at moderate angles of 

attack which became somewhat more apparent when the inboard 

wing fences were removed from the slats fully extended configuration. The pilots thought that the airplane configuration with slats 

fully extended were [sic] a definite improvement over the airplane 

configurations flown with the slats retracted.

Because the soft bungee had little or no effect on the stick 

forces, the character of the stick-free and stick-fixed instability of 

the airplane appeared to the pilot to be about the same as when 

no bungee was used. In both instances, the lightening of the stick 

forces at moderate angles of attack tended to increase the control 

rate, which in turn would aggravate any pitching. With the stiff 

bungee, however, the character of the decay in stability appeared 

much improved, for now the stick-free stability was improved and 

the airplane appeared to have a lower pitch divergence rate than 

previously; thus, the change in stick-fixed stability was somewhat 

less apparent and less objectionable.
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106

In the configuration with slats half extended, the pilot thought 

the airplane behavior was similar to that encountered with slats 

retracted, and the pitch-up encountered was equally uncontrollable. Although data obtained in this configuration were limited 

to two high-speed maneuvers, the pilot reported that pitch-up was 

also encountered in other maneuvers performed at lower speeds 

(down to M < 0.7), at lower values of normal acceleration as the 

Mach number was decreased.

...If the stick-fixed stability is made acceptable, the provision 

of a bob weight, bungee, or artificial feel system to supply more 

satisfactory stick-force characteristics would be desirable.37

When pitch-up flights resumed in February 1953, with Crossfield as the 

sole project pilot, a new wing modification had been made to the airplane.38

Wind tunnel tests indicated that a chord increase on the outer 32 percent of 

the wing might eliminate the tendency to pitch up under high lift conditions 

at around Mach 1.2. During the winter of 1952–53, new outer wing panels 

were added, effectively giving the Skyrocket a “sawtooth” leading edge. 

The first flight with the chord extensions was made on February 27, 1953, 

in a jet-only flight. Crossfield made windup turns and 1-g stalls. The maneuvers 

were halted when longitudinal or lateral instability occurred. Additional tests of 

the modified wing shape were made on April 8 and 10 at speeds between Mach 

0.45 and Mach 1.0 and altitudes from 18,000 to 34,000 feet. Crossfield made 

windup turns, aileron rolls, sideslips, and 1-g stalls during the flights. The tests were 

repeated on the final flight of the series, on June 15, bringing the tests to a close. 

Jack Fischel and Cyril D. Brunn subsequently wrote a research memorandum on the chord extensions. Their summery concluded:

Addition of wing chord-extensions had only a minor effect on the 

decay in stick-fixed stability (pitch-up) and stick-free stability experienced by the airplane at moderate angles of attack. The chordextensions alleviated the pitch-up to a small degree, but the pilot 

still considered the airplane unsatisfactory for controlled accelerated flight in this region. However, at higher angles of attack, the 

airplane appeared to retrim and regain some stability.... 

The buffeting of the airplane was of such a nature that the 

increase in buffeting intensity induced by the chord-extensions 

became a major problem, in addition to the longitudinal instability problem.

A comparison of wind tunnel with flight data showed good 

agreement in the reduction of stability evident at moderate angles 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

107

of attack. The results indicate that an abrupt reduction of stability to a region of neutral or even slight stability could tend to 

cause pitch-up.39

Following the April 10, 1953, flight, the chord extensions were removed 

from the aircraft and the slats were reinstalled and locked in an open position. 

The pitch-up research flights resumed on June 15, using both the jet and rocket 

engines. Accelerated longitudinal-stability maneuvers began in earnest, and the 

aircraft showed decay in stability at all speeds except Mach 1. Following the 

flight, a stiff bungee was installed on the control stick. This change was tested 

on the June 25 jet-only flight and showed improvement, sufficient at least to 

make the Skyrocket controllable at high angles of attack. The reduction in 

stability was judged by Crossfield to be less objectionable. After this flight, the 

aircraft was restored to its basic configuration.

One approach tested on the D-558-2 was that of a sawtooth chord extension on the outer leading edge designed to prevent spanwise airflow across the wing. (NASA)

Crossfield made five more flights between September 9 and December 22, 

1953, in the third D-558-2, with both the jet and rocket propulsion systems. 

These looked at lateral, longitudinal, and directional stability and control at 

transonic speeds (Mach 0.4 to Mach 1.08), as well as turns and stalls. Two 

of the flights suffered rocket malfunctions; on the September 22 flight only 

two rocket cylinders fired, and on the final flight the rockets failed to ignite 
Probing the Sky

108

altogether. This completed pitch-up flight testing. After that final attempt, the 

group terminated tests with that aircraft.

A summation of the different wing modifications applied to the Skyrocket 

was published in 1956. The research memorandum noted: 

None of the wing modifications had an appreciable effect on the 

decay in stick-fixed stability (pitch-up) exhibited by the airplane 

at moderate angles of attack, particularly over a Mach number 

range from about 0.8 to 0.95. All configurations were considered unsatisfactory and uncontrollable in the pitch-up region by 

the pilots. On the basis of these tests and other flight and tunnel investigations, it is felt the position of the horizontal tail on 

this airplane should be lowered appreciably to obtain substantial 

improvements in longitudinal handling qualities. 

Wing fences had no apparent effects on the buffeting characteristics with slats retracted; however, unlocking the wing slats 

raised the buffet boundary, below a Mach number of 0.70, above 

that for the retracted slats condition for the basic-wing, one-fence, 

and two-fence configurations. Wing chord-extensions lowered the 

buffet boundary, compared with unmodified airplane configuration, up to a Mach number of 0.80 and caused an increase in 

buffet intensity which was objectionable to the pilot. Moderate 

buffeting appeared to exist over most of the lower and moderate 

lift range with the slats fully extended; however, this configuration 

did alleviate some of the pitch-up divergent rate and appeared to 

the pilots to provide the greatest improvement in the longitudinal 

handling characteristics of the airplane.40

None of the wing modifications had an appreciable effect on 

the trim-stability characteristics of the airplane and all configurations exhibited similar trends over the Mach number range. 

The airplane was stable at Mach numbers below about 0.82, and 

exhibited characteristic nose-down and nose-up trim changes 

between Mach numbers of about 0.87 and 1.03.41

Clearly, the various wing modifications failed to solve the pitch-up issue. 

For all the advantages of sweptback wings, pitch-up represented a major problem that required a solution. This was found not exclusively in modifying the 

wings, but rather in moving the location of the horizontal tail. The original 

placement of the horizontal stabilizer midway up on the D-558-2’s vertical 

tail was to avoid the wing wake. This, however, placed the stabilizer in the 

downwash from the wings when the pilot pulled back on the stick, initiating 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

109

a sharp pull-up maneuver that increased the aircraft’s angle of attack relative 

to the airflow around it. The report continued:

On the basis of wind-tunnel tests performed on a model of 

the D-558-2 airplane, as well as other wind-tunnel and flight 

investigations, it has been concluded that with the present tail 

configuration of the D-558-2 airplane, a real cure of the pitchup is not feasible. Lowering the horizontal tail to approximately 

the height of the wing-chord plane extended would be required 

to obtain substantial improvement in airplane longitudinal 

handling qualities.42

The combined results of the D-558-2 research flights and wind tunnel testing altered subsequent aircraft designs. The first generation of jet fighters had 

horizontal stabilizers located at the base of the vertical fin. Later aircraft had 

their stabilizers located on the lower aft fuselage, even with or below the wing 

centerline. This cured the pitch-up issue, and it is why the low-placed horizontal tail is a standard feature of transonic and supersonic combat aircraft design. 

Exploring Operational Limits, Lateral 

Stability, and Vertical-Tail Loads

At the same time that the third D-558-2, NACA 145, was making its pitchup research flights, the all-rocket second D-558-2, NACA 144, was also 

flying research missions. After the latter had been turned over to the NACA, 

Crossfield had made a series of flights to explore its operational limits. Once 

these were completed, NACA researchers undertook a new effort focused on 

the aircraft’s lateral stability and control at high Mach numbers. This had been 

the cause of the violent rolling motions experienced by Bridgeman during the 

Douglas contractor flights in 1951. Additionally, loads on the vertical tail were 

also to be recorded. 

Flights in the new research effort began on June 13, 1952. The first of these 

reached a speed of Mach 1.36. Two more flights were made in June, reaching 

low supersonic speeds of Mach 1.05 and Mach 1.35. The July 10 flight reached 

Mach 1.68 at 55,000 feet, the peak speed for this initial test series. As with 

the other Skyrocket, the flights did not always go smoothly. The July 15 flight 

reached only Mach 1.05 because of an engine malfunction, and the August 

13 flight was aborted after launch from the P2B-1S when a liquid-oxygen 

valve stuck in the open position. The aircraft did not return to flight until 

October 10, 1952. This flight measured longitudinal stability at speeds of up 
Probing the Sky

110

The second D-558-2, NACA 144, parked on the ramp at South Base. Modifications made to 

allow air launch included the removal of the jet engine and fuel tanks. Between the air launch 

and the increased rocket burn time, the craft’s maximum speed was doubled. On November 20, 

1953, this aircraft made the first Mach 2 flight. (NASA)

The modified B-29 used to launch the Skyrockets had striking nose art. This included its nickname, “Fertile Myrtle,” and a scoreboard that tallied the number of launches made with each of 

the D-558-2s. Each of the launch aircraft was used for a single rocket plane, as modifications 

made to the mother ships’ underside were specific to each aircraft type. Fertile Myrtle survived 

damage it sustained when the no. 4 propeller tore free. (NASA)
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

111

to Mach 1.65. The year’s activities ended with a flight on October 23, and the 

research effort was then placed on hold until the following spring. The first 

flight of 1953, made on March 26 by Crossfield, brought the longitudinalstability flights to a close.

The level of D-558-2 flight activity was lower than that of any new jet 

fighter undergoing testing at Edwards. This highlighted the difference between 

specialized flight research and production and acquisition-oriented test operations. Extensive effort went into planning for each research flight. Research 

aircraft also required extensive ground testing before a flight, including careful 

calibration of instrumentation. Once the flight was completed, malfunctions 

had to be identified, understood, and corrected before the next flight was 

attempted. Onboard systems had to be inspected and tested after the flight 

was completed. Finally, the data collected during the flight had to be reduced 

and then analyzed to assess results, detect any problems, and identify data 

points that would have to be repeated or would be used in preparing the next 

flight’s plan. 

The next phase in research activity with the second D-558-2 was a series of 

flights to collect data on supersonic lateral stability at low and moderate angles 

of attack, in both pushover and straight flight, at altitudes between 50,000 

and 70,000 feet. Crossfield made six flights between April 2 and August 5, 

1953. One of the flights, on June 18, was aborted after launch due to a rough 

running rocket engine. During the August 5 flight, Crossfield reached a speed 

of Mach 1.878, just below Bridgeman’s maximum speed flight.43

The research memorandum noted:

The first flights to Mach numbers near 1.8 were performed by 

climbing the airplane to altitudes of about 55,000 feet and pushing over to nearly zero lift (angle of attack ≈ –2°) with the rudder 

locked. During these flights, violent lateral oscillations occurred, 

with side-slip angles reaching ±6° and roll angles reaching about 

±60°.... It was decided that the low angle of attack, and consequent low inclination of the principal axis of inertia with respect 

to the flight path, may have aggravated the motion. A subsequent 

flight was made in which the pilot did not push over to low lift 

but maintained an angle of attack greater than 0°. In this condition, the lateral oscillation was much less pronounced. It should 

be noted that the two flights were made at different altitudes and 

at different constant angles of attack.44
Probing the Sky

112

The memorandum also noted the Skyrocket pilots’ experiences:

During one of the flights with the rudder locked, the pilot 

attempted to hold the ailerons fixed for a period of time in order 

to determine the lateral motion that would ensue.... The airplane 

tended to roll off at a fairly rapid rate until the roll angle reached 

about 90°, at which time the pilot stopped the motion with the 

ailerons. During another flight at a low angle of attack with the 

rudder locked...the lateral motion was of such a nature that the 

airplane rolled nearly 140° although the pilot was trying to control 

the airplane with the ailerons. At one time during the flight when 

the pilot felt the controls were ineffective in stopping the rolling 

motion...the pilot reversed the aileron control in order to make 

the airplane complete the roll to 360° in order to recover from the 

inverted attitude. This action was also ineffective as the airplane 

at the same time apparently began to recover of its own accord 

against the control motion supplied by the pilot. It is pertinent to 

point out here that this condition was not caused by a complete 

lack of aileron effectiveness. Unpublished flight data indicate that 

aileron effectiveness at these speeds is indeed low but that an 

appreciable amount still remains....

The motions obtained during these two flights suggest the 

possibility that the control-fixed transient oscillation at high 

supersonic speeds at low angles of attack may be one in which 

the roll angles reach values in excess of 90°, whereas the transient 

sideslip angles remain at relatively low values. However, because 

of the difficulty of flying the airplane at these speeds and angles 

of attack, it has not been possible to check this hypothesis further 

in flight.45

The Road to Mach 2

Crossfield’s flights were approaching twice the speed of sound. Though reaching Mach 2 was the next landmark in the quest for higher altitudes and faster 

speeds, it did not have the significance of breaking the sound barrier. Unlike 

the transition from transonic to supersonic flow, no fundamentally different aerodynamic phenomena appeared going from Mach 1.88 to Mach 2.0. 

The D-558-2 had been reaching speeds of about Mach 1.8 to 1.9 during 

both Bridgeman’s and Crossfield’s flights made between 1951 and 1953. But 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

113

although a Mach 2 flight seemed to be only an incremental increase over earlier 

flights, it was, in fact, far more difficult, and, if nothing else, it had tremendous 

symbolic importance as the next important milestone on a path that, increasingly, many saw as eventually leading to flight into space. 

Crossfield noted that “even at Mach 1.8 we were pressing [the Skyrocket] 

far beyond rational limits.”46 That the NACA team could even reach Mach 

1.8 was due to experience gathered in earlier flights. Recalled Crossfield, “The 

plane was by now almost completely debugged.”47 The team had also learned 

the “many little tricks to save time and gain an edge on the unknown.”48 Even 

so, Crossfield said, “the best any ordinary team could hope for, with luck, was 

a speed of Mach 1.9.”49

A second factor working against the prospect of a Mach 2 flight was the 

traditional NACA mindset. NACA flights were made to collect data, not set 

speed records. Crossfield’s flights in the D-558-2 had exceeded the existing 

world speed record, but the NACA had not highlighted the achievement in 

any public announcements. The research memorandums on the flights were 

classified “Confidential” and were limited to distribution among the NACA, 

contractors, and the military. Most important, Hugh L. Dryden had told 

Crossfield not to attempt a Mach 2 flight. 

But much larger issues were at hand than whether research for its own sake 

was justified. The cost of research activities was going up, and construction of 

new, more advanced facilities was required to explore new realms of flight. The 

NACA required political support for this, as money was short and the new 

Eisenhower administration, elected in November 1952, was skeptical about 

devoting scarce resources into research activities at a time when the Nation 

was at war in Korea and facing extremely serious global challenges in Europe 

and Asia.50

The NACA contributions were highly technical ones, easily appreciated 

by the science and engineering community but not so readily discerned by 

others outside those fields. What others could understand, however, was a new 

world airspeed record. Aviation had always been about flying faster, higher, 

and farther. In the 1930s, air races had been major spectator events. In both 

commercial activities and military aviation, better aircraft performance was 

equated with better aircraft. Additionally, aircraft technology continued to 

undergo rapid change in the early 1950s. In late 1953, the F-80 Shooting Star, 

the hottest American airplane in 1945, was being phased out of service; the 

F-86 Sabre was approaching old age; the supersonic F-100 Super Sabre was 

beginning production; and the Mach 2 F-104 was in advanced development.51

So Hugh Dryden gave Walt Williams approval for a single attempt at a 

Mach 2 flight. Exceeding Mach 2 in the Skyrocket would require careful planning and special preparations. Herman O. Ankenbruck, the project engineer, 
Probing the Sky

114

developed a flight plan that would offer the best chance of reaching the goal. 

Success would depend on Crossfield’s ability to fly the plan, the thrust of the 

rocket engine, and the amount of fuel that could be carried. 

Aft view of NACA 144. The four nozzles of the LR-8 rocket engine are visible. This engine was 

essentially the same one used in the X-1 series of aircraft. (NASA)

None of this would prove easy. Lieutenant Colonel Marion Carl, a superb 

Marine test pilot, had made two attempts in the summer of 1953 to reach 

Mach 2 and fallen short both times. Crossfield noted that “the slightest overpressure on the stick would cut the speed back drastically.” Crossfield did have 

an advantage over Carl, however. The D-558-2 had been fitted with cone-like 

nozzle extensions following Carl’s Mach 2 attempts, to increase thrust at high 

altitude. These allowed Crossfield to reach Mach 1.96 during an October 14, 

1953, flight, on the sixth anniversary of the first supersonic flight by Chuck 

Yeager in the first XS-1.52

The nozzle extensions were critical to Crossfield’s eventual success in reaching Mach 2. The reason literally came down to rocket science. High-pressure 

gas in the LR-8 combustion chambers expanded out the nozzles to produce 

an equal and opposite force, which accelerated the D-558-2. As the exhaust 

gas left the nozzles, it expanded and its pressure dropped. In an ideal expansion, exhaust pressure drops to that of the outside atmospheric pressure, but 

no lower. These conditions would achieve maximum thrust. But if the exhaust 

gas over-expanded, it created drag. 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

115

Because atmospheric pressure varied according to the altitude at which the 

D-558-2 was flying, the extended nozzle was shorter than the length needed 

for ideal expansion. The excess energy was lost, as the pressure was not fully 

recovered. If Crossfield were to reach Mach 2, however, he would have to fly 

at a higher altitude than that for which the rocket nozzles had originally been 

designed. Moreover, he would need to get every bit of thrust he could from 

the rocket. He needed the larger, extended nozzles to capture more of the ideal 

expansion of exhaust gas.53

From left are Walter C. Williams, High-Speed 

Flight Research Station director; A. Scott 

Crossfield, D-558-2 pilot; and Joe Vensel, HSFRS 

official in charge of operations, in front of a 

D-558-2, circa 1953. (NASA)

This was not the only advantage that would be needed to reach Mach 2. 

The second D-558-2 underwent special preparation. To minimize drag, the 

ground crew sanded and polished the aircraft, and every panel seam was taped 

over. The two stainless-steel fuel-jettison tubes were replaced with aluminum 

tubes, which were bent into the rocket’s exhaust. Once the rockets fired, the 

tubes were no longer needed and 

would burn off, reducing drag and 

weight by a small amount. But 

even with the reductions in drag 

and weight, a longer rocket burn 

time would also be needed. To 

enable this, the water/alcohol fuel 

would be cold-soaked in a refrigerator the night before the flight, 

increasing the amount that could 

be carried by 10 or 15 gallons. 

The liquid oxygen was also loaded 

into the aircraft the night before, 

cold-soaking the tank and airframe 

around it, which enabled the crew 

to add a little more oxidizer. Some 

of the liquid oxygen would boil off 

during ground preparations and 

the flight under the P2B-1S to the launch point, they knew; such was always 

the case. Just before the drop, the plane’s liquid-oxygen tank would be topped 

off from a tank on the P2B-1S, replacing the lost oxidizer. 

The attempt was scheduled for November 20, 1953. Crossfield was not 

in the best condition. He had a bad case of the flu but was determined to fly. 

With the Skyrocket’s preflight preparations completed, the P2B-1S launch 

aircraft took off. Stanley Butchart piloted the P2B-1S and was in charge of a 

precise sequence of events. Each of the rocket planes had an individual launch 

area. The second D-558-2 (NACA 144) was typically launched over Lake 

Elizabeth. The jet-and-rocket-powered third D-558-2 was usually launched 
Probing the Sky

116

west of Rosamond. The X-1s were mostly launched over Victorville, southeast 

of Edwards. 

The rocket-powered X-planes underwent regular engine test firings. The shock diamonds in the 

rocket exhaust are visible. Engineers monitoring the test are standing close to the D-558-2. In 

the early 1950s, ear protectors, blast shields, and similar safety measures were not required. 

The panels attached to the horizontal stabilizers were there to prevent any damage. (NASA)

Butchart later described the Skyrocket launch procedures. After takeoff, he 

would head out over Big Bear Lake in the long, slow climb to launch altitude. 

With no control room then in use, the launch plane pilot directed the operation. The first step in the sequence came after about 20 to 30 minutes, when 

Butchart called for fire trucks to be deployed on the lakebed. This was followed 

a few minutes later by a call for the chase planes to take off and join up with the 

P2B-1S. When the launch plane reached 10,000 feet, Crossfield climbed into 

the D-558-2, closed the canopy, and began launch preparations. The maximum 

power of the P2B-1S engines was used for the climb, for which an hour to an 

hour and a half was allotted. This put a heavy strain on the engines. Once the 

launch altitude was reached, Butchart began the final 6-minute sequence before 

launch. He would then fly out for 2 minutes, make a turn lasting 2 minutes, 

and take the final 2 minutes to return to the launch point. The release would 

then be pulled. 

On November 20, 1953, Crossfield was successfully launched from the 

P2B-1S, and he followed the flightpath Ankenbruck had calculated. Reaching 

72,000 feet, he gently nosed over into a shallow dive, reaching the maximum 

Mach number at 62,000 feet. In doing so, Crossfield avoided too low a g-force 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

117

on the aircraft, which had caused the uncontrolled rolls Bridgeman had experienced 2 years before. Following burnout, he made a sweeping approach to the 

lakebed for a deadstick landing, the usual chase plane trailing behind. 

Flight data indicated that Crossfield had reached a speed of Mach 2.005 ± 

0.02. Calculated in a standard atmosphere, the true airspeed was determined 

to be 1,327 miles per hour. However, weather balloon data indicated the air 

temperature was 22° lower than that of the standard atmosphere. With this 

correction, the D-558-2 had reached 1,291 miles per hour ± 17 miles per hour. 

This was the highest speed yet reached in the Skystreak, and no subsequent 

attempts were made to exceed Mach 2. Crossfield later recalled that it was 

decided that the extraordinary efforts needed to exceed Mach 2 would not be 

justified by the value of the data.54

After the brief interlude of the Mach 2 flight, research with the second 

D-558-2 resumed. For the next 3 years, engineers and pilots put the aircraft 

through a wide range of research activities. These included examinations of 

dynamic stability and control, structural temperature and loads, wing and tail 

pressure distribution, buffeting data, and vertical-tail loads.

A D-558-2 being loaded into its launch aircraft. The B-29 was lifted up on jacks inside a hangar, 

the Skyrocket was rolled underneath, and the launch plane was then lowered. Executing the 

process inside a hangar eliminated problems posed by desert winds. (NASA)

Though Crossfield made most of those flights, he left the NACA HighSpeed Flight Station in the late summer of 1955 to join North American 
Probing the Sky

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Aviation as a technical adviser and eventually became a corporate test pilot on 

the X-15 program.55 His replacements on the Skyrocket were NACA research 

pilots Joseph A. Walker and John B. “Jack” McKay. And while the D-558-2 

research flights had been an ongoing NACA project since 1949, they were never 

routine. On McKay’s pilot-familiarization flight, made on September 16, 1955, 

the landing gear failed to extend, and he had to use the emergency hydraulic 

system to lower the wheels.56

McKay also experienced a far more dangerous situation that nearly resulted in 

the loss of his own life, the second D-558-2, the P2B-1S launch aircraft, and the 

P2B-1S crew. On March 22, 1956, the P2B-1S had reached drop altitude when 

its number 4 engine abruptly failed. The flight engineer reported the problem 

to Butchart, who responded by feathering the propeller, that is, adjusting the 

pitch of the blades so that it would stop and present a minimal-drag profile to 

the airflow. The propeller slowed to a stop but then unfeathered and began spinning again. Butchart feathered the propeller again and again, but it continued 

to unfeather and spin back up. At this speed and altitude, the propeller was 

spinning so fast that centrifugal force was inevitably going to cause the blades to 

disintegrate. On his third try, Butchart was still unable to feather the propeller. 

(He later recalled that the airplane’s manual warned that there was only enough 

hydraulic fluid for three in-flight feathering attempts.) At that point, McKay 

called out that he had a problem with the Skyrocket and directed the P2B-1S 

crew not to drop him. 

But Butchart and P2B-1S copilot Neil Armstrong had no choice, as the 

propeller would soon disintegrate. They nosed the P2B-1S down, and Butchart 

pulled the emergency jettison handle. The D-558-2 separated; McKay jettisoned 

the fuel and glided to a lakebed landing. That aircraft was undamaged and McKay 

was not harmed.

Just 10 to 15 seconds after Butchart dropped the Skyrocket, the P2B-1S 

propeller disintegrated. A blade from the number 4 engine passed through the 

number 3 engine, destroying it, then sliced all the way through the fuselage, 

exiting to strike the number 2 engine. Had Butchart not released the D-558-2, 

the blade would have hit the Skyrocket, likely triggering an explosion that would 

have destroyed both planes and all aboard them. Shrapnel from the disintegrating propeller hub and blades severely damaged engine number 3 and severed 

the aileron cables to Butchart’s yoke, leaving him with no control. Armstrong’s 

aileron cables had only one or two remaining strands. The severed wires rendered 

the yoke difficult to move. Despite the loss of engines 3 and 4 and the damaged 

controls, Armstrong was able to retain control and limp home to land on the 

north lakebed. The P2B-1S was grounded for nearly 5 months of repairs.57

The second D-558-2 returned to flight on August 17, 1956, with McKay 

as pilot. This was the first of eight flights following the near tragedy. The final 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

119

flight was on December 20, 1956, and it was the last flight by a Skyrocket. 

Today, the historic second D-558-2 is suspended in honor at the National Air 

and Space Museum of the Smithsonian Institution, Washington, DC.

Denouement: Skyrocket External 

Stores Research Flights

The rocket-powered X-planes were commonly viewed by the public as exotic 

aircraft being used to probe the boundaries of the unknown, traveling ever 

higher and faster. In reality, their role also included the collection of flight 

data on more mundane aspects of high-performance aircraft. While the second 

D-558-2 was flying at high Mach numbers, the third Skyrocket was testing the 

effects of “external stores,” such as bombs and fuel tanks, on aircraft handling 

qualities at subsonic and transonic speeds. Fighter aircraft had long carried 

these items under their wings. These payloads increased drag and affected flight 

characteristics. As part of the development effort of operational fighters, extensive wind-tunnel testing had been done to verify the safety and aerodynamics 

of these fixtures. In contrast, little effort had been made to assess the effects of 

external stores on longitudinal and lateral handling. 

A close-up of the pylon/bomb on NACA 145. Data collected on their effects was applied to 

operational Air Force and Navy strike aircraft. (NASA)

To explore these effects, the third Skyrocket was used for a series of flight 

tests involving three different streamlined configurations: with an empty 
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mid-semispan under-slung pylon on each wing; with a simulated 1,000-pound 

bomb shape attached to the pylon; and with a simulated 150-gallon fuel tank 

attached. The three were respectively referred to as the “pylon configuration,” 

the “small store configuration,” and the “large store configuration,” all built 

by Douglas Aircraft Company.58

The aircraft was modified with the pylons in mid-May 1954 and made its 

first flight in the stores project on June 2, 1954, with Crossfield as pilot. The 

aircraft was in the pylon configuration for the test and used the jet engine only. 

Top speed was Mach 0.72. The flight was repeated on June 16, again using only 

the jet engine and also at Mach 0.72, but with the small store configuration. 

The flight went smoothly, with no apparent complications. 

The same was not true for the next flight, on July 8. Top speed was Mach 

1, using both the jet and rocket engines. The small store configuration resulted 

in a decrease in transonic performance and an increase in buffeting. These 

problems caused the stores project to temporarily be put on hold, resuming 

on October 8. The first flight made once research resumed used the large store 

configuration’s simulated 150-gallon fuel tank. As on the initial flights, the jet 

engine only was used for this mission, and speed was limited to Mach 0.74. 

The way was now cleared for testing the large store configuration at higher 

speeds. Crossfield reported no adverse effects but noted a rise in drag and heavier 

buffeting in longitudinal maneuvers. More serious, the data from the wing strain 

gauges indicated problems. The stores flights were again halted until Douglas 

engineers completed rechecking the strength of both the wings and pylon.

The stores flights resumed in December 1954 with McKay in the third 

D-558-2. These were made in the clean configuration, without the pylons 

or stores shapes, and with the jet engine only. Once this series was complete, 

however, another 4 months would pass before more stores flights would be 

made. Flights resumed again on April 27, 1955, using the pylon configuration 

and both the jet and rocket engines. Reaching a speed of Mach 1, McKay made 

sideslips, rolls, and elevator and rudder pulses to collect data on the Skyrocket’s 

handling qualities, wing and pylon loading, and buffet levels. 

Nearly a month later, on May 23, McKay repeated the maneuvers, this 

time with the large stores configuration. He noted that the simulated fuel 

tanks caused high buffet levels compared to those of the pylon configuration. 

June 1955 was a busy month, with five flights—two with the pylon configuration and three with the large stores configuration. Identical maneuvers were 

performed on all five.59

Data on the static longitudinal-stability and control characteristics were 

obtained during speed runs made at about 35,000 feet, along with windup 

turns between Mach 0.50 and 1.03 and altitudes ranging from 22,200 and 

39,400 feet. Elevator pulses provided data on the aircraft’s longitudinal-dynamic 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

121

characteristics. These were made at Mach numbers between 0.49 and 0.75, 

at altitudes between 22,000 and 27,000 feet. Data were also collected on 

low-speed characteristics during 1-g stalls with the slats both locked and 

unlocked and in the landing configuration with the slats unlocked and flaps 

and gear down.

The D-558-2’s static lateral- and directional-stability characteristics were 

determined through incremental increases in constant flightpath sideslips, 

abrupt aileron rolls, and trim runs at Mach numbers of 0.44 to 1.04 and altitudes of 22,000 to 36,000 feet. Lateral damping characteristics were derived 

from rudder pulses made at Mach numbers between 0.50 and 0.87, at 20,000 

and 32,000 feet. Both the longitudinal and lateral pulses were abrupt inputs, 

with the pilot attempting to return and hold the controls at the trim position 

while the oscillation damped out.60

The Skyrocket following a landing on the lakebed. Ground support personnel would remove the 

film used to record the flight data and prepare the aircraft to be towed back to the hangar. The 

dress code at the High-Speed Flight Research Station was relaxed but ran the gamut from suits 

and ties to work clothes, short-sleeve shirts, baseball caps and sun hats, and Hawaiian shirts. 

Crossfield was the pilot for the flight. (NASA)

Following the series of flights in June, tests were halted and did not resume 

until August 30. As with previous flights, these were made with large store 
Probing the Sky

122

configurations, and, though the maneuvers performed were the same, the 

outcome was different. McKay landed nose high, and the underside of the 

aft fuselage was damaged when it scraped on the lakebed. The aircraft was 

grounded for 2 months for repairs. 

Butchart flew the next mission with the D-558-2 on November 2, and 

McKay made the next three flights on November 8 and 17 and December 8, 

1955. As with all the flights since May, the large store configuration was used 

and the same maneuvers were performed. This brought the stores research 

project to a close. Two flights remained, however. The Skyrocket was stripped 

of the pylons and restored to the original clean configuration. This took place 

during the winter months, while the lakebed was typically closed to flight 

anyway. Once the work was completed, the two additional flights would be 

made to collect wing-load data. The lateral, directional, and longitudinal data 

from the two clean flights would identify the effects caused by the stores. 

The first of these, on February 1, 1956, was successful, reaching a speed of 

Mach 1. The final flight, planned for Mach 0.9, was attempted on February 

3. However, a turbopump overspeed aborted the flight. Because the flight data 

were needed to complete the analysis of the stores data, a final flight of the third 

D-558-2 was added. It did not take place until August 28, 1956, more than 6 

months after the initial attempt. This time the flight was successful, reaching 

Mach 0.96. The aircraft was permanently grounded, and the stores tests ended.

The results of the external stores tests were summarized in a research memorandum issued in October 1957: 

The results presented herein are somewhat limited—particularly 

in regard to the dynamic characteristics of the airplane—because 

only a few flights were obtained with each external-store configuration. Sufficient data were obtained, however, to establish 

trends of the effects of the stores on the stability and control characteristics of the airplane at subsonic and transonic speeds. Since 

most of the tests were performed in the large-stores configuration 

and only small differences were noted in the data for the various 

configurations, most of the data presented here are for the largestore configuration.

The incremental lift and drag effects for the large-store configuration...indicate the appreciable increment of drag produced by the stores, especially at M > 0.9. In addition, the pilots 

commented that they detected an increased penalty in airplane 

performance with an increase in size of the stores investigated, 

particularly at transonic speeds.61
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

123

The report’s concluding remarks noted:

The results of a flight investigation, from a Mach number of 

approximately 0.45 to a Mach number of approximately 1.05, 

of the Douglas D-558-2 research airplane equipped with several 

configurations of external stores have indicated that the smaller 

mid-semispan store installations generally had a small or negligible effect on the handling qualities of the airplane. With the 

large-store configuration, the trends exhibited in the longitudinal 

and lateral trim, stall approach, dynamic and static stability and 

control, and buffeting characteristics were generally the same as 

for the clean airplane; however, significant changes in the magnitude of the parameters measured were sometimes apparent, 

particularly at the higher speeds.

The large-store configuration effected: a decrease in the subsonic static longitudinal stability; an improvement in the lateral 

damping characteristics at subsonic speed; an appraisable leftwing drop at a Mach number greater than 0.9; an appreciable 

decrease in lift-curve slope between a Mach number of about 

0.85 and 0.96; a slight decrease in the level of lift for the onset 

of buffeting at all subsonic speeds; and an appreciable increase 

in apparent dihedral parameter dδa / dβ particularly at a Mach 

number greater than 0.94. The side force parameter CYβ increased 

and the aileron control parameter (pb/2V)/ δα decreased with an 

increase in the size of the store.62

Originally, this was not to be the end of the stores testing. A second series 

of tests, at supersonic speeds, was planned. The aircraft to be used for these 

flights was to have been the first D-558-2, used in the Douglas test flights. 

As the others were, NACA 143 (as it was designated) had been transferred 

by the Navy to the NACA in August 1951. It spent the following 3 years in 

storage at Edwards. NACA managers decided in 1954 to have it modified to 

an all-rocket/air-launched configuration. (The same as the second D-558-2.) 

The Skyrocket was returned to Edwards on November 15, 1955, and checkout began. It was not until the fall of 1956 that preparations with NACA 143 

were complete. McKay made the modified aircraft’s first flight on September 

17, 1956. And though the flight was successful, time had run out. The NACA 

canceled the planned supersonic stores test project. As a result, this was the first 

and only flight NACA 143 would ever make following modifications. The third 

Skyrocket had made its final flight in August 1956. The second D-558-2 would 
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124

continue to fly until December 1956 before being grounded. This brought the 

8-year Skyrocket project to a close.63

The NACA’s focus was shifting, and despite the sad irony of the end of the 

Skyrocket project after many productive years, a new set of challenges was 

on the horizon. Flights with the X-15 were due to begin in 3 years. The goals 

of the X-15 program were not simply to fly slightly faster, but to triple both 

the speeds and altitudes reached with the D-558-2. The X-15 would not just 

reach Mach 6 and fly to the edge of Earth’s atmosphere. It would enter space. 

An Assessment

Although the D-558-1 Skystreak’s achievements in the development of highspeed flight were mixed, the importance of the D-558-2’s accomplishments 

was clear. Ironically, like its jet-powered predecessor, the D-558-2’s original 

performance was designed only for Mach 0.9 to 0.95 due to having to make 

a ground takeoff. Had the Skyrocket not been modified for air launch, its 

accomplishments would have been minimal and would have probably been 

seen as a major disappointment. 

NACA researchers had opposed air-launching the XS-1, arguing it should 

be capable of normal takeoff and landing from Langley’s runway. By the time 

the Douglas test flights on the Skyrocket were starting, however, the advantages 

of air-launching research aircraft had been made clear. Converting the second 

and third Skyrockets to an air-launched configuration greatly expanded their 

performance envelope and, as a result, their research value. Another consequence of the decision to use an air launch was that the second and third 

Skyrockets ultimately were assigned different research roles. The earlier objections of the Langley researchers were forgotten. 

Removal of the second D-558-2’s jet engine allowed the aircraft to carry a 

larger rocket fuel supply, doubling its maximum speed. It repeatedly flew at 

speeds of Mach 1.8 to 1.9. This allowed testing of the swept wings to speeds 

higher than could be reached with operational service aircraft. The third 

D-558-2 retained the jet/rocket configuration, which reduced its top speed. 

Work with this aircraft focused on transonic and low-supersonic research, with 

top speeds ranging from Mach 0.72 to 1.04. As with the XS-1 and D-558-1, 

this provided a complementary research capability. 

An interrelated series of factors in the D-558-2 wing design contributed to 

one of its major accomplishments. The idea that swept wings could reduce the 

detrimental effects of transonic and supersonic flight was new. The means of 

ground-testing the Skyrocket’s wing design were limited by wind tunnel choking, which referred to the inability of wind tunnels to provide reliable data at 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

125

high transonic and supersonic speeds. Finally, the D-558-2 wing design focused 

on good low-speed/stall characteristics without loss of high-speed performance. 

Thanks to Van Every’s careful design, the result was a wing with good low-speed 

handling, low drag, and excellent transonic and supersonic characteristics. 

Aft upper view of NACA 144 at South Base. Though a variety of methods were tried in attempts 

to reduce or eliminate pitch-up, their effects were limited. The solution was to place the horizontal 

stabilizer low on the fuselage, where it would not be affected by airflow from the wings. (NASA)

The Skyrocket wing’s susceptibility to pitch-up, coupled with its comprehensive instrumentation package, ironically rendered the aircraft ideal for 

research on the problem. To find solutions, various configurations of wing 

fences, slats, leading-edge chord extensions, and bungee cords were developed 

in wind tunnel tests. These tests looked at ways of reducing the flow of air that 

caused the separation of flow at the tips. Separation triggered both the pitch-up 

and the tip stalls, which were then exacerbated by the swept-wing configuration. Crossfield described the test procedures: “The technique would be to go 

up there and pull g at a fairly constant rate, trying to maintain as constant an 

air speed as possible. And incidentally, there was something we really re-learned 

with these kinds of wings. And that is that the old CMCl was a bunch of garbage 

as far as this goes. We had to go back to the CMa because CL

 was dropping so 

fast that it looked like the airplane was going stable—when really it was going 

quite unstable at the time. And it would pitch.”64
Probing the Sky

126

Pitch-up was the most significant stability problem plaguing the 

D-558-2, but it was not the only one. 

The aircraft was originally designed 

for transonic and low-supersonic 

speeds due to the ground takeoff profile. When Bridgeman made the early 

air launches, he experienced severe 

oscillations and directional divergence from straight and level flight. 

This became known as “supersonic 

yaw,” and it had not been expected 

as other high-speed phenomena such 

as dynamic and static instability had 

been. Crossfield said later, “Bill 

Bridgeman found that by manipulating the G, you could control the 

rate of this divergence, and give yourself time to get in very soft controls 

to hold it on almost a knife edge.... 

And we managed to take the airplane 

out substantially beyond its expected 

design speed.”65 Originally built to fly 

Mach 0.9 after a ground takeoff, the second D-558-2 ultimately broke Mach 2.

The D-558-2 and other X-planes existed 

expressly for the collection of flight data. 

Shown here on a D-558-2 wing are some of 

the instruments used to record measurements 

of the forces and moments acting on the 

aircraft. (NASA)

The Skyrocket also served as a training ground for the engineers and pilots 

who later worked on the X-15 rocket plane in the 1960s. Crossfield made a 

total of 87 rocket-powered flights in the X-1 and D-558-2, along with another 

12 D-558-2 jet-only flights, all of these between 1951 and 1956. He subsequently made another 14 X-15 flights (1 glide flight and 13 powered flights) 

as the North American Aviation contractor pilot. This made him the most 

experienced rocket-plane pilot in the Nation. Jack McKay was the second-most 

experienced rocket pilot, with 46 flights in the X-1B, X-1E, and D-558-2. He 

subsequently made another 29 X-15 flights. Joe Walker made 26 flights in the 

X-1, X-1A, X-1E, and D-558-2 (3 glide flights and 23 powered), which he 

followed with 25 X-15 flights. Additionally, McKay and Walker qualified for 

astronaut wings, as they exceeded an altitude of 50 statute miles in the X-15. 

Another measure of the Skyrocket’s success was that all three aircraft survived the unknowns of research work. In contrast, of the six X-1 aircraft completed, only three—two of the first X-1s (the NACA aircraft, the second XS-1, 

was rebuilt as the X-1E) and the X-1B—survived. The third X-1, X-1A, and 

X-1D were all lost in explosions caused by the use of so-called “Ulmer” leather 
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

127

gaskets in the liquid-oxygen tanks. Both of the X-2 aircraft were lost, the first 

in a gasket explosion during a captive flight, the second when its pilot lost 

control during a Mach 3 flight.66

In the end, however, it was the data that counted. The Skyrocket research 

activities covered a wide range of fields: aircraft control and stability at speeds 

approaching Mach 2, measurements of structural temperature and loads, wing 

and tail pressure distribution, buffeting data, vertical-tail loads, possible solutions to pitch-up, and data on the effects of external stores. Though it was 

overshadowed by both the X-1 and the X-15, the D-558-2 played a major 

role in exploring the unknowns of supersonic flight during the critical years 

between the late 1940s and the mid-1950s. 
Probing the Sky

128

Endnotes

1. Opening quotation from William Bridgeman and Jacqueline

Hazard, The Lonely Sky (New York: Henry Holt and Company,

Inc., 1955), p. 154.

2. Hallion, Supersonic Flight, pp. 68–69, 75–76.

3. Ibid., pp. 76–79.

4. The turbopump delay was, of course, why the first two XS-1s had

been completed with high-pressure blow-down nitrogen fuel and

oxidizer-feed systems. The turbopump issue thus prevented both

the X-1 and the D-558-2 from achieving their greatest possible

performance, which, in both cases, was approximately Mach 2 and

more than 70,000 feet.

5. The change is very noticeable when comparing photos taken during

the initial versus the later Douglas flights.

6. Hallion, Supersonic Flight, pp. 157–159; Libis, Skystreak, Skyrocket,

& Stiletto, pp. 48–53, 123–126. According to legend, John Martin

became the first D-558-2 project pilot due to the Douglas test pilots’

being reluctant to fly the aircraft. After discussion, the story goes,

the Douglas test pilots all decided to submit very high bids that they

knew the company would turn down. Martin was away delivering

an aircraft and did not know about the plan. Martin submitted a

reasonable bid and “won” the job.

7. Sigurd A. Sjoberg, “Preliminary Measurements of the Dynamic

Lateral Stability Characteristics of the Douglas D-558-2 (BuAero

No. 37974) Airplane,” NACA RM L9G18 (August 18, 1949), p.

2; Hallion, Supersonic Flight, p. 159; Libis, Skystreak, Skyrocket, &

Stiletto, p. 128.

8. Sjoberg, “Preliminary Measurements of the Dynamic Lateral

Stability Characteristics of the Douglas D-558-2 (BuAero No.

37974) Airplane,” pp. 2–3.

9. Ibid., p. 3.

10. Ibid., p. 3.

11. Hallion, Supersonic Flight, p. 159.

12. J.D. Hunley, ed., Toward Mach 2: The Douglas D-558 Program

(Washington: DC: NASA SP-2003-4531, 1999), p. 48.

13. Peebles, ed., The Spoken Word: Recollections of Dryden History, The

Early Years, pp. 50–51; Hallion, Supersonic Flight, pp. 159–160.
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

129

14. Michael H. Gorn, Expanding the Envelope Flight Research at NACA 

and NASA (Lexington, KY: The University Press of Kentucky, 2001), 

p. 220.

15. S.A. Sjoberg, James R. Peele, and John H. Griffith, “Static 

Longitudinal Stability and Control Characteristics at Mach 

Numbers up to 0.87,” NACA RM L50K13 (January 17, 1951), 

p. 11.

16. Those seeking more information on the results of early flights with 

the D-558-2 can refer to these NACA research memorandums: S.A. 

Sjoberg, “Static Lateral and Directional Stability Characteristics 

as Measured in Sideslips at Mach Numbers up to 0.87,” NACA 

RM 50C14 (May 19, 1950); J.V. Wilmerding, W.H. Stillwell, 

and S.A. Sjoberg, “Lateral Control Characteristics as Measured in 

Abrupt Aileron Rolls at Mach Numbers up to 0.86,” NACA RM 

L50E17 (July 20, 1950); John P. Mayer and George M. Valentine, 

“Measurements of the Buffet Boundary and Peak Airplane NormalForce Coefficients at Mach Numbers up to 0.90,” NACA RM 

L50E31 (August 28, 1950); W.H. Stillwell, J.V. Wilmerding, and 

R.A. Champine, “Low-Speed Stalling and Lift Characteristics,” 

NACA RM L50G10 (September 5, 1950); John P. Mayer, George 

M. Valentine, and Beverly J. Swanson, “Measurements of Wing 

Loads at Mach Numbers up to 0.87,” NACA RM L50H16 

(December 26, 1950); John P. Mayer and George M. Valentine, 

“Measurements of the Distribution of the Aerodynamic Loads 

Among the Wings, Fuselage, and Horizontal Tail at Mach Numbers 

up to 0.87,” NACA RM L50J13 (January 19, 1951); and W.H. 

Stillwell and J.V. Wilmerding, “Dynamic Lateral Stability,” NACA 

RM L51C23 (June 18, 1951).

17. S.A. Sjoberg, James R. Peele, and John H. Griffith, “Static 

Longitudinal Stability and Control Characteristics at Mach 

Numbers up to 0.87,” NACA RM L50K13 (January 17, 1951), pp. 

5, 11, 13–14.

18. Bridgeman and Hazard, The Lonely Sky, p. 145.

19. Hallion, Supersonic Flight, p. 160.

20. Libis, Skystreak, Skyrocket, & Stiletto, p. 133

21. Hallion, Supersonic Flight, pp. 226–231.

22. Ibid., pp. 162–163.

23. Ibid.

24. Bridgeman and Hazard, The Lonely Sky, p. 248.

25. Ibid., pp. 257–261; Hallion, Supersonic Flight, p. 231.

26. Libis, Skystreak, Skyrocket, & Stiletto, p. 130.
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130

27. Hallion, Supersonic Flight, pp. 165–167; Bridgeman and Hazard, 

The Lonely Sky, pp. 304–305; Libis, Skystreak, Skyrocket, & Stiletto, 

p. 130; Theodore E. Dahlen, “Maximum Altitude and Maximum 

Mach Number Obtained with the Modified Douglas D-558-2 

Research Airplane During Demonstrations Flights,” NACA RM 

L53B24 (April 20, 1953), pp. 4–5.

28. Dahlen, “Maximum Altitude and Maximum Mach Number D-558-

2,” p. 4.

29. Herman O. Ankenbruck and Theodore E. Dahlen, “Some 

Measurements of Flying Qualities of a Douglas D-558-2 Research 

Airplane During Flights to Supersonic Speeds,” NACA RM L53A06 

(March 10, 1953), pp. 2, 8–9.

30. Letter, Chief, Bureau of Aeronautics, “Transfer of Aircraft to the 

NACA under Public Law 266 of the 81st Congress,” December 

27, 1950; letter, Chief of Naval Operations, “Transfer of Aircraft to 

the NACA under Public Law 266 of the 81st Congress,” January 5, 

1951. These letters covered the transfer of the two D-558-1s, the 

three D-558-2s, the ex-B-29–now–P2B-1S launch aircraft, and spare 

parts to the NACA.

31. Rogers Dry Lake is often flooded by winter rains, turning it into an 

actual lake.

32. Hallion, Supersonic Flight, pp. 175, 184–185, 232, 237.

33. Jack Fischel and Jack Nugent, “Flight Determination of the 

Longitudinal Stability in Accelerated Maneuvers at Transonic Speeds 

for the Douglas D-558-2 Research Aircraft Including the Effects of 

an Outboard Wing Fence,” NACA RM L53A16 (March 13, 1953), 

pp. 1, 5–8.

34. Ibid.

35. Hallion, Supersonic Flight, pp. 175, 184–185, 232, 237.

36. Jack Fischel, “Effects of Wing Slats and Inboard Fences on the 

Longitudinal Stability Characteristics of the Douglas D-558-2 Research 

Airplane in Accelerated Maneuvers at Subsonic and Transonic Speeds,” 

NACA RM L53L16 (February 8, 1954), pp. 13–14.

37. Ibid., pp. 13–14.

38. This period also saw the High-Speed Flight Research Station pilot’s 

office undergo a number of changes. Robert Champine transferred 

back to Langley, and John Griffith took a job with Chance Vought 

in Texas. Walter P. Jones left for a job as a test pilot with Northrop 

in August 1952, during the pitch-up tests. Jones was killed in the 

crash of a YF-89D prototype on October 20, 1953. Crossfield made 

nearly all the Skyrocket flights for the next 2 years.
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

131

39. Jack Fischel and Cyril D. Brunn, “Longitudinal Stability 

Characteristics in Accelerated Maneuvers at Subsonic and Transonic 

Speeds of the Douglas D-558-2 Research Airplane Equipped with 

a Leading Edge Wing Chord-Extension,” NACA RM H54H16 

(October 27, 1954), pp. 1–2.

40. Jack Fischel and Donald Reisert, “Effects of Several Wing 

Modifications on the Subsonic and Transonic Longitudinal 

Handling Qualities of the Douglas D-558-2 Research Airplane,” 

NACA RM H56C30 (June 5, 1956), p. 12.

41. Ibid., p. 13.

42. Ibid., p. 7.

43. Libis, Skystreak, Skyrocket, & Stiletto, p. 75.

44. Herman O. Ankenbruck and Chester H. Wolowicz, “Lateral 

Motions Encountered with the Douglas D-558-2 All-Rocket 

Research Airplane During Exploratory Flights to a Mach Number of 

2.0,” NACA RM H54I27 (December 17, 1954), p. 7.

45. Ibid., pp. 8–9.

46. A. Scott Crossfield and Clay Blair, Jr., Always Another Dawn: 

The Story of a Rocket Test Pilot (New York: Arno Press, 1972), pp. 

171–172.

47. Ibid., p. 160.

48. Ibid., p. 172.

49. Ibid.

50. Ibid.

51. Marcelle Size Knaack, Post–World War II Fighters (Washington, DC: 

Office of Air Force History, 1986), pp. 10, 60–64, 114, 175–176.

52. Crossfield and Blair, Always Another Dawn, pp. 160–161.

53. E-mail, Albion H. Bowers to Curtis Peebles, “Rocket Science,” July 

14, 2010. In possession of the author.

54. Hunley, ed., Toward Mach 2: The Douglas D-558 Program, 

pp. 54–56; Cyril D. Brunn and Wendell H. Stillwell, “Mach 

Number Measurements and Calibrations During Flights at High 

Speeds and at High Altitudes Including Data for the D-558-2 

Research Airplane,” NACA RM H55J18 (March 12, 1956), p. 

15. Crossfield was not the only person who was not in the best 

condition that morning. Jack Moise, a B-29 launch panel operator, 

was sprayed with highly concentrated hydrogen peroxide during 

preflight preparations. A mechanic named Kinkaid hosed him 

down with water, and both were taken to the flight-line dispensary. 

Moise suffered several minor chemical burns on his face, which 

cleared up within a few days. Crossfield saw Kinkaid sitting on a 
Probing the Sky

132

bench, wearing wet clothing. When Crossfield asked if he was cold, 

Kinkaid replied that he was warm. Crossfield realized Kinkaid was 

also soaked in hydrogen peroxide and could catch fire. Crossfield 

ripped off the clothing and found that chemical burns had bleached 

Kinkaid’s arms and legs white.

55. Crossfield’s departure roughly coincided with another change in the 

NACA’s High Desert test team: the High-Speed Flight Research 

Station dropped “Research” from its name in 1954, becoming 

simply the High-Speed Flight Station.

56. Hallion and Gorn, On the Frontier, p. 392.

57. Peebles, ed., The Spoken Word: Recollections of Dryden History, The 

Early Years, pp. 121–123.

58. Jack Fischel, Robert W. Darville, and Donald Reisert, “Effects of 

Wing-Mounted External Stores on the Longitudinal and Lateral 

Handling Qualities of the Douglas D-558-2 Research Airplane,” 

NACA RM H57H12 (October 28, 1957), pp. 1–2.

59. Hallion, Supersonic Flight, p. 240. Those seeking detailed 

information on the results of later D-558-2 research missions 

can refer to these NACA research memorandums: Jack Nugent, 

“Lift and Drag Characteristics of the Douglas D-558-2 Research 

Airplane Obtained in Exploratory Flights to a Mach Number of 

2.0,” NACA RM L54F03 (August 4, 1954); Wendell H. Stillwell, 

“Results of Measurements Made During the Approach and Landing 

of Seven High-Speed Research Airplanes,” NACA RM H54K24 

(February 4, 1955); Gareth H. Jordan and Earl R. Keener, “FlightDetermined Pressure Distribution over a Section of the 35° Swept 

Wing of the Douglas D-558-2 Research Airplane at Mach Numbers 

up to 2.0,” NACA RM H55A03 (March 25, 1955); and Glenn 

H. Robinson, George E. Cothren, Jr., and Chris Pembo, “Wing 

Load Measurements at Supersonic Speeds of the Douglas D-558-2 

Research Airplane,” NACA RM H54L27 (March 30, 1955).

60. Fischel, Darville, and Reisert, “Effects of Wing-Mounted External 

Stores on the Longitudinal and Lateral Handling Qualities of the 

Douglas D-558-2 Research Airplane,” pp. 2–3, 6. The apparent 

dihedral parameter dδa /dβ is defined as the “rate of change of 

aileron deflection with sideslip angle.” The side force parameter CYß

is defined as the “rate of change of lateral force coefficient with angle 

of sideslip.” The aileron control parameter (pb / 2V)/ δα is defined as 

the “variation of wing-tip helix angle with total aileron deflection” 

and is measured in radians/degrees.

61. Ibid., p. 4.
Proving the Swept Wing: The Douglas D-558-2 Skyrocket

133

62. Ibid., p. 13.

63. Hallion, Supersonic Flight, p. 230. Once retired, the three D-558-2s 

were put on display. The first D-558-2, used by Douglas and later 

modified to an all-rocket configuration, is at the Planes of Fame 

Museum in Chino, CA. The second D-558-2 (all-rocket configuration) is in the Smithsonian Institution National Air and Space 

Museum in Washington, DC. The third D-558-2 (jet-and-rocket 

configuration) is mounted outdoors on a pole at Antelope Valley 

College in Lancaster, CA.

64. J.D. Hunley, ed., Toward Mach 2: The Douglas D-558 Program, pp. 

48–49. CMCl is the static stability in pitch of an aircraft. CM is the 

pitching-moment coefficient. CMa is the partial derivative of the 

pitching-moment coefficient with respect to the angle of attack.

65. Ibid., p. 54.

66. For details, see Hallion, Supersonic Flight, pp. 169–172, 175–177, 

189–191.
134

Mockup of the X-3 at the Douglas factory. The X-3 was the sleekest of the early X-planes but 

delivered the most disappointing performance. (USAF)
135

Unfulfilled Promise,

Serendipitous Success:

The Douglas X-3 Stiletto

CHAPTER 4

Boy, it’s hard to keep your speed and altitude in this thing.

—William “Bill” Bridgeman1

An ambitious jet-powered research aircraft meant to cruise at Mach 2, the 

twin-engine Douglas X-3 Stiletto was the sleekest and most radical-appearing 

of the early X-planes. Like the D-558-1, the Stiletto’s accomplishments were 

mixed. The X-3 never achieved its intended design speed but did furnish critical 

data on a new stability and control peril spawned by the design requirements 

of supersonic flight. The X-3’s failure to attain Mach 2 stemmed largely from 

the failure of Westinghouse, its engine manufacturer, to successfully develop its 

Westinghouse J46 turbojet. As a consequence, the X-3 had to employ smaller 

and far less powerful Westinghouse J34 engines, inhibiting its potential performance and making a mockery of its sleek supersonic shape.

The X-3 had its origins in December 1943, when the Army Air Forces 

requested that the Douglas Aircraft Company study the feasibility of an aircraft 

capable of sustained speeds in excess of Mach 1. This was a bold request, as 

many (as discussed previously) considered the “sound barrier” as presenting an 

insurmountable challenge. A team of Douglas engineers working for Frank N. 

Fleming took on the challenge, submitting a proposal in January 1945, and the 

AAF responded by issuing Douglas a contract in June 1945 for the design of a 

supersonic jet-powered research aircraft capable of attaining Mach 2 at 30,000 

feet and having a flight endurance of 30 minutes. The project was designated 

MX-656, the plane to be known as the XS-3 (later simply X-3).2

 As with the 

X-1 and D-558 aircraft, initial flights with the X-3 would be made by the 

contractor to demonstrate that the aircraft could meet the basic performance 

requirements. The Air Force would then conduct an abbreviated performance 

evaluation up to the X-3’s maximum design speed and altitude. Finally, NACA 

pilots would make a long-term series of research flights to extract the maximum 
Probing the Sky

136

amount of data on aerodynamics, stability and control, handling qualities, and 

other aspects of the aircraft.

Designing the Stiletto

Douglas Aircraft’s Frank N. Fleming was named project engineer on the X-3 

and faced a demanding task. The design had to be a conventional one capable 

of operating without special launch and handling equipment, taking off and 

landing under its own power using standard landing gear, and flying its entire 

speed range without external assistance, and it would be required to demonstrate flying and handling qualities like those of modern high-speed aircraft. 

Finally, it would have to have conservative thrust margins, fly for a reasonable 

amount of time at supersonic speeds, perform satisfactorily at transonic speeds, 

and withstand aerodynamic heating.3

Thanks largely to the visual power of the V-2 missile and Douglas’s shapely 

Skyrocket, the popular image of a supersonic aircraft in the late 1940s was 

one featuring an elongated, needle-shaped fuselage with highly swept wings 

and tail surfaces. Actually making such airplanes was extremely challenging. 

The basic requirement would call for the engines’ thrust capacity to be greater 

than the airplane’s drag up to the Mach 2 design goal. If the thrust and drag 

curves intersected below this speed, the X-3 would not perform as required. 

The engines would need to have a frontal area that was as small as possible yet 

still capable of producing the necessary amount of thrust. In the late 1940s, 

this meant turbojet engines fitted with afterburners.

To minimize drag, the engines had to be enclosed within a fuselage likewise 

having the minimum possible frontal area. But jet engines typically had a considerable amount of external equipment attached to their cylindrical casings. 

This necessarily increased frontal area. Designers also determined that in order 

for the X-3 to achieve its performance goals, two engines would be needed, both 

for safety and to improve thrust-to-drag ratio. The two engines represented the 

greatest mass of the aircraft, so the center of lift would need to be near the engines. 

This meant the wing spar had to be under the engines, which would further 

enlarge the fuselage. The X-3’s main landing gear, like that of the D-558-2, also 

had to be housed in the fuselage. And as with the wings, the main landing gear 

had to be near the center of the greatest mass, reinforcing yet again the fact that 

the fuselage would have to be made bigger.

Auxiliary equipment also had to be included. Air had to be supplied to the 

engines. Protruding air inlets were added to the fuselage, and diffusers of the 

proper length and expansion ratio connected these to the engines. The two 

turbojet engines and afterburners generated considerable heat. Countering this 
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

137

effect required having ducting 

sufficient for carrying a large 

volume of cool air, the source 

of which was the boundary 

layer bled off ahead of the 

air intake scoops. Air passed 

through ducts between the 

two engines. To make room 

for the ducts, the two engines 

were spread apart. Shrouds and 

insulation were required to 

direct the cooling air and keep 

the aircraft structure at proper 

temperatures. Still more space 

would be needed to allow for 

thermal expansion, structural 

deformation, vibration, and 

manufacturing tolerances. A 

tunnel was added on top of the 

fuselage to contain the flight 

control cables, hydraulic lines, 

tank vents, wiring, instrumentation tubing, and other items.

An overhead photo of the completed X-3. The aircraft 

had a very long fuselage and a short wingspan. The 

fuselage was tightly packed with the two engines, 

fuel tanks, landing gear, and other systems required. 

These changes resulted in an (USAF)

increase in the X-3’s frontal area 

and thus the amount of drag. The ratio of the engine area as a percentage of total 

fuselage frontal area had been 93 percent in the minimal fuselage design. With 

all the added equipment, the ratio of engine area to total fuselage frontal area 

dropped to 33 percent for the two-engine design.

The next design issue was the selection of a wing planform combining the 

optimum values of high-speed drag, stability, weight, maximum lift, structural 

strength, and minimum drag for a given amount of lift. The X-3’s wings would 

need both a high critical Mach number and the ability to cope with the effects of 

compressibility over the full speed range up to Mach 2. The design that emerged 

was a slightly swept low-aspect-ratio wing that was very thin and had a sharpwedge airfoil shape and leading and trailing-edge flaps.

Beyond wind tunnel tests of the proposed wing shape, a series of free-flight 

rocket launches were made by the NACA to compare the coefficient of drag for 

various wing shapes. These shapes included the X-1 (high-aspect-ratio, straight 

wings), D-558-2 (low-aspect-ratio, swept wings), X-2 (high-aspect-ratio, swept 

wings), and X-3 (low-aspect-ratio, swept wings). All the wings had identical areas 
Probing the Sky

138

and fuselage bodies, so the lone variable was wing drag due to compressibility. 

Of the four wing shapes, the X-3s had the lowest drag coefficient over the range 

of Mach 0.9 to Mach 1.7. The highest-drag wing was the X-1’s. Although the 

main fuselage and wing designs were complete, the X-3 still lacked a cockpit, 

fins, rudder or horizontal control surfaces, fuel tanks, or accommodations for 

an instrument payload.

Early rocket-powered model flights and drop tests had indicated that drag 

on an optimum ogival body was reduced when the shape had a high fineness 

ratio (the body’s length divided by its diameter). The optimum ratio for a 

subsonic aircraft was 3:1, versus 14:1 for a supersonic aircraft. With the frontal area established, it was clear that the X-3’s fuselage length would need to 

be extended to minimize supersonic drag. This was accomplished by adding 

a long nose that held the pilot, data instrumentation, controls, radio, and 

other equipment.

The next step was the addition of tail surfaces. There were long-established 

relationships between the positions of the tail surfaces and the wing, relationships shaped by structural and aerodynamic factors. In the early X-3 design 

studies, the tail area was increased and moved downward and forward relative 

to the wing. This also allowed the tailpipes and supporting structure to be 

extended to support the tail. The flaw in this design was added weight, but 

engineers reduced the weight by keeping the tailpipe short. Additionally, experience held that to maximize the tail surfaces’ stability and control effectiveness, 

they should be located as far back as practical. As a result, a tail boom was used 

on the final X-3 design, the Model 499D. The X-3’s horizontal stabilizer also 

had an all-moving, one-piece design, rather than the two-part movable stabilizer and elevator used on the X-1, D-558-1, and D-558-2. The X-3 marked 

the first application of an all-moving “slab” tail to an American supersonic 

aircraft. Testing in Langley’s 7-by-10-foot High-Speed Wind Tunnel resulted 

in Douglas’s enlarging the horizontal tail by 40 percent.

The X-3 design was fitted with an air conditioning system installed between 

the air intakes; additional flight data equipment placed below the wing; and the 

hydraulic units, fire extinguishers, electrical components, and other items fixed 

in various nooks and crannies. Finally, fuel tanks were fitted into the remaining 

fuselage volume. A fuel tank holding 500 gallons of kerosene was located in 

the lower fuselage, below the diffusers, and another 170-gallon fuel tank was 

added to the aft fuselage, under the engine exhaust. The final 330-gallon fuel 

tank was located in the tail boom, above the jet engines. Douglas engineers 

estimated this would enable a 3-hour flight at high-subsonic speeds or 10 

minutes at Mach 2. To protect the fuel tanks and control cables in the event of 

an engine compressor or turbine wheel failure, armor plating was installed in 

critical locations. This was similar to the modifications made to the D-558-1.4
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

139

Reaching a sustained speed of Mach 2 required considerable thrust. 

Consequently, the X-3 had a pair of Westinghouse J46 axial-flow jet engines. 

These were derivations of the firm’s earlier J34 engine, but with a greatly redesigned turbine and compressor section. Though the J34 jet could produce 

a maximum thrust of 3,000 pounds, the J46’s basic thrust was to be 4,200 

pounds. Use of the afterburner raised this to 6,600 pounds. Douglas engineers 

estimated that the X-3 could reach Mach 2 at 35,000 feet using only 80.6 percent of the J46’s anticipated maximum thrust. The top speed of the X-3 was 

limited not by engine thrust, but by structural heating of its largely 24S-T81 

aluminum alloy airframe.5

The result was an aircraft that seemed as if it had flown in from the future. 

None of the other early X-planes had the X-3’s look of raw power. The bullet 

shapes of the X-1 and D-558-2 looked conventional by comparison, while 

the cylindrical D-558-1 was old-fashioned. Not until the hypersonic X-43A 

a half century later was there another experimental aircraft that so embodied 

the concept of speed.

In Thrust, Not Enough

Because its designers drastically underestimated the challenges they would face 

creating several new high-performance engines, Westinghouse’s efforts to build 

upon its combat-proven J34 experience failed miserably, crippling a number of 

aircraft programs, one of which was the X-3. “The history of the Westinghouse 

J34,” RAND analyst Thomas A. Marschak wrote in 1964, “[gives] a rather clear 

impression of a ‘classic’ case of engine-airframe commitments being made in 

the face of great uncertainty about all engine magnitudes.”6 By June 1951, 

development of the J46 was a year behind schedule, and Westinghouse, in 

desperation, decided to spin off a lower-powered variant of the J46 that could 

be installed in the X-3, with the higher-performance J46 variant installed later. 

But a year later, by August 1952, the problems had grown worse, and even the 

lower-powered engine was now 14 months behind schedule and not expected 

to be ready until 1955, 11 years after the X-3 program had begun.7

Douglas managers had already considered using the less-powerful J34-WE17 engines as an interim measure pending the availability of the more powerful 

J46. These could produce 4,850 pounds of thrust with afterburner and had 

dimensions similar to those envisioned for the more powerful J46, minimizing 

the difficulties created by the switch. The assumption was that initial flights 

would be made with these J34 engines. Then, once the J46 engines were ready, 

they would be installed in the pair of X-3s being built. It was a reasonable 

strategy, and one followed subsequently for other programs, such as the North 
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140

American X-15 (which used two RMI XLR-11 interim engines until the more 

powerful Thiokol XLR-99 was ready) and the Lockheed A-12 Oxcart program 

(which employed two Pratt & Whitney J75 engines until its more powerful 

P&W J58s were available).

But now, the J46 was not only behind schedule, it was growing too large 

to permit two to fit within the X-3, forcing Douglas to turn to an alternative: 

the earlier J34. The transonic thrust-to-drag ratio of the X-3 with the interim 

J34-WE-17 engines was much lower than that calculated for the J46 engine. 

Various means of boosting the J34’s thrust were examined. NACA engineers 

at the High-Speed Flight Station went even further and considered modifying 

the X-3 to an all-rocket/air-launched configuration.8

This, however, would have involved removing the fuel tanks, air inlets, and 

jet engines and replacing the latter with a pair of Reaction Motors 6000C4 

four-chamber rocket engines. The eight rocket cylinders would also have to be 

fitted with nozzle extensions, resulting in exhaust expansion to the ambient air 

pressure at 50,000 feet. The amount of propellant to be carried, based on the 

assumed consumption rate, was approximately equal to the available volume 

of the jet engine compartment.

A second prohibitive issue was the choice of a launch aircraft for use with an 

all-rocket/air-launched configuration. The candidates were the Douglas C-74 

and C-124 transports and the B-36, B-52, and YB-60 bombers. The C-74 

and C-124 were swiftly eliminated because their maximum launch speed was 

slower than the X-3’s anticipated stall speed. (As well, the C-124 was also too 

small to carry the X-3). The B-36 could carry and launch the X-3 but was 

unavailable because the Air Force required all available examples of this huge 

6- (and later 10-) engine bomber for its global strategic nuclear bombing 

mission. The B-52’s wheelbase was too short to allow the X-3 to be mounted 

beneath the fuselage (air-launch advocates did not yet conceive of attaching 

the X-3 to a launch shackle under the wing, as with the later North American 

X-15). The only possibility was the YB-60, a jet-powered version of the B-36 

that met the performance requirements. Ultimately, however, both the acquisition of a launch aircraft and the necessary modifications to the X-3 were 

deemed too costly, ending any prospects for the program’s future entirely.9

The failure to produce the J46 engine in time and in a size to be incorporated on the aircraft crippled the X-3 program. The X-3’s best speed in level 

flight with the J34-WE-17 engines was a meager Mach 0.96. If it entered a 

dive, its smooth high-fineness-ratio fuselage and low transonic drag ensured 

that it rapidly accelerated into the supersonic, to Mach 1.2—but the pilot 

then had to pull out, and its speed would swiftly drop back to the “other 

side” of the sonic divide. Although hopes lingered for a time that a rocketpowered/air-launched conversion might be approved, the NACA Research 
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

141

Aircraft Projects Panel rejected both options as too expensive and unjustified. 

The panel reasoned that “other aircraft are coming along which operate in 

the same speed range as that for which the X-3 was intended,” a reference to 

the Mach 2 Lockheed F-104, which, ironically, benefited greatly from the 

X-3’s development experience.10 By this time, the first X-3 was nearly ready 

for delivery to Edwards Air Force Base; the second airframe was only partially 

completed. The Air Force ended construction of the nearly complete vehicle 

and transferred the unfinished airframe to the NACA for spare-parts support.

A view of the X-3 from the rear. The fuselage was painted white but the wings, horizontal stabilizer, jet exhaust, and parts of the tail boom were left in bare metal. (NASA)

The X-3 research aircraft had been intended for the exploration of the 

unknowns of sustained Mach 2 flight, such as aerodynamic heating, intake 

scoop and air duct flow characteristics, and high-speed stability and control. 

But the failure to produce a J46 that could fit in the airplane effectively transformed the X-3 into a low-speed test bed for aircraft having low-aspect-ratio 

wings joined to high-fineness-ratio fuselages, nothing more.11
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142

The completed X-3 was delivered to Edwards on September 11, 1952, and 

final checkout by Douglas engineers began in preparation for high-speed taxi 

tests. This involved making engine test runs, replacing failed components, 

and installing NACA instrumentation. The instruments recorded airspeed 

and altitude; normal and transverse acceleration; roll, pitch, and yaw angular 

velocity; angle of attack and sideslip; control column, control wheel, and 

rudder pedal positions; stabilizer, aileron, and rudder positions and control 

forces; and leading and trailing-edge flap positions.12

The X-3 high-speed taxi tests began on September 30, 1952. William 

Bridgeman, having made the air-launched high-speed/high-altitude flights 

in the second D-558-2 the previous summer for Douglas, now conducted 

the manufacturer acceptance flights with the X-3. A series of problems were 

encountered in early tests, including a failure of the afterburner hydraulic 

actuator; a primary engine control system malfunction; a malfunction of the 

drag chute, which was jettisoned immediately after deployment on several 

runs; the loss of all tread on the nosewheel; and problems with the main 

landing gear brakes and tires. By the sixth taxi test, made on October 14, 

the problems had been largely resolved. The following day, Bridgeman lifted 

the X-3 off the lakebed and remained airborne for about a mile and a half. 

The way had now been cleared for the first flight, scheduled for October 20.

The eighth and final taxi test was made to check the X-3’s longitudinal control 

with hydraulic power off. Bridgeman found the resulting control forces were very 

high, and the dead spot around neutral made control difficult. Despite this, the 

first flight went ahead. The weekly status report noted:

Flight No. 1 was made on 10-20-52 right after taxi test No. 8. Loss 

of some power on the right-hand engine and loss of the right-hand 

hydraulic pump output was caused by a malfunction of the righthand fire shut-off control. Longitudinal pitching of the airplane 

was excessive. Duration of the flight was nineteen minutes.13

Troubleshooting of the problems took nearly 2 weeks, and the second flight 

was made on October 31, 1952. Again, as the progress report noted, “[During 

t]he longitudinal control sensitivity oscillations in which fairly large attitude 

changes occurred, a buffet-like shutter was felt in the airplane. Flight duration 

was 13 minutes.”14

A considerable amount of troubleshooting, repairs, testing, and modification had to be undertaken. The resulting delays were made worse when 

a new nozzle actuator on the right engine afterburner failed twice. Then in 

mid-November 1952, an inch of rain fell, closing the lakebed. The initial 

estimate was that the lakebed would be closed for 3 weeks. But coupled with 
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

143

the weather, continuing repair work on the X-3 delayed the third flight until 

late April 1953.

Normally, aircraft were taxied or towed to the runway. The X-3’s tires had to withstand very high 

takeoff and landing speeds. To avoid tire damage, a special vehicle was built to carry the aircraft 

out to the lakebed. Here it is seen on the back ramp behind Hangar 4802, at the new NACA 

facility at Edwards (still in use more than a half-century later). (NASA)

The X-3 was ready, but delays persisted. The first attempt was made on April 

21, 1953. As with the first flight, a taxi run was made to check out the aircraft. 

This was followed by the attempt at the third flight. The taxi test resulted in 

a loss of tread from all three tires. The tires were replaced, but by the time the 

work was finished, wind conditions had changed, and the flight was postponed. 

The tire failures were later traced to the high takeoff speeds. Underpowered 

engines and small wings meant the X-3 had to reach 260 knots before it could 

take off. Existing tire technology and materials were incapable of withstanding 

the stresses this produced.

A second try came on April 23, but high-altitude turbulence forced another 

cancellation. The following day, Bridgeman attempted to take off, but just before 

leaving the ground he felt the aircraft vibrating, so he aborted the takeoff. An 

inspection showed that the nose tire’s tread and most of the main tires’ treads 

were missing. Again, the high-speed takeoff had resulted in tire failure. Engineers 

concluded that the loss of tread had caused the main tires to be unbalanced, 

resulting in the vibration.15

Flight 3 was rescheduled for April 27, 1953, but rain fell the night before, 

closing the lakebed for a day and a half. Bridgeman successfully made a 

34-minute flight on April 30. Among changes that had been made to the X-3 

in the months since its last flight was an increase of the elevator power-valve 
Probing the Sky

144

linkage ratio to 5:1, which resulted in some improvement in longitudinal 

control. However, the aircraft had been fitted with a “load feel control” that 

allowed the control setting to be changed either automatically or manually. The 

weekly report noted, “But it was still very difficult, if not impossible, to avoid 

over-controlling with the load feel control operating automatically. Manual 

increase of the load feel gradient to its maximum value improved control considerably at the speeds encountered during the flight.... With the exception of 

longitudinal control, airplane and engine operation was satisfactory.”16

A subsequent report expanded on the handling problems:

During the climb following take-off on Flight No. 3, while the 

control system was still on automatic load feel, longitudinal oscillations periodically appeared. These oscillations were similar to 

those encountered on Flight No. 2 following afterburner shutdown.... It is apparent that at oscillation frequencies as low as 

1⁄2 cps [cycles per second], the time lag between aircraft normal 

acceleration response and pilot’s elevator force makes it difficult 

for the pilot to control these longitudinal oscillations.17

Again, extensive work and checkout were required before the fourth flight 

could be made on June 5. This one lacked most of the problems of earlier 

flights. The X-3 Weekly Status Report for the week ending on June 5, 1953, 

stated, “Flight no. 4 was made on 6-5-53. A maximum indicated airspeed of 

525 knots was reached. Airplane and control stability was good.”18

X-3 Flight 5 was made on June 11, 1953, and reached an altitude of 25,000 

feet. In a level flight run at this altitude, a maximum observed Mach number 

of 0.943 was reached. The X-3’s control and stability appeared satisfactory.19

Flight number 6 was made on June 25. The report for the week ending on 

June 27, 1953, noted:

The airplane climbed to 34,000 feet and a level run was made at 

approximately 33,000 feet. The maximum observed Mach No. 

during the run was approximately .94. An accelerated turn at a 

load factor of 2.0 was made at approximately 30,000 feet and an 

observed Mach No. between .93 and .90. At approximately 28,000 

feet, a pushover at a load factor between 0.2 and 0.5 was made at an 

observed Mach No. of approximately .94. A lateral and directional 

stability check made during the pushover indicated no reduction in 

damping at the reduced load factor. During the recovery from the 

pushover, a maximum observed Mach No. of 1.018 was reached....20
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This cleared the way for supersonic dive flights. The NACA X-3 weekly 

report noted:

Flight No. 7 was made on 7-15-53. The airplane climbed to 

36,000 feet from which a dive at an angle of approximately 

15° was started. During the dive, a rudder pulse was made at 

an observed Mach No. of .96, an aileron pulse was made at an 

observed Mach No. of 1.05 and an elevator pulse was made at an 

observed Mach No. of 1.09. Immediately following the elevator 

pulse, a +2.5g pullout was made and the minimum observed altitude reached was 23,790 feet. A maximum Mach No. of M=1.10 

was reached at 26,150 feet during the dive. All pulses during the 

dive were satisfactory.21

Bridgeman commented after the flight in “Douglas Flight Report No. 7”:

In the initial part of the dive, the rudder was pulsed. The damping 

of the lateral-directional oscillations was slow and, because of the 

necessity of waiting for the damping before increasing the dive 

angle, the requested twenty degree attitude was never reached.22

The first attempt to make flight 8 on July 21 was aborted during the takeoff 

roll when the nose wheel again lost tread. The flight was successfully made 

on July 22. Bridgeman climbed to about 36,160 feet, accelerated in level 

flight to Mach 0.89, and pushed over into a 25° dive angle. During the dive, 

Bridgeman made rudder and elevator pulses and reached Mach 1.19 before 

pulling out of the dive. The X-3 pitched around the lateral axis several times. 

Bridgeman commented, “This was mostly pilot induced [while] coping with 

the tender longitudinal control.” Low-speed stability and control tests were 

made before landing.23

Bridgeman made the X-3’s fastest flight on July 27, 1953. The weekly 

report stated:

A dive was made from 36,440 feet to 19,320 feet at a maximum 

dive angle of approximately 30°. A maximum observed Mach No. 

of 1.21 was attained during the dive and a maximum load factor 

of 6.7 was reached during the pullout. Low-speed stability and 

control was investigated during three approaches to stall and two 

stalls using “military” power with the leading edge flaps full down. 

In each stall, the airplane rolled to the right.24
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Despite the successful supersonic flight, it was now clear that the X-3 was 

a disappointment in terms of performance. Even in a steep dive, its top speed 

was little different from that of an F-86. Bridgeman continued to make X-3 

flights, with his 25th and final flight coming on December 1, 1953, but most 

were in the low-supersonic speed range. Nevertheless, given the X-3’s lack of 

thrust and stability, the flights were never routine. On Bridgeman’s 23rd flight, 

on October 21, he made a rolling pullout from a dive. Both afterburners were 

already shut down due to high temperatures in the tail area. Bridgeman felt 

severe vibrations, and the right engine switched to emergency mode, its revolutions dropped, and the outlet temperature increased. He was unable to restart 

the engine and had to use the left engine’s afterburner just to stay aloft. He 

was able to make a lakebed landing on one engine without damaging the even 

more severely underpowered aircraft, which itself constituted a tremendous 

tribute to his extraordinary abilities as an experimental test pilot.25

A postflight report noted the scale of the engine malfunction:

The failure was due to the root failure of one blade in the secondstage turbine and shearing at approximately the mid-span of an 

adjoining blade. It is the opinion of the engine manufacturer’s representative that the whole blade was ejected downstream through 

the engine and the afterburner and subsequently lost to the free 

stream but that the half-blade segment or a fragment of it traveled 

forward, finally becoming wedged between the first-stage turbine 

and the first-stage stator vanes. Further, it is his opinion that the 

half-blade segment being wedged between the turbine wheel and 

the nozzle vanes was responsible for the failure of 10 doweled 

bolts which allowed all of the first-stage nozzle vanes to fall to the 

bottom of the nozzle, from where they were recovered.26

A replacement engine was installed in the X-3, and Bridgeman made contractor flights 24 and 25, completing Douglas’s obligations to the customer. 

On December 8, 1953, Douglas turned the X-3 over to the Air Force.

Launching the Air Force–NACA Program

Originally, the Air Force had planned to conduct an extensive evaluation of the 

aircraft. But the X-3’s disappointing performance caused Air Force testers to 

drastically scale back their planned evaluation, particularly as the service had 

major flight-testing responsibilities reflecting its participation in—and lessons 

learned from—the Korean War, which had ended the previous July.
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147

Charles E. Yeager beside the X-3, in which he made three flights. The aircraft was designed to 

reach Mach 2, but the jet engines that powered it were never capable of delivering the required 

amount of thrust. As a result, the X-3 barely exceeded Mach 1 in a dive. The aircraft’s design, 

with its mass concentrated in the fuselage and small, razor-blade-like wings, made it susceptible to inertial coupling. Ironically, data collected on this phenomenon constituted the project’s 

most important results. (USAF)

Accordingly, Lieutenant Colonel Frank K. Everest and Major Charles E. 

“Chuck” Yeager each made only three flights in the X-3, then washed their 

hands of the project. Although these were made by Air Force pilots, they were 

listed as NACA flights 1 through 6 because NACA instrumentation was aboard 

the X-3, enabling the taking of flight research data. Everest made his first flight 

on December 23, 1953, with Yeager following on December 29. The flights 

included windup turns, stalls, level runs, dives, and rudder pulses. The top 

speed reached during NACA flights 1 and 2 was Mach 1.09. NACA flights 3 
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through 6 were made between July 2 and July 29, 1954. Like the two initial 

flights, these were primarily for evaluating handling qualities and performance. 

Strain gauges were also checked out, and measurements were made of wingpressure as well as of wing, tail, and landing gear loads.27

Colonel Everest later recounted his experiences with the X-3, which he 

called “one of the most difficult airplanes I have ever flown”:

Distinct control problems resulted from its combination of very 

long fuselage and extra-short wings. Longitudinal control was 

sensitive and the airplane pitched up and down on the slightest 

provocation.... The wings had both leading and trailing edge 

flaps to give it added lift during take-off and landing. In fact, 

leading edge flaps were required after take-off until the airplane 

had attained an air speed close to 350 knots. I found this out on 

my first flight, when I retracted them at 300 knots and at the same 

time began a turn back toward the base. The X-3 immediately 

began to buffet and stall because the wings could not support the 

weight, and I had to level out and continue flying straight ahead. 

I climb[ed] to about 37,000 feet, where I made some maneuvers, 

put the airplane in a dive and went supersonic.... After more 

maneuvers I returned to Edwards for my landing. At 5,000 feet 

I extended leading edge flaps and chopped the engines back to 

a low power setting. Then I turned into my downwind leg, and 

extended the trailing edge flaps and landing gear. As I did so I 

began sinking like a rock. At once I applied full power and pulled 

my gear up, and not until I was turning into my final approach 

did I again extend the gear.28

On a more detailed level, the stability and control data from the Douglas 

contractor flights and those made by Everest and Yeager were summarized in 

an NACA research memorandum:

Longitudinal control deflection required to trim the airplane 

over the Mach number range was generally similar to that of 

other airplanes, characterized by a stable variation at Mach numbers below 0.92 and a slight nose-down trim change at Mach 

numbers above 1.07. Data obtained during turns and pull-ups 

indicated that throughout the Mach number range from 0.65 

to 1.21, the apparent static longitudinal stability was positive at 

low lifts and increased by a factor of about 21⁄2 as Mach number 

was increased from 0.9 to 1.2. The apparent stability exhibited a 
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149

gradual decrease as lift increased and mild pitch-ups occurred at 

Mach numbers above 0.95....

Difficulty was experienced in performing smooth longitudinal maneuvers. This condition appeared to result from the combination of control system, pilot, airplane, and their dynamic 

characteristics; however, additional tests are required to determine the primary cause of the lag and oscillations experienced. 

Unaccelerated stalls appeared stable in all configurations tested, 

except at large angles of attack in the landing configuration where 

some instability was evident. Roll-off tendencies, which became 

more severe as the speed was decreased, were apparent in all configurations. Data obtained during sideslips at Mach numbers 

from 0.84 to 0.98 showed the apparent directional stability to be 

positive and to increase with increase in Mach number. A smaller 

degree of apparent stability existed for smaller angles of sideslip 

than existed for larger angles.

Meager aileron effectiveness data obtained at Mach numbers of 0.89 to 0.98 indicate that the control effectiveness was 

generally linear with deflection and exhibited little change with 

increase in Mach number. Comparison of flight data with windtunnel and rocket-model tests showed similar trends and good 

quantitative agreement.29

With the completion of the six Air Force flights, the X-3 was turned over 

to the NACA for planned research flights. NACA pilot Joseph A. Walker was 

assigned as X-3 project pilot. After an engine inspection, instrument calibration, and maintenance work, Walker made his first X-3 flight (NACA flight 

7) on August 23, 1954. During the flight, he made two windup turns, stalls 

in both clean and landing configurations, and a dive to Mach 1.05. After the 

flight, the vertical-tail strain gauges were calibrated. With both Walker and the 

X-3 checked out, the NACA research flights could begin.30

On September 3, Walker successfully made NACA flight 8, which involved 

speed runs, longitudinal maneuvers, and stalls. Data were collected at speeds 

between Mach 0.70 and 1.1. A second flight by Walker was planned but was 

canceled due to lakebed cross winds. Walker then completed NACA flights 9 

and 10 on September 9. Despite the flight series’ successful start, the basic unreliability of the X-3 continued to hamper progress. Two flights were planned 

for September 16. Walker took off and began to climb on NACA flight 11 

but noticed that the right afterburner’s fuel consumption was too high. This 

suggested a problem with the afterburner’s fuel-flow control, and Walker cut 

the afterburners. Despite the failure, some useful data were salvaged from the 
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aborted flight. He performed 1-g stalls in a clean configuration with the leading 

edge flaps in the 10° and full-down 30° positions and landed on the lakebed 

without mishap. To correct the fuel-consumption problem, the ground crew 

had to replace the afterburner and repair the fuel-flow control.31

The X-3 parked on the ramp after ownership was transferred to the NACA. The Air Force markings were removed and NACA insignia added. Joe Walker, who also flew the X-1E, X-4, X-5, and 

X-15, among other aircraft, was the only NACA research pilot to fly the X-3. He made 20 flights 

and experienced its demanding flight characteristics. (NASA)

Walker made five NACA flights in late October, collecting data on stability 

and control; wing pressure distribution pattern; lift and drag; buffeting; and 

wing, horizontal stabilizer, and vertical-tail loading. Walker also made fixedrudder aileron rolling maneuvers at Mach 0.9 and 1.05. On the plane’s 10th 

NACA flight, on October 27, 1954, Joe Walker and the X-3 entered aviation 

safety’s history book, and their milestone came nearly at the price of both 

plane and pilot.32

The Inertial Coupling Crisis

In June 1948, NACA Langley researcher William Hewitt Phillips published a technical note innocuously entitled “Effects of Steady Rolling on 

Longitudinal and Directional Stability.”33 It soon proved a remarkably prescient 
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151

and important report, one of particular significance for the future design of 

practical high-speed aircraft.

In his introduction, Phillips set forth why studying the inertial characteristics of the new generation of high-speed airplanes—airplanes that had long 

fuselages and increasingly smaller wings—was important:

When an airplane rolls around an axis...not aligned with its 

longitudinal axis, inertial forces are introduced which tend to 

swing the fuselage out of line with the flight path. These forces are 

ordinarily neglected when the usual theory of lateral stability of 

aircraft is used to calculate the motion of an airplane in a roll. This 

assumption is probably justified for the case of most conventional 

airplanes because inertial forces involved are small compared with 

aerodynamic forces on the airplane. Design trends of very highspeed aircraft, however, which include short wing spans, fuselages 

of high density, and flight at high altitude, all tend to increase the 

inertial forces due to rolling in comparison with the aerodynamic 

restoring forces provided by the longitudinal and directional stabilities. It is therefore desirable to investigate the effects of rolling 

on the longitudinal and directional stabilities of these aircraft.34

As a consequence of these phenomena, Phillips wrote, “The rolling 

motion introduces coupling between the longitudinal and lateral motion of 

the aircraft. An exact solution of this problem is very complicated because 

of the large number of degrees of freedom involved.”35

At the same time Phillips was writing his paper, Fleming and the Douglas 

engineers were in the midst of designing the X-3. The Douglas aircraft had a 

fuselage 66.75 feet long containing two jet engines, fuel tanks, air conditioning, research instrumentation, ducts, control cables, landing gear, and a host 

of other systems. Yet its wingspan was just 22.69 feet. Douglas, in short, was 

building the airplane Phillips had imagined.36

Phillips’s idea was still considered a theoretical one at the time of his 

writing, but there was already a crucial data point from the XS-1 program 

that applied. In the summer of 1947, during falling-body tests of the XS-1, 

a model of the rocket plane veered wildly off course and disappeared after 

having been dropped from a high-flying B-29. Postflight analysis of optical and telemetric records indicated that, as Phillips recalled, “some kind 

of gyroscopic effect” had taken place, causing the XS-1 to roll, pitch, and 

tumble, falling so far off its predicted trajectory that it was never found.37

Out of this incident, Phillips began an analytical study that resulted in his 
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1948 technical note, which introduced the aeronautical world to the expression “inertial coupling.”38

A hint of problems to come was mentioned briefly in a memorandum by 

Hartley Soulé about Douglas engineer Harold F. Kleckner’s visit to Langley 

on September 5, 1951. Soulé’s text read:

Early computations of the lateral stability of the X-3 airplane at 

supersonic speeds indicate that the relation between rolling and 

yawing motions would be different from...past experience, primarily because of the high airplane length to wingspan ratio with 

the resulting large differences in the moment of inertia and rolling 

as compared with yaw and pitching. For this reason, these characteristics have been the subject of concern although it is not definitely 

known that the different characteristics would be undesirable.39

(Emphasis added.)

Slightly less than a year later, in August 1952, technicians at the NACA’s 

Pilotless Aircraft Research Division (PARD) located on Wallops Island, VA, 

fired a rocket-boosted model of an early short-fin D-558-2 model fitted with 

a small rocket thruster in its nose to induce combined rolling, pitching, and 

yawing motions typical of inertial coupling as the model decelerated below the 

speed of sound. After it fired, the D-558-2, as expected, “coupled” and experienced the combined roll, pitch, and yaw motions. But they did not dampen 

out. Rather, they grew in severity, and the D-558-2 model was completely 

out of control by the time it impacted in the Atlantic Ocean off the Virginia 

coast. As the XS-1 model had shown 6 years earlier, and the D-558-2 model 

confirmed, roll-coupling was a far from innocuous phenomenon.40

On December 12, 1953, during a Bell X-1A flight to Mach 2.44 (1,612 

mph) at 74,200 feet, Air Force test pilot Major Chuck Yeager nearly perished 

when the speeding rocket plane violently coupled, tumbling over 50,000 

feet before Yeager managed (in a feat of superlative airmanship unequaled in 

flight-testing history) to recover the airplane safely to level flight and return to 

Edwards.41 Inertial coupling had struck a piloted aircraft and, had it not been 

for its extraordinary pilot, likely would have destroyed it. In any case, during 

his glide earthwards, Yeager confided to listeners that “if I’d had [an ejection] 

seat, you wouldn’t still see me in this thing.”42 Because the X-1A was, like its 

XS-1 predecessor, thoroughly instrumented, the NACA was able to analyze 

the flight and the onset of the coupling departure in detail, issuing a thorough 

report on the episode.43

While NACA analysts pondered their experience with models and the 

X-1A, events were moving swiftly and ominously in the fast-paced world of 
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153

fighter aircraft development. North American had designed a new jet fighter, 

the F-100A Super Sabre, for the supersonic era. Highly streamlined, with an 

afterburning J57 turbojet and a 45° swept wing, it first flew at the end of May 

1953. Slightly over a year later, the Tactical Air Command activated its first 

F-100A fighter wing, at George AFB, Victorville, CA, just 35 miles southeast 

of Edwards, over the objections of Frank Everest and other Edwards test pilots, 

who thought the plane needed more study and possible modifications to increase 

its stability at supersonic speeds. Almost immediately, some pilots encountered 

disturbing, unsettling motions at high speed. On October 12, 1954, disaster 

struck. North American Aviation test pilot George “Wheaties” Welch took 

off from Palmdale, CA, in an F-100A, climbed high over Rosamond, dove to 

supersonic speed, and then began a rolling dive pullout. The F-100A abruptly 

yawed, rolled, and pitched out of control, disintegrating and killing its pilot. 

Inertial coupling had claimed its first, but not its last, victim.44

Another overhead shot of the X-3 on the back ramp. Unlike the D-558-I and D-558-II, the X-3 

was fitted with a downward-firing ejection seat. The seat was lowered to allow the pilot to enter 

the cockpit then returned to its original position once he was seated. A similar design was used 

in the early F-104s, with poor results. (NASA)

As accident investigators analyzed the wreckage of Welch’s ill-fated F-100A, 

Joe Walker and the X-3 continued probing the behavior and performance 

of the sleek research jet. On October 27, Walker initiated an abrupt left roll 
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at Mach 0.92 at an altitude of 30,000 feet. For 5 wild seconds, as the X-3 

rolled, its nose pitched up and simultaneously yawed until its motions finally 

damped out. As NACA researchers Richard E. “Dick” Day and Jack Fischel of 

the High-Speed Flight Station dryly reported afterwards (describing the time 

history of the maneuver):

While the ailerons are deflected for the aileron roll, a favorable 

sideslip angle is generated, together with a rather large increase in 

angle of attack. (The initial decrease in angle of attack is probably 

attributable to pilot stabilizer control input.) At time 3.8 seconds, 

even though the pilot is now applying 10° right aileron control, 

left rolling velocity increased and exceeded 5 radians/sec accompanied by violent pitching and sideslipping motions. During this 

uncontrollable phase of the maneuver, an angle of attack of 20° and 

left sideslip angle of 16° were encountered. It might be of interest 

to note that the onset of the violent maneuver coincided with the 

attainment of the angle of attack (α = 80) at which unpublished 

flight data indicates the occurrence of reduction of longitudinal 

stability. Also, the angle of attack of 8° corresponds to the angle of 

attack at which a reduction in the measured wing lift slope occurs; 

therefore large wing loads were not experienced at the maximum 

angles of attack. After the primary rolling motion has subsided 

at t = 5 seconds, the large lateral and longitudinal motions damp 

fairly well.45

Walker’s next foray with inertial coupling—during another left roll at Mach 

1.05—was far more violent, and even the dry language of aeronautical engineering cannot mask the danger and drama attending his encounter:

During this maneuver the favorable sideslip builds up rapidly 

with roll velocity and peaks at 21° at the time the airplane ceases 

rolling left. This large sideslip angle results in about 2g transverse 

acceleration. Near the time at which maximum sideslip occurs 

(t ≈ 4.0 sec) a large divergence in pitch develops in the negative 

direction which attains about –6.7g. The pilot applied control to 

stop this pitch-down and immediately reduced the control deflection, but was unable to avoid obtaining 7g when the airplane 

pitched up. Maximum wing loads measured during the maneuver 

did not approach or exceed the design limit load; however, the 

fuselage load obtained from airplane weight and acceleration, and 

horizontal tail loads, and wing load showed maximum values of 
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155

63,000 pounds. These maximum values approximated the limit 

design total load of the fuselage. The measured horizontal tail 

loads were near limit design load. The maximum measured vertical tail loads reached approximately 50 percent of the limit load at 

sideslip angles of 21°. It may be noted that a reduction in vertical 

tail load with sideslip was experienced at sideslip angles about 

8°. In this maneuver...when the rolling stopped, the airplane 

motions quickly damped.46

Fortunately, the plane was at such a high pitch angle that the wing was 

effectively robbed of lift, thus not exceeding its limit loads. But the fuselage, 

as noted above, had reached, though not exceeded, its limit load. Following 

this flight, the X-3 was grounded for an extensive structural inspection, not 

flying again for 11 months.

In the wake of both the F-100A crash and the near loss of the X-3, Scott 

Crossfield was assigned to fly inertial coupling research missions in an F-100A 

delivered to the High-Speed Flight Station for research. Crossfield made a total 

of 45 flights to probe the phenomenon. During one flight, the forces were so 

severe that Crossfield suffered a cracked vertebra in his neck.47

In the research memorandum detailing the X-3’s inertial coupling incidents, 

there is a description of an F-100 flight, this one made at 30,000 feet and Mach 

0.70, initiated by an abrupt left aileron roll:

As peak aileron deflection is attained, there is a steady development of left (adverse) sideslip and a progressive decrease in angle 

of attack. Between t = 3 seconds and t = 4 seconds, the divergence 

rates are accelerated considerably and negative angles of attack 

greater than 16° (–4.4 g) and the left sideslip angles as large as 

26° were reached in the more violent stages of this maneuver. 

Maximum vertical tail loads of 5,500 pounds were measured at 

a sideslip angle of 26°.48

Day and Fischel summed up the results of the X-3 and F-100 inertial coupling tests by noting that “[t]he behavior of the two airplanes in the aileron 

rolls is similar in that large cross-coupling effects are evidenced. The airplanes 

are loaded primarily along the fuselage, particularly in the case of [the X-3], 

so that considerable inertial coupling is expected.”49

In addition to analyzing the data from the X-3 and Crossfield’s F-100 flights, 

a new tool was used for the first time in the inertial coupling investigation: 

the first computer flight simulations. Analog computers were programmed by 

Richard E. Day to simulate the inertial conditions that led to the incidents. 
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In an interview, Day commented on the value of Phillips’s theoretical paper 

on inertial coupling:

It was almost essential. It was great, because nobody had any idea 

what had happened.... So once the F-100 and X-3 got into roll 

coupling, somebody either at Edwards or back at Langley said 

Bill Phillips had written a theoretical report on this. So Walt 

Williams sent Hubert Drake and Joe Weil back to Langley to 

talk with Phillips.50

The inertial coupling problem with the F-100 was solved by making the 

vertical tail taller and increasing the wingspan by 2 feet. The grounding order 

was partially lifted in February 1955. Subsequently, automatic dampers and 

stability augmentation systems were added to supersonic aircraft control systems 

to prevent the inertial coupling from occurring. Even so, until the advent of 

advanced stability augmentation and electronic flight controls, the challenge 

of inertial coupling imposed serious handling-quality limitations on new highperformance aircraft such as the McDonnell F-101 Voodoo and the Lockheed 

F-104 Starfighter. The legacy of the inertial coupling crisis could be found in 

the proliferation of ventral fins and large vertical fins on advanced fighters (and 

eventually twin vertical fins, beginning with the MiG-25 and the Grumman 

F-14A Tomcat, which are now found on many, though not all, of the world’s 

high-performance supersonic military aircraft).51

The X-3 did not fly again until September 20, 1955, and when it did, it 

marked the beginning of the final phase of the aircraft’s useful life. Walker 

continued as project pilot. Research goals included collecting data on static 

longitudinal stability and control, wing and tail loads, and pressure distribution, but NACA researchers were careful not to probe further into the inertial 

coupling arena. Six more flights were made in October of 1955 focusing on 

directional stability and control and tail loads. The October 12 flight was 

marred by the inadvertent deployment of the drag chute in flight, although 

the plane landed safely.

The X-3 made its next flight on December 13, 1955, but the flight was cut 

short when a pressure probe broke off and damaged an engine. The aircraft 

was grounded for repairs until the following spring. It next took to the air on 

April 6, 1956, for pressure-distribution measurements. That day, there was an 

in-flight abort when Walker smelled smoke in the cockpit and landed quickly. 

The problem was traced to an electrical failure of test instrumentation in the 

nose. The damage was limited to charred wiring, and the airplane was again 

repaired. The final flight of the project was made May 23, 1956, to collect 

lateral control data. With the research flights complete, the aircraft was retired 
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157

and sent to the Air Force Museum, where, even today, it impresses visitors with 

its sleekness and purity of line.52

An Assessment

Before the X-3 flew, researchers already realized that the ambitious goals set for 

the aircraft would never be met, not through the fault of its manufacturer or 

its creators, but because of the regrettable failure of its engine manufacturer to 

deliver suitable engines. Thus, the desired data on aerodynamic heating, inlet 

duct and scoop airflow, stability and control of high-fineness-ratio fuselages, 

and aerodynamics of low-thickness-chord-ratio wings at Mach 2—the raison 

d’être of the program—were never collected.53

The X-3 was difficult to fly—John McTigue, a former High-Speed Flight 

Station engineer, recalled that following one landing, Joe Walker had climbed 

out of the aircraft and thrown his helmet to the lakebed in apparent frustration 

with the plane—though this reflected as much the unknowns and challenges 

of control system design in the early supersonic era as it did any inherent flaw 

in the system itself. Specifically, the system suffered from control lag, a flawed 

automatic load feel system, longitudinal oscillations that were difficult to control, and poor damping of oscillations. In light of these control system shortcomings, a question arises. If the J46 jet engines had been available or the pair 

of LR-8 rocket engines had been fitted, could the pilot have even controlled 

the aircraft at Mach 2? Inertial coupling came close to destroying the X-3 at 

just over Mach 1, nearly destroyed the X-1A at Mach 2.44, and did destroy the 

Bell X-2 at Mach 3. At Mach 2, the loads on the X-3 would have been much 

higher; it is reasonable to wonder how long the airplane would have remained 

intact at that speed at the first hint of roll coupling.

The relative “research worth” of the X-3 compared with that of the three 

Douglas D-558-2 research aircraft can also be measured by the number of 

flights made with each aircraft. The single X-3 accumulated only 52 flights, 

and the three D-558-2s made 313 flights in total.54

Given all this, the X-3 might be easily dismissed as an outright failure—

underpowered, exhibiting poor control characteristics, and unable to meet 

performance or research goals. Yet the X-3 provided critical data on inertial 

coupling, a goal never planned and one involving an aircraft design issue no 

one knew existed. Its contribution was a happy accident. The D-558-2’s sweptwing design had inadvertently allowed the testing of methods for preventing 

pitch-up. In the same way, Douglas’s efforts to build a Mach 2 aircraft resulted 

in a design that concentrated mass in the fuselage. Entirely by accident, this 

resulted in production of an aircraft susceptible to inertial coupling. This, in 
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turn, facilitated the discovery of an as yet unknown problem with high-speed 

aircraft. Since modern jet fighters would be built to resemble the F-100 in 

their concentration of mass in the fuselage, with swept, low-aspect wings, the 

X-3’s contributions were invaluable to virtually every new fighter coming off 

the drawing boards.

The transport vehicle on the lakebed with the X-3. Once at the takeoff point, the X-3 was rolled 

off the trailer and prepared for flight. (NASA)

The best illustration of this is seen in the iconic 1950s fighter, the Lockheed 

F-104, to whose design the X-3 contributed immediately. Both aircraft had 

thin, short, razorbladelike wings and heavy fuselages. The development of the 

F-104 overlapped with the X-3 research flights, and the Air Force insisted that 

Douglas provide Lockheed with the X-3 plans. The F-104 was designed as a 

lightweight air-superiority fighter capable of reaching high altitudes and engaging Soviet fighters. The initial contract for two XF-104 aircraft was signed on 

March 11, 1953, and the first XF-104 flight was made on February 28, 1954. 

The speed with which the XF-104 went from drawing board to flying aircraft 

was due to the X-3’s development, which proved the aerodynamic validity of 

the low-aspect-ratio wing, performance predictions, and other details.

A RAND study of the F-104’s development underscored this assessment, 

observing:

The F-104 history illustrates that research and development in one 

program can have a great carry-over value in another. Lockheed’s 

success in building and flying a prototype less than a year after 
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

159

go-ahead would very probably not been possible without the 

knowledge derived from the Douglas X-3 program. Although 

the value of this experimental effort in the F-104 could hardly 

have been anticipated when Air Force money was advanced to 

finance the program, nevertheless the value to the Air Force of the 

X-3 program extended far beyond the immediate results achieved 

with it.55

The F-104 was used by the U.S. Air Force and by allied air forces in West 

Germany, Canada, Belgium, the Netherlands, Italy, and Japan. More than 

1,400 F-104s of all variants were built by Lockheed and under license. The lowaspect-ratio wing design was incorporated into other fighter designs as well, 

including the Northrop F-5, which saw service in 15 air forces, including those 

of South Vietnam, Greece, Iran, South Korea, the Philippines, Nationalist 

China, Turkey, Norway, and Libya.56 In its own way, this, too, represented a 

legacy of the X-3.
Probing the Sky

160

Endnotes

1. Opening quotation from the pilot transcript of the X-3’s second 

flight, October 31, 1952, DFRC archives.

2. The AAF development contract was W33-038-ac-10413, dated 30 

June 1945, and the aircraft received project designation MX-656. 

For the detailed development history of this airplane, see Richard 

P. Hallion, “Serendipity at Santa Monica: The Story of the Douglas 

X-3,” Air Enthusiast Quarterly 4 (October 1977): 129–136.

3. Douglas Aircraft Company, “Model X-3 Mock-Up Conference 

Airplane Descriptions and Illustrations,” December 6, 1948, p. 7.

4. Ibid., pp. 8, 13–14, 38, 42–43; Frank N. Fleming, “Evolution of 

the Configuration of the X-3 Supersonic Research Aircraft” (Santa 

Monica, CA: Douglas Aircraft Company, August 1, 1949), pp. 1, 

4–18.

5. Douglas Aircraft Company, “Model X-3 Mock-Up Conference 

Airplane Descriptions and Illustrations,” pp. 8–10.

6. Thomas A. Marschak, The Role of Project Histories in the Study of 

R&D, Report P-2850 (Santa Monica: The RAND Corporation, 

January 1964), p. 43.

7. Marschak, Role of Project Histories, p. 44; see also James St. Peter,

The History of Gas Turbine Development in the United States...A 

Tradition of Excellence (Atlanta, GA: International Gas Turbine 

Institute of The American Society of Mechanical Engineers, 1999), 

pp. 139–140. Nor was the X-3 the J46’s only “victim”: it helped 

limit the Navy’s Vought F7U-3 Cutlass, rendered the entire first 

production run of McDonnell F3H-1 Demons both dangerous 

and useless, led to the abandonment of the Lockheed XF-90, nearly 

crippled the Navy’s F3D-2 Skyknight, and forced Vought to lay off 

over 2,500 production workers, resulting in widespread criticism 

of Westinghouse. Though the company ostensibly remained in the 

jet engine business for another decade, it dropped out of the first 

rank of engine manufacturers and never matched the success it had 

enjoyed with its J34, an excellent design. The tragedy is that the 

X-3 appeared too early to employ a magnificent small afterburning turbojet, the General Electric J85 (the power plant for the later 

Northrop F-5/T-38 family). At half the weight and smaller even 

than the J34, the J85 had substantially higher performance. With 

it, the X-3 could clearly have reached Mach 2—but the J85 was not 

ready for flight testing until almost the end of the decade.

8. Hallion, “Serendipity at Santa Monica,” pp. 131–132.
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

161

9. Memorandum for Chief of Research, “Proposed Modifications of 

X-3 Airplane to Rocket Power and Air Launch,” October 24, 1952.

10. Hallion, “Serendipity From Santa Monica,” p. 132; for the F-104 

benefit, see Marschak, Role of Project Histories, pp. 85–86, which 

notes, “Lockheed’s task was made substantially easier by the availability of information, particularly wind-tunnel data, gathered in 

the course of the Douglas X-3 program [whose experience] had a 

carry-over value for the F-104 program.” Richard P. Hallion recalls 

a conversation with Melvin B. Zisfein, Deputy Director of the 

National Air and Space Museum, who recalled seeing X-3 wind tunnel reports handed to Lockheed staff for reference as they worked on 

the F-104’s design.

11. Hallion, “Serendipity at Santa Monica,” pp. 132, 136.

12. Richard E. Day and Jack Fischel, “Stability and Control 

Characteristics Obtained During Demonstration of the Douglas X-3 

Research Airplane,” NACA RM H55E16 (July 21, 1955), p. 6.

13. NACA HSFRS, “X-3 Status Report for Period 14 September to 18 

October 1952,” DFRC Archives. Unless otherwise noted, all NACA 

status reports are located in the DFRC archives.

14. NACA HSFRS, “X-3 Status Report for Week Ending 25 October 

1952.”

15. NACA HSFRS, “X-3 Weekly Status Report for Week Ending 1 

November 1953”; “X-3 Weekly Status Report for Week Ending 

15 November 1953”; “X-3 Weekly Status Report Week Ending 25 

April 1953.”

16. NACA HSFRS, “X-3 Weekly Status Report Week Ending 2 May 

1953.”

17. NACA HSFRS, “X-3 Weekly Status Report Week Ending 20 June 

1953,” p. 1.

18. NACA HSFRS, “X-3 Weekly Status Report Week Ending 5 June 

1953,” p. 1.

19. NACA HSFRS, “X-3 Weekly Status Report Week Ending 13 June 

1953,” p. 1.

20. NACA HSFRS, “X-3 Weekly Status Report Week Ending 27 June 

1953,” p. 1.

21. NACA HSFRS, “X-3 Weekly Status Report Week Ending 18 July 

1953.”

22. Douglas Aircraft Company, Flight Test Division, “Flight Report No. 

7,” 1953, DFRC Archives.
Probing the Sky

162

23. NACA HSFRS, “X-3 Weekly Status Report Week Ending 25 July 

1953”; Douglas Aircraft Company, Flight Test Division, “Flight 

Report No. 8 (1953).”

24. NACA HSFRS, “X-3 Weekly Status Report Week Ending 1 August 

1953.”

25. Douglas Aircraft Company Testing Division, “Flight Report No. 

23,” 1953, DFRC Archives.

26. Memorandum for Research Airplane Projects Leader, “Progress 

Report for the X-3 Research Airplane for the Period November 1 

to November 30, 1953”; “Bridgeman,” http://www.astronautix.com/

astros/brigeman.htm, accessed October 6, 2010.

27. Memorandum for Research Airplane Projects Leader, “Progress 

Report for the X-3 Research Airplane for the Period July 1 to July 

31, 1954.”

28. Lt. Col. Frank K. Everest, as told to John Guenther, The Fastest Man 

Alive (New York: E.P. Dutton, 1958), pp. 158–160.

29. Day and Fischel, “Stability and Control Characteristics Obtained 

During Demonstrations of the Douglas X-3 Research Airplane,” pp. 

1–2.

30. Memorandum for Research Airplane Projects Director, “Progress 

Report for the X-3 Research Airplane for the Period August 1 to 

August 31, 1954.”

31. Memorandum for Research Airplane Projects Director, “Progress 

Report for the X-3 Research Airplane for the Period September 1 to 

September 30, 1954.”

32. Memorandum for Research Airplane Projects Director, “Progress 

Report for the X-3 Research Airplane for the Period October 1 to 

October 31, 1954.”

33. NACA Technical Note 1627. For this remarkable aeronautical 

scientist, see W. Hewitt Phillips, Journey in Aeronautical Research: A 

Career at NASA Langley Research Center (Washington, DC: NASA, 

1998).

34. William H. Phillips, “Effects of Steady Rolling on Longitudinal and 

Directional Stability,” NACA Technical Note No. 1627, June 1948, 

pp. 1–2.

35. Ibid.

36. Fleming, “Evolution of the Configuration of the X-3 Supersonic 

Research Aircraft,” pp. 10, 20–23; High-Speed Flight Station, 

“Flight Experience with Two High-Speed Airplanes Having Violent 

Lateral-Longitudinal Coupling in Aileron Rolls,” NACA RM 

H55A13 (February 4, 1955), pp. 7–8.
Unfulfilled Promise, Serendipitous Success: The Douglas X-3 Stiletto

163

37. Quoted in Richard P. Hallion, “Sweep and Swing: Reshaping the 

Wing for the Jet and Rocket Age,” in Richard P. Hallion, ed., 

NASA’s Contributions to Aeronautics, vol. 1: Aerodynamics, Structures, 

Propulsion, Controls (Washington, DC: NASA SP-2010-570-Vol 1. 

2010), p. 38.

38. As well, the more descriptive “roll coupling” was used.

39. Memorandum for Chief of Research, “X-3 Airplane Visit of Harold 

F. Kleckner of the Douglas Company to Langley on October 18, 

1951,” p. 2.

40. Hallion, “Sweep and Swing,” pp. 39-40; James H. Parks, 

“Experimental Evidence of Sustained Coupled Longitudinal and 

Lateral Oscillations from a Rocket-Propelled Model of a 35° Swept 

Wing Airplane Configuration,” NACA RM L54D15 (May 28, 1954).

41. Hallion, Supersonic Flight, pp. 180–182.

42. Yeager transcript and pilot report, 23 December 1953, and J.L. 

Powell, “X-1A Airplane Contract W33-038-ac-20062, Flight Test 

Progress Report No. 15, Period from 9 December through 20 

December 1953,” Bell Aircraft Corporation Report No. 58-980-019 

(February 3, 1954), in AFFTC History Office archives.

43. Hubert M. Drake and Wendell H. Stillwell, “Behavior of the Bell 

X-1A Research Airplane During Exploratory Flights at Mach 

Numbers Near 2.0 and at Extreme Altitudes,” NACA RM H55G25 

(September 1, 1955).

44. Everest and Guenther, The Fastest Man Alive, pp. 19–22; Bill 

Gunston, Early Supersonic Fighters of the West (New York: Charles 

Scribner’s Sons, 1975), pp. 146–154; Knaack, Post–World War II, 

pp. 115–116.

45. NACA High-Speed Flight Station (HSFS), “Flight Experience with 

Two High-Speed Airplanes Having Violent Lateral-Longitudinal 

Coupling in Aileron Rolls,” NACA RM H55A13 (February 4, 

1955), p. 4.

46. Ibid.

47. Crossfield and Blair, Always Another Dawn, pp. 198, 199.

48. HSFS, “Flight Experience with Two High-Speed Airplanes,” p. 5.

49. Ibid.

50. Peebles, ed., The Spoken Word, pp. 68–69.

51. Ibid.

52. Hallion, “Serendipity at Santa Monica,” p. 136; Hallion and 

Gorn, On the Frontier, p. 398. More information on the X-3 

research results can be found in the following NACA research 

memorandums: Harriet J. Stephenson, “Flight Measurements of 
Probing the Sky

164

Horizontal-Tail Loads on the Douglas X-3 Research Airplane,” 

NACA RM H56A23 (April 18, 1956); William L. Marcy, Harriet 

J. Stephenson, and Thomas V. Cooney, “Analysis of the Vertical-Tail 

Loads Measured During a Flight Investigation at Transonic Speeds 

of the Douglas X-3 Airplane,” NACA RM H56H08 (November 9, 

1956); Jack Fischel, Euclid C. Holleman, and Robert A. Tremant, 

“Flight Investigation of the Transonic Longitudinal and Lateral 

Handling Qualities of the Douglas X-3 Research Airplane,” NACA 

RM H57I05 (December 5, 1957); William L. Marcy, “High-Speed 

Loads Measured on the Douglas X-3 Research Airplane,” NACA 

RM H57L08 (February 24, 1958); and Earl R. Keener, Norman 

J. McLeod, and Norman V. Taillon, “Effect of Leading-Edge-Flap 

Deflections on the Wing Loads, Load Distribution, and Flap Hinge 

Moments of the Douglas X-3 Research Airplane at Transonic 

Speeds,” NACA RM H58D29 (July 15, 1958).

53. Hallion, “Serendipity at Santa Monica,” p. 136.

54. “D-588 II Skyrocket,” http://www.globalsecurity.org/military/systems/

aircraft/d-588-ii.htm, accessed May 24, 2010; John McTigue interview notes, March 9, 2010.

55. Marschak, Role of Project Histories, pp. 85–86; Knaack, Post–World 

War II Fighters, pp. 175–176; Hallion and Gorn, On the Frontier, 

pp. 57–58.

56. Knaack, Post–World War II Fighters, pp. 185–187, 288–289.

166

The X-4 Bantam, the second X-4 flown by the NACA between 1950 and 1953, after restoration 

at the National Museum of the Air Force. The semi-tailless swept-wing design was influenced by 

Northrop’s work on flying-wing aircraft, the belief that a lack of horizontal stabilizers would prevent transonic handling problems, and the Me-163B Komet rocket-powered fighter developed 

by Nazi Germany. (USAF)
167

CHAPTER 5

Versatile Minimalist:

The Northrop X-4 Bantam

We did a lot of things with the X-4 that 

weren’t only involved in its being a tailless airplane.

—A. Scott Crossfield1

The tiny Northrop X-4 Bantam, one of the smallest piloted jet aircraft ever 

built and flown, represented the confluence of several technological threads. 

First was aviation pioneer and industrialist John Knudsen “Jack” Northrop’s 

longstanding interest in flying-wing aircraft, dating to the late 1920s. By the 

early postwar years, his dream had culminated in the graceful and futuristic 

multiengine Northrop XB-35 and YB-49 flying wing bombers, the former 

piston-powered and the latter a pure turbojet, undergoing flight testing at 

Muroc. Another thread was the transonic revolution itself: studies by both 

the Army Air Forces and the NACA indicated that a semi-tailless aircraft with 

swept wings and a vertical tail, devoid of horizontal stabilizer and elevators, 

might avoid the buffeting and controllability problems afflicting conventional 

“tailed” aircraft (a byproduct of the deleterious interaction between the turbulent flow streaming behind a wing and the tail surfaces located at the rear of 

the aircraft). Instead, to compensate for the lack of conventional elevators for 

pitch control, combined elevator and aileron control surfaces (called elevons) 

were built into the wing. Operating symmetrically (both up or both down), 

they controlled pitch, like elevators; operating differentially (one up, the other 

down), they controlled roll, like ailerons.

Tailless aircraft (pure flying wings with no tail surfaces whatsoever) and 

semi-tailless aircraft (ones having only a vertical tail) had a long heritage dating 

to the pre–World War I era, and while (by the late 1940s) never securing a 

dominant position among widely accepted aircraft configurations, they nevertheless had undergone significant technical evolution, rendering them increasingly practicable.2

 The advent of the swept-wing semi-tailless Messerschmitt 

Me 163 Komet rocket-propelled interceptor had added to their allure. In an 

era when 400-mph propeller-driven fighters climbing at 4,000 feet per minute 
Probing the Sky

168

were considered remarkable, the 600-mph Komet—climbing to 30,000 feet 

in little over 2 minutes and then streaking through Allied bomber formations 

so rapidly that bomber gunners could not bring their weapons to bear—was 

fantastic. At first glance, to a postwar world entranced by German wartime 

aerodynamic and design accomplishments, the Me 163 seemed to point the 

way to future flight. What was missed was that the Me 163 had demonstrated 

extremely dangerous transonic handling qualities, as a result of its highly deficient high-speed stability and control, and that this had severely limited its 

military effectiveness. Coupled with other weaknesses, including an unreliable, 

unstable, and highly dangerous propulsion system and limited endurance, 

these deficiencies as a group ensured that the Me 163 remained more a curiosity than a serious threat.3

Fascination with the semi- or completely tailless configuration had spawned 

some remarkable concepts for a variety of military and civil aircraft. In Britain, 

this had led to the de Havilland DH 108 Swallow, a transonic swept-wing 

research aircraft intended to function as a technology demonstrator for the 

proposed DH 106 jet airliner.4 Inspired by the Me 163 and launched in late 

July 1945, the Swallow took the fuselage and turbojet engine installation of 

the de Havilland DH 100 Vampire jet fighter and joined it to a graceful swept 

wing, with an equally graceful vertical fin installed on the aft fuselage. The 

DH 108 had a more sharply swept wing compared to the Me 163’s gentler 

sweep. Flight testing of the first of three DH 108s began in May 1946, with 

the aircraft flown by Geoffrey de Havilland, Jr., son of the firm’s founder. The 

first Swallow (TG 283) was reserved for low-speed trials, but the second (TG 

306) was a high-speed machine. It achieved Mach 0.895 at 34,000 feet on its 

fourth flight. During its early trials, it experienced a “short period” longitudinal pitching oscillation at higher Mach numbers that was poorly damped. 

On September 27, 1946, as young de Havilland practiced for a world airspeed 

record attempt, TG 306 violently pitched at Mach 0.875 at only 7,500 feet (an 

altitude that imposed very high structural loadings on the aircraft), breaking up 

and killing him instantly. Though a third DH 108 Swallow (VW 120) eventually did exceed the speed of sound during a high-speed dive from 45,000 feet 

to 23,500 feet on September 6, 1948 (becoming the first British-designed jet 

airplane to exceed the sound barrier), it entailed another wild ride. At one point 

(as test pilot John Derry noted), the plane exhibited an “extremely rapid and 

completely unstable nose-down pitch” during which the VW 120 briefly went 

past the vertical, attaining –3 g’s.5

 Derry, an unusually skilled and courageous 

airman (even by the standards of test pilots) managed to both exceed Mach 1 

and then return safely to Earth. The Swallow had earned a more favorable place 

in aviation history, but it still clearly was a dangerous airplane, and, indeed, all 

three were eventually lost in uniformly fatal accidents.6
Versatile Minimalist: The Northrop X-4 Bantam

169

The first X-4 was used only for contractor demonstration flights, as it had a number of deficiencies. After the flights were completed, it was turned over to the NACA for spare parts. (USAF)

The X-4: Concept, Design, and Construction

On June 11, 1946, slightly over 3 months before Geoffrey de Havilland, Jr.’s 

fatal crash in the second DH 108, Northrop and Army Air Forces’ representatives had signed a contract to build and flight-test two examples of a semitailless swept-wing aircraft. This marked the beginning of the XS-4 (later X-4) 

program, which was undertaken as Air Force research and development project 

MX-810. As with Britain’s luckless Swallow, the inspiration for the Northrop 

design was largely that of the Me 163. Northrop constituted a natural contractor for the experimental airplane, as no other American company had such 

extensive experience and insight into the special challenges and problems of 

tailless and flying-wing aircraft. Unlike any of its X-series predecessors, the X-4 

was specifically intended for subsonic testing. 

Northrop entrusted design of the X-4 to a team led by chief project engineer Arthur Lusk. Assisting Lusk were aerodynamicist Irving Ashkenas; chief 

of structures A.M. Schwartz; and a team of specialists in weight and balance, 

wing construction, propulsion, hydraulics, landing gear, instrumentation, 

and avionics. Basic design work for the aircraft was completed at Northrop’s 

Hawthorne, CA, plant by the fall of 1946, and the mockup was finished in 

mid-November. Results of the inspection of the mockup by Air Force and 

NACA engineers were generally favorable. The exception was one item found 

by NACA personnel, who noted that no space had been allotted for specialized research instrumentation. In response, the Air Force and Northrop 
Probing the Sky

170

(at the NACA’s suggestion) reduced the fuselage fuel tank’s size to provide the 

requisite room.7

The aircraft employed two Westinghouse J30-WE-7-9 nonafterburning 

turbojet engines, each producing just 1,600 pounds of thrust at sea level—the 

same propulsion package used earlier in an unsuccessful Northrop flying-wing 

fighter, the XP-79B, that had fatally crashed on its very first flight. Use of the 

low-thrust engines underscored that the airframe had to be both small and 

light; in its final configuration, a person could look into the cockpit without having to use a ladder. To minimize weight, the X-4’s wings were made 

of magnesium and incorporated integral fuel tanks that, when combined to 

the tanks in the fuselage, carried 230 gallons of useable fuel, giving the little 

jet a 45-minute endurance.8

 In configuration, the X-4 resembled the British 

Swallow, but with a more angular and lower-aspect-ratio 200-square-foot wing. 

Its leading-edge sweep was 41.57°, it had an overall span of 26.83 feet (its 

fuselage length was 23.25 feet), and it had an aspect ratio of 3.6. For safety, 

the X-4 design team prudently incorporated hydraulically actuated split flaps 

on the wing’s inner trailing edge. If the X-4 encountered dangerous transonic 

pitching, its pilot could actuate the flaps, which would immediately extend 

both above and below the wing, increasing its frontal area and generating such 

drag that the X-4 would rapidly decelerate to firmly subsonic velocities before 

it emulated the unfortunate DH 108 and pitched to destruction. Overall, the 

little X-4 had a maximum weight of 7,050 pounds.9

Northrop completed the first X-4 (AF serial number 46-676) in June 1948, 

and the aircraft underwent an engineering inspection with mixed results. Issues 

surfaced regarding the landing gear up-and-down locks, the fuel system, and 

the level of protection for control cables against engine failure or fire. Howard 

Lilly had been killed the previous month in the second D-558-1 when its 

engine disintegrated, causing inspectors to regard new aircraft more stringently. 

The required changes to install armor shielding for its flight control system 

and around the engines to prevent a disintegrating engine from puncturing 

the fuselage or wing tanks took 4 months to make. The first X-4 finally arrived 

at Muroc aboard a flatbed trailer on November 15, 1948. After taxi tests were 

completed, it was ready for its first flight.10

Early Flights

Northrop test pilot Charles Tucker flew the customary proving flights before 

the two X-4s were turned over to the Air Force and the NACA. The first 

Northrop flight was made on December 16, 1948. Instrumenting the aircraft 

promptly proved problematic due to the aircraft’s small size and Northrop’s 
Versatile Minimalist: The Northrop X-4 Bantam

171

structural and engine-temperature-measurement requirements. As a result, 

NACA stability and control instrumentation on the first flight was minimal. 

Standard NACA instrumentation recorded altitude, airspeed, angle of sideslip, right and left elevon position, and rudder position. The instrumentation 

transmitted some data to a ground station, where they were recorded for later 

analysis. (Monitoring in real time, however, was not possible.) Data taken 

included rates of normal acceleration, altitude, airspeed, right and left elevon 

position, and rudder position. The data were synchronized with a common 

time stamp. 

During the first flight, Tucker took data during takeoff and landing and 

made in-flight records of the X-4’s speed as it went from 250 to 275 miles 

per hour and back down to 225 miles per hour indicated air speed (IAS). A 

subsequent research memorandum on the flight noted:

These data show that in the clean condition the airplane is slightly 

unstable as shown by an upward deflection of the control required 

for increasing speed. The pilot stated that it was impossible to trim 

the airplane in the clean condition. With gear down the airplane is 

stable for both center-of-gravity positions. There is an indication 

that there may be some instability at high normal-force coefficients with the gear down. However, the data at 145 miles per 

hour were obtained in the landing approach just before contact 

so there may be some effects of the proximity of the ground on 

these data. Although these data are rather sketchy, they indicate 

that the center of gravity should be moved forward.11

One issue that appeared on the first flight was the effectiveness of the rudder 

control. The X-4 originally was fitted with an electronically operated system 

with a four-speed actuator, an early example of applying electronic flight control (though not a computerized flight control) to an experimental airplane. 

The maximum rate of rudder movement was 25° per second. Available evidence 

indicates that this system was designed to serve as a yaw-damper stabilityaugmentation system. Controlling yaw excursions was far from an innocuous 

issue. Should too large a sideslip angle develop, the aircraft could stall and then 

enter a dangerous spin. 

Flying-wing aircraft had inherently poor spin characteristics, the danger 

exacerbated by often violent longitudinal pitch changes that could result in 

structural failure. Several Northrop flying wings already had been lost in spins 

and pitching accidents, including an MX-324 glider, an N-9M subscale piloted 

demonstrator for the XB-35, an experimental XP-56 tailless propeller-driven 

fighter, and the sole XP-79B jet-powered flying-wing fighter. Then, on June 5, 
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172

1948, as Northrop readied the first X-4 for flight, came the dramatic loss of a 

test crew aboard an eight-engine Northrop YB-49, resulting in the renaming 

of Muroc as Edwards Air Force Base, after the ill-fated plane’s copilot, Captain 

Glen Edwards. An investigation indicated that the YB-49 had stalled, entered 

a spinning dive, and then had broken up as the crew tried to recover. Thus, in 

the X-4, Northrop, the Air Force, and the NACA all had a compelling desire 

to ensure that yaw rates could be limited, thus reducing the risk of the little 

jet’s entering an unrecoverable or otherwise destructive spin.12

For that reason, researchers noted with concern the apparent lag test pilot 

Charles Tucker reported in the X-4’s rudder response: 

The pilot stated that the rudder control seemed to have considerable lag and the motion of the control was too slow.... [T]he rate 

of rudder motion was about 25° per second, which [shows that 

the] rate is considerably slower than the rate at which a pilot is able 

to move the rudder pedals. The electrical system which operates 

the rudder is arranged to give several rates of control movement 

corresponding to the rate at which the pilot moves the pedals. The 

rate of 25° per second is the maximum rate that is available to the 

pilot at the present. The flight records showed this rate was used 

in virtually all rudder applications indicating that motion to the 

pedals was applied at a rate of 25° per second or greater. During 

the first attempt for take-off, it was also indicated that this rate of 

rudder movement is too slow for maintaining directional control. 

The pilot reported excessive friction in the elevon control system[,] which is an irreversible hydraulic system with artificial feel 

for the pilot. He also reported that the aileron forces seemed very 

heavy relative to the elevator forces[,] which on occasion caused 

him to apply elevator control as well as aileron when attempting to 

move only the ailerons. Since the aileron forces are about normal 

it is believed that the pilot was given this impression by the excessively light elevator forces. Since the elevator-force system depends 

primarily upon elevator position, it could be expected that with a 

change in center-of-gravity location sufficient to provide adequate 

stick-fixed longitudinal stability, the aileron-elevator forces would 

be proportioned satisfactorily. 

Inspection of the sideslip records showed no evidence of snaking oscillation over the range of speeds covered.13

Changes were made to the X-4 that reflected the results of the first flight. 

Postflight analysis indicated that the aft position of the center of gravity was 
Versatile Minimalist: The Northrop X-4 Bantam

173

the cause of the poor longitudinal stability. Consequently, the airplane’s center 

of gravity was shifted to 19.7 percent of the mean aerodynamic chord (MAC) 

from 22.4 percent of the MAC. To resolve the rudder issues, the theoretically more advanced electrically operated rudder actuator was removed and 

replaced with a much more reliable (and, somewhat counterintuitively, faster) 

mechanical cable and bell-crank system, akin to the rudder control in a model 

airplane. This doubled the rate of rudder travel to 50° per second. The control 

system friction and springs were also checked by mechanics to eliminate the 

excessive friction reported by the pilot. A check of engine vibrations during 

ground run-up was planned. The date of the second flight was dependent on 

lakebed conditions. In late January 1949, it was not expected that it would be 

usable for about 4 weeks.14

Head-on view of the first X-4. The aircraft was the smallest of the early X-planes. The bubble 

canopy was different from the V-shaped windshields of most other research aircraft. Like the 

X-3, X-5, and XF-92A, the X-4 offered the advantage of an ejection seat. (NASA)

In fact, because of lingering water on the lakebed, the X-4’s second flight 

was not made until April 27, 1949. Tucker collected data during stabilized 

speed runs at 170, 210, and 290 miles per hour IAS and at altitudes from 

12,000 to 15,000 feet, but he had to terminate the flight prematurely because 

of fuel transfer difficulties.15 The approach and landing data were also analyzed 

from the second flight; the results reflected both good and bad characteristics. 

Williams’s postflight memorandum noted, “The airplane possesses adequate 
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174

position stick-fixed longitudinal stability with the center of gravity at 19.7 

percent mean aerodynamic chord as compared with the slight instability with 

the center of gravity at 22 percent mean aerodynamic chord.”16

As for the approach and landing, Williams wrote, 

[T]he airplane possessed slight positive longitudinal stability 

with the landing gear down and flaps up as evidenced by the 

increase in upward elevon deflection as the speed was decreased. 

Approximately 14° of longitudinal control was used for landing[,] 

which left adequate control for lateral motions of the airplane. 

It should be pointed out, however, that this landing was made 

well above minimum speed without flaps. It should be noted that 

during the approach to landing, the small movements of the rudder caused a lateral oscillation that was slow to damp and even 

continued at small amplitudes with the rudder held fixed.... [The 

control was] held essentially fixed while the airplane oscillated in 

sideslip.... The lateral oscillation has a period of approximately 

2 to 3 seconds and...the oscillation damps to half-amplitude in 

approximately 4 to 5 seconds.

The pilot was satisfied with the longitudinal stability with the 

center of gravity at 19.7 percent mean aerodynamic chord. The 

rudder control was considered adequate. Although the pilot did 

not consider the poor damping of the lateral oscillation objectionable at the speeds for which data are presented herein, he 

encountered a poorly damped lateral oscillation at 290 miles per 

hour which he considered very objectionable.17

Clearly, the X-4 had stability and control issues, reflecting its close-coupled 

configuration, which virtually guaranteed poorly damped control response, low 

inherent stability, and a greater than usual susceptibility to pilot-induced oscillations (PIOs). But that it did is unsurprising since so much of what aeronautical engineers were doing at the time was new, and it was flying, as well, in both 

the pre-stability augmentation and pre-fly-by-wire flight control eras. Between 

April 27 and June 1, 1949, flights 2 through 6 were completed with the first 

X-4. The flights generated data, but problems appeared with Northrop’s temperature recorder and radio. After the fourth flight, the engineers initially 

concluded that the interference was caused by the NACA telemeter system. 

The telemeter was not used on the fifth and sixth flights, to verify this, but the 

problems remained. Other difficulties were experienced during the sixth flight. 

After takeoff, Tucker mistakenly left the recording instruments on through the 

climb, leaving no recording film for the rest of the planned maneuvers, so they 
Versatile Minimalist: The Northrop X-4 Bantam

175

were aborted. Northrop engineers also reported the left engine had problems 

during the flight, requiring that it be replaced. This grounded the aircraft for 

2 months. 

Another view of the first X-4. The aircraft had a functional design, but it suffered from major 

stability problems that the technology of the early 1950s could not correct. (NASA)

Such delays were often experienced with research aircraft. These were often 

due, as in this case, to technical problems with equipment and systems. The 

data instrumentation system was particularly critical. If it was not working, 

the flight was pointless. Another cause was the flooding of the lakebed, which 

occurred during most winters. Another was processing a backlog of data. This 

was the job of female “computers,” who had to measure the traces on the film 

manually, apply corrections and calibrations that transformed the traces into 

numerical data, and plot these on graphs. 

The first and second flights of the second X-4 (AF serial number 46-677) 

were made in June. During the aircraft’s preflight preparation, all NACA instruments were recalibrated. The telemeter in the X-4 burned out before the first 

flight, however, and was not replaced before the flight was made. Some data 

were collected, including a rudder release oscillation and a 2-g pull-up. But the 

lack of telemetry didn’t matter since the primary goals for the flight were pilot 

familiarization and instrumentation checkout. And in any event, both flights 

were cut short due to fuel leaking from the wing filler caps.18
Probing the Sky

176

The second X-4 underwent modifications to correct the fuel siphoning. 

That aircraft’s third flight was made on June 23 to test the fix. But as on the 

two previous flights, fuel continued to be lost and the mission had to be cut 

short. Despite the aborted flight, data were collected on a snaking oscillation 

and two pull-ups of 2 g’s. 

To identify the cause of the continuing fuel siphoning, instrumentation 

was added to measure vent-line pressures under varying flight conditions. 

Measurements were taken during the fourth X-4 flight, on June 30. In addition to diagnostic measurements, records were taken at speeds of 300 and 160 

mph at different speed brake angles and at altitudes of 10,000 and 8,500 feet.19

While the first X-4 still remained grounded, Tucker made flights 5 and 6 

in the second X-4 on July 8 and 12, respectively. The fifth flight provided data 

on accelerated maneuvers at 30,000 feet and at 180, 260, and 300 mph; the 

sixth was primarily a photographic mission, but data were also obtained on 

different dive brake angles at 140 mph. Following this flight, the second X-4 

was grounded, pending engine changes. The first X-4 resumed flights on July 

26, 1949, the second aircraft on the following day. These constituted the seventh flight for each aircraft. Again, a significant shortcoming was identified: a 

series of directional stability runs was undertaken with the second X-4, but data 

indicated that hands-off stability runs could not be made, as the time required 

to trim the aircraft was excessive. As well, Northrop engineers revealed they 

were “concerned about some component of the X-4 airplane which has proven 

unsatisfactory in another installation.” But they “were unwilling to state at 

this time what the exact trouble is.” A meeting between Northrop and NACA 

personnel was held, and the Northrop contingent indicated that the landing 

gear door locks were not satisfactory. The risk was that the doors could open 

in flight at high speeds. Work began on developing a fix for the problem.20

To allow Tucker’s contractor flights to continue, the X-4s were restricted 

from exceeding 300 mph IAS and 1.5 g’s of normal acceleration. Each aircraft 

made its eighth and ninth flights in August. The landing gear door issue was 

not the only problem the project faced. The second X-4 made a lateral-stability 

flight on August 3 and a roll-rate measurement flight on August 5. When the 

film from onboard instruments was developed following the flight, it was 

discovered that the film drums had been improperly loaded and no data had 

been collected. Additionally, the aircraft had experienced excess vibrations on 

both flights, requiring that both engines be removed and replaced.21

This was successfully carried out, but Northrop managers requested funding to investigate the vibrations. Pending a decision on their request, the 

second aircraft was temporarily grounded. The first X-4 remained on flight 

status, but no flights were made through mid-September. This was due to the 

removal of the NACA telemetry transmitter so as to allow the installation of 
Versatile Minimalist: The Northrop X-4 Bantam

177

engine-temperature-measurement instruments requested by Northrop. Another 

reason no flights were made was that Northrop engineers were still working 

on a fix for the landing gear doors. Not until this was completed would the 

aircraft make another flight.22

The research airplanes were not typically designed with ease of servicing in mind. The X-4 was 

an exception. Its aft fuselage could be removed from the forward section, allowing access to the 

jet engines. (NASA)

The second X-4 finally resumed flights on September 30. During flight 10, 

Tucker made two stalls in the clean configuration and with the gear down. 

Additionally, an accelerated stall was performed. Based on the flight data, it 

was clear that modifications would have to be made to the landing gear doors 

on both aircraft. The task was expected to take 3 weeks. 

During the halt in flights, Northrop submitted a revised contractor test plan. 

This sharply pared the original plan to a level sufficient only for proving the guaranteed performance requirements. This would involve six more flights totaling 

about 15 hours. In reviewing Northrop’s proposed revision, Melvin Sadoff, an 

NACA aeronautical research scientist at Muroc who wrote the X-4 report, noted 

several requirements that would be necessary in order to provide the data that the 

NACA needed for its research efforts: “With regard to the NACA’s final acceptance of the abbreviated Northrop program on the X-4, it is considered essential 

that directional stability data be obtained up to the level flight high speed of the 
Probing the Sky

178

airplane. The tests should be made at sufficiently large sideslip angles to provide 

satisfactory proof of the structural integrity of the vertical tail at high speeds.”23

Sadoff also summarized the data from the second X-4’s 10th flight:

The results indicate that the 1 g “stalls” in both the clean and the 

gear down configurations were mild and were accompanied by a 

slight dropping of the right wing. The maximum lift coefficients 

obtained were about 0.71 in both cases. There was no warning 

prior to these stalls. Rapid recovery was effected with down elevons. From the relative mildness of the airplane motion subsequent 

to these stalls and because no appreciable buffeting was obtained, 

it is believed that the stalls were not quite complete. Conceivably, 

higher values of CLmax could be obtained by holding the right 

wing up with the rudder control. An accelerated stall to about 

1.6 g was made at about an indicated speed of 165 mph, (25 mph 

higher than the 1 g stalls), and the data showed that the stall was 

essentially complete. The right wing dropped fairly abruptly and 

large up-elevons at the stall were ineffective in increasing the CLmax

above about 0.83. Moderate buffeting set in at the stall in the case 

and persisted throughout the recovery. Recovery was again rapid 

and complete with the down elevon movement.24

Sadoff also raised a more significant issue: whether the X-4 would be suitable 

from an operational and maintenance standpoint. He listed a series of concerns: 

(a) Engine vibration

(b) Engine availability and maintenance (including accessories)

(c) Fuel system peculiarities and sources of trouble

(d) Hydraulic system setup, including protection from vibration and 

engine failure

(e) Availability of control in case of hydraulic system failure, etc.

Sadoff concluded, “From information available at present, item (a) will 

not be remedied and will probably be a source of pilot discomfort, although 

Westinghouse assures us the vibration does not affect the structural soundness 

of the engine. Items (b) through (e) are currently being looked into by Messrs. 

Collins, Griffith and myself.”25

The second X-4 did not make its 11th flight until November 29, 1949. The 

results were a disappointment; after takeoff, Tucker retracted the landing gear, 

but it did not lock properly. He aborted the flight, and the only data recorded 

were of the approach and landing. The Northrop contractor test flights were 
Versatile Minimalist: The Northrop X-4 Bantam

179

drawing to a close, with only four flights planned with the second X-4. Sadoff 

stated that NACA and Air Force representatives needed to meet before the 

completion of the Northrop flights and make a decision on the number of 

additional flights the company would need to complete before the X-4s could 

be accepted by the Air Force and the NACA.26

Northrop X-4 Flights Continue

After the delays in the early contractor flights, activity with the second X-4 picked 

up in mid-December 1949. Flights 12 and 13 were both made on December 7, 

flight 14 on December 9, and flight 15 on December 14. The most significant 

event occurred on flight 15. As Sadoff summarized in the biweekly report: 

During Flight 15 at about 0.80 Mach number and about 5g, an 

inadvertent pitch-up occurred and the airplane acceleration built 

up to about 6.5g before the pilot was able to regain control of the 

airplane. During the recovery, the airplane went through a series 

of violent pitching, rolling, and yawing oscillations, which apparently were associated with the pilot’s manipulation of the controls 

rather than an inherent dynamic instability of the airplane.

Flight 16 scheduled for Friday, December 16 was cancelled so 

that the nose wheel door may be reworked preparatory to higher 

speed flights. The interval before the next flight will also be used 

to analyze the data obtained during Flight 15 so that we won’t 

be proceeding to higher Mach numbers entirely ignorant of the 

airplane’s behavior at lower speeds.27

The rapid flight schedule continued into the new year. With the nosewheel 

door up-lock modified, the second X-4 returned to flight on January 13, 1950. 

The day’s first mission, flight 16, was aborted soon after takeoff after the outside 

canopy lock opened about an inch. The aircraft landed, the lock was repaired, 

and flight 17 was made, which collected static-stability data at 35,000 feet at a 

Mach number of about 0.8. Similar tests had been successfully made at 20,000 

feet without incident. At the higher altitude, Tucker reported difficulty achieving 

the higher acceleration in the test plan due to insufficient longitudinal control. 

Once the test runs had been completed, Tucker began a rapid descent with the 

dive brakes open and immediately encountered violent rolling and yawing oscillations he could not control. Tucker closed the dive brakes, and the X-4 rapidly 

sped up. The aircraft pulled away from the F-86 chase plane and reached a speed 

of about Mach 0.91. Unfortunately, no data were recorded during the dive.
Probing the Sky

180

Engineers decided that the flight would have to be repeated, as the normal 

acceleration data and the telemeter record were not useful. Further complications were engine malfunctions that occurred on both flights 16 and 17 

and caused the aircraft to be grounded for 10 days pending an inspection. 

Examination revealed that the right engine had sustained damage due to excessive temperatures during flight 17. 

While the second X-4 was grounded pending delivery of a new engine, 

the first X-4 made its 10th and final flight on January 24, 1950. The flight’s 

primary research goal was to check the aircraft’s climb performance at 25,000 

feet. A secondary goal was the collection of limited longitudinal-stability data 

in steady, straight flight at 25,000 and 10,000 feet. Finally, data on an oscillation that had occurred with the dive brakes open was also on the flight-test 

card during the descent. Tucker reported after landing that the friction in 

all three controls was excessive and made the X-4 very unpleasant to fly. The 

ground crew began an investigation of the control friction on both aircraft.28

Northrop flights with the second X-4 resumed in mid-February 1950 with 

the completion of flights 18, 19, and 20. The two initial flights investigated the 

aircraft’s longitudinal-stability characteristics in accelerated flight at speeds of 

up to about Mach 0.84 at 30,000 feet. During the last run on flight 19, Tucker 

reached Mach 0.88 in steady, straight flight. He reported “a very noticeable 

buffeting, porpoising, and a yawing and rolling oscillation which were uncontrollable.”29 Tucker compared the buffeting to that of a washboard road, an oftrepeated judgment that quickly became convenient shorthand to describe the 

X-4’s transonic qualities. Sadoff noted in his report: “Although true buffeting 

existed at this speed, what the pilot probably felt was the porpoising motion 

which had a period of about 0.6 seconds. The yawing oscillation reached maximum double amplitude of about 6 degrees.”30

Flight 20 was made on February 17, 1950, but no data were collected 

because the landing gear failed to lock up. At the time, Northrop and NACA 

personnel expected the contractor flights to continue, but events intervened. 

Changing of the Guard

Northrop contractor flights in the X-4 ended abruptly when the Air Force 

issued an order on February 20, 1950, halting the flights. The following day, 

Colonel E.W. Richardson (author of the halt order) and Major L.K. Cox came 

to the High-Speed Flight Research Station to discuss with NACA engineers 

their desires regarding the acceptance and future of the two aircraft. 

The NACA engineers shared their concerns about remaining work. 

Pull-up tests had been made by Tucker to the NACA’s satisfaction during 
Versatile Minimalist: The Northrop X-4 Bantam

181

the contractor flights. The strength of the vertical tail, however, had not yet 

been demonstrated. This involved making steady sideslips up to maximum 

level-flight speed at 20,000 feet and was accomplished using either full rudder 

deflection or 300 pounds of rudder pedal force. 

NACA engineers felt that these sideslips would still not give a true indication of the vertical tail’s structural integrity until the diameter of the rudder 

cables was increased. One-sixteenth-inch-diameter cables had been used in the 

X-4s and were prone to excessive stretching, making it very difficult to obtain 

reasonable values of sideslip at high speeds. While Northrop engineers argued 

that the small cables were sufficiently strong, Colonel Richardson agreed with 

NACA engineers who felt that larger-diameter cables were necessary. As was 

often the case, money entered the picture. Colonel Richardson told the NACA 

that the Air Force lacked the funding it would require to change the rudder 

cables. The NACA agreed to install the larger cables but insisted the sideslip 

tests had to be completed.31

The X-4 rudder cables were not the only safety issue. Ralph Sparks, a 

Northrop engineer working on the aircraft, later recalled his concern about 

the two dive brakes, each of which was operated by a separate hydraulic system. 

If one of the systems failed and the pilot activated the speed brakes, one would 

remain closed while the other opened, inducing dangerous asymmetrical loads 

and forces that could not only throw the X-4 out of control, but perhaps break 

it up as well. Sparks feared the tail might be torn off the aircraft. He discussed 

his concerns with Walt Williams; Williams told Colonel Richardson and Major 

Cox that it would be a simple matter to fix the problem and the repair could 

be done at the same time the rudder cables were replaced.32

The repeated problems with the X-4 engines were also a concern. Williams 

noted that the NACA was not happy about the “burping” of the J30 engines 

or their excessive vibrations. However, discussions with Westinghouse and the 

Air Force’s Power Plant Laboratory clarified that these conditions were not 

inherently dangerous. And so the NACA engineers decided that the engines 

could be operated without modification. They did insist that all engines used 

on the X-4 have annealed turbine shafts, as both Westinghouse and the Navy 

had issued technical orders requiring shafts to be annealed to reduce failures. 

By that time, about one-half of the X-4’s engine stock had the annealed shafts. 

Major Cox asked the NACA engineers if they had a preference as to which 

of the two X-4 aircraft they would receive. They replied that “it would be highly 

desirable” to get the second X-4. The primary reason for their choice was that 

the second aircraft had already been fitted with the NACA instrumentation, 

so no time would be lost reequipping it. Perhaps a more significant reason was 

that the first X-4 was in poor mechanical condition compared to the second, 

and the NACA personnel knew it.33
Probing the Sky

182

The second X-4, photographed from a chase plane. As with Northrop’s other flying wings, the 

X-4 had a sleek appearance. It also shared the stability problems that these aircraft displayed. 

Not until development of the computer fly-by-wire control system would semi-tailless aircraft be 

practical. (USAF)

By the spring of 1950, the status of the X-4 aircraft was settled. The Air 

Force originally requested that the NACA maintain both aircraft on flight 

status, one to be flown by Air Force pilots and the other by the NACA, but the 

NACA rejected this proposal. The first X-4 would be grounded and used for 

spare parts, and the second would be kept on flight status. In addition, NACA 

personnel would replace the existing 1/16-inch rudder cables with 1/8-inch cables 

and modify the dive brakes’ hydraulic system on the second X-4. Once that 

work was complete, Air Force test pilots would make a short series of evaluation flights and make the high-speed sideslip tests of the vertical fin’s structural 

integrity. NACA pilots would then begin research missions.34

Work on the aircraft modifications began in mid-June 1950. In addition to 

the cable replacement and modifications to the hydraulic system, the engines 

were replaced. The only major problem was a delay in the engine installation that was due to a fuel leak in the left wing. This was fixed in August, 

and the installation was nearly complete. Separate from the X-4 modification 

was preparation for the vertical fin structural tests. Before making any flights, 

ground load tests would be made using the first X-4. Flight tests of the vertical 

fin structure would be made once these tests were completed.35

The Air Force X-4 evaluation began on August 18 with flight 1 by Captain 

Charles “Chuck” Yeager. (As with the X-3, these were listed as NACA flights 

since NACA instrumentation was used.) The second and third flights, by Major 
Versatile Minimalist: The Northrop X-4 Bantam

183

Frank Everest, were made on August 22. The third flight was aborted when 

the main landing gear did not lock up, a problem the team thought it had 

resolved. (The solution was to increase the cycling time of the main landing 

gear). Flight 4 was made without incident. On flight 5, both Everest and Yeager, 

the chase pilot, noticed oscillations of the wingtips and the outboard portions 

of the elevons. This was especially noticeable on the left side. After Everest landed, 

an examination showed the left elevon had about 11⁄2° of play, the right elevon 

1⁄2°. The X-4’s elevons were not mass balanced, and this caused them to flutter. 

NACA ground personnel began work to eliminate the play in the elevons, for 

aerodynamic flutter could pose an insidious threat, particularly if encountered at 

higher speeds and with such magnitude that the ailerons were torn from the plane.

The Air Force evaluation now moved to tests of the X-4’s stall behavior. As 

a safety measure, an 8-foot-diameter spin chute was added to the aircraft so 

that if the X-4 got into a spin from which the pilot could not recover, he could 

deploy the spin chute. As the chute opened, it pulled the tail up, forcing the 

nose down into a dive. Airflow over the wings would be restored and speed 

increased, allowing the pilot to recover.36

The Air Force flights were made in rapid succession. Flights 6 through 11 

came between September 13 and 22, 1950, all of them made by Yeager. Flight 

6 involved 1-g stalls, an accelerated stall to about 2 g’s, stall approaches with 

the dive brakes at 20° and 30°, and some longitudinal and lateral dynamic 

stability. Due to instrumentation failure, the flight had to be repeated. After 

that flight, the spin chute was removed. 

The Air Force pilots did have problems: flight 7 had to be aborted when the 

canopy and landing gear did not lock closed, and flight 8 was a repeat of flight 

6. On flight 9, a maximum-speed run was made to Mach 0.89, static-stability 

data were collected, and an accelerated maneuver was made to the instability 

boundary at Mach 0.7. Flight 10 suffered an instrumentation failure and had to 

be repeated as flight 11. This involved accelerated maneuvers to the instability 

boundary at Mach numbers of 0.6, 0.65, and 0.76; directional, longitudinal, 

and lateral dynamic-stability data at Mach 0.5 and 0.7; and aileron rolls to the 

right and left at Mach 0.7. Flight 11 marked the effective end of the Air Force 

X-4 flights, although eight more flights were made, short ones on which no data 

were taken. NACA pilot John Griffith made flight 15. The Air Force evaluation was completed with flight 19 and totaled about 13 hours of flight time.37

Data from the Northrop and Air Force flights of both X-4 aircraft were 

summarized in a December 1950 research memorandum: 

The airplane was almost neutrally stable in straight flight at low 

Mach numbers with the center of gravity located at about 21.4 percent of the mean aerodynamic chord for the clean configuration. 
Probing the Sky

184

Lowering the landing gear had no significant effect on the longitudinal stability. There was some indication that the stability 

tended to increase for both configurations as the normal-force 

coefficient was increased. 

The airplane was longitudinally stable in accelerated flight 

over a Mach number range of 0.44 to about 0.84 up to a normalforce coefficient of about 0.4. At higher values of normal-force 

coefficient and at Mach numbers of about 0.8 a longitudinal 

instability was experienced. 

The airplane does not meet the Air Force specifications for the 

damping of the longitudinal oscillations. The pilot, however, did 

not object to the low damping for small amplitude oscillations. 

However, an objectionable undamped oscillation about all three 

axes was experienced at the highest test Mach number of about 

0.88[,] which may well limit the X-4 to this speed.

The theory predicted the period of short-period longitudinal 

oscillation fairly well, while, in general, the theoretical damping 

indicated a higher degree of stability than was actually experienced. This disagreement was traced to a large error in the estimation of the rotational damping factor.

The directional stability of the airplane was high and essentially 

constant over the speed range considered, while the effective 

dihedral increased considerably with an increase in normal-force 

coefficient. The lateral- and directional-stability characteristics 

estimated from wind-tunnel data compared favorably with the 

flight results.

The damping of the lateral oscillation does not meet the Air 

Force requirements for satisfactory handling qualities.

The dynamic lateral-stability characteristics were estimated 

fairly well by the theory at low Mach numbers at a pressure altitude of 10,000 feet. At 30,000 feet, however, and at [a] Mach 

number above about 0.6, the theory indicated a higher degree of 

stability than was actually experienced.

For the conditions covered in these tests, the stalling characteristics of the airplane at low Mach numbers were, in general, 

satisfactory. The stall was characterized by a roll-off to the right 

and by moderate buffeting[,] which served as a stall warning. 

The buffet boundary for the X-4 airplane, which was almost 

identical to that for the D-558-2 airplane, showed a sharp dropoff in the normal-force coefficient for the onset of buffeting as the 

Mach number exceeded about 0.8.38
Versatile Minimalist: The Northrop X-4 Bantam

185

In addition to the Northrop/Air Force/NACA flights, the ground staticload tests of the first X-4’s vertical tail, fin, and rudder were also completed on 

September 21, 1950. The fin was tested to its design limit of 6,600 pounds. 

The rudder was loaded to 1,500 pounds at the hinge line near the center hinge. 

This was approximately 65 percent of a hypothetical combination of maximum 

loads on the three rudder hinges and about equal to the maximum load on any 

one hinge in a critical load condition. The results showed that the vertical tail 

and rudder could withstand the aerodynamic forces they would experience on 

future NACA research flights.39

The NACA’s X-4 Research Flights

With the Air Force flights completed, the second X-4 was grounded for instrumentation changes. In addition, several engine problems had appeared that 

required both engines to be replaced, including governor and starter troubles. 

This work grounded the aircraft until early November.40

Flights resumed on November 7, 1950, with NACA flight 20 flown by 

Griffith. The pilot made a maximum-speed run to about Mach 0.88 to determine whether elevon motion was contributing to the aircraft’s porpoising. 

Additional test maneuvers included gradual turns to the stall or instability 

boundary at Mach 0.60 to 0.80, to further define that boundary, and rudder 

kicks at Mach 0.70 to 0.80, to confirm deterioration of lateral damping. All 

the tests were made at 30,000 feet. Though instrument malfunctions occurred 

on the flight, they did not impair the collection of data. 

NACA flight 21 was not made until November 17, 1950, again with Griffith 

as pilot. This time, tests included turns to the instability boundary at speeds of 

up to Mach 0.88. In addition, aileron rolls were made at Mach 0.40, 0.50, and 

0.60. Problems were found in the postflight inspection: minor instrumentation errors had occurred, fuel leaks appeared in the wings, and the left engine 

suffered from governor problems. These issues were fixed by early December.41

Flight 22, on December 6, 1950, was a pilot check for Major R.L. Johnson, 

and no data were collected. NACA research pilot A. Scott Crossfield made 

flight 23, which was for aircraft familiarization, the same day. Crossfield later 

described his first experience with the X-4: 

As the X-4 wobbled down the long, bumpy runway, I gingerly 

felt out the controls. Then churning jets took hold, and the small 

X-4 abruptly lunged into the air. Backing off the stall point, I 

nosed her over gently and leveled out. Then I eased back on the 

stick and the tiny tailless craft zoomed skyward like a winged 
Probing the Sky

186

rocket. As predicted, at Mach .88 the X-4 broke into its gentle 

but potentially dangerous porpoising motion. I opened the speed 

brakes, and the X-4 slowed instantly, throwing me forward against 

my shoulder restraining straps.

After about fifteen minutes in the air, I felt at home in the 

X-4. The plane responded so well, in fact, that it was hard for me 

to keep in mind that I was flying a marginally stable, experimental 

race horse.42

The flight was cut short when the left engine suffered a flameout. The X-4’s 

biweekly report stated that Crossfield was making a prolonged 4-g maneuver 

at an altitude of 23,000 feet when the problem occurred. In his biography, 

Crossfield wrote that he had pulled into a loop and, when going over the top, 

both engines flamed out. He was able to restart the right engine but not the 

left one. He made an emergency landing on the lakebed rather than the South 

Base runway.43

Despite these difficulties, Crossfield began a series of flights to explore 

the X-4’s basic handling qualities. The series began with NACA flight 24 

on December 15, 1950. Sideslip and aileron roll data were supposed to 

be collected, but the flight was cut short due to instrument problems. The 

year’s activities came to a close with flight 25, made on December 28, for 

lateral directional-stability, accelerated longitudinal-stability, and lateral control data at Mach numbers between 0.50 and 0.84. This was the first of 20 

flights made to collect data on flight qualities, a series that lasted until May 

29, 1951. All but three were made by Crossfield, the others by NACA pilot 

Walter P. Jones.44

The initial results were written up by Walt Williams and Scott Crossfield 

in 1952 as part of an overview of handling qualities of the X-1, D-558-1, 

D-558-2, X-4, XF-92A, and F-86A. By this time, Crossfield understood 

how difficult the X-4 was to fly. He and Williams described the X-4’s most 

serious shortcoming: 

With the X-4 airplane, an undamped oscillation about all three 

axes at a Mach number of 0.88 has been experienced.... The 

pitching appears predominant to the pilot. The small undamped 

yawing oscillation at this speed induces a pitching oscillation at 

twice the frequency of the yawing oscillation. The pitching is 

apparently amplified because the natural frequency in pitch is 

twice that of yaw.... These data were obtained from rudder kicks 

and stick impulses, and incidentally, in all maneuvers where there 

is yawing there is pitching at twice the frequency.... [A]t 0.9 
Versatile Minimalist: The Northrop X-4 Bantam

187

Mach number the yawing oscillation diverges, rolling becomes 

large, and the whole motion is intolerable. The violence is attested 

by the control motions which result from accelerations on the 

pilot. Lateral accelerations reached ±1 g. The irregularity of pitching is probably caused by the control motion.”45

This handling characteristic was judged to be a “Category I” problem, 

as it was dangerous and imposed serious limitations on the operation of the 

aircraft.46 The research memorandum continued, “Also in the first category, 

the X-4 oscillations about the three axes are determined as having their origin 

in very low to zero damping in yaw and by the fact that the ratios of natural 

frequencies and coupled oscillations are similar. Largely because of these oscillations it was considered unreasonable to extend the speed beyond the maximum 

Mach number reached, nearly 0.93.”47

A more detailed report of the X-4’s handling was written by Melvin Sadoff 

and Scott Crossfield in 1954. The conclusions stated:

At low speeds marginal stability restricted the aft center-of-gravity 

travel to 19 percent mean aerodynamic chord and low longitudinal control power restricted the forward limit to 16.5 percent 

mean aerodynamic chord yielding less than 3 percent permissible center-of-gravity travel. The low longitudinal control power 

within this center-of-gravity range limited the approach to 1g 

stalls which was characterized by mild instability roll-off and normal response to recovery control. 

Throughout the speed range, typical swept-wing instability 

and buffet characteristics occurred at lower normal-force coefficients than with tail-on airplanes of similar sweep.

At high speeds the X-4 characteristics deteriorated as follows:

• At Mach numbers above 0.76 a residual yawing and rolling motion 

persisted at all times.

• At Mach numbers above 0.75 loss of total elevon effectiveness with 

speed and acceleration severely restricted maneuverability and maximum attainable lift.

• At Mach numbers above 0.85 elevon effectiveness began to decline 

rapidly in rolling maneuvers.

• At a Mach number of 0.88 the yawing and rolling coupled with 

the longitudinal motions resulting in persistent oscillation about 

three axes.
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• At a Mach number of 0.90 a high-frequency short-period longitudinal oscillation appeared at normal acceleration greater than 1g.

• At Mach numbers above 0.90 elevon effectiveness had virtually 

disappeared, angles required for trim in level flight were high and 

maneuverability was only slight. 

• Also, at Mach numbers above 0.90 the lateral-directional oscillation 

diverged to unsafe values. The tests were limited by the lack of control power to trim and maneuver and the divergent oscillation.48

Additional Research Activities

The NACA basic handling qualities program was completed with flight 45, 

made on May 29, 1951. This included a climb to about 42,300 feet, a speed 

run at about 38,000 feet, and windup turns at 30,000 feet over a speed range 

from Mach 0.40 to 0.88. After the flight, the aircraft was grounded for the 

repair of an oil leak and installation of new turbine blades. The work involved 

the assembly of a new right engine. During its thrust-stand check, the new 

engine suffered a failure of the accessory drive.49

In the wake of the problems with the right engine, plans for the aircraft 

were changed. Project managers decided a modification of the wing shape 

would be made. Like those on most aircraft, the X-4 wing had a rounded 

leading edge and a thin, knife-edge trailing edge. The modification increased 

the dive brake’s trailing-edge thickness to half that of the hinge-line thickness. 

Engineers thought a blunt trailing edge on the speed brakes would improve 

aircraft stability. Accordingly, they blocked the dive brakes open at an angle 

of ±5°. The brakes could be opened to angles greater than ±5° but could no 

longer be closed completely.50 The modifications to the X-4’s dive brakes 

were completed in late July 1951. Strain gauges were also added to the upper 

and lower segments of the left dive brakes to record hinge-moment data. The 

research flights were delayed when a test run of the replacement right engine 

indicated that the throttle mechanism required adjustment. This pushed the 

start of research flights back to late August.51

Walter P. Jones made the first tests of the dive brake modifications on 

NACA flight 46 on August 20, 1951. The tests involved an unaccelerated 

stall at 35,000 feet and aileron rolls and turns at Mach 0.71, 0.83, and 0.86. 

Jones then dove and made a turn and aileron roll at Mach 0.91 at 28,000 feet. 

Jones reported a general improvement in the aircraft’s handling. In particular, the porpoising and lateral oscillations that had limited the X-4 at speeds 

of Mach 0.88 to 0.90 were not apparent, with the thickened trailing edges, 

to a Mach number of 0.91.52
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Once more, engine problems appeared, delaying flight 47 until October 

2, 1951. Crossfield made this flight, which continued the investigation of the 

effects of the thickened dive brakes on stability and control. The improvements were significant; the X-4’s longitudinal oscillations with the thin trailing edges were undamped above Mach 0.88. In contrast, with the thickened 

speed brakes at a speed of Mach 0.90, the longitudinal oscillations damped 

out after about two cycles. 

Crossfield executed windup turns at Mach numbers from 0.79 to 0.90 for 

static-stability data and reported after the flight that slight porpoising and 

instability occurred at about Mach 0.86. When he added a rudder impulse at 

Mach 0.90, it caused an undamped lateral oscillation of about ±51⁄2° of sideslip 

and ±10° of angle of bank, with slight porpoising. A similar maneuver had 

been made with the thinner trailing edges at about the same altitude and Mach 

number, but subsequent comparison showed that the longitudinal oscillations 

were less pronounced with the thicker trailing edges.53

With initial tests of the thickened dive brake complete, focus shifted 

to obtaining lift-to-drag ratios for various dive brake settings during landings. Crossfield piloted NACA flights 48 through 51 between October 5 

and October 12, 1951. The dive brake deflections ranged from 0° to ±40° 

and provided landing data on lift-to-drag ratios between 8 to 1 and 3.5 

to 1. Jones made flight 52 on October 17, making constant speed runs to 

determine lift-to-drag ratios at dive brake deflections of 0° to ±60° in the 

landing configuration. 

Joe Walker now joined the X-4 research effort, making NACA flights 

53 and 54 on October 18 and 19. These were both pilot-familiarization 

and research flights involving speed runs, rudder kicks, low-speed stalls at 

10,000 feet, dive brake deflections, and the collection of landing pattern data. 

Research with the X-4 ended for 1951 with NACA flight 55, made by Jones 

on October 24. This flight provided additional stability and control flight data 

on the thickened dive brakes. 

After the flight, an inspection found that fuel leaks had reappeared. 

Engineers initially estimated that these could be quickly fixed and flights could 

be resumed on November 9. The leaks were more serious than first thought, 

however, and the aircraft was grounded indefinitely.54

After considerable effort, NACA flight 56 was finally attempted on January 

29, 1952, but was aborted before takeoff due to excessive fuel overflow. The 

fuel regulator was replaced and set at a lower limiting pressure. This was not 

the only maintenance problem X-4 personnel faced in the winter of 1951–52. 

The sideslip transmitter had to be repaired, and the left engine was replaced due 

to “torching” between the tailpipe and the shroud. This was traced to leaking 

oil around the front bearing seal.55
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NACA research pilot Joe Walker discusses his upcoming X-4 flight with a ground crewman. 

Walker made only one flight in the aircraft, on October 18, 1951. The bulk of the NACA flights 

were made by Crossfield and Walter P. Jones. (NASA)

Not until March 6, 1952, did Jones finally complete NACA flight 56. 

This involved measurements of lift-over-drag characteristics at different drag 

brake deflections, as well as unaccelerated stalls and constant speed runs at 

various throttle settings. The engine problems reoccurred, this time on the 

right engine. Flame instabilities, which required investigation, also occurred 

during the flight. Unlike previous engine difficulties, these were soon fixed, 

and NACA flight 57 was made on March 13. It was primarily an engine check 

flight, but data were recorded on directional trim changes at Mach 0.79 and 

during the landing.

Flight 58, on March 17, was intended to be a continuation of the lift-overdrag studies, but an open circuit in the dive brake actuating solenoid prevented 

the dive brakes’ use. Flight 59, during which lift-and-drag data were collected 

with the dive brakes at ±50° deflections, was completed March 21. NACA 

flight 60 was made on March 26 and entailed rudder-fixed aileron rolls, as well 

as testing of stick and rudder impulses at true Mach numbers between 0.80 and 

0.90. Flight 61 was made on March 27, with stick impulses recorded at true 

Mach numbers of 0.50, 0.60, and 0.70. Landing data were also collected on 
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191

the flight. In all, Jones made seven consecutive X-4 research flights in March 

1952. This brought the dive brake tests to a close.56

The X-4’s hectic March flight schedule was concluded with a pilot checkout 

flight on March 27. Stanley P. Butchart made NACA flight 62. The dive brake 

research was now completed, the X-4 was grounded, and the dive brake data 

were under analysis.57

Landing data initially seemed a less consequential area of study but emerged 

as a significant issue. Supersonic flight required an entirely new set of design 

features. These included thin wings, low-aspect-ratio wings, and swept wings, 

all with high wing loading. As a result, the aircraft had lower lift-to-drag ratios 

and higher stall speeds than earlier designs. The resulting vertical velocities 

made it difficult for a pilot to land safely and accurately. 

The low lift-to-drag ratio and high stalling speed of high-performance aircraft meant the excess speed ratio required at the start of the flare increased 

considerably as lift-drag ratio decreased. The flare also had to begin at a relatively high altitude. Additionally, past flight tests had shown that a vertical 

velocity in excess of 25 feet per second at the start of the flare put too high a 

demand on pilot skill and was regarded to be impractical. The X-4’s large dive 

brakes enabled a landing maneuver at lift-drag ratios from 8 to 1 down to 3.5 

to 1. A research memorandum discussed the results:

These landings were started at an altitude of approximately 3,000 

feet with the engines maintaining zero thrust and with a constant dive-brake angle during the landing maneuver. The patterns 

become smaller as the lift-drag ratio decreases[,] which requires 

an increase in acceleration during the approach turn from 1.1g 

at a lift-drag ratio of 8 to about 1.5g at a lift-drag ratio of 3.5. 

The higher acceleration results also from the fact that part of the 

landing flare is made during the final approach turn at the lower 

lift-drag ratios. This has prevented landings from being made at 

dive-brake settings greater than 35° because the largest portion 

of the flare is made during the turn at these settings and there is 

insufficient elevon control to enable the maneuver to be accomplished at larger dive-brake settings. One factor noted by the 

pilots was the short length of time, 50 seconds, at an approach 

lift-drag ratio of 3.5 as compared with about 140 seconds at a liftdrag ratio of 8 during which the pilot could correct and modify 

his landing approach. 

The poor longitudinal control at large dive-brake settings was 

the pilots’ greatest complaint during these flights. They felt that, 

if sufficient longitudinal control were available, landings could 
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192

be performed at still lower lift-drag ratios. Landings at the lowest 

lift-drag ratios were not felt to require exceptional piloting skill 

or a great deal of practice. However, it should be remembered 

that for these landings the lift-drag ratio increased with decreasing speed and although landings were started at a lift-drag ratio 

of 3.5 the lowest lift-drag ratio at contact was about 6.2 even 

neglecting ground effect. At high lift-drag ratios ground effect was 

very noticeable to the pilots, whereas at the lower lift-drag ratios 

ground effect was not nearly so pronounced.58

The memorandum concluded, “Landings of the X-4 airplane to determine 

the effect of lift-drag ratio showed that the largest portion of the landing flare 

was made at altitudes above 50 feet at low lift-drag ratio and that, although the 

vertical velocities during the approach varied from 30 to 90 feet per second, 

the vertical velocities at contact were less than 5.5 feet per second.”59

The X-4’s Twilight

In the spring of 1952, the X-4 was again grounded for a prolonged period. 

Several modifications had to be completed before flights resumed. One of these 

was the fabrication of jigs to support the aircraft during planned moment-ofinertia measurements. A more significant activity was the modification of the 

elevons’ trailing edges. As with the dive brakes, their thickness was increased to 

50 percent of hinge-line thickness. Again, this was done to determine whether 

blunt trailing edges would improve aircraft handling and was completed by 

the end of April. However, a May 1 ground engine run was unsuccessful. The 

left engine overheated, requiring replacement.60

A successful engine run was made on May 16, and NACA flight 63 was 

made May 19 with Jones as pilot. Of the planned test maneuvers, the stick 

impulses and one-half-deflection aileron rolls, performed at 30,000 feet and at 

Mach numbers of 0.55 and 0.60, were successfully completed. At this point, 

the right engine began to overheat. Jones aborted the rest of the tests and 

landed safely. This flight also marked the end of Jones’s research flying with the 

NACA. He left the High-Speed Flight Research Station in July 1952 to work 

as a Northrop test pilot. Tragically, he died the following October in the crash 

of a Northrop YF-89D Scorpion.

The malfunctioning engine was replaced, but a ground run on May 26 was 

unsuccessful due to a stuck fuel valve. The problem was fixed, and a second 

ground test was made on May 29. Excessive vibrations caused this test to fail 

as well. Their cause was unknown, and the X-4 remained grounded until a 
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193

replacement engine could be installed and successfully tested. This proved 

more difficult than expected. All of the spare J30 engines proved faulty when 

ground tested, due either to vibration or to excessive bearing heating. To produce a functional engine, maintenance personnel had to assemble parts from 

spare jets.61 The effort proved frustrating, as a progress report noted: “In each 

instance the reassembled engine has experienced excessive vibration. Assembly 

of another engine is underway.”62 Finally, on July 25, an engine was tested that 

seemed to be fault-free. 

After being grounded for more than 2 months, NACA flight 64 was made 

on August 6 with Crossfield as pilot. The first test of the thickened elevons was 

made without incident. Crossfield’s opinion was that elevon control had been 

much improved at higher Mach numbers by the modification. Postflight examination showed the windup turn data were unusable, however, due to instrumentation failure. The maneuvers would have to be repeated on a later flight. 

More bad news followed: NACA flight 65 was delayed following a preflight 

engine run. An investigation showed that an object had passed through the right 

engine, presumably during landing on flight 64. The aircraft was grounded 

until the engine was rebalanced and a successful ground run completed. While 

the aircraft was being serviced, the thickening material was removed from the 

dive brakes, restoring their original configuration. Subsequent research flights 

would be made with the thickened elevon configuration and the original dive 

brake configuration.63

After the series of problems, the X-4 made NACA flights 65 through 68 

during September 1952. Data from the initial flight were lost due to instrumentation malfunction. The three other flights were successful, with windup 

turns and aileron rolls at Mach 0.65 to 0.94 and stick and rudder impulses from 

Mach 0.70 and 0.90. Postflight analysis indicated that the increased elevon 

thickness improved aileron effectiveness over the entire Mach number range.64

Research missions with the X-4s had been flown without serious mishaps, 

despite each aircraft’s having exhibited poor stability at high transonic speeds 

and unreliable engines. It was therefore ironic that the most serious damage 

either aircraft suffered happened on the ground, at the hands of a person 

assigned to protect it. Over the weekend of October 4–5, an Air Force guard 

on duty at the NACA hanger at South Base accidentally drove an airplane tug 

into the second X-4’s right wing. The impact damaged the wing’s underside. 

The extent of the damage was not immediately evident; an inspection was 

made to determine whether the wing could be repaired and the X-4 returned to 

flight, or if the damage was so severe that the aircraft would have to be retired.65

Inspection determined that the damage to the aircraft was limited to the 

wing’s skin. Repair was complete by mid-December 1952. The X-4 remained 

off flight status for several weeks while the moments of inertia about the 
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three axes at various fuel levels and attitudes were determined. This work 

continued through the end of February 1953, and the aircraft then resumed 

research flights.66

The first of these was made on March 27 with Crossfield as pilot. NACA 

flight 69 included a speed run to Mach 0.91 and successful windup turns 

at Mach 0.78 and 0.88. At that point, the fuel gauge malfunctioned and 

Crossfield had to cut the flight short.67

NACA flights 70 and 71 followed at the end of April. The first flight 

involved tower passes at a low level to check airspeed calibration. The second 

flight undertook speed runs and windup turns between Mach 0.77 and 

Mach 0.90, both to check the airspeed calibration and to collect additional 

data for the longitudinal control study with the thickened elevons. Flight 

70 marked the end of this investigation. After it was concluded, the elevons 

were restored to their original configuration. Work was completed by late 

May, when Crossfield made NACA flight 72. This flight provided dynamicstability data without the thickened elevons, using rudder pulses at Mach 

0.50 and 0.88 speeds.68

X-4 research activity continued with NACA flights 73 and 74 on July 1 

and 3. Both flights were made by Butchart to acquire additional data on the 

aircraft’s dynamic stability without the thickened elevons. This was accomplished using elevon and aileron pulses at Mach numbers from 0.50 to 0.88.69

Although the X-4 project was entering its final phase, activity increased 

substantially. During August 1953, seven flights were made beginning with 

flight 75 on August 4, a checkout flight for Ames Aeronautical Laboratory 

research pilot George Cooper. Flight 76 on August 11 was a checkout flight 

for John B. McKay, a new High-Speed Flight Research Station research pilot. 

McKay also made NACA flights 77 and 78, on August 13 and 19, collecting 

dynamic-stability data from aileron pulses. However, data from several runs 

were lost due to instrumentation failures. Once again, this required another 

research flight to be added to the schedule. 

The fast pace continued to the end of the X-4 project. Two days after his 

previous mission, McKay again piloted the X-4 on NACA flight 79. Crossfield 

went aloft on August 24 for flight 80. The X-4 was checked out over the next 

2 days and flown by McKay on August 26 for the 81st NACA mission. These 

flights were for dynamic-stability data. The 4 years of X-4 flights by Northrop, 

the Air Force, and the NACA then came to a close.

The final research mission, NACA flight 82, was made on September 29, 

1953. McKay’s flight plan involved aileron pulses across the Mach number 

range and low-speed turns to 2 g’s at different dive brake settings. This brought 

the X-4 project to an end. At the time the mission was flown, there were plans 

to use the aircraft to check out a number of Air Force and NACA pilots, but 
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195

plans were dropped, and the second X-4 joined the first in storage. Eventually, 

the first X-4 was put on display at the U.S. Air Force Academy in Colorado 

Springs, CO. It subsequently became part of the Air Force Flight Test Museum 

collection at Edwards Air Force Base, where it awaits restoration. The second 

X-4 was sent in 1955 to Maxwell Air Force Base, Montgomery, AL. It remained 

there until March 17, 1972, when it was transferred to the U.S. Air Force 

Museum at Wright-Patterson Air Force Base, OH, where it is on display, having 

been magnificently restored.70

An Assessment

The X-4 Bantam was a highly productive research aircraft, though one that, 

at the time, was of impracticable configuration for commercial or military 

use. More research flights were made with the X-4 than with the X-3, and the 

X-4 provided data in a wide range of disciplines. Yet the X-3 influenced the 

design and development of the F-104, one of the most widely used fighters of 

the mid–20th century. In contrast, the X-4’s semi-tailless swept-wing design 

did not see significant use until much later, following the advent of electronic 

fly-by-wire flight control technology. The only U.S. aircraft to feature a similar 

configuration in the 1950s was the Navy’s severely underpowered and, indeed, 

highly dangerous F7U Cutlass, which blended prolonged development with 

a brief operational service life, entering service in 1954 and retiring almost 

exactly 5 years later. Of the 320 F7Us built, some 80—fully 25 percent—were 

lost in accidents.71

Crossfield summed up the X-4 project during a 1998 interview:

Oh, the X-4 was a whole buildup of stability and handling, stability and control, and handling qualities of a tailless airplane. With 

the X-4, we pretty much found out why we may not have solved 

the problem.... The X-4 was a very productive program. We did 

a lot of things with the X-4 that weren’t only involved in it’s [sic] 

being a tailless airplane. It was a good test bed for other purposes, 

as we did thickened trailing edges on the X-4. We did variable 

Lift over Drag (L over D) landings from probably an L over D of 

two to an L over D of nine with the airplane, which gave us good 

insight into pilots preferences and that sort of thing, which were 

useful in coming up with new airplanes like the X-15 and some 

of the other low L over D airplanes [like] the lifting bodies. But 

the X-4 was a very good research airplane, for purposes of looking 

into a tailless configuration, and for other reasons.72
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Though the configurations of the D-558-2, X-3, X-5, and XF-92A became the basis for numerous U.S. aircraft, only the F7U Cutlass featured the X-4’s semi-tailless swept-wing configuration. 

An early prototype XF7U-1 is shown here on the Langley ramp in 1948. The aircraft entered U.S. 

service after a prolonged development, but it suffered a high loss rate and was soon retired from 

service. (NASA)

But for all the flights made with it and all the data returned, the X-4 was 

conceptually a flawed aircraft. Its instability was described by research pilots 

as being akin to driving on a washboard road. The X-4 displayed increasingly 

severe combined yaw, pitch, and roll motions, as well as inadequate damping 

as Mach number increased. In an attempt to correct the instability, NACA 

engineers decided to modify the dive brakes and later the elevons, giving 

them a blunt trailing edge. X-4 research flights indicated that this produced a 

25-percent increase in roll rate and improved longitudinal control effectiveness. 

Above Mach 0.9, however, instability reoccurred. The severity of the porpoising increased rapidly; at Mach 0.94, vertical accelerations reached ±11⁄2 g’s. 

The basic cause of the instability could not be solved by Band-Aid fixes such 

as thickening the trailing edges of dive brakes and elevons.73

Until the advent of the fly-by-wire Northrop B-2A Spirit stealth bomber, 

the various direct or hydromechanically controlled semi-tailless and flyingwing designs, such as the Horten Ho 229; the Northrop N-9M, XP-79B, and 

YB-49; and the British de Havilland DH 108 and Armstrong Whitworth AW 
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52, all suffered from serious stability and control deficiencies that limited their 

practical use and even endowed them with vicious, life-endangering quirks 

(indeed, between them, the six types listed here killed a dozen highly skilled 

airmen). For all their perceived advantages from an aerodynamic viewpoint, 

all were thus impractical from the start. The semi-tailless X-4 had been built to 

determine whether eliminating the horizontal stabilizer would prevent interactions between the shock waves formed by the wings and horizontal stabilizers. 

In fact, the lack of horizontal stabilizers made the instability worse compared 

with that of a conventional swept-wing/horizontal stabilizer design. The DH 

108 demonstrated the risks of what deficiencies could do if encountered at 

high speed and low level. The X-4 demonstrated the problem but, flown at 

higher altitudes and lower dynamic pressures, it avoided the disastrous experience of its British cousin. Nevertheless, it clearly had such deficiencies that it 

did not warrant further development. Northrop played around with the swept 

semi-tailless configuration, even employing it on a long-range, jet-propelled, 

nuclear-tipped cruise missile (the disappointing Northrop SM-62 Snark), but, 

overall, swept-wing semi-tailless aircraft remained impractical for nearly three 

decades, until the advent of computer-controlled flight.

Ironically, the original X-4 rudder control system gave a hint of what would 

be needed to correct the aircraft’s control problems. Rather than using a cable 

and bell-crank system, the rudder was electrically operated by a four-speed 

actuator. This was a proto-fly-by-wire system, which served as a yaw-damping 

stability-augmentation system. The X-4 system, as noted earlier, was limited to 

a maximum speed of 25° per second. This system was too slow to the point of 

being dangerous, and it was replaced with a conventional mechanical system. 

As long as an aircraft had to be inherently stable, the swept-wing semi-tailless 

configuration remained impractical. The technology for providing a solution 

did not exist in the early 1950s.74

It was not until the 1970s that practical digital fly-by-wire systems were 

developed. These systems supplied an artificial inherent stability by providing 

control inputs faster than human pilots could. The faster inputs eliminated 

washboard and other unstable motions, allowing the pilot to maneuver the 

aircraft rather than struggle to control it. The first aircraft to use a digital flyby-wire control system was a modified F-8 flown at the Dryden Flight Research 

Center (now called the Armstrong Flight Research Center). In an important 

innovation, the airplane’s mechanical control system was removed. Should 

the digital system fail, an analog backup computer would take over. NASA 

engineers realized that fly-by-wire would never be accepted as proven as long 

as a mechanical backup was still being used. 

Once the aircraft was totally dependent on a fly-by-wire system, inherent 

stability was no longer a design limitation. Among the first applications of this 
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198

was the Lockheed XST Have Blue stealth test aircraft, first flown in December 

1977. This was a swept-wing, semi-tailless aircraft with two inwardly canted tail 

fins mounted at mid-span of each wing. The design was unstable in the pitch 

axis and relied on a modified F-16 fly-by-wire system for stability.75 Even so, 

for various reasons, its successor, the Lockheed F-117A stealth fighter, had a 

different vertical fin configuration, effectively giving it a V-tail (like the Beech 

Bonanza general aviation airplane).

A photo illustrating the rapid changes in aviation technology at Edwards in the mid-1950s. In 

the background is a pair of B-36 bombers, originally designed for the bombing of Nazi Germany 

from U.S. bases. The XB-52 prototype, with the original tandem cockpit design, is in the 

foreground. Its descendants are still in service a half century later. Beside the XB-52 is the tiny, 

futuristic X-4. Visible are its control surfaces, which were designed to overcome the absence of 

horizontal stabilizers. (USAF)

Over the decades to follow, fly-by-wire control systems were fitted to aircraft 

ranging from business jets, airliners, fighters, and research aircraft to bombers 

and heavy transports. But for the X-4, such a solution was not yet available. 

A computer in the early 1950s took up a large room, was slow and unreliable, and required a large maintenance staff to keep it operating. Today, the 

basic X-4 configuration can be discerned in the lines of a variety of remotely 

piloted, computer-controlled aircraft. Impractical in the 1950s, the little X-4 

was ahead of its time. 
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A famous photo showing the family of X-planes that were being flown at the High-Speed Flight 

Research Station in the early and mid-1950s. In the center is the X-3, with the X-1A, D-558-1 

Skystreak, XF-92A, X-5, D-558-2 Skyrocket, and X-4 arrayed around it. (NASA)
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Endnotes

1. Opening quote from A. Scott Crossfield interview transcript, DFRC 

Archives.

2. Richard P. Hallion, “Before B-2: The History of the Flying Wing,” 2 

parts, Air Power History 41, nos. 3 and 4 (fall–winter 1994): 4–13, 

40–51.

3. Richard P. Hallion, “Rocket Dreams,” World War II 23, no. 4 

(October–November 2008): 57–58. For reminiscences by members of the Komet team, see Wolfgang Späte, Top Secret Bird: The 

Luftwaffe’s Me 163 Comet [sic] (Missoula, MT: Pictorial Histories 

Publishing Company, 1989); and Mano Ziegler, Rocket Fighter 

(Garden City, NY: Doubleday and Co., Inc., 1961).

4. The DH 106, which evolved into the Comet I jetliner, was initially 

conceived as a swept-wing semi-tailless design. However, it ultimately emerged as a conventional wing-fuselage-tail configuration, 

with four jet engines buried in its wing roots.

5. Quote from Derry’s pilot notes, reprinted in Charles Burnet, 

Three Centuries to Concorde (London: Mechanical Engineering 

Publications Limited, 1979), p. 112.

6. See also Captain Eric Brown, RN FAA, “An Ill-Fated ‘Swallow’...

but a Harbinger of Summer,” Air Enthusiast 10 (July 1979): 1–7; 

and Brian Rivas, A Very British Sound Barrier: DH 108—A Story 

of Courage, Triumph, and Tragedy (Newark, U.K.: Red Kite/Wing 

Leader Publishers, 2012).

7. For the detailed history of the X-4’s design, development, and flight 

testing, see Richard P. Hallion, “X-4: The Bantam Explorer,” Air 

Enthusiast Quarterly 3 (June 1977): 18–25.

8. Total fuel load was 240 gallons, but 10 gallons were “trapped” 

within the vehicle’s plumbing and pump system and unusable.

9. Technical details from Hubert M. Drake, “Stability and Control 

Data Obtained from First Flight of X-4 Airplane,” NACA RM 

L9A31 (February 7, 1949), Table 1, “Physical Characteristics of X-4 

Airplane,” p. 6.

10. Jay Miller, The X-Planes X-1 to X-45 (Hinckley, England: Midland 

Publishing, 2001), pp. 79–81; see also Hallion, “X-4: The Bantam 

Explorer,” pp. 20–21.

11. Drake, “Stability and Control Data Obtained from First Flight of 

X-4 Airplane,” p. 3.

12. Discussion with NASA research pilot Mark Pestana, Dryden Flight 

Research Center, July 19, 2010.
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13. Drake, “Stability and Control Data Obtained from First Flight of 

X-4 Airplane,” p. 4.

14. Memorandum for Chief of Research, “Progress Report on 

Acceptance Tests of X-4 Airplanes from January 1 to January 14, 

1949.” Readers are advised that this and all other flight-test documents and reports mentioned in this chapter are from the DFRC 

historical archives unless otherwise noted.

15. Hallion, “X-4: The Bantam Explorer,” p. 20.

16. Walter C. Williams, “Results Obtained from Second Flight of X-4 

Airplane (A.F. No. 46-676),” NACA RM L9F21 (July 18, 1949), p. 3.

17. Ibid., pp. 3–4.

18. Memorandum for Chief of Research, “X-4, January 14, 1949 to 

June 10, 1949”; Memorandum for Chief of Research, “X-4, June 

20, 1949.”

19. Memorandum for Chief of Research, “X-4, June 17 through July 1, 

1949.”

20. Memorandum for Chief of Research, “X-4, July 16 through July 29, 

1949”; Memorandum for Chief of Research, “X-4 Airplanes, July 30 

through August 12, 1949.”

21. Memorandum for Chief of Research, “X-4 Airplanes, July 31 

through August 12, 1949.”

22. Memorandum for Chief of Research, “X-4, August 27 to September 

14, 1949.”

23. Memorandum for Chief of Research, “X-4 Airplanes, October 8 to 

October 21, 1949.”

24. Ibid.

25. Ibid.

26. Memorandum for Chief of Research, “X-4, November 19 to 

December 2, 1949.”

27. Memorandum for Chief of Research, “X-4, December 3 to 

December 16, 1949.”

28. Memorandum for Chief of Research, “X-4, December 31, 1949 

to January 1, 1950”; Memorandum for Chief of Research, “X-4, 

January 14 to January 27, 1950.”

29. Memorandum for Chief of Research, “X-4, January 27 to February 

24, 1950.”

30. Ibid.

31. Memorandum for Research Airplanes Project Leader, “Acceptance of 

the X-4 Airplane.”

32. Ibid.; Peebles, The Spoken Word, p. 28.
Probing the Sky

202

33. Memorandum for Research Airplanes Project Leader, “Acceptance of 

the X-4 Airplane.”

34. Memorandum for Chief of Research, “X-4, February 24, 1950 to 

March 24, 1950”; Memorandum for Chief of Research, “X-4, May 

5 to May 19, 1950”; Memorandum for Chief of Research, “Test 

Program for X-4 Aircraft Evaluation, May 3, 1950.”

35. Memorandum for Chief of Research, “X-4, July 15 to July 28, 

1950.”

36. Memorandum for Chief of Research, “X-4, August 26 to September 

8, 1950.”

37. Memorandum for Chief of Research, “X-4, September 22 to 

October 6, 1950.”

38. Melvin Sadoff and Thomas R. Sisk, “Summary Report of Results 

Obtained During Demonstration Tests of the Northrop X-4 

Airplanes,” NACA RM A50I01 (December 13, 1950), pp. 11–12.

39. Memorandum for Chief of Research, “X-4, September 8 to 

September 22, 1950.”

40. Memorandum for Chief of Research, “X-4, October 6 to October 

20, 1950.”

41. Memorandum for Chief of Research, “X-4, November 3 to 

November 17, 1950”; Memorandum for Chief of Research, “X-4, 

November 17 to December 1, 1950.”

42. Quote from Crossfield and Blair, Always Another Dawn, pp. 41–44.

43. Memorandum for Chief of Research, “X-4, November 17 to 

December 1, 1950.”

44. Memorandum for Chief of Research, “X-4, December 15, 1950 to 

December 29, 1950.”

45. W.C. Williams and A.S. Crossfield, “Handling Qualities of HighSpeed Airplanes,” NACA RM L52A08 (January 28, 1952), p. 5.

46. Indeed, tailless aircraft in general, and the X-4 in particular, inherently had very low damping-in-pitch compared to conventional 

“tailed” aircraft. The conventional F-86 had a damping-in-pitch 

factor of +10. A 60° delta like the XF-92A or F-102 had a dampingin-pitch of +3. The X-4 had a damping-in-pitch factor less than +1, 

and as one study concluded, it “dropped to almost zero at Mach 

numbers slightly below 1.0.” See Joseph Adams Shortal, A New 

Dimension—Wallops Island Flight Test Range: The First Fifteen Years, 

NASA RP 1028 (Washington, DC: NASA, 1978), p. 278.

47. Williams and Crossfield, “Handling Qualities of High-Speed 

Airplanes,” p. 7.
Versatile Minimalist: The Northrop X-4 Bantam

203

48. Melvin Sadoff and A. Scott Crossfield, “A Flight Evaluation of the 

Stability and Control of the X-4 Swept-Wing Semitailless Airplane,” 

NACA RM H54G16 (August 30, 1954), pp. 13–14.

49. Memorandum for Chief of Research, “X-4, May 18 to June 1, 

1951”; Memorandum for Chief of Research, “X-4, June 16 to June 

29, 1951.”

50. Memorandum for Chief of Research, “X-4, June 30 to July 13, 

1951.” The concept of a blunt trailing wing edge was also tested at 

supersonic speeds using rocket-launched models launched from the 

Pilotless Aircraft Research Division over the Atlantic. The test-wing 

configuration had an NACA 0010-64 airfoil (identical to that of 

the X-4) with flat-side ailerons with a trailing-edge thickness equal 

to one-half that of the hinge line. NACA researchers H. Kurt Strass 

and Edison M. Fields analyzed the data from the rocket flights and 

found that the increased trailing-edge thickness eliminated control reversal and prevented a large and abrupt decrease in aileron 

effectiveness in the transonic speed range. The downside was that 

the aileron modifications increased wing drag at subsonic speeds. 

Above Mach 1, however, the drag increase was small. See H. Kurt 

Strass and Edison M. Fields, “Flight Investigation of the Effect 

of Thickening the Aileron Trailing Edge on Control Effectiveness 

for Sweptback Tapered Wings Having Sharp- and Round-Nose 

Sections,” NACA RM L9L19 (May 2, 1950), pp. 1–2, 5.

51. Memorandum for Chief of Research, “X-4, July 14 to July 27, 

1951”; Memorandum for Chief of Research, “X-4, July 28 to 

August 10, 1951.”

52. Memorandum for Chief of Research, “X-4, August 11 to August 25, 

1951.”

53. Memorandum for Chief of Research, “X-4, September 22 to 

October 5, 1951.” Initially, flight 47 was made for further investigation of the effect of the thickened dive brakes. Subsequently, this 

was changed to obtaining data on the X-4’s landing characteristics. 

Finally, the plan was again changed back to collecting data on the 

thickened dive brakes. The tests of the landing configurations followed the completion of the dive brake tests.

54. Memorandum for Chief of Research, “X-4, October 6 to October 

19, 1951”; Memorandum for Chief of Research, “X-4, October 20 

to November 2, 1951”; Memorandum for Chief of Research, “X-4, 

November 3 to November 16, 1951.”
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204

55. Memorandum for Chief of Research, “X-4, January 26 to February 

8, 1952”; Memorandum for Chief of Research, “X-4, February 9 to 

February 22, 1952.”

56. Memorandum for Chief of Research, “X-4, March 8 to March 21, 

1952”; Memorandum for Chief of Research, “X-4, March 22 to 

April 4, 1952.”

57. Memorandum for Chief of Research, “X-4, March 22 to 

April 4, 1952.”

58. Wendell H. Stillwell, “Results of Measurements Made During the 

Approach and Landing of Seven High-Speed Research Airplanes,” 

NACA RM H54K24 (February 4, 1955), pp. 1–2, 9–10.

59. Ibid.

60. Memorandum for Chief of Research, “X-4, April 5 to April 18, 

1952”; Memorandum for Chief of Research, “X-4, April 19 to May 

2, 1952.”

61. Memorandum for Chief of Research, “X-4, May 17 to May 30, 

1952”; Memorandum for Chief of Research, “X-4, May 31 to June 

13, 1952.”

62. Memorandum for Chief of Research, “X-4, June 28 to July 11, 

1952”; Memorandum for Chief of Research, “X-4, July 12 to July 

25, 1952.”

63. Memorandum for Chief of Research, “X-4, August 9 to August 22, 

1952”; Memorandum for Chief of Research, “X-4, September 5 to 

October 1, 1952.”

64. Memorandum for Chief of Research, “X-4, September 5 to October 

1, 1952.”

65. Memorandum for Chief of Research, “X-4, October 1 to November 

1, 1952.”

66. Memorandum for Chief of Research, “X-4, November 1 to 

December 1, 1952”; Memorandum for Chief of Research, “X-4, 

January 1, 1953 to January 31, 1953”; Memorandum for Chief of 

Research, “X-4, February 1 to February 28, 1953.”

67. Memorandum for Chief of Research, “X-4, March 1 to March 31, 

1953.”

68. Memorandum for Chief of Research, “X-4, April 1 to April 30, 

1953”; Memorandum for Chief of Research, “X-4, May 1 to May 

31, 1953.”

69. Memorandum for Chief of Research, “X-4, July 1 to July 31, 1953.” 

The monthly report included an item regarding analysis of the X-4 

flight data. It read, “Frequency response analysis of the last two data 

flights is awaiting access to automatic computing equipment here at 
Versatile Minimalist: The Northrop X-4 Bantam

205

Edwards Air Force Base.” Before this, data reduction had been done 

by women who were referred to as “computers.” It was at this point 

that the term’s meaning was changed from a human to a machine.

70. Memorandum for Chief of Research, “X-4, August 1 to August 

31, 1953”; Memorandum for Chief of Research, “X-4, September 

1 to September 30, 1953.” The August 1 to August 31, 1953, 

report indicates that NACA flights 79, 80, and 81 were pilotfamiliarization flights. However, Crossfield had made numerous X-4 

flights, and McKay made the majority of the final flights. It appears 

that the familiarization flights entry was an error and that these were 

research flights.

71. “Vought F7U Cutlass,” http://aviationtrivia.info/Vought-F7U-Cutlass.

php, and “F7U Cutlass,” http://www.absoluteastronomy.com/topics/

F7U_Cutlass, accessed October 5, 2010.

72. Peebles, The Spoken Word, p. 79.

73. Hallion and Gorn, On the Frontier: Experimental Flight at NASA 

Dryden, p. 50.

74. Sadoff and Crossfield, “A Flight Evaluation of the Stability and 

Control of the X-4 Swept-Wing Semitailless Airplane,” p. 4; 

Malcolm J. Abzug and E. Eugene Larrabee, Airplane Stability 

and Control: A History of the Technology that Made Aviation 

Possible (Cambridge, U.K.: Cambridge University Press, 1997), 

pp. 175, 311.

75. David C. Aronstein and Albert C. Piccirillo, Have Blue and the 

F-117: Evolution of the “Stealth Fighter” (Reston, VA: American 

Institute of Aeronautics and Astronautics, 1997).
206

The initial wind tunnel tests of variable sweep wings were made using an X-1 fitted with movable wings. The design proved impractical, but the results were sufficient to encourage the 

approval of the X-5. (NASA)
207

CHAPTER 6

Transformative Pioneer: 

The Bell X-5

It was like having a whole stable of swept-wing airplanes in one.

—A. Scott Crossfield1

The development of swept wings was a major step in making routine supersonic 

flight a reality.2 But while swept wings reduced the drag that occurs at transonic 

and supersonic speeds, they also introduced new problems, principally at low 

speeds and at high angles of attack, particularly during takeoff and landing. 

The wing design had to balance the conflicting demands of the two ends of the 

performance envelope. Engineers tried to capture the beneficial characteristics 

of both low- and high-speed wing shapes. This, though, could potentially 

make the aircraft difficult to fly, increasing the pilot’s workload and the risks 

that aviators faced. 

Additionally, jet air combat required multiple mission profiles during a 

single flight. A long-range strike mission might involve a long-range cruise, 

requiring maximum fuel efficiency, followed by a brief supersonic dash to bomb 

a target and escape, and, finally, a long cruise back to base. The Navy also faced 

additional problems. The high landing speeds of swept-wing aircraft would 

make already-dangerous carrier landings on narrow straight-deck carriers even 

more difficult. Tactically, swept wings posed a problem for fighters on combat 

air patrol over a naval task force. A capability for low-speed loitering was needed 

to maximize coverage. If incoming bombers were detected, fighters would have 

to accelerate to high speeds to intercept the bombers before the bombers got 

close enough to attack the carriers. 

The shape of aircraft carriers complicated matters further; the vessels were 

still being designed with straight decks, as they had been from their origins. 

Before the advent of powerful steam catapults, launching these fighters was 

difficult. Deck activities were also more difficult since the new, larger aircraft 

occupied more of the available deck space, which was limited. Of course, the 

new jet fighters approached and landed at faster speeds than had their propellerdriven antecedents, and this complicated matters on straight decks even more, 
Probing the Sky

208

to say nothing of circumstances created for pilots making a landing approach in 

the rain, at night, to an airport rolling and pitching in the middle of the ocean.

A potential solution was changing the sweep angle of the wings during 

flight. For takeoff, landing, and cruise flight, the wings’ sweep angle could 

be nearly straight. As an aircraft’s speed increased, the wings could be swept 

back to reduce drag. The aircraft’s aerodynamics could be adjusted to meet the 

demands of speed and altitude at any time during a flight. Initial research on 

this innovation had begun previously, as the X-1 was being prepared for flight. 

After the Second World War, the NACA moved rapidly to study the application of variable-sweep wings to aircraft. Early NACA studies examined both 

symmetrical wing sweeping (as with the X-5 and subsequent aircraft such 

as the F-111) and asymmetrical “oblique” wing sweeping (as later incorporated on the Ames-Dryden AD-1 research aircraft).3

 In early 1946, Charles 

J. Donlan and William C. Sleeman tested an X-1 model fitted with variablesweep wings in Langley’s 7-by-10-foot High-Speed Wind Tunnel. The wings 

could be adjusted to sweep angles of 0°, 15°, 30°, and 45°. The design assumed 

that the wings moved on a fixed pivot point on the fuselage centerline. Results 

were mixed. Tests showed that variable-sweep wings reduced the inherent 

low-speed handling problems experienced with high-speed aircraft. Engineers 

also saw limited success in controlling longitudinal stability by changing the 

sweep angles. A problem was identified regarding the pivot design, however: A 

fixed wing-pivot point was not workable. This was because both the airplane’s 

center of gravity and center of pressure shifted as the wings changed position. 

The net result was too much stability, resulting in excessive trim drag and 

limited maneuverability.4 

The wing-pivot system would have to be capable of changing the wingsweep angle and translating it forward and aft to compensate for the resulting 

changes in center of gravity and center of pressure caused by the wing-sweep 

movement. The amount of translation was also dependent on the mass distribution of the airplane and the location of the wing pivot point. The weight of 

the wings, the locations of the wing and fuselage fuel tanks, and the procedures 

for emptying the fuel tanks in flight also had to be factored into any assessments of the aircraft’s stability. This was a major issue in the view of researchers. 

In their conclusions, they wrote, “It seems unlikely that satisfactory stability 

for all flight conditions can be achieved with a variable-sweep wing without 

recourse to relative translation between the wing and the center of gravity of 

the airplane.”5
Transformative Pioneer: The Bell X-5

209

Nose view of the X-5 with the wings at the 20° sweep angle. Unlike those of later variable-sweep 

aircraft, the X-5’s mechanism moved the wings forward as the sweep angle increased. On the 

F-111, B-1, and F-14, the pivot point was in a stub wing outboard of the fuselage. This eliminated 

the need for dual motions. (NASA)

The Evolution of the X-5

Just as the X-4 was influenced by the Me 163, Bell’s X-5 design was influenced 

by an unsuccessful aircraft design that originated in Nazi Germany. This was 

the P.1101, designed by Waldemar Voigt and built by Messerschmitt’s advanced 

projects development group outside Oberammergau, Bavaria, near the end of 

the war. Unlike the Me 163, however, the P.1101 never flew. Rather, having 

been rejected by the Luftwaffe as unsuitable for further development, it had 

been retained by Messerschmitt as a possible test bed for swept wings. The 

single prototype, nearing completion, was captured in April 1945 when the 

Oberammergau development center was overrun by U.S. Army forces. Shortly 

afterwards, a U.S. technical intelligence team arrived, including Robert Woods 

of Bell Aircraft. Woods arranged to have the unfinished aircraft shipped to 

the United States for analysis and eventually turned over to Bell Aircraft in 

August 1948. 
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210

As originally conceived, the wing-sweep angle of the P.1101 could be 

adjusted on the ground to three fixed settings of 35°, 40°, and 45°, and Bell 

engineers originally suggested to the Army Air Forces that the P.1101 be used 

in similar fashion, recommending that it be repaired and fitted with a U.S. 

turbojet engine and then test-flown. A potential next step would have been 

to modify the P.1101 wings to allow the sweep range to increase from 20° to 

50°, again at three fixed positions. But Bell’s P.1101 advocates also suggested 

redesigning the P.1101 so that its sweep angle could be changed in flight. This 

capability would be useful in an operational fighter, as the wings could be fully 

extended for takeoff and landing, then swept back to achieve high-speed flight. 

Another Bell proposal was to use the P.1101 as an engine test bed. The jet 

engine was attached beneath the aircraft, at its center of gravity. This made engine 

changes relatively easy. Among the engines proposed for tests on the P.1101 were 

the J34, J35, J46, and J47. Another possibility was a 36-inch-diameter ramjet. 

Finally, a 4,000-pound liquid-fuel rocket also could be tested, by placing a fairing 

over the nose inlet and fitting the aircraft with new fuel tanks. 

In the end, use of the P.1101 itself as a research aircraft proved impractical. 

The aircraft was too small, major technical problems existed with the proposals, and the aircraft had been damaged when it was dropped during delivery 

to Bell Aircraft. Rather, Woods and Bell engineers concluded that designing 

a new aircraft based upon its basic configuration would be preferable. The 

proposed model would be similar in design configuration to the P.1101 but 

larger and more advanced. Bell also hoped that the resulting research plane 

could be modified into a lightweight production jet fighter. (Eventually, the 

X-5 would be proposed as a NATO export fighter but rejected on grounds 

remarkably similar to those given by the Luftwaffe in its rejection of the earlier 

P.1101.) Given the collapse in aircraft orders following the end of World War 

II, however, such approval was unlikely.6

Bell’s Robert Wood and his design engineers began work on the new aircraft. 

A major problem was the development of a practical wing-sweep mechanism. 

The sweep angle could be varied from 20.25° to 58.7°. Limit switches prevented 

the wings from exceeding these angles, which would cause interference between 

the wing root and fuselage. As the wings swept back and forward, the X-5’s 

center of gravity and center of pressure shifted. These characteristics had to be 

kept in close proximity to prevent stability problems. 

The solution was complex. As the wings’ sweep angle increased (reducing 

the wingspan and aspect ratio), the X-5’s center of gravity moved aft. To counter this, the complete wing-root assembly moved forward along a track inside 

the fuselage. When the sweep angle was reduced (increasing the wingspan 

and aspect ratio), the wing assembly moved backward as the wingtips moved 

forward. The net result was to keep both the center of gravity and center of 
Transformative Pioneer: The Bell X-5

211

Nose view of the X-5 with the wings swept back to 60°. The aircraft was a low-speed demonstrator for variable-swept wings, as it lacked the supersonic-speed capacity necessary to take 

full advantage of the maximum sweep angle. (NASA)

pressure within limits, no matter the sweep angle. As the wing’s sweep angle 

changed, so did the wing characteristics. At a 20° wing sweep, the X-5 had a 

wing area of 184.3 square feet and an aspect ratio of 6.09. With the wings swept 

back to the maximum 58.7° angle, these values were reduced to a 166.9-squarefoot area and 2.16 aspect ratio.7

Each wing also required a special fairing “glove” where the leading edge of 

the wing met the fuselage. This was designed to be movable to ensure that the 

wing had a smooth airfoil whatever the sweep angle. The wings’ leading edges 

also had slats, which could be extended to increase lift and reduce stall speed. 

One unusual feature was the speed brakes. Normally, these are located on the 

aft fuselage, as on the D-558-1. The X-5 had its speed brakes on the nose, just 

behind the inlet. This proved to be a very poor location, however, and thus the 

brakes proved generally ineffective. 

As on the P.1101, the X-5’s engine, an Allison J35-A-17A axial flow jet 

engine, was slung under the fuselage. The engine produced 4,900 pounds of 

thrust. Unlike the exotic flush inlets on the D-558-2 or the wing inlets of the 
Probing the Sky

212

X-4, the X-5 had a simple, straight nose inlet that ran directly to the front of the 

engine. “This design,” a Bell Aircraft press release noted, “holds air-duct losses 

to a minimum, scooping greater quantities of air at high altitudes where the 

decreased oxygen content of the atmosphere lowers jet engine performance.”8

Like the X-3 and X-4, the X-5 had an emergency ejection seat. The long main 

landing gear struts retracted into the fuselage above the engine, and the nose 

gear retracted into the nose, under the air inlet tunnel.

Unlike the other early X-planes, the X-5 was not particularly sleek, having 

a configuration more akin to then-contemporary transonic jet fighters than 

exotic supersonic research airplanes. Bell described it as having a “flying guppy” 

configuration, with an underslung engine, fat forward fuselage, and boom tail. 

The X-5 was 33.6 feet long and stood 12.2 feet tall from the ground to the tip 

of the fin. Its wing employed an NACA 64(10)A011 airfoil at the wing pivot 

point, changing to an NACA 64(08)A008.28 section at the wingtip. Maximum 

wingspan was 32.75 feet; with the wings fully swept back, the wingspan was 

just 19.4 feet. The aircraft weighed 10,006 pounds fully loaded and 7,894 

pounds without fuel.9

Bell Aircraft made a formal proposal to the Air Force for building a pair 

of X-5 research aircraft on February 1, 1949. (The first aircraft was to be 

operated by the NACA, the second by the Air Force.) The response was rapid; 

just 3 days later, Air Force Headquarters directed that a contract be written 

and signed. The “X-5” was to be used to demonstrate the best sweep angle for 

lightweight interceptors and the tactical advantages of variable-sweep wings. 

But the USAF had no interest at that time in funding an operational aircraft, 

and, indeed, variable sweep would not appear on an Air Force combat aircraft 

for another 15 years.10

The contract signing initiated a series of reviews of Bell’s design. Engineers 

from the Air Force Power Plant Laboratory issued their report on May 9, 

1949. They objected to the placement of the fuel tank directly over the 

jet engine and requested a redesign. This would have meant designing an 

entirely new aircraft, delaying the project significantly. And so the Power 

Plant Laboratory engineers withdrew their objections, instead agreeing on 

August 5 to approve the existing configuration. (However, they asked that 

shielding of the fuel tank be improved, which it was.) They stressed that their 

approval applied only in the case of the X-5’s use as a research aircraft and 

not as a production fighter.

These were not the only doubts about the X-5 design. Engineers at the 

Wright Field Aircraft Laboratory in Ohio issued a critical report on August 

29, 1949, in which they urged numerous design changes to make the aircraft 

both more airworthy and compliant with Air Force design specifications. 

More serious was the engineers’ questioning of the project’s value. The 20° 
Transformative Pioneer: The Bell X-5

213

minimum angle was judged to be too low to be of any real use. More substantive, it seemed, was that the X-5 data would duplicate data from aircraft 

either already available or under construction. The engineers concluded, “In 

other words, the X-5 does not fill any particular gap in either high-speed or 

sweep-back research.”11 What was missed in this was that the greatest value 

of the X-5 would be in simply demonstrating the act of in-flight variable 

wing sweep itself.12 The NACA, in contrast, strongly supported the X-5’s 

development. NACA researchers had decided by mid-1948 that the aircraft 

would complement their earlier wind tunnel work. Following Bell’s initial 

proposal that a variable-sweep research aircraft be designed and built, a team 

from Langley went to Wright Field to argue for Air Force approval. This did 

much to win needed military support for the project at a time when the 

service faced many competing demands for scarce funding. 

Contractor and Air Force Proving Flights

Ground tests with the first X-5 (AF Serial 50-1838) began on February 15, 

1951, at Bell’s Niagara Falls facility. The next several months entailed aircraft 

system testing, load and structural testing, and taxi runs. The Air Force also 

completed engineering inspections. On June 9, 1951, the disassembled aircraft 

was crated and loaded onto a Fairchild C-119 cargo plane for the long flight to 

Edwards. The test flights and research plan for the pair of X-5s were similar to 

those of earlier X-planes. Initial demonstration flights would be made by Jean 

L. “Skip” Ziegler, Bell’s chief test pilot. Once the first X-5 had been proven 

airworthy, Air Force familiarization flights would follow. With these complete, 

the first X-5 would then be turned over to the NACA, which would begin 

research flights. The second X-5 (AF Serial 50-1839) would be flown by Air 

Force pilots in a separate test effort. 

Ziegler took the first X-5 aloft on June 20, 1951. After takeoff, he climbed 

to 15,000 feet, using reduced power to keep his speed below the wheels-down 

limit. Once at altitude, he tried out the X-5’s control response and found it 

to be stiff. He reached a top speed of Mach 0.56 and a maximum altitude of 

15,974 feet. So far, the flight had been a success, and Ziegler began to descend. 

At that point, he discovered the fuel cells were showing negative pressure. 

This meant their internal pressure was lower than that of outside air. If the 

reading was valid, as the X-5 descended the increasing outside air pressure could 

cause the fuel cells to rupture. Fuel would then spill onto the engine, triggering 

a fire. Ziegler made maneuvers to determine whether the reading was correct, 

and after several minutes he determined that the fuel-cell pressure reading was 

false. He landed several minutes later without incident. 
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214

The fuel-cell pressure gauge error and several other problems had to be 

addressed, and the aircraft was grounded for 5 days. The June 25 flight went 

off but yielded no data. The next two flights, on June 27 and 28, reached Mach 

0.638 and 0.696. Zeigler undertook the first wing-sweep test on July 27. This 

involved only a partial movement, rather than the full range of 20.25° to 58.7°. 

The Bell contractor tests continued into late August 1951, with nine flights made. 

Brigadier General Albert Boyd, the Air Force Flight Test Center commander, received permission from the Wright Air Development Center to 

make an evaluation flight in the X-5 on August 23. General Boyd took off, 

climbed to 40,000 feet, and moved the wings back to their full 58.7° sweep 

angle. He then descended to 30,000 feet, reached a speed of Mach 0.92, and 

made several more test maneuvers. Boyd’s total flight time was 28 minutes. 

The NACA progress report on the X-5 stated that the primary goal of Boyd’s 

flight was to demonstrate a landing with a 40° sweepback. Data were obtained 

on three landing configurations:

• Landing at alternate gross weight, slats out, and with wings at a 

20° sweep

• Landing with slats out and wings at a 20° sweep

• Landing with flaps at 35°, slats out, and at a sweepback of 40°

The first two landings were made without incident, but during a landing 

approach at 40°, Boyd accidently raised the flaps by striking the flap switch. 

The X-5 experienced a stall roll-off, striking the ground on its nose and right 

main wheels before Boyd could recover. The impact caused minor damage to 

the tire tread. No structural damage was listed.13

The Bell test effort with the first X-5 ended on October 8, 1951, when 

Ziegler made his final flight in the NACA aircraft. The first X-5 was grounded 

pending the installation of NACA instrumentation. The initial payload measured the following quantities:

• Vertical, longitudinal, and transverse acceleration

• Sensitive longitudinal acceleration

• Rolling angular velocity

• Pitching angular velocity and acceleration

• Yawing angular velocity and acceleration

• Airspeed and altitude

• Angle of sideslip and angle of attack

• Control positions

• Wing-sweep angle

• Elevator and aileron stick forces14

Along with instrument installation, the X-5 was inspected. The fuel cells 

were replaced, and functional checks of them were made. These preparations 

were expected to be completed by the week of December 17, 1951. Once the 
Transformative Pioneer: The Bell X-5

215

work was complete, Air Force demonstration flights could begin. These were to 

be made by Lieutenant Colonel Frank K. “Pete” Everest.15

Everest’s first two flights were made on December 20. The first was for pilot 

familiarization, and no data were collected. A second flight the same day reached 

Mach 0.844 and 22,380 feet. Everest made two more flights on December 

21 and 27. His final pair of flights came on January 7 and 8, 1952. The tests 

involved clean stalls with wing sweeps of 20°, 40°, and 60° and multiple accelerated turns at the three sweep settings of clean, slats out and flaps down, and 

flaps down. Thomas W. Finch, NACA X-5 project manager, summarized the 

initial results of the flights in two research memorandums. In the first, he said:

At low speed for 20° sweepback an increase in longitudinal stability 

occurred as maximum lift was approached. At 40° and 60° sweep a 

reduction in stability occurred at the higher lift coefficients. During 

the tests at 30,000 feet to investigate the effects of lift coefficients 

at Mach number of approximately 0.8—for the 20° sweep, there 

occurred a reduction in longitudinal stability at lift coefficient of 

approximately 0.4. No change in stability was noticed at 40° sweep 

up to lift coefficient of 0.5 (the limit of tests to date). For 60° sweep 

an abrupt pitch-up occurred at a lift coefficient of 0.6. Stick forces 

were of the order of 20 and 35 lbs/g at the high Mach numbers. 

Structural integrity demonstration required 5.86 g at Mach of 

0.8 at 12,000 feet which corresponds to a lift coefficient of approximately 0.5. Pull-ups made at 20° and 60° sweep at this speed and 

altitude were limited to load factors less than 4gs due to the high 

stick forces.16

And in the second, he said:

There was a slight increase in stability at 20° sweep from M = 0.75 

to M = 0.79, and the stability at 60° sweep remained essentially 

the same for a Mach number range of 0.7 to 0.8. At 40° sweep 

there is an appreciable increase in stability at M = 0.81 above that 

for M = 0.71 and M = 0.76. During the tests to investigate the 

effects of lift coefficients up to M ≈ 0.8, instability was found for 

the following conditions: (1) for 40° sweep there was a decrease in 

stability at M ≈ 0.75 and at M ≈ 0.81 a mild pitch up occurred at 

a CAn [airplane normal-force coefficient] of 0.5. (2) for 60° sweep 

an abrupt pitch up occurred at a CAn of 0.6 and M ≈ 0.8.

The only indication of instability occurred at 20° sweep and 

20,800 feet with a decrease in stability above a CAn [of] 0.4.
Probing the Sky

216

A load factor of 5.86g could not be demonstrated at M = 0.8 

and 12,000 feet because of high stick forces. At lower Mach numbers of 0.7 and 0.75 the stick forces were essentially the same, 

and the lift coefficients for instability were being approached or 

exceeded. In view of these results it was decided to accept the airplane since it became apparent that the longitudinal stability evaluation required a more complete analysis, and the flight testing 

was entering into the research program planned for the airplane.17

NACA X-5 Research Flights Begin

With the X-5 officially transferred to the High-Speed Flight Research Station, 

Joe Walker was selected as the initial NACA X-5 pilot. He made his checkout 

flight on January 9, 1952. Walker’s first research flight came on January 14, 

1952, and dealt with static and dynamic longitudinal and lateral stability and 

control. Ultimately, the bulk of the flights in the X-5 focused on this type of 

research. Walker was kept busy flying the X-5, completing 17 flights between 

January 9 and April 1. Though most of the flights were for stability and control, 

three were for airspeed calibration and another was for a gust loads investigation 

at 20° and 60° sweep angles. Finally, on February 12, Air Force pilot J.C. Meyer 

had a check flight in the NACA’s X-5. (This was the only flight not made by 

Walker during this period.)

Not everything went smoothly. The March 13 flight was aborted after takeoff, and Walker’s first flight on March 17 was aborted as well due to “poor voltage, no data.”18 The problem was corrected, and a second flight that day was 

made for lateral and longitudinal stability and control data at a 60° sweep angle. 

The test data from the Bell acceptance flights had been analyzed, and a 

research memorandum emerged. The report offered a more detailed look at 

the data than had initial observations in Finch’s progress reports. The memorandum on the X-5’s horizontal-tail load measurements noted:

During sweep changes at an altitude of 20,000 feet, the trends 

of the balancing tail loads variation with sweep angle at Mach 

numbers of 0.50, 0.56, and 0.85 were similar with the larger 

down tail load occurring at approximately 36° sweep angle at each 

Mach number tested. The largest balancing tail load in a down 

direction over the entire sweep range from 20° to 59° occurred at 

a Mach number of 0.85. 
Transformative Pioneer: The Bell X-5

217

During level-flight-trim points at 20° sweepback at an altitude of about 20,000 feet and an airplane weight of about 8,800 

pounds, the tail load increased in a down direction as the Mach 

number was increased from 0.54 to 0.84.

During pull-ups at a Mach number of about 0.83 and at 

sweep angles of 20°, 45°, and 59°, the wing-fuselage combination 

was stable at 20° and 45° sweepback and unstable at 59° sweepback at normal-force coefficients less than 0.3. At normal-force 

coefficients near 0.3 the stability of the wing-fuselage combination changed for these sweep angles, becoming unstable at a sweep 

angle of 20°, experiencing a reduction in stability at 45°, and 

became stable at 59° as the normal-force coefficient increased. 

For the normal-force-coefficients range covered in these tests the 

aerodynamic center of the wing-fuselage combination moved 

rearward as the sweep angle was increased from 20° to 59°. A larger 

change in the wing-fuselage aerodynamic center was experienced 

with the sweep for the high normal-force-coefficient range than 

was experienced for the low normal-force-coefficient range.19

In April 1952, A. Scott Crossfield and Walter P. Jones began making X-5 

flights. Crossfield’s check flights came on April 2, Jones’s on April 29. Crossfield 

had already talked with Walker about the X-5’s handling. Crossfield recalled 

later, “The X-5 handled in the air like a three-wheeled automobile. It was loose 

and danced crazily. Even so, we thought it would make a fine research tool.”20

The X-5’s most serious shortcoming was its spin characteristics. Both Walker 

and Crossfield would experience these in full fury.

Crossfield’s flight plan called for making several accelerated stalls. After 

reaching 25,000 feet, he pulled back on the throttle and the stick. He wrote later: 

As the X-5 slowed, she began to buffet. Suddenly her nose veered 

sharply to the left. In a split second, the X-5 turned 180 degrees. 

Then she dropped precipitously into a spin.

A kaleidoscope of brown desert, blue sky, and white clouds 

passed dizzily in review in my windshield as the X-5 wound up 

steadily towards the desert floor. I pressed the stick hard to forward left and bent on full right rudder—the prescribed spin recovery maneuver—but the X-5 stubbornly refused to conform. After 

a drop of over 10,000 feet, the X-5 pulled out.21

Walker had experienced the X-5’s spin behavior the month before; he had 

entered a spin at 40,000 feet and had not recovered until reaching 20,000 
Probing the Sky

218

feet. He described the experience in his pilot report: “As the airplane pitches, 

it yaws to the right and causes the airplane to roll to the right. At this stage 

aileron reversal occurs; the stick jerks to the right and kicks back and forth from 

neutral to full right deflection if not restrained. It seems that the airplane goes 

longitudinally, directionally, and laterally unstable in that order.”22

During the spring and summer of 1952, pilots flew the X-5 research plane for 

longitudinal control effectiveness, static lateral stability and control characteristics, and dynamic longitudinal and lateral stability and control data. Between 

May 3 and May 16, four research flights were made, three by Crossfield and 

one by Jones. Maneuvers included speed runs, aileron rolls, sideslips, elevator 

and rudder pulses, and stall approaches. The bulk of the maneuvers were flown 

with the wings at a 60° sweep angle. Speeds ranged from Mach 0.47 up to 

0.94, altitudes from 36,000 to 42,000 feet. The initial results were as follows:

Preliminary evaluation of the records and pilot’s notes indicated 

that all speed runs were easily controllable with elevator. The nose 

down trim change reported in the speed run...occurred about 

0.93 which is in agreement with previously reported data.

The aileron effectiveness is low and approximately constant 

up to M = 0.9. The rudder effectiveness determined from sideslip data is noticeably improved below M = 0.70 as compared 

to an approximate constant effectiveness between M = 0.70 and 

M = 0.90. Almost total aileron control available is required for 

sideslip angles of about 5° at Mach numbers below 0.70.

The values of the period and time-to-damp of the longitudinal 

and lateral oscillations obtained from pulses below M = 0.90 were 

in agreement with previously reported data. Data have not yet 

been reduced from maneuvers performed above M = 0.9.23

Crossfield and Jones continued to make longitudinal and lateral stability 

data flights into late June 1952. Walker made his next flight on June 25, 1952. 

These flights continued static and dynamic longitudinal and lateral stability 

data collection. Walker made flights on July 2 and 10. The July 2 flight and 

most of the July 10 flight collected additional static longitudinal and lateral 

stability data at 59° wing-sweep angles. These involved speed runs, pushovers 

and pull-ups, aileron rolls, and gradually increasing sideslips. 

Walker’s July 10 flight also generated data on lift and drag at a 20° sweep 

angle. He made speed runs as well as pushovers and pull-ups with the wings at 

a 20° sweep. The pushover was made to a g-meter reading of 0 g, followed by a 

pull-up into the longitudinal instability boundary. These runs were made to 

obtain accelerated maneuver data starting from zero lift. During this flight, 
Transformative Pioneer: The Bell X-5

219

Walker also made fast-rate stabilizer pull-ups into the instability region while 

flying in a speed range of Mach 0.70 to 0.96. He performed these maneuvers 

to get baseline data, which would be compared to data collected following 

the installation of modified wing-root fillets. 

By July 25, Walker had made another three research flights, with all the tests 

conducted at a 59° sweep angle. As on his earlier flights, he performed gradually increasing sideslips, pushovers and pull-ups, and a speed run. One of the 

flights was made with the modified wing-root fillets. A wind tunnel test with 

a model that resembled the X-5 indicated that changing the wing-root shape 

would prevent the reduction of longitudinal stability that characterized the X-5. 

For these tests, stabilizer pull-ups were made from Mach 0.70 to 0.965, in 

a 2° dive from 40,000 to 38,000 feet. Mindful of how fast the aircraft could 

lose altitude in a spin, pushovers and pull-ups were made at Mach 0.70 to 

0.84 and 40,000 feet, pull-ups at Mach 0.965 and 38,000 feet. The results 

were summarized as follows:

Static longitudinal instability was encountered on all pull-ups 

both in the original configuration and with modified wing root 

fillets. The pilot reported increased stability at lower normal-force 

coefficients with the modified wing root fillets at the higher Mach 

numbers tested while the instability seemed to occur at slightly 

higher load factors.

Data from the speed run performed in a 20° degree dive 

has not been reduced but the pilot’s Machmeter reading of 0.93 

indicates a Mach number greater than 0.98 has been reached. 

The speed run was easily controlled with elevator with the pilot 

reporting maximum stick forces of about 10 pounds push at the 

trim change and about 15 to 20 pounds pull at the maximum 

Mach number.24

Walker expanded the longitudinal and lateral stability and control flights 

in late July and early August 1952. To extend the speed range of the X-5, 

shallow dives were made to exceed Mach 1. He began the dives at angles 

of 20° and 30°, starting at 45,000 feet, and pulled out at 38,000 feet at a 

wing-sweep angle of 59°. The test maneuvers were much the same as those 

on earlier flights: aileron rolls, gradually increasing sideslips, pushovers and 

pull-ups, and stabilizer pull-ups. (The latter were done at both 59° and 20° 

wing-sweep angles.) During the July 25 flight, Walker reached a maximum 

speed of Mach 0.986. He achieved Mach 1.006 on both the August 1 and 7 

flights. The preliminary results of the three flights were summarized in the 

X-5 progress report:
Probing the Sky

220

Preliminary inspection of the records and pilots comments indicate that near M = 1.0 the maneuvers were performed without 

any difficulties. About 35 pounds aileron force was required for 

a full deflection roll and about two-thirds the available aileron 

control was required in sideslips to β ≈ 1.5°.

The dives were easily controlled with elevator with a maximum push force of about 60 pounds being encountered in the 

dive at...–3°. Elevator forces up to about 180 pounds was required 

in the pullout from the dives near M = 1.0.

Longitudinal instability was encountered in all pull-ups. The 

trend of the instability boundary above M = 0.96 will not be 

known until the data has been reduced from the pullouts.25

Additional research flights were made in September at the 59° sweep angle 

before being halted. The X-5 did not fly again until October 21, with Walker 

at the controls. Pilots of the era would likely have described Walker’s experience 

that day as “eventful.” The progress report noted:

At 59° sweepback a 30° dive was made to M = 1.04 with a stabilizer setting of –2.5°. Trim force for 1g was about 45 pounds 

push at maximum speed. An elevator pull-up to about 2g was 

completed above M = 1.0 without encountering any reduction 

in stability. A stabilizer pull-up was started above M = 1.0 with 

a reduction of stability being encountered at better than 2g as 

the Mach number decreased. The remainder of the flight was to 

be composed of pushovers to 0g followed by pull-ups through 

the reduction of stability boundary starting from the trimmed 

stabilizer deflection. However, a violent spin occurred following 

the reduction of stability during a pull-up at a Mach number near 

0.7. As the airplane yawed to the right following the reduction 

in longitudinal stability it abruptly snap-rolled and went into a 

spin at an altitude of about 44,000 feet. There was a high rate 

of roll (greater than 3 radians/sec) about the longitudinal axis. 

The elevator hinge moments caused the elevator to float up; the 

forces were too high for the pilot to apply full down elevator. The 

recovery was not affected until the stabilizer control had been in 

the full leading-edge up position (+4.4°) for several seconds with 

ailerons and rudder opposing the spinning direction. Recovery 

was completed at an altitude of about 37,000 feet.26
Transformative Pioneer: The Bell X-5

221

Following the spin, mechanics inspected the aircraft and the flight data 

were analyzed for any indication that the aircraft had been overstressed. The 

progress report noted:

Strain gage data indicated that compression loads imposed on 

the sweep mechanism jackscrews approached limit load. It may 

be necessary to examine and test the various components of the 

sweep mechanism. The engine was removed for inspection and 

cracks were found in the nozzle diaphragm as well as evidence 

of rubbing between the turbine wheel and the diaphragm. The 

engine will be replaced as soon as the oil tank can be modified to 

accommodate a –B engine.

A “g” limitation that has existed for the X-5 wing is being 

lifted by replacement of a number of rivets on each wing panel 

aft of the pivot point by high sheer bolts. The present limitation 

of 5.65g will be then lifted to 7.33g.27

Given the complexity of the inspection and the resulting repair work, it is 

not surprising that the X-5 was grounded until December 5, 1952. The aircraft 

made a total of seven flights in December, another five in January 1953. These 

flights, unlike those made earlier in the year, had a wider range of goals. Only 

three of the December flights dealt with longitudinal- and lateral-stability and 

control data; two were checkout flights for Stanley P. Butchart; another was a 

photographic flight; and one was aborted after takeoff due to an inoperative 

stabilizer motor. The January 1953 flights involved two made for vertical-tail 

load measurements during maneuvers, another for stalls and maneuvers at a 

20° wing sweep, and the check flight of Major Arthur A. “Kit” Murray. The 

last was made to compare the behavior of the first X-5 with that of the second, 

which was being flown by the Air Force.28

The other two flights in January and the first flight in February all had an 

unusual pair of goals: “investigation of flight at speeds below minimum drag of 

the test airplane and testing the controllability of a highly swept-wing aircraft 

in the wingtip wake of a straight-wing aircraft.”29 The tests originated with a 

verbal request made by Wright-Patterson engineers to Walter Williams during a 

visit to Edwards. The data they sought would be used in the design of a tanker 

capable of refueling an interceptor aircraft in flight. Originally, the delta-wing 

XF-92A was suggested as the test aircraft. Analysis indicated that preliminary 

results could be obtained at a much earlier date using the X-5 flying with a 59° 

sweepback behind an F-80 and a B-29 bomber. 
Probing the Sky

222

Multiple exposures showing the full range of travel of the X-5 wings. The sweep angle could be 

adjusted in flight depending on test requirements. One unusual effort undertaken with the aircraft involved the X-5 flying behind a B-29 in a test of how a swept-wing aircraft would handle 

while taking on fuel from a straight wing/propeller-powered tanker. (NASA)

Walker made the research flights. The first, on January 27, 1953, was flown 

with the X-5 trailing an F-80 Shooting Star, the first production U.S. jet fighter. 

The X-5’s wing sweep was 59° at an altitude of 25,000 feet. The first part of 

the formation tests involved stabilized test points recorded at intervals from 

290 to 190 mph. A second set was done in a continuingly decreasing speed 

run between 290 and 190 mph. Finally, a continuously increasing speed run 

was made from 190 to 290 mph. In the X-5, Walker took a position 10 to 15 

feet behind the F-80’s wingtip, which he used as a reference point. His pilot 

report on the January 27 flight noted:

Accurate position could be maintained at any of the test speeds 

and during the accelerating and decelerating runs once the position was established. Getting into position was difficult at these 

test speeds. Throttle control of speed was effective only for minor 
Transformative Pioneer: The Bell X-5

223

position changes once position was established. At other times, 

except at high speed, power change was so slow that speed changes 

were difficult and slow. A fast acting effective speed brake combined with a control switch on the throttle is the only practical means of speed control for join up. If the speed brakes were 

capable of variable positioning, throttle regulation would be made 

easier in case of two dissimilar aircraft attempting to maintain an 

accurate position relative to each other.

The variations in power required to maintain position at different speeds was most apparent during the deceleration and acceleration speed runs. At minimum speed of 190 mph there was still 

ample reserve power. The power required had not started increasing rapidly down to 190 mph. It was relatively less difficult to 

maintain position at 190 mph than at 250 mph because of larger 

airplane drag changes with slight changes of angle of attack.30

The other two flights, on January 29 and February 6, 1953, were with a 

B-29 at 40,000 feet. The X-5’s wing sweep was again 59°, with the stabilizer 

incidence at –2°. Level formation runs were made with the B-29 at speeds of 

230, 220, 210, 200, and 190 mph. Walker commented later that he experienced “less difficulty” maintaining position with the B-29 than with the F-80, 

as the bomber “didn’t bounce around as much,” adding, “It was not difficult to 

maintain fore and aft and vertical position. There was plenty of elevator and 

rudder control. However, lateral control with the ailerons gradually deteriorated as speed was reduced.”31 A problem appeared at 190 mph, when Walker 

extended the X-5’s wing slats. He later wrote, “This configuration is not recommended. As soon as slat extension started, extreme sensitivity in roll developed. 

The airplane gained nose up trim. The ailerons and elevator became noticeably 

stiffer. It appeared that the static longitudinal stability stick-fixed and stick-free 

decreased markedly. The result was an all out effort to maintain the wings level 

and increased difficulty maintaining vertical position.... It should be possible to 

reduce speed below 190 mph in the clean configuration without encountering 

as much control difficulty as with the slates extended at 190 mph.”32

Walker flew the third and final formation test on February 6, 1953, again 

trailing a B-29. The flight went smoothly, with Walker’s postflight report noting:

Because the airplane [X-5] buffeted at 176 mph and the stability 

decreased at that speed, 180 mph was set as the minimum speed 

for tests, clean configuration, of ability to fly formation below 

minimum drag. A record was obtained while getting into position 

on the lead airplane which was flying at 190 mph. No particular 
Probing the Sky

224

difficulties were encountered which would not be present at any 

speed during the join up.

The only noticeable handicap to maintaining position at 180 

mph was the small amount of control remaining and large deflection required. This was minimized for the X-5 because of the 

component of thrust acting as an additional longitudinal control.

It actually appeared easier to maintain position at 180 mph 

than at 240 mph because of greater drag at trim and larger drag 

changes with small angles of attack changes. It was found, for 

example, that a slight tendency to overshoot could be quickly 

stopped merely by raising the nose of the airplane slightly. 

Based upon observations during these flights, for the purposes of aerial refueling it would be feasible to fly below the speed 

for minimum drag of a highly swept wing airplane and may be 

more easily done than at the speed for minimum drag, providing 

reserve power is available for maneuvering.33

Initial X-5 Research Results

As NACA research flights with the X-5 began, the flight data were being 

simultaneously analyzed. This was a complex process that began with data 

reduction and was followed by the writing of a rough draft, a review by 

High-Speed Flight Research Station staff, and further reviews and comments 

from other NACA laboratories. Not until early 1953 were the initial research 

memorandums published. 

The first of these dealt with flight measurements of the X-5’s stability characteristics in sideslips with the wings at a 59° sweep angle. The flight conditions were at Mach 0.62 to 0.97 between 35,000 and 40,000 feet. The results 

indicated the following:

Throughout the Mach number range the apparent directional 

stability is positive. It is constant from a Mach number of 0.62 

to 0.90 and increases to a value about 60 percent higher as the 

Mach number increases to 0.97.

The apparent effective dihedral is positive and high, doubling 

in value between Mach numbers of 0.75 and 0.97. The dihedral 

is so high at the highest Mach number that the ailerons can only 

trim out about 2.5° of sideslip and can be easily overpowered by 

the rudder.
Transformative Pioneer: The Bell X-5

225

The cross-wind force coefficient per degree of sideslip is stable 

and constant to a Mach number of 0.94, above which it increases 

rapidly to a Mach number of 0.97.

There is little or no change in pitching moment due to sideslip.34

Another research memorandum looked at the lift and drag measurements 

at different sweep angles, including a comparison of the lift and drag at 59° 

and 20° sweep angle configurations. The data indicated the following:

At an altitude of about 42,000 feet the drag force for the 20°

sweep configuration in unaccelerated flight was considerably 

less than that for the 59° sweep configuration for flight speeds 

below a Mach number of 0.81; above a Mach number of 0.82 

the reverse is true.

At an altitude of 42,000 feet and a Mach number of 0.74 the 

total drag force for the 59° configuration was more than twice 

that for the 20° configuration at any given lift coefficient below 

maximum lift.35

Other important research was devoted to determining the X-5’s buffeting 

characteristics, as these produced bending stresses and moments, shear loads, 

and torque on the aircraft. Buffeting also affected controllability. Flight measurements of the X-5’s buffeting were made at a 58.7° sweep angle, at a speed 

range of between Mach 0.65 and 1.03, and at altitudes between 37,000 and 

43,000 feet. Maximum airplane normal-force coefficients were reached for 

Mach numbers up to 0.96. The research memorandum’s concluding remarks 

on the X-5’s buffeting read: 

At all airplane normal-force coefficients the horizontal tail was 

found to experience buffeting.... At angles of attack below maximum airplane normal-force coefficient the peak buffet-induced 

tail-bending stresses were 20 percent of the maximum observed 

tail steady-state bending stress; the peak buffet-induced tail shear 

loads were approximately 5 percent of the tail design limit load. 

Wing buffeting began at moderate angles of attack but above 

the wing buffet boundary there was no appreciable increase in 

wing buffet intensity with increase in Mach number or angle of 

attack. At angles of attack below maximum airplane normalforce coefficient, the peak buffet-induced wing bending stresses 

were 5 percent of the maximum observed wing steady-state 

bending stress.
Probing the Sky

226

Coefficients of incremental normal acceleration greater than 

±0.05, considered to be high-intensity buffeting on other research 

airplanes, were not experienced by the X-5 airplane below normalforce coefficient. The pilot considered the buffeting to be “unobjectionable” throughout the entire test region.36

Though most of the research was focused on the X-5’s basic aerodynamics, 

stability and control, along with the effects of the swept wings, changing the 

shape of the wing roots to improve stability was also examined. This effort 

illustrated the value of actual flight testing as a means of verifying wind tunnel 

results. Replacing the original 52.5° sweepback leading edges with rounded 

leading-edge fillets showed improved stability in the wind tunnel, but flight 

tests produced different results. The research memorandum noted:

A comparison was made between two configurations of the Bell 

X-5 research airplane at 59° sweepback, one with the original 

wing-root fillets and the other with the wing-root fillets shown by 

low-speed wind-tunnel to eliminate the loss of stability at highlift coefficients. The data obtained from the flight investigations, 

however, show that the longitudinal stability characteristics, as 

well as the buffet and drag characteristics, were essentially unaffected by the modification.37

Dark Day: The Loss of Ray Popson and the Second X-5

While NACA research pilots were flying missions in the first X-5, Air Force test 

pilots were conducting their own flight tests with the second aircraft. As the first 

X-5 underwent contractor flights in 1951, the second X-5 (AF serial number 

50-1839), meant for use in the Air Force test effort, was under construction. It 

was completed in early October and arrived at Edwards aboard a C-119 cargo 

plane on October 9, 1951. It was test-flown by Ziegler on December 10 and 

delivered to the Air Force 8 days later. The Air Force test effort was an extension of Bell Phase I testing and involved stall testing with wing slats closed. 

Subsequently, the X-5 would make takeoffs with slats closed. The goal was to 

determine whether they could be closed permanently. Other tests involved the 

nose-mounted speed brakes. The X-5 had experienced severe buffeting during 

the Bell flights at speeds at which the aircraft were to be used. The Air Force 

wanted more data on the cause.38
Transformative Pioneer: The Bell X-5

227

On October 13, 1953, Major Raymond A. “Ray” Popson took off from 

Edwards for a familiarization flight in the second X-5. Such flights were 

common Air Force practice for test pilots at Edwards. Popson was an experienced and skilled test pilot. Major Arthur A. “Kit” Murray was the chase 

pilot, flying an F-86. The takeoff and climb to 40,000 feet were uneventful. 

Popson then began a series of stalls with the wing sweep at 20°, 40°, and 60°, 

as well as level flight accelerations. These maneuvers were followed by a dive 

that incorporated a 20° sweep angle and pull-up. During the pull-up, Popson 

swept the wings back to 60°. He then entered a dive, reaching a speed of Mach 

0.96 and leveling off at 35,000 feet. 

The first indication of trouble appeared at that point. Popson radioed 

Murray that the X-5 had lost oxygen pressure. Murray answered that Popson 

should immediately descend, which he did. During the descent, made with the 

speed brakes extended, Popson indicated he had 25 to 50 pounds of oxygen 

pressure. This was sufficient for continuing the flight. 

The NACA X-5 parked on the ramp at South Base. The X-5 aircraft had a bulky design that led 

to poor flight characteristics. Spins were a particular problem and resulted in the loss of the 

second X-5 as well as the death of Air Force pilot Ray Popson. (NASA)
Probing the Sky

228

The two aircraft leveled out at 10,000 feet. Popson radioed Murray that the 

X-5 had 120 gallons of gas remaining and he wanted to “feel the aircraft out 

some more” before landing. The pair climbed to 12,000 feet above sea level 

(about 10,000 feet above ground level), and Popson began a series of stalls at 

20° and 40° sweep angles. He then began a third series of stalls at 60°, but the 

X-5 slowly rolled over onto its back and entered a spin to the right. Murray 

radioed Popson to eject during the third turn, but received no reply. The X-5 

struck the ground after four to six turns, in a 60° diving turn. Popson made 

no ejection attempt and died on impact.39

Although the accident investigation board concluded that the cause of the 

crash was loss of control leading to an unintentional spin at an altitude too low 

for successful recovery, board members felt that several factors had contributed 

to the loss of control. One of these factors was that analysis of crash debris indicated that the wings had been at a 45° to 47° sweep angle when the X-5 hit the 

ground. As the sweep angle was known to be 60° at spin entry, the board believed 

Popson had attempted to move the wings to a 20° sweep angle. The reaction to 

sweeping the wings forward during a spin was unknown, but in straight and 

level flight, a trim change had been noted during wing-sweep changes. 

The X-5’s cockpit pressurization system was not operational at the time 

of the flight. This meant Popson would have been using 100-percent oxygen 

from takeoff until reporting the loss of oxygen pressure. However, the supply 

of oxygen would have lasted about 35 minutes. Popson reported the loss of 

oxygen pressure after 26 minutes of flight time. Roughly 2 minutes later, he 

radioed that he had 25 to 50 pounds of oxygen pressure. The X-5’s crew chief 

told the accident board he was certain a full supply of oxygen was on board 

the aircraft before it took off. If the numbers Popson reported were an accurate 

reading, this would indicate excessive leakage or a partial malfunction within 

the oxygen system.40

The medical official on the accident panel expressed the opinion that the 

conditions described could induce some degree of hypoxia, but to an extent that 

was unknown. Although Popson’s actions and radio communications during 

the 20° and 40° sweep-angle stalls had seemed normal, hypoxia could have been 

a contributing factor in the accident, he asserted. Popson’s intentions for the 

60° stalls at 12,000 feet were unknown; they had not been part of the original 

familiarization flight plan. Full exploration of the X-5’s stall characteristics at 

so low an altitude “would have been unwise,” the accident report noted.41 But 

Popson had extensive experience and should have been able to recover successfully. The board considered the possibility that Popson had made mistakes in 

attempting to recover from the spin, such as holding the ailerons against the 

spin rather than with it, or using excessive elevator trim. The accident report 

stated, “This is considered less likely, however, than the assumption of partial 
Transformative Pioneer: The Bell X-5

229

hypoxia[,] which would reduce the pilot’s ‘keenness’ or ‘edge’ of reaction.” (But 

the report did not officially endorse the hypoxia theory.)42

In-flight photo of the X-5 over Edwards Air Force Base with the wings in the midrange position. 

As many of the other X-planes did, the X-5 suffered from poor stability and control. The stall/

spin behavior was very poor, requiring 10,000 feet to make a recovery. This was the cause of 

the loss of Major Popson during a familiarization flight in the second aircraft. (NASA)

As was routine in such incidents, panel members questioned Major Murray. 

He was asked if Popson’s voice had sounded normal during radio communication, or if there had been any indication he was suffering from hypoxia. Murray 

responded, “At the time I did not [think there was anything unusual], he was 

perfectly normal, it seemed....” At a later point, a panel member asked, “Again, 

he seemed normal, his words were used properly, no slurring of speech?” Murray 

replied, “When talking through an oxygen mask a voice sounds garbled, at the 

time I was under the impression that he was in a normal condition, gave no 

hesitation in answering.”

One member also asked Murray: “Have you ever thought there was any 

indication that the flight characteristics of this aircraft differed from the 

NACA aircraft?”

Murray, who had about 40 hours of flight time piloting both of the X-5s, 

replied, “They differ quite a bit in the high speeds in that [the Air Force’s] 

airplane tends to become uncontrollable [at] .93 to .96 and we have noticed a 

great difference in the stall characteristics. In accelerated stalls and unaccelerated stalls the airplane will recover as soon as the elevator was put forward and 

after speed was regained lateral control came back.” 
Probing the Sky

230

Joe Walker also testified before the accident panel. One of the members 

noted that Popson had stalled the X-5 with a 60° wing sweep, in a clean configuration, and entered a spin. He asked Walker if he had experienced any 

similar conditions. Walker replied: 

In a 1g stall, clean configuration, 60° sweep I have experienced 

several different conditions. The airplane exhibited increasing loss 

of lateral control and abrupt yaw to the left. The most vicious stall 

approach was with slats extended and occurred with no warning. 

This took the form of a half snap roll to the left. If excess right 

rudder were used it would probably have been to the right. It 

seemed like I had perfect control of the airplane before the snap 

roll. Application of trim by actuation of the stabilizer with the 

electrical motor results in excess control which pitches the airplane 

into a spin condition near the stall.43

Walker was asked later if he had anything he wanted to add to his testimony. 

He replied, “I would like to say a 1g stall approach at 60° is accompanied by 

large loss of altitude. You get an increase of drag with an increase of angle of 

attack. Below 170 mph use of full throttle still requires about 5,000 feet altitude 

to gain sufficient speed to pull out of the dive.”44

The final conclusion was, “The Board is of the opinion that this accident 

was the result of loss of control of the aircraft resulting in an unintentional 

spin at insufficient altitude for recovery but it is felt that several other factors or 

conditions could have contributed to this loss of control. They are as follows: 

‘Wing Sweep Control and Oxygen System and Pressurization.’ The panel recommended that stalls only be performed above 20,000 feet in research aircraft 

with unknown qualities.”45

Although not mentioned in the accident investigation, a breakdown in communications between the Air Force and the NACA had occurred. Crossfield 

recalled later that following Walker’s and his own X-5 spin, a note had been 

added to the X-5 pilot’s handbook prohibiting any maneuvers that could result 

in a spin at an altitude below 20,000 feet. When Crossfield learned that Popson 

had made aggravated stalls at 12,000 feet, he later recalled, “I was sick. We 

had failed in a basic NACA mission—getting information to the right place 

in time.... If there had been better coordination between [the] Air Force and 

NACA, [Popson] might be alive today.”47

The medical safety division had its own comments: “One fact apparently 

overlooked is that even after descending to 12,000 feet...where stalls were practiced for about 10 minutes, the cabin pressure would have been near 13,000 

feet to 14,000 feet and hypoxia possibly induced at a higher altitude would not 
Transformative Pioneer: The Bell X-5

231

be completely recovered from since mild hypoxia is actually induced at these 

altitudes. Concur generally with findings.”48

The first X-5 in January 1952, after it was transferred to the NACA. A total of 122 NACA 

research flights were made with the X-5, most of them piloted by Joe Walker. In contrast, 

A. Scott Crossfield made 10 flights between 1952 and 1954. He later commented that the lowslung engine resulted in a misalignment of the drag axis, the principal axis, and the thrust axis. 

“So,” he later said, “it could get into some interesting maneuvers and motions....”46 (NASA)

The X-5’s Later Years—1953–1955

The NACA continued to fly the surviving X-5 following the Popson crash. The 

number of flights made in the aircraft each year gradually declined, however. 

In 1953, a total of 31 flights were made with the X-5 (four by Crossfield, one 

by Murray, and the rest by Walker). Research goals included collecting data 

on vertical-tail loads, gust loads, effects of wing transition on aircraft trim, 

longitudinal stability and control, and wing twisting and bending tail loads.

The X-5 made a total of 27 flights in 1954, with Walker, Crossfield, Stanley 

P. Butchart, and John B. “Jack” McKay as pilots. Much of the research work 

continued to focus on longitudinal and lateral stability and control, as well 

as vertical-tail loads. Additional research included measuring the effects of 

dynamic pressure on buffet. Most of the flights were made between January and 

June of that year. The X-5 then was grounded, awaiting delivery and installation 

of a new nose gear housing from the Electrol Company. The aircraft did not fly 
Probing the Sky

232

again until December 14, when Butchart conducted an instrumentation check 

flight. The following day, McKay made the first of four pilot checkout flights. 

The X-5 project made its final research flights in 1955, totaling 19 flights 

in all. The first flight, made on January 27, was an instrumentation check by 

Butchart and reached Mach 0.922. The next eight flights, made by Butchart 

and McKay between January 28 and March 23, were for longitudinal-stability 

and control data. Six more flights, starting on March 23 and continuing 

through April 8, were for lateral-stability and control measurements. Flights 

then halted until the fall.49 A limited number of additional flights were still 

necessary to collect lateral control effectiveness data and to obtain an angle-ofattack calibration. McKay made these on October 19, 20, and 24, 1955. The 

134th and final NACA flight was made on October 25 by Neil A. Armstrong. 

During the pilot check flight, a landing gear door separated. Armstrong landed 

safely, but the aircraft never flew again.50 The X-5 was put into storage for the 

next several years. In March 1958, it was sent to the U.S. Air Force Museum 

at Wright-Patterson Air Force Base. The aircraft was subsequently restored 

and now is on display in the museum’s Research and Development Gallery.51

As the X-5 flights were winding down, the final set of research memorandums on the project was completed, reviewed, and approved. The first of these, 

which dealt with wing loads at a sweep angle of 58.7°, stated: 

Flight measurements on the Bell X-5 research airplane at a sweep 

angle of 58.7° have shown the wing loads exhibit nonlinear trends 

over the angle-of-attack range from zero to maximum wing lift. 

These nonlinearities were, in general, more pronounced at angles 

of attack above the “pitch-up” where there is a reduction in the 

wing-panel lift-curve slope and an inboard and forward movement in the center of load. These characteristics have been found 

to exist in the results of wind-tunnel tests of swept wings and 

emphasize the need of model testing for accurate wing design 

data when nonlinearities exist.

No apparent effects of altitude on the wing loads were evident 

over the comparable lift ranges of these tests at altitudes from 

40,000 to 15,000 feet.52

The second memorandum looked at the loads on the X-5’s horizontal tail. 

Its conclusions read:

An investigation of the horizontal-tail loads on the Bell X-5 

research airplane at a sweep angle of 58.7° has indicated that the 

horizontal-tail loads are nonlinear with lift throughout the lift 
Transformative Pioneer: The Bell X-5

233

ranges tested at all Mach numbers except at a Mach number of 

approximately 1.00. The balancing tail loads reflected changes 

which occur in the wing characteristics with increasing angle of 

attack. The nonlinearities of the horizontal-tail loads were generally more pronounced at the higher angles of attack near the pitchup where the balancing tail loads indicate that the wing-fuselage 

combination becomes unstable.

No apparent effects of altitude on the balancing tail loads were 

evident over the comparable lift ranges of these tests at altitudes 

from 40,000 feet to 15,000 feet.

Comparisons of balancing tail loads obtained from flight and 

wind-tunnel tests indicated discrepancies in absolute magnitudes, 

but the general trends of the data agree. Some differences in absolute magnitude may be accounted for by the tail load carried 

inboard of the strain-gage station and the load induced on the 

fuselage by the presence of the tail. These loads were not measured 

in flight.53

Research with the X-5 was primarily focused on longitudinal and lateral 

stability and control. Most of the research flights were made for these purposes, 

and the resulting research memorandums were both lengthy and detailed. One 

such memorandum noted:

An investigation of the dynamic stability of the X-5 research airplane at 58.7° sweepback at altitudes of 40,000 feet and 25,000 

feet over a Mach number range of 0.5 to 0.97 shows the following: 

The longitudinal motions were well damped over the entire Mach 

number range tested except for residual oscillations resulting principally from engine gyroscopic coupling with the lateral oscillatory mode. This engine gyroscopic coupling results in motion on 

both the longitudinal and lateral oscillatory modes for either a 

longitudinal or lateral disturbance. The lateral oscillatory mode 

exhibits moderately good damping except for nonlinear damping 

characteristics above a Mach number of 0.80 where the motion is 

well damped at large amplitudes and poorly damped for double 

amplitude sideslip angles less than 2°. Small inadvertent aileron 

control motions often produced apparently undamped small 

amplitude oscillations in the Mach number range above 0.80.54

Another memorandum expanded on this, describing lateral-stability 

issues such as roll coupling, pitching characteristics aggravated by directional 
Probing the Sky

234

divergence and aileron overbalance, an abrupt wing-dropping tendency, wing 

heaviness, and rudder flutter. Direction divergence, for example, meant the X-5 

had suddenly entered a sideslip of 25°, resulting in a spin. The memorandum 

also noted, “The problem of aileron overbalance occurred less frequently but 

was no less disconcerting to the pilot because the stick would jerk from side 

to side unless restrained.”55

Perhaps the most damning criticism of the airplane was reserved for the 

section “Pilots’ Impressions,” which made clear the demands of flying the X-5 

(emphasis added):

The X-5 airplane at 58.7° sweepback is considered to have the least 

desirable lateral stability and control characteristics of any of the 

airplanes tested, including straight-wing, swept-wing, semitailless, 

and delta-wing configurations. One pilot, while checking out in 

the X-5 airplane, discontinued a speed run at M = 0.85 and an 

altitude of 35,000 feet because he strongly doubted his ability to keep 

the airplane right side up.

56

The outstanding deficiency of the X-5 airplane is the lateraldirectional oscillation or “Dutch roll” caused by the high positive dihedral effect. This oscillation is annoying but tolerable for 

research flying over the entire speed range at 40,000 feet, except 

over the range of M = 0.86 to M = 0.88 where the residual, 

small amplitude, virtually undamped oscillation is most noticeable. The dihedral effect decreases with a decrease of altitude but 

never reaches a satisfactory value. The airplane exhibits positive 

lateral stability during sideslip maneuvers and requires large aileron deflections for small rudder deflections; however, it is impossible to maintain a steady sideslip without rolling oscillations. 

Normal turning maneuvers tend to be jerky with abrupt increases 

and decreases of bank angle, apparently caused by small yawing 

motions and angle-of-attack changes. In straight and level flight, 

lateral-directional oscillations can be initiated by control motions, 

power changes, or turbulent air.

The aileron effectiveness is low at all Mach numbers and, 

except for the adverse dihedral effects in some conditions, the rolling characteristics are normal with rolling velocities proportional 

to aileron deflection and increase as Mach number increases. 

The rolling characteristics improve with decrease of altitude, but 

maximum rolling velocity is limited because of the excessive force 

necessary to obtain large aileron deflections. Near the 1g stall there 

is little or no lateral control and nearly zero stick force.57
Transformative Pioneer: The Bell X-5

235

An Assessment

The X-5 left a mixed legacy. Like its progenitor, the Messerschmitt P.1101, 

the aircraft was flawed, with poor stability and vicious stall/spin behavior that 

cost the life of one of its pilots. At the same time, while the wing-sweeping 

mechanism used on the X-5 was impractical, the aircraft had proven the basic 

fundamental value of in-flight variable wing sweeping (in contrast to the X-4, 

whose stability problems had proven the semi-tailless configuration impractical in the pre-fly-by-wire era). Additionally, because it could vary its sweep, 

researchers gained a wide range of data on wing-sweep at various angles; NACA 

test pilot Scott Crossfield memorably recalled that having the X-5 was like 

“having a whole stable of swept-wing airplanes in one.”58 Absent the X-5, 

collecting such a quantity and variety of data would have required multiple 

aircraft with different wings. 

Though the X-5, despite its stability problems, had demonstrated that the 

concept of variable-sweep wings was workable, manufacturers were slow to 

embrace the idea of variable-sweep wings, in part because of traditionalist 

design tendencies, but in large measure because the technology was still immature and not yet ready for practical application. The mechanism needed for 

the complex wing movements added weight and complexity, and the benefits 

did not justify this burden. 

A lone prototype U.S. fighter was built in the 1950s using variable-sweep 

wings of a configuration similar to that of the X-5. This was the Grumman 

XF10F-1 Jaguar, which began flight testing in early 1952. A “shoulder wing” 

design like the X-5, it, too, used a single pivot point and shifted the wing roots 

fore and aft, though coupling this with an imaginative delta-wing “flying tail” 

to help control trim changes. But again like the X-5, it was impractical for 

routine use. Worse, the XF10F-1 was in any case a thoroughly bad design, 

plagued by miserable flying qualities and an unreliable engine. These deficiencies, coupled with changes in aircraft carrier design (including the angled deck 

and mirror landing system and the steam catapult) and better aircraft exemplified by the forthcoming Vought XF8U-1 Crusader, resulted in its termination. 

The cancellation must have come as a relief to company test pilot Corwin H. 

“Corky” Meyer, who stated revealingly afterward, “I had never attended a 

test pilots’ school, but for me the XF10F-1 flight test program provided the 

complete curriculum.”59

The NACA, and later NASA, played the leading role in transforming 

the variable-sweep wing concept into a viable configuration.60 The change 

came about as the result of a British proposal by famed Vickers corporation 

designer Barnes Wallis for a supersonic airliner with variable-sweep wings 

called the “Swallow.” Wallis’s imaginative study was largely impractical but did 
Probing the Sky

236

incorporate a novel form of pivot location that ultimately hastened the development of practical, operational variable-sweep aircraft. Vickers requested that 

NASA review the design, and at the behest of John Stack and project engineer 

Charles Donlan, four different Swallow configurations were eventually tested. 

Configurations I through III had better stability margins than the X-1 model 

had shown but were unacceptable. Configuration IV incorporated a different 

concept for the variable-sweep wing. Unlike on the X-5 and the XF10F-1, the 

pivot point was outboard of the fuselage, at the extremities of a small, fixed 

forward wing. The outboard pivot, combined with the geometry of the fixed 

forward wing, favorably changed the way the aerodynamic load shifted as the 

wing sweep changed. The means of building a variable-sweep aircraft without 

the complications of the X-5 or XF10F-1 wing designs was now at hand.61

The first U.S. variable-sweep aircraft to enter service was the F-111A, first 

flown in 1964.62 It had originally been designed as a strike aircraft for the U.S. 

Air Force and a fleet air defense interceptor for the Navy. (The Navy version 

was subsequently canceled.) It was followed by the Navy’s F-14 fleet air defense 

fighter and the Air Force B-1B strategic bomber. The NATO and Saudi air 

forces operated the Panavia Tornado strike aircraft. The Soviets also operated 

and exported a number of variable-sweep aircraft, including the MiG-23/27 

fighter, the Su-17/22 and Su-24 strike aircraft, and the Tu-22M and Tu-160 

strategic bombers. The original Boeing supersonic transport (SST) design also 

featured variable-sweep wings, though fixed-sweep wings replaced these during 

the design process. (The American SST project ended in 1971 when Congress 

withdrew the funding.) 

But variable-sweep wings subsequently proved to be of largely transitory 

value, however, for they added cost and weight to a design and complicated the 

designer’s challenge in the era of stealth. Today, while “legacy” variable-sweep 

aircraft such as the B-1B and Tu-160 remain in service, few designers look to 

them as viable options for future aircraft. As Richard P. Hallion noted in 2010:

On the whole, the variable-geometry wing has not enjoyed the 

kind of widespread success that its adherent hoped. While it may 

be expected that, from time to time, variable-sweep aircraft will 

be designed and flown for particular purposes, overall the fixed 

conventional planform, outfitted with all manner of flaps and 

slats and blowing, sucking, and perhaps even warping technology, 

continues to prevail.63
Transformative Pioneer: The Bell X-5

237

Endnotes

1. Opening quote from Crossfield and Blair, Always Another Dawn, 

p. 155.

2. The best survey of the advent and significance of variable sweep 

is Robert L. Perry’s Innovation and Military Requirements: A 

Comparative Study, RAND Report RM-5182PR (Santa Monica, 

CA: The RAND Corporation, 1967). See also Richard P. Hallion, 

“Sweep and Swing: Reshaping the Wing for the Jet and Rocket Age,” 

in Richard P. Hallion, ed., NASA’s Contributions to Aeronautics, vol. 

1: Aerodynamics, Structures, Propulsion, Controls, NASA SP-2010-

570-Vol 1 (Washington, DC: NASA, 2010), pp. 57–64; and James 

R. Hanson, The Bird Is on the Wing: Aerodynamics and the Progress of 

the American Airplane (College Station, TX: Texas A&M University 

Press, 2004), p. 123.

3. NACA-NASA interest in variable-sweep wings is documented 

in NASA Langley Research Center, “Summary of NACA/NASA 

Variable-Sweep Research and Development Leading to the F-111 

(TFX),” NASA LRC Working Paper LWP-285, 1966; and Edward 

C. Polhamus and Thomas A. Toll, “Research Related to Variable 

Sweep Aircraft Development,” NASA TM 83121, 1981, pp. 5–6. 

For the earliest example of oblique-wing wind tunnel testing, see 

John P. Campbell and Hubert M. Drake, “Investigation of Stability 

and Control Characteristics of an Airplane Model with Skewed 

Wing in the Langley Free-Flight Laboratory,” NACA TN No. 1208, 

May 1947.

4. Charles J. Donlan and William C. Sleeman, “Low-Speed WindTunnel Investigation of the Longitudinal Stability Characteristics of 

a Model Equipped with a Variable-Sweep Wing,” RM L9B18, May 

23, 1949.

5. Hanson, The Bird Is on the Wing, p. 123; Donlan and Sleeman, 

“Low-Speed Wind-Tunnel Investigation,” pp. 1, 3, 8; Jay Miller, The 

X-Planes: X-1 to X-45 (Hinckley, U.K.: Midland Publishing, 2001), 

pp. 87–88.

6. Hanson, The Bird Is on the Wing, p. 124; Miller, The X-Planes: X-1 to 

X-45; Bell Aircraft Corporation, “Bell X-5 Ready For Flight Tests,” 

for release June 14, 1951.

7. John T. Rogers and Angel H. Dunn, “Preliminary Results of 

Horizontal-Tail Load Measurements of the Bell X-5 Research 

Airplane,” NACA RM L52G14 (August 15, 1952), p. 4; Donald 

R. Bellman, “Lift and Drag Characteristics of the Bell X-5 Research 
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238

Airplane at 59° Sweepback for Mach Numbers from 0.60 to 1.03,” 

NACA RM L53A09c (February, 17, 1953), p. 7. It should be noted 

that reports and documents frequently give X-5 wing-sweep angles 

of 59° or 60°. This was apparently rounding up, rather than an 

actual modification to the wings.

8. Bell Aircraft Corporation, “Bell X-5 Ready for Flight Tests,” 

press release.

9. Data from Jack Nugent, “Lift and Drag of the Bell X-5 Research 

Airplane in the 45° Sweptback Configuration at Transonic Speeds,” 

NACA RM H56E02 (July 11, 1956), Table I, pp. 10–11.

10. The Air Force role in developing the X-5 is minutely and definitively detailed in Warren E. Green’s The Bell X-5 Research Airplane 

(Wright-Patterson AFB, OH: Wright Air Development Center, 

March 1954).

11. Miller, The X-Planes: X-1 to X-45, p. 89.

12. See Perry’s Innovation and Military Requirements: A Comparative Study.

13. Memorandum for Chief of Research, “Progress Report of 

Engineering and Instrumentation for the X-5 Airplane During the 

Period August 25 to September 7, 1951.”

14. Thomas W. Finch and Donald W. Biggs, “Preliminary Results 

of Stability and Control Investigation of the Bell X-5 Research 

Airplane,” NACA RM L52K18b (February 11, 1953), p. 4.

15. Memorandum for Chief of Research, “Progress Report of 

Engineering and Instrumentation for the X-5 Airplane During the 

Period December 1 to December 14, 1951.”

16. Memorandum for Chief of Research, “Progress Report of 

Engineering and Instrumentation for the X-5 Airplane During the 

Period December 15, 1951 to December 28, 1951.”

17. Memorandum for Chief of Research, “Progress Report of 

Engineering and Instrumentation for the X-5 Airplane During the 

Period December 29, 1951 to January 11, 1952.”

18. Hallion and Gorn, On the Frontier, p. 402 (quoting flight records).

19. Rogers and Dunn, “Preliminary Results of Horizontal-Tail Load 

Measurements of the Bell X-5 Research Airplane,” pp. 8–9.

20. Crossfield and Blair, Always Another Dawn, p. 155.

21. Ibid.

22. Hallion and Gorn, On the Frontier, pp. 47–48.

23. Memorandum for Chief of Research, “Progress Report for the X-5 

Research Airplane for the Period May 3 to May 16, 1952.”

24. Memorandum for Chief of Research, “Progress Report for the X-5 

Research Airplane for the Period July 12 to July 25, 1952”; James 
Transformative Pioneer: The Bell X-5

239

A. Martin, “Longitudinal Flight Characteristics of the Bell X-5 

Research Airplane at 59° Sweepback with Modified Wing Root,” 

NACA RM L53E28 (August, 10, 1953), p. 1.

25. Memorandum for Chief of Research, “Progress Report for the X-5-1 

Research Airplane for the Period July 26 to August 8, 1952.”

26. Memorandum for Chief of Research, “Progress Report for the X-5-1 

Research Airplane for the Period October 1 to November 1, 1952.”

27. Ibid. At the time, 7.33 g’s was the Air Force loads design standard.

28. Hallion and Gorn, On the Frontier, p. 403 (flight records).

29. De Elroy Beeler, Acting Chief, NACA High Speed Flight Research 

Station, to NACA Liaison Officer, Wright Patterson Air Force Base, 

“Preliminary Investigation of Handling the X-5 Airplane Below 

Minimum Drag in Close Proximity with Other Aircraft,” March 

5, 1953, and Joseph A. Walker, NACA HSFRS, “Investigation of 

Flight at Speeds Below Minimum Drag, Flight No. 60,” January 27, 

1953, p. 1, DFRC Archives. These tests were not an investigation 

specifically into wingtip vortices. Later, pilots and engineers would 

realize that the wingtip vortices from a jumbo jet could cause a 

smaller airplane to lose control and crash during a landing approach. 

This led to increases in separation between aircraft on final approach.

30. Joseph A. Walker, NACA HSFRS, “Investigation of Flight at Speeds 

Below Minimum Drag, Flight No. 60,” January 27, 1953, p. 1, 

DFRC Archives.

31. Joseph A. Walker, NACA HSFRS, “Investigation of Flight at Speeds 

Below Minimum Drag, Flight No. 61,” January 29, 1953, p. 1, 

DFRC Archives.

32. Ibid.

33. Joseph A. Walker, NACA HSFRS, “Investigation of Flight at Speeds 

Below Minimum Drag, Flight No. 63,” February 6, 1953, p. 1, 

DFRC Archives. Aerial refueling became a standard practice in both 

the Air Force and Navy. In particular, aerial refueling had become 

critical to Strategic Air Command operations within a few years of 

the X-5 tests. Aerial refueling extended the range of bombers and 

allowed airborne alert missions. Subsequently, tactical aircraft and 

even transport aircraft were outfitted for aerial refueling.

34. Joan M. Childs, “Flight Measurements of the Stability 

Characteristics of the Bell X-5 Research Airplane in Sideslips at 59° 

Sweepback,” NACA RM L52K13b (February 11, 1953), pp. 4–5.

35. Donald R. Bellman, “Lift and Drag Characteristics of the Bell X-5 

Research Airplane at 59° Sweepback for Mach Numbers from 0.60 

to 1.03,” p. 8.
Probing the Sky

240

36. Donald W. Briggs, “Flight Determination of the Buffeting 

Characteristics of the Bell X-5 Research Airplane at 58.7° 

Sweepback,” NACA RM L54C17 (May 24, 1954), pp. 1, 10–11.

37. Martin, “Longitudinal Flight Characteristics of the Bell X-5 

Research Airplane at 59° Sweepback with Modified Wing Root,” 

pp. 6–7.

38. Miller, The X-Planes: X-1 to X-45, p. 91; Hallion and Gorn, On the 

Frontier, p. 401 (flight records).

39. USAF, “Report of Major Aircraft Accident—X-5, 50-1839A, 

Section 0—Description of Accident” (Edwards AFB, CA: AFFTC, 

October 27, 1953), first page. This section of the accident report has 

no page numbers. It is typed text on a blank page.

40. Ibid.

41. Ibid., first and second pages.

42. Ibid., second page.

43. Ibid., eighth page.

44. Ibid., ninth page.

45. Ibid., “Accident Board Action Findings, Recommendations,” no 

page number.

46. NASA, Fact Sheet “X-5,” http://www.nasa.gov/centers/dryden/news/

FactSheets/FS-081-DFRC_prt.htm, modified July 3, 2013.

47. Crossfield and Blair, Always Another Dawn, pp. 155–156.

48. USAF, “Report of Major Aircraft Accident—X-5, 50-1839A,” 

October 27, 1953, “Remarks Medical Safety Division,” no page 

number.

49. Hallion and Gorn, On the Frontier, pp. 402–404 (flight records).

50. Ibid., p. 404. See also Memorandum for Research Aircraft Project 

Leader, “Progress Report for the X-5-1 Research Airplane for the 

Period July 1 to July 31, 1955.”

51. X-5 Fact Sheet, http://www.nationalmuseum.af.mil/factsheets/factsheet.

asp?id=630 (accessed October 5, 2010).

52. Richard D. Banner, Robert D. Reed, and William L. Marcy, “WingLoad Measurements of the Bell X-5 Research Airplanes at a Sweep 

Angle of 58.7°,” NACA RM H55A11 (April 5, 1955), p. 8.

53. Robert D. Reed, “Flight Measurements of Horizontal-Tail Loads on 

the Bell X-5 Research Airplane at a Sweep Angle of 58.7°,” NACA 

RM H55E20a (April 4, 1955), pp. 6–7.

54. Edward N. Videan, “Flight Measurements of the Dynamic Lateral 

and Longitudinal Stability of the Bell X-5 Research Airplane at 58.7° 

Sweepback,” NACA RM H55H10 (October 6, 1955), p. 1.
Transformative Pioneer: The Bell X-5

241

55. Thomas W. Finch and Joseph A. Walker, “Flight Determination of 

the Lateral Handling Qualities of the Bell X-5 Research Airplane 

at 58.7° Sweepback,” NACA RM H56C29 (May 31, 1956), pp. 

12–13.

56. Ibid., p. 12.

57. Ibid., p. 13.

58. Crossfield and Blair, Always Another Dawn, p. 155.

59. Corwin H. Meyer, Corky Meyer’s Flight Journal: A Test Pilot’s Tales 

of Dodging Disasters—Just in Time (North Branch, MN: Specialty 

Press, 2006), p. 216.

60. The NACA, which existed as an agency from 1915 to 1958, gave 

way to the National Aeronautics and Space Administration (which 

had a broader mandate encompassing both aeronautics and astronautics) in the reorganization of Federal aerospace taking place 

after the shock of Sputnik. See Roger D. Launius, “An Unintended 

Consequence of the IGY: Eisenhower, Sputnik, and the Founding 

of NASA,” AIAA Paper No. 2008-860, 2008; and T. Keith Glennan 

(edited by J.D. Hunley and in association with Roger D. Launius), 

The Birth of NASA: The Diary of T. Keith Glennan, NASA SP-4105 

(Washington, DC: NASA, 1993).

61. Polhamus and Toll, “Research Related to Variable Sweep Aircraft 

Development,” pp. 9–12.

62. Robert W. Kress, “Variable-Sweep Wing Design,” AIAA Paper No. 

83-1051 (1983) is an excellent survey of this and other efforts.

63. See the previously cited Hallion, “Sweep and Swing: Reshaping the 

Wing for the Jet and Rocket Age,” p. 64.
242

The XF-92A is shown parked on the lakebed soon after final assembly. The aircraft spent most 

of its research career in a bare-metal finish. The fuselage was wider at its midpoint than at the 

nose or tail, which was long the standard design used in high-speed aircraft. (USAF)
243

CHAPTER 7

Progenitor of the Delta: 

The Convair XF-92A

The pilot never really flew that airplane, he corralled it.

—A. Scott Crossfield1

The Consolidated-Vultee (Convair) XF-92A was not built as a research airplane but as a flying mockup for a proposed supersonic interceptor, hence 

its prototype fighter designation. It was not, however, intended as a combat 

aircraft, and thus it occupies a similar place in research aircraft development 

as the Bell XP-59A, the first American turbojet, which itself had a fighter designation while actually functioning as a technology demonstrator. Its design 

evolution shows the ambitious plans of the immediate post–World War II 

period, the unknowns of supersonic flight, and the impractical concepts proposed to overcome them. From this environment came the birth of the semitailless delta-wing aircraft. This configuration would eventually be used by 

nations around the world in fighters, strike aircraft, reconnaissance airplanes, 

and bombers, as well as in a pair of supersonic airliners—but it was proven 

first by the Convair XF-92A. 

The Birth of the XF-92A

The war against Imperial Japan had barely ended in the first week of September 

1945 when the Army Air Forces announced a design competition for three 

different fighters: the first was for an all-weather fighter capable of attacking enemy aircraft at night or in bad weather; the second was for a longrange escort fighter to protect bombers; and the third was for a short-range 

point-defense supersonic interceptor, intended as a last-ditch defense against 

enemy bombers. This third project became the XP-92. Consolidated-Vultee 

Aircraft Corporation (CVAC, subsequently Convair) was among potential 

contractors the Army approached on the three projects. In response, the firm 

assembled a small design team at their Downey, CA, plant. The team consisted 
Probing the Sky

244

of Jack Irvine, Frank W. Davis, Adolph Burstein (chief of design), Thomas 

Hemphill, and Ralph H. Shick, chief of aerodynamics.2

Out of this team came the Convair Model 7002, later known as the XF-92A. 

The first challenge the four men faced was the point-defense interceptor’s propulsion system. Existing jet engines lacked the thrust necessary for reaching 

supersonic speeds, and rocket engines lacked sufficient burn time to take off, 

climb to altitude, engage an enemy bomber, and land. As a result, engineers 

came up with a hybrid propulsion system that used a ramjet combined with 

an auxiliary jet engine, similar to that employed a decade later on the French 

Mach 2+ Nord 1500-02 Griffon II experimental fighter. Several ducted rocket 

motors were mounted within the XP-92’s ramjet both to provide thrust and 

to act as igniters and flame holders. The auxiliary jet engine was fitted in the 

aft fuselage for the return to base and landing.

The initial XP-92 airframe design featured a cylindrical fuselage, large 

swept wings, and a V-tail. The wing sweep was 35° on the leading edge. The 

Consolidated-Vultee design was submitted to the Army Air Forces, where 

it was selected for development in May 1946. The first step in refining the 

design involved wind tunnel testing. These tests showed a number of problems. The wing suffered from tip stall at angles of attack as low as 5°, as well 

as poor lateral control characteristics. The design team set to work modifying 

the XP-92 configuration. 

Replacing the V-tail with a conventional design resolved some of the stability problems, but the wings required more work. The Convair design team 

looked at wing slots, fences, and gloves as possibilities. The solution proved 

far simpler. The wing’s trailing edge was changed to a straight line, making the 

wing a 45° triangle rather than a conventional swept wing. This was the first 

time the shape had been used on a jet aircraft. The design was further refined 

to a pure 60° delta wing in a series of July 1946 wind tunnel tests.3

The adaptation of a delta wing for the XP-92 thus was a byproduct of sweptwing refinement, not a choice from the outset based on theoretical work or 

emulation, despite NACA research aerodynamicist Robert T. Jones’s having 

enunciated the thin, sharply swept delta more than a year previously.4 More 

importantly, in contrast to popular myth, the XP-92 (and the XF-92A and, for 

that matter, all subsequent Convair delta aircraft) owed nothing to the work 

of German aerodynamicist Alexander Lippisch, who had designed the Me 163 

and who had conceived a very thick-wing 60° delta glider, the Lippisch DM-1, 

intended as a low-speed test bed for a proposed ramjet-powered delta-wing 

fighter, the P.13. The Convair team rejected his concept of a delta wing, which 

envisioned a very thick wing with a very thick vertical fin (in contrast, the delta 

wing that they chose had—for its time—a very thin 6.5-percent thicknesschord ratio). Lippisch’s DM-1 had more than twice the thickness-chord ratio 
Progenitor of the Delta: The Convair XF-92A

245

of the XP-92, and its vertical fin was so large that the pilot sat within it, the 

clear cockpit windscreen being formed by its leading edge. More significantly, 

when the DM-1 glider was tested in Langley’s Full-Scale Tunnel in 1946, the 

results were “very disappointing; the lift coefficient was low, the drag was high, 

the directional stability was unsatisfactory, and the craft was considered unsafe 

for flight tests.”5 The Convair thin-wing approach was altogether more satisfactory, and for it, Convair’s XP-92 design team deserves great—and sole—credit.

Another early photo of the XF-92A. The canopy was considered problematic by pilots because 

the framework hampered visibility. The sensitivity of the flight-control system, which required a 

light touch by pilots, also presented difficulties. (USAF)

Model 7002—From Flying Mockup 

to Research Airplane

The final design of the XP-92 was striking, with the 60° delta wing, a triangular vertical fin, and a tubular fuselage that extended well behind the fin.6

 The 

cockpit was in a bullet-shaped nose mounted within the circular ramjet-intake 

duct. In the event of an emergency, the forward fuselage could be jettisoned 

as an escape capsule in a manner reminiscent of the D-558-1 and -2. After 

separation, a parachute would slow the capsule, and the pilot would then bail 
Probing the Sky

246

out using his own parachute. Ironically, the airplane, which resembled a craft 

straight out of the world of Buck Rogers, was armed with conventional weapons—four 20-millimeter cannons.

A research aircraft in wolf’s clothing. During the Air Force test program, the XF-92A was painted 

in bogus Soviet markings as a “MiG 23” for the movie Jet Pilot. The aircraft apparently did not 

appear in the final film. (USAF)

The new design retained the rocket-augmented ramjet engine. Because a 

ramjet must be in flight before it can produce thrust, a set of eight 1,500-poundthrust rockets, burning liquid oxygen and alcohol/water, would be used for the 

takeoff. The XP-92 did not make use of conventional landing gear for takeoff 

but rather was mounted on an eight-wheel cart containing the liquid-oxygen 

and alcohol/water tanks necessary for the takeoff run. Once the plane lifted 

off, a pair of large wing drop tanks supplied the fuel to boost the XP-92 to a 

top speed of Mach 1.65. 

At that point, 16 50-pound ducted rockets fueled by liquid oxygen and 

gasoline and mounted inside the ramjet ignited. The rockets started the ramjet 

and stabilized the flame within the ramjet combustion chamber. The ramjet 

increased the airplane’s speed to Mach 1.75 at 50,000 feet, speed and altitude that could be maintained for 5.4 minutes. The XP-92 was designed as a 

last-ditch/point-defense interceptor to destroy high-speed bombers and cruise 

missiles before they could reach their target. Once the mission was complete, 
Progenitor of the Delta: The Convair XF-92A

247

the XP-92 would fly back using the auxiliary jet to land on a runway with its 

tricycle landing gear. 

A full-scale metal mockup of the XP-92 was built, but Army Air Forces 

personnel had already begun to have doubts about its design. The rocketaugmented ramjet engine was seen as too complex for use in a near-term 

project, and the sled launch system seriously limited its utility. Other doubts 

were raised about Convair’s proposed hydraulic-powered, nonreversible control 

system, and weight increases had reduced performance. Even if these issues 

could be dealt with, the Air Force was in the midst of postwar budget cuts, 

and the number of personnel and aircraft were at a low point. No funding was 

available for so advanced and risky a concept as the XP-92. The interceptor 

program ended in June 1948.

But construction of the subscale demonstrator went ahead, for it had value 

even if completely separated from the futuristic interceptor program that had 

spawned it. The research aircraft that eventually became the XF-92A originated 

in meetings between Air Force and Convair personnel in September 1946. 

Building a subscale transonic test vehicle would provide data on the behavior 

of the delta wing at low and high speeds. Delta wings offered a number of 

advantages, including large wing area, a thin airfoil cross section, and a low 

aspect ratio. In addition, the wings were light in weight relative to their surface 

area while being very strong, an added benefit in any aircraft design. 

On November 7, 1946, the Army Air Forces contract with Convair was 

amended. Two XP-92 prototypes were to be built. In addition, a single Model 

7002 would be constructed to test the wing and tail configuration. The Model 

7002 that emerged had little in common with the exotic XP-92. The cylindrical 

fuselage had a small diameter at the nose and tail, but the center section was 

much larger in diameter to accommodate its centrifugal-flow jet engine. The 

Model 7002’s wing was the same size, airfoil, and configuration as that planned 

for the XP-92, though its tail fin was larger than the planned interceptor’s. To 

keep development costs down, existing components and systems were used in 

building the aircraft. The Model 7002 was powered by an Allison J33 jet engine 

from a Lockheed P-80 and had the canopy and ejection seat from Convair’s 

YP-81, the nose gear of a Bell P-63, the brakes of a Lockheed P-80, the pilot 

stick and rudder pedals from a Vultee BT-13, and the main landing gear of a 

North American FJ-1. 

But in other ways, the Model 7002 was as exotic as its ramjet-powered 

predecessor. The Model 7002 had the 100-percent hydraulically powered, nonreversible flight controls that were planned for the XP-92. Because the Model 

7002 was a semi-tailless configuration—lacking a conventional horizontal 

stabilizer—the plane employed elevons combining pitch and roll control, similar to the X-4 then under development. Convair closed its Downey facility in 
Probing the Sky

248

the summer of 1947, as a result of tightening military budgets following the 

end of World War II, and transferred the unfinished Model 7002 airframe to 

the company’s San Diego facility between June and July. The final assembly 

of the Model 7002 was completed in San Diego on October 31, 1947. In 

November, Convair sent the aircraft by Navy cargo ship to San Francisco for 

wind tunnel testing in the full-size tunnel at the Ames Aeronautical Laboratory. 

This testing was completed in early January 1948, and the aircraft returned to 

San Diego for final testing and installation of its J33-A-21 turbojet engine. The 

completed Model 7002, now designated the XF-92A, was then shipped from 

the San Diego harbor aboard the Navy landing craft LST-827 and arrived at 

the harbor in Los Angeles on March 26, 1948. The aircraft was then loaded 

on a truck and shipped over the San Gabriel Mountains to Muroc Air Force 

Base, where it arrived on March 31, 1948.7

Phase I Flight Tests

A delta-wing aircraft model attached to a rocket 

booster for a high-speed test. Such tests were used 

for preliminary research on new wing configurations. (NASA)

Convair’s chief test pilot, Ellis D. “Sam” Shannon, started tests of the XF-92A 

on May 25, 1948, by making high-speed taxi runs across the Rogers lakebed. 

The first “hop” came on June 9, 1948, during the fifth taxi test. The aircraft 

reached 10 or 15 feet above the 

lakebed, and he flew straight for 

about 2 miles before touching 

down again. Shannon found the 

controls were very sensitive to 

the smallest inputs. Touching 

the stick and rudder pedals was 

enough to create control problems. With the first taxi tests 

and short hop behind them, the 

team took the XF-92A back to 

the hangar, where the plane was 

fitted with a more powerful J33-

A-23 jet engine that had water 

injection to increase thrust. 

The aircraft resumed taxi tests 

in September. These were successfully completed, and with 

the aircraft systems checked 

out, Shannon could make the 

first flight. 
Progenitor of the Delta: The Convair XF-92A

249

Shannon made his first actual flight in the XF-92A on September 18, 1948, 

and it lasted 18 minutes. He found that the hydraulic control system was 

overly sensitive and had a lag in its control movements. Tiny movements of the 

stick and rudder pedals caused the XF-92A to wallow in all three axes, he told 

engineers following the flight. During a second flight, on September 29, he 

found the same conditions. After another flight, on October 13, the XF-92A 

was grounded through November 1948 while the ground crew harmonized 

its flight controls. 

Testing resumed once sufficient modifications had been made, and by the 

end of 1948, Shannon had made a total of 10 flights. A second Convair test 

pilot, Bill Martin, joined the Phase I test effort, making his first taxi test on 

December 21, 1948. Martin’s first flight was on February 9, 1949. The contractor flights initially focused on general handling qualities and performance. 

An unusual characteristic of the XF-92A that the pilots quickly identified 

was its remarkable low-speed stability. Shannon discovered to his surprise that 

even with the indicated airspeed below 90 mph and the angle of attack as high 

as 30°, the aircraft would not stall. This was due to the delta wing’s lack of an 

abrupt break in its lift curve at the stall. Instead, the XF-92A exhibited a stabilized high sink rate due to the increase in drag. This loss of altitude continued 

despite the use of full throttle. 

Another unusual characteristic was that the aircraft would not spin. Tests 

of a model in the Langley spin tunnel had indicated that if the Model 7002’s 

center of gravity was between 24 and 29 percent of the mean aerodynamic 

chord, a spin was impossible. By August 1949, the Phase I tests moved to 

transonic flights. To reach these speeds, the pilots had to put the underpowered 

XF-92A into steep dives. In the end, neither pilot exceeded Mach 0.925, as 

the nonafterburning J33 could not power the XF-92A through the speed of 

sound. Phase I tests ended on August 26, 1949, after a total of 47 flights by 

Shannon and Martin. 

With the contractor flights completed, the Air Force now began Phase II 

flights. The two pilots assigned to the tests were Captain Charles E. Yeager 

and Major Frank Everest. Phase II flights began on October 13, 1949, and 

concluded on December 28. The two pilots completed a total of 25 flights and 

were highly enthusiastic about the little delta. The Phase II flights also included 

very steep dives attempting to reach speeds slightly above Mach 1. Even assisted 

by a dive, however, the J33-A-23 engine lacked sufficient thrust to reach Mach 

1. Despite this, Everest was impressed with the aircraft. He later noted, “We 

found that the delta planform handled very well during the transition from 

subsonic to supersonic speed, in comparison with the straight-wing F-94 and 

the swept-wing F-86, which encountered severe buffet and loss of control 

effectiveness in this speed range.”8
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While Phase II flights were underway, the Air Force issued a contract to 

Convair to replace the J33-A-23 engine with an afterburner-equipped J33-

A-29 engine and lengthen the aft fuselage. This would increase the available 

thrust with afterburner to 7,500 pounds, compared to the 4,250 pounds with 

water injection available with the J33-A-23. Convair engineers estimated that 

the XF-92A’s level speed would be increased to Mach 0.98, higher than the 

maximum dive speed with the J33-A-23 engine. Moreover, the maximum dive 

speed would be Mach 1.2. To improve visibility for the photo-tracking cameras, 

the XF-92A’s bare metal exterior was also painted white. Original plans called 

for Yeager to make a ferry flight of the XF-92A from Edwards to Convair’s San 

Diego facility. Yeager took off on May 12, 1950, but suffered an engine failure 

requiring an immediate emergency landing on the lakebed. The aircraft was 

not damaged but had to be taken south by truck. The aircraft spent the next 

14 months in San Diego undergoing the engine change. The XF-92A did not 

return to Edwards until July 1951, and Yeager made the first flight with the 

new engine on July 20.

The XF-92A in flight over a cloud deck late in the Air Force test flight program. The airplane had 

been painted white, as had other X-planes, to allow the aircraft to be photo-tracked against the 

dark blue desert sky. The dark orange or dark red finishes originally used on the X-1 and the 

D-558-1 Skystreak were difficult to spot against the dark sky. (USAF)

Yeager and Everest now embarked on a second series of flights to evaluate 

the XF-92A’s performance with the new engine, which proved to be a disappointment. They found that the afterburner would flame out above 38,000 

feet, and the J33-A-29’s increased thrust failed to raise the aircraft’s Mach 
Progenitor of the Delta: The Convair XF-92A

251

number. In fact, the XF-92A’s performance was not better than it had been 

with the J33-A-23 engine. To compound matters, while the J33-A-23 engine 

had few maintenance issues, the J33-A-29 was beset by reliability problems. 

Completing the initial 18 flights of the power plant demonstration program 

required six engine changes. 

Problem areas included the fuel control system, the improper sequencing 

of the afterburner, and the backward flow of the turbine cooling air. The last 

issue resulted in molten metal in the aft compressor inlet, which triggered fire 

warning lights in the afterburner and faulty turbine wheel seal clearances.9

Despite the unreliable engine, Yeager and Everest made 10 research flights 

in the XF-92 by the end of the year. Peak altitude achieved with the aircraft was 

41,443 feet on August 8, 1952, and the top speed was Mach 1.01 on February 

2, 1953. The only serious failure occurred on September 14, 1951, when the 

engine suffered a turbine failure shortly after takeoff, forcing an emergency 

landing. The aircraft was repaired and test flights resumed. Another 11 flights 

were made in 1952 before the Air Force flights ended.10

Initial Results

As they had with earlier contractor and Air Force test flights of research aircraft, 

NACA personnel wrote research memorandums on the XF-92A’s handling and 

flight qualities. The NACA had access to the data collected and conducted the 

data reduction and analysis. From this, the XF-92A’s characteristics were determined. The first of these, on preliminary measurements of static longitudinal 

stability and trim, covered the power plant demonstration flights following 

the replacement of the original J33-A-23 engine with a J33-A-29 fitted with 

an afterburner, as well as Air Force performance testing. The flights covered 

Mach numbers up to 0.75 to 0.97, at altitudes from 11,000 to 40,000 feet. 

The research memorandum noted: 

The airplane had a stable variation of control angle with speed 

up to some Mach number between 0.75 and 0.87. A nose-down 

trim change extended from a Mach number of 0.87 to a Mach 

number of about 0.93, at which point a nose-up trim change was 

encountered.

The apparent longitudinal stability was constant up to a Mach 

number of about 0.75. As the Mach number increased above 

0.75 the apparent stability increased rapidly to approximately five 

times the low-speed value at a Mach number of 0.96. The static 

longitudinal stability increased threefold as the Mach number 
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252

increased from approximately 0.75 to 0.94; this result indicates 

that most of the increase in apparent stability was caused by an 

increase in static longitudinal stability.11

Air Force Phase II flights tested the XF-92A’s performance, and, as a result, 

no maneuvers were made during Phase II flights specifically to obtain dynamicstability data. However, several random disturbances occurred that could be 

analyzed to identify dynamic longitudinal-stability characteristics. The situation was complicated by the fact that under certain flight conditions, pilots 

observed “undesirable lateral and longitudinal oscillations” during this stage of 

the flights.12 NACA researchers thought these could indicate “the possibility 

of cross coupling between the lateral and longitudinal modes of motion.”13

To obtain the stability data, two different techniques were used. The first was 

simple: the period of the oscillations and time it took to damp out the aircraft’s 

motions were measured. The other technique, involving the use of a Reeves 

Electronic Analog Computer (REAC), represented a glimpse of the future. 

The actual control deflections were used as the REAC input. A solution, in the 

form of a time history, was produced for a transfer-function equation with a 

particular set of transfer-function coefficients. The coefficients were varied on 

each run until the output most nearly matched the actual flight record. 

Though the REAC represented an advance in data processing, it was significantly limited by the primitive computer technology of the early 1950s. 

The effects of trim changes due to Mach number and altitude changes were 

not included in the calculations. Agreement was considered good between 

the period and static-stability measurements acquired using the two methods, 

though there were differences in the time and cycles needed to damp and in 

the damping factor. These differences were thought to be due to such factors 

as the method of analysis, small control motions during the maneuver, or 

nonlinear damping. 

An example of the analysis and the information that could be determined 

from it was a time history of an XF-92A dive from 38,000 feet to 34,000 feet 

at Mach 0.94. The control deflections were small during the dive. The REAC 

was used to analyze the dive from 2 to 16 seconds, using only the longitudinal 

control inputs. The REAC calculations showed good agreement with actual 

flight data. From this, the NACA researchers concluded that no serious coupling was occurring between lateral and longitudinal modes, as the aircraft 

underwent both lateral and longitudinal motion. (An earlier concern had been 

raised over the possibility that the XF-92A’s lateral and longitudinal motions 

were coupled, indicating major stability problems.)14

After Phase II flights were completed, several Air Force test pilots made 

familiarization flights in the XF-92A. These included Colonel Albert Boyd, 
Progenitor of the Delta: The Convair XF-92A

253

Major Jack Ridley, Captain J.E. Wolfe, Captain Arthur “Kit” Murray, and 

Lieutenant Colonel Richard L. Johnson. After one landing, Colonel Boyd, 

the Edwards base commander at the time, was asked by Shannon, “Colonel, 

didn’t you find the airplane a little sensitive laterally?” Boyd replied, “No, not 

sensitive, but if I find the guy that didn’t show me where the lateral trim switch 

is, I’ll shoot him. I had to hold five pounds pressure on the stick for the whole 

flight, and my arm is about to fall off.”15

Following XF-92A tests with the J33-A-29 engine, the Air Force test team 

noted that, “Since the XF-92A was designed purely as a research airplane and 

never intended for operational use, the aircraft systems, maintenance, and 

accessibility were not stressed in the design.”16 The Air Force report also stated 

that “the ‘delta-wing’ configuration exhibits characteristics that could be used 

advantageously in future Air Force tactical aircraft.” Given the advantages 

shown by the delta wing, the report recommended improvements for a new 

delta-wing aircraft:

• Install a higher rated power plant in conjunction with the design of 

a more efficient inlet duct

• Improve the centering and breakout forces of the power 

control system

• Improve the over-sensitivity of the longitudinal and lateral 

control system

• Improve the directional stability characteristics and decrease the 

excessive dihedral effect (the rolling moment of an aircraft due 

to sideslip)

• Decrease the high side forces produced by the vertical tail being 

located too near the airplane’s center of gravity 

• Improve the poor low-speed flying characteristics during approach 

and landing

• Modify the directional control system to eliminate the trim changes 

caused by a change in temperature

• Improve directional control on the ground and cross-wind handling 

characteristics by increasing the landing gear tread and the wheel size

• Improve general ground handling characteristics and reduce landing 

ground roll by installation of an adequate brake system

• Improve forward visibility at high angles of attack

• Improve the restricted visibility resulting from the 

“greenhouse” canopy

• Relocate the landing gear position relative to the center of gravity to 

eliminate tail heaviness on the ground 

• Redesign the aft fuselage to permit engine accessibility 

for maintenance 
Probing the Sky

254

• Redesign the landing gear control system for easier operation by 

the pilot

• Install an improved hydraulic system17

Crossfield Flies the First Delta

The final three flights of the Air Force power plant demonstration program were 

completed in early 1953. Crossfield and the NACA engineers then began a 

separate series of research flights. The NACA personnel were eager to try out the 

new wing configuration. They were also aware of the aircraft’s shortcomings, 

in particular, those of the J33-series engines. One of the first things ground 

personnel did was replace the J33-A-29 jet with an afterburner-equipped J33-

A-16 engine capable of producing a maximum thrust of 8,600 pounds. The 

modification gave the XF-92A a lengthened tail cone enclosing the afterburner.

Crossfield was named NACA project pilot, and his introduction to the aircraft was not encouraging. Crossfield had a “first flight jinx”; he always seemed 

to have difficulties on his first flight of a new airplane, and the XF-92A “hop” 

was no exception. Pete Everest was originally scheduled to make the final Air 

Force flight before the XF-92A was turned over to the NACA. The XF-92A was 

towed out onto the lakebed and pointed into the wind. But the wind shifted and 

quickly grew too strong, so the final Air Force flight was canceled. Crossfield 

and Everest were meeting with Williams at the time, and Williams suggested 

that Crossfield taxi the airplane back to the NACA hangar at the main base and 

lift it off the lakebed to get a feel for its handling. Crossfield agreed. 

He climbed into the XF-92A, started the engine, closed the canopy, and 

began a fast taxi toward the main base. The XF-92A was heavy with fuel and 

wobbled into the air. By this time, the lakebed shoreline was fast approaching. 

Crossfield pulled back on the stick to raise the nose and slow the airplane. The 

decrease in speed was only slight, and he thought the XF-92A was going to 

hit a sand dune and be destroyed. Crossfield then spotted a dirt road heading 

into the desert and pressed the left brake to turn the aircraft toward it. The 

brake seized up, then failed, but not before turning the aircraft toward the 

road. When the XF-92A reached the road, the tires burst, but the airplane 

continued straight ahead. After about a hundred yards, it came to a stop. 

Crossfield triggered the fire extinguishers and jumped out. Crossfield and the 

XF-92A were unharmed. The dirt road was promptly, if unofficially, dubbed 

“Crossfield Pike.”18

The NACA XF-92A research effort involved 25 flights focused on two 

areas. The first 15 flights would collect the stability and control characteristics 

of the basic aircraft design. The subsequent 10 flights would then look at the 
Progenitor of the Delta: The Convair XF-92A

255

effectiveness of wing fences positioned at 60 percent of the wing semispan. 

Data on drag; static longitudinal, lateral, and directional stability and control; 

buffeting; wing loads; and airfoil section characteristics would be obtained 

both with and without the fences.19

The XF-92A in the desert after Crossfield’s first high-speed taxi test. The dirt road was soon 

named “Crossfield Pike.” The aircraft sustained only minor damage. (NASA)

Crossfield made his first XF-92A flight on April 9, 1953, for pilot checkout 

and static longitudinal-stability data. He then flew the airplane three times 

in April, including one flight that reached the highest altitude by an NACA 

pilot in the airplane—41,364 feet. Following one flight in May, the research 

program hit its stride. A total of eight flights were made between June 3 and 

June 24 and included the fastest NACA XF-92A flight, which reached Mach 

0.962. All these flights were made for longitudinal-stability and control data. 

The procedure was to make sideslips, aileron rolls, and rudder pulses at altitudes 

from 18,000 to 30,000 feet at indicated airspeeds of 160 to 420 mph. 

In the case of the rudder pulses, Crossfield stabilized the aircraft at the 

planned test speed and altitude and then made an abrupt pulse of the rudder 

while the other control surfaces were held in a fixed position until the aircraft 

returned to stabilized flight. Test maneuvers were done in both a clean configuration and with landing gear down. The month’s final flight, on June 26, 

was for low-speed stability and control data.20
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After that, the aircraft was grounded to undergo the installation of the 

wing fences. Wing fences had been installed on the D-558-2 as one of several 

potential cures for pitch-up. The fence itself was simple, running from the 

wing’s front edge back to the 60-percent chord. The leading and trailing edges 

of the fence were fairing curves.21

The first two flights made with the fences installed took place on July 3. 

The fences were then modified and reinstalled on the XF-92A, which was 

again flown on July 22. The changes proved unsuccessful; one of the fences 

buckled under the flight stresses. Following the incident, the aircraft remained 

grounded for nearly a month. Crossfield took the airplane aloft on August 

17, but the engine suffered a malfunction and forced the research flight to be 

cut short. 

In contrast to the long delays the Air Force endured during test flights, the 

engine problem was repaired in 3 days and the XF-92A was back in the air on 

August 20. Crossfield made two flights that day, both for longitudinal-stability 

and control data with the modified fences. 

Not until September 30 did the aircraft again go aloft. The two flights on 

this date marked the start of flights to collect data on the airplane’s low-speed 

lateral and directional control with the original wing fences. Two more flights 

followed with the same research goals, on October 2 and October 5. The 

fences were removed as the research program drew to a close. The earlier test 

maneuvers were repeated to identify what effects the fences had on the aircraft’s 

low-speed lateral and directional control. 

A pair of flights was made on October 14, 1953. At the end of the second 

mission, Crossfield touched down on the lakebed. As the XF-92A rolled out, 

the nose landing gear collapsed. The aircraft ground-looped and came to a 

stop on its nose and one wingtip. Once Crossfield was sure the aircraft would 

not tip over, he climbed out of the airplane. Inspection revealed considerable damage to the nosewheel, forward fuselage, and right wingtip. Taken 

together, the extent of the damage, the amount of time and money that would 

have been needed to restore the XF-92A to flight status, and the fact that 

the research program was nearly complete meant that the aircraft would be 

grounded permanently.22

Even as the XF-92A project ended, the data generated during the NACA 

research flights were being analyzed. There was quite a bit of information, and 

because data reduction in that era took time, results were somewhat slow in 

coming. But come they did. And since the XF-92A was the first delta-wing 

aircraft from which any data had been collected in flight, what the Air Force 

and NACA engineers and pilots produced was of considerable value. A 1955 

research memorandum on the aircraft’s lateral-stability and control characteristics noted: 
Progenitor of the Delta: The Convair XF-92A

257

The static lateral stability characteristics as measured in sideslips 

appear satisfactory although there is a rapid increase in apparent 

dihedral effect at the lower speeds. There is adequate rudder power 

over the entire speed range tested.

The lateral control as measured in aileron rolls appears adequate over the entire speed range tested although there is a considerable reduction in aileron effectiveness at the lower speeds.

The dynamic lateral stability of the airplane was generally 

in the unsatisfactory region when compared to U.S. Air Force 

requirements for satisfactory values of reciprocal of cycles to damp 

to half amplitude and ratio of roll angle to sideslip velocity.

Although the airplane appears to have satisfactory static lateral 

stability and control characteristics, the pilots object to the overall lateral handling characteristics primarily because of the high 

roll-to-sideslip ratios which probably resulted from the relatively 

low static directional stability and relatively high effective dihedral. These adverse characteristics were aggravated at low speeds 

by high adverse yaw and rough air and at high speeds by high 

airplane response to small control deflections. The apparent high 

side force and poor hydraulic control system added to the objectionable characteristics.23

The research memorandum also included comments on the control system 

issue: “Part of the difficulties encountered on the XF-92A may be attributed to the poor hydraulic control system. Evaluation of ground calibration 

has shown the control system to have high friction and breakout forces and 

appreciable lag of surface-to-stick motion. These characteristics are particularly objectionable at low speeds where large control deflections are required 

to maneuver and also at high speeds where the airplane is sensitive to small 

control displacements.”24

Though the bulk of the NACA flights had been made for longitudinaland lateral-stability and control data, the tests of the wing fences proved 

successful. The research memorandum stated: “Installing wing fences on the 

airplane improved the handling characteristics at the lower speeds, in the 

pilots’ opinion, probably because of the increase in static directional stability 

attributed to the fences.”25

The XF-92A was eventually patched up as a static display for air shows 

and was later sent to the University of the South, at Sewanee, TN, where it 

remained until 1969. It was returned to the U.S. Air Force Museum at WrightPatterson AFB, OH, and thoroughly restored; it is now on display, along with 

the X-3, X-4, and X-5, in the museum’s Research and Development Gallery.26
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In-flight photo of the XF-92A during one of its NACA flights. The only significant difference in finish was the X’s on the nose and tail, which were put on for photo reference. The right wing had 

tufts attached to the upper surface to make visible changes in the airflow that occurred under 

different flight conditions. (USAF)

An Assessment

The XF-92A occupies an honored place among the world’s pioneering aircraft, but it was not an airplane that was popular with its pilots. An NACA 

research memorandum noted: “All the pilots who have flown the XF-92A 

(during the joint NACA-Air Force program and the NACA research program) 

have reported that the airplane exhibited poor lateral handling characteristics, particularly at low speed.”27 Crossfield later said of the airplane, “It was 

under-powered, under-geared, under-braked, and overweight.” After the gear 

collapsed on Crossfield’s last flight, he noted, “It was finished and no one shed 

any tears.”28

At the same time, the potential that the XF-92A represented was clear. The 

Air Force technical report noted, albeit in understated tones, “The ‘delta-wing’ 

configuration exhibits characteristics that could be used advantageously in 

future tactical aircraft.”29 Yeager was impressed with the high roll rate of the 

aircraft, which would be useful in a dogfight. He commented, “When I’m 
Progenitor of the Delta: The Convair XF-92A

259

doing a maximum rate of roll, it feels like 

the seat of my pants are pushing down 

into the seat while at the same time the 

centrifugal force is pulling my head up 

toward the canopy.”30 Though Crossfield 

did not lament the airplane’s demise, his 

Air Force counterparts found elements 

of it to their liking. And the Air Force 

itself saw the planform’s potential. 

On another note, the shortcomings 

of the XF-92A’s control system underscored that the building of a successful 

supersonic aircraft was not merely an 

issue of aerodynamics. A proper control 

system had to provide handling qualities that would allow the pilot to fly his 

aircraft to its limits rather than struggle 

against it. Some of the most skilled pilots 

in America flew the XF-92A, and even 

they found it a difficult airplane to control in spite of its desirable qualities. It is 

clear in retrospect that control systems 

evolved in the same way other aircraft 

elements did in this new era of aviation and that fits and starts were part of 

the process.

The solution to performance problems that 

plagued the F-102 was a major redesign 

of the aircraft. On the top is the original 

YF-102. Like the XF-92A, it had a fuselage 

that was widest at the midsection. This 

produced excessive drag. The solution, 

developed by Richard T. Whitcomb, was 

the “area rule.” The fuselage was reduced 

in width, forming a shape referred to as a 

“Coke-bottle” or “wasp-waist.” The production F-102 saw considerable improvement 

in performance as a result of the modification. (NASA)

Of course, the story of delta-wing 

aircraft development did not end with 

XF-92A. On October 24, 1953, just 

10 days after the XF-92A’s final flight, 

Convair’s YF-102 prototype took off 

from Edwards. The new design was based 

on what Convair engineers had learned 

from the XF-92A. The new delta-wing 

aircraft featured a pointed nose to house 

the radar system along with side air 

inlets. The NACA was eager to test the 

new aircraft and had begun negotiations 

with the Air Force to receive an early prototype. But like the XF-92A, the YF-102 

had its problems. The first prototype 
Probing the Sky

260

Ground photo of the XF-92A during NACA research flights. The poor visibility from the canopy is 

evident. So is the forward position of the main landing gear, which made the aircraft tail-heavy, 

and a pole, visible under the aft fuselage, that was necessary to prevent the aircraft from resting 

on its tail. (USAF)

The YF-102 prototype interceptor was developed using data acquired with the XF-92A. The 

new interceptor was beset by performance problems and crashed only 9 days after its first 

flight. (NASA)
Progenitor of the Delta: The Convair XF-92A

261

crashed on November 2, only 2 weeks after its first flight, and the brief test 

program was just long enough to demonstrate the design’s poor performance. 

Test flights with the second YF-102A began on January 11, 1954, and 

although Convair had expected it, they found that the plane could not exceed 

Mach 0.98 and had a 

maximum ceiling of only 

48,000 feet, well below 

the required performance 

set by the Air Force. 

The design underwent 

extensive changes, which 

included cambered wing 

leading edges and new 

wingtips. The wings were 

also moved farther aft 

on the fuselage, which 

itself was lengthened by 

11 feet. The vertical fin 

was also moved aft, and, 

most important, the 

center fuselage was redesigned (thanks to NACA 

research aerodynamicist 

Richard Whitcomb) with 

a “Coke-bottle” shape to 

reduce drag. This last feature represented one of 

the great breakthroughs 

made during the early 

supersonic era.31

Two NACA ground personnel with a prototype F-102. The 

family resemblance between the XF-92A and the F-102 is 

apparent. Both had a bulky fuselage that resulted in excessive drag and poor performance. (NASA)
Probing the Sky

262

Endnotes

1. Opening quotation from Crossfield and Blair, Always Another Dawn, 

p. 167.

2. For the XF-92A’s evolution and flight research program, see Richard 

P. Hallion, “Convair’s Delta Alpha,” Air Enthusiast Quarterly 2 (June 

1976): 177; Robert E. Bradley, “The Birth of the Delta Wing,” 

American Aviation Historical Society (winter 2003): 243.

3. Hallion, “Convair’s Delta Alpha,” pp. 177–178; Bradley, “The Birth 

of the Delta Wing,” pp. 245–247.

4. See Hallion, “Lippisch, Gluhareff, and Jones,” passim.

5. Letter, Major Howard C. Goodall, USAF, to Paul E. Garber, “DM-1 

Glider Disposal,” November 28, 1949, in the Gluhareff Dart 

accession file, Registrar’s Office, National Air and Space Museum, 

Smithsonian Institution, Washington, DC.

6. For details, see Hallion, “Convair’s Delta Alpha,” passim.

7. Bradley, “The Birth of the Delta Wing,” pp. 254–258. The pointdefense role would eventually be filled by surface-to-air missiles 

(SAMs), which provided quicker response and could be built, 

deployed, and operated far more cheaply than aircraft. 

8. Hallion, “Convair’s Delta Alpha,” p. 181.

9. “High-Speed Flight Research Station: Research Airplane Project 

Panel 1952 Annual Report,” pp. 8–9.

10. Hallion, “Convair’s Delta Alpha,” pp. 181–182; Bradley, “The Birth 

of the Delta Wing,” p. 259. The following chronology of reports 

offers some idea of the engine problems faced by the XF-92-A:

10/3/1951: Installed in airframe.

10/4–8/1951 Ground runs—would not start on “normal,” had 

to use emergency fuel supply.

10/10/1951 AF flight 10—afterburner would not light normally.

10/11/1951 Engine removed—turbine rotor #5-6-7 replaced.

11/28/1951–1/9/1952 Reinstalled—experienced fuel control 

problems, changed controls several times/two small fuel fires 

(no damage).

1/10/1952 Removed from aircraft—nozzle vanes cracked/diaphragm replaced.

6/4/1952 AF flight 11—two cracks found in aspirator.

6/5/1952 AF flight 12—molten metal found in aft impeller area.

6/9–11/1952 Ground runs—cracks in afterburner.

6/11/1952 Engine removed from aircraft—cooling air diffuser 

overheated.
Progenitor of the Delta: The Convair XF-92A

263

7/18/1952 Test cell run.

7/22/1952–8/5/1952 Installed in airframe/ground runs. 

8/6–8/1952 AF flights 13–14

8/15/1952 AF flights 15–16

8/18/1952 Removed from aircraft—air baffles cracked—metal 

found on aft impeller area.

11. Thomas R. Sisk and John M. Mooney, “Preliminary Measurements 

of Static Longitudinal Stability and Trim for the XF-92A DeltaWing Research Airplane in Subsonic and Transonic Flight,” NACA 

RM L53B06 (March 27, 1953), p. 5.

12. Euclid C. Holleman, John H. Evans, and William C. Triplett, 

“Preliminary Flight Measurements of the Dynamic Longitudinal 

Stability Characteristics of the Convair XF-92A Delta-Wing 

Airplane,” NACA RM L53E14 (June 30, 1953), p. 1.

13. Ibid., pp. 4–5.

14. Ibid., p. 5.

15. Letter, William F. Chana to James Young, April 4, 2001; 

“Comments on Derek Horne Convair XF-92A Website,” Edwards 

AFB History Office F-92 file. The quoted text is from the letter.

16. Joseph W. Redd, Jr., Major Charles E. Yeager, and Lieutenant 

Colonel Frank K. Everest, “Performance Flight Tests of the XF-92A 

Airplane, USAF S/N 46-682, with a J33-A-29 Power Plant,” 

AFFTC Technical Report No. 53-11, Edwards AFB, CA: Air Force 

Flight Test Center, 1953, pp. 4–5.

17. Ibid.

18. Crossfield and Blair, Always Another Dawn, pp. 165–167. When a 

pilot was killed in a crash at Edwards Air Force Base, a street was 

named for the pilot as a memorial. Crossfield was the only living 

pilot at the time with a street named after him.

19. “High-Speed Flight Research Station: Research Airplane Project 

Panel 1953 Annual Report,” pp. 10–11.

20. Hallion and Gorn, On the Frontier, p. 405 (flight records); Thomas 

R. Sisk and Duane O. Muhleman, “Lateral Stability and Control 

Characteristics of the Convair XF-92A Delta-Wing Airplane as 

Measured in Flight,” NACA RM H55A17 (May 26, 1955), p. 1; 

Euclid C. Holleman, “Flight Measurements of the Lateral Response 

Characteristics of the Convair XF-92A Delta-Wing Airplane,” 

NACA RM H55E26 (August 5, 1955), p. 5.

21. Sisk and Muhleman, “Lateral Stability and Control Characteristics 

of the Convair XF-92A Delta-Wing Airplane as Measured in Flight,” 

p. 19.
Probing the Sky

264

22. “High-Speed Flight Research Station: Research Airplane Project 

Panel 1953 Annual Report,” p. 10; Hallion and Gorn, On the 

Frontier, p. 405 (flight records).

23. Sisk and Muhleman, “Lateral Stability and Control Characteristics 

of the Convair XF-92A Delta-Wing Airplane as Measured in Flight,” 

p. 12. Other research memorandums on the XF-92A include 

Clinton T. Johnson, “Flight Measurements of the Vertical-Tail 

Loads on the Convair XF-92A Delta-Wing Airplane,” NACA RM 

H55H25 (October 27, 1955); and Earl R. Keener and Gareth H. 

Jordon, “Wing Pressure Distributions over the Lift Range of the 

Convair XF-92A Delta-Wing Airplane at Subsonic and Transonic 

Speeds,” NACA RM H55G07 (November 30), 1955.

24. Ibid.

25. Ibid.

26. Crossfield and Blair, Always Another Dawn, p. 167; Convair XF-92A 

Fact Sheet, National Museum of the United States Air Force.

27. Sisk and Muhleman, “Lateral Stability and Control Characteristics 

of the Convair XF-92A Delta-Wing Airplane as Measured in Flight,” 

p. 1.

28. Crossfield and Blair, Always Another Dawn, pp. 165, 167.

29. Redd, Yeager, and Everest, “Performance Flight Tests of the XF-92A 

Airplane, USAF S/N 46-682, with a J33-A-29 Power Plant,” p. 4.

30. Fred Stoliker, Bob Hoey, and Johnny Armstrong, Flight Testing at 

Edwards: Flight Test Engineers’ Stories 1946–1975 (Edwards, CA: 

Flight Test Historical Foundation, n.d.), pp. 29–30.

31. In late 1949 and early 1950, Langley researchers conducted wind 

tunnel tests of models of aircraft with swept wings and optimized 

fuselage shapes. The results were disappointing. The models had 

a higher total drag than transonic theory had predicted for the 

combined drag of the fuselage and wings. Richard T. Whitcomb, 

one of the researchers working on the study, realized he needed to 

understand the details of the flow fields around different wing/fuselage combinations at transonic speeds. The first clue was found in 

Schlieren photos, which showed shock waves forming at the model’s 

nose; a second where the wing and fuselage pushed the air out of 

their way; and a third at the wings’ trailing edge. This investigation was followed in November 1951 by another test series focused 

on measuring the transonic drag caused by interference between 

the wings and fuselage. Whitcomb drew two conclusions from 

this series. The first was that even small changes in the shape of the 

fuselage caused disproportionate changes in the amount of drag. The 
Progenitor of the Delta: The Convair XF-92A

265

second was that the drag from the wings and fuselage was the result 

of both interacting together and not, as had been assumed, the result 

of simple addition. In thinking about these conclusions, Whitcomb 

realized that if the middle of the fuselage were pinched in, the air 

would flow over the area and would not form the strong shock waves 

shown in the Schlieren photos. 

Convair had learned of the “area rule” and hurriedly applied it to 

their YF-102A prototype. Whitcomb assisted with the effort. On 

December 20, 1954, a YF-102A modified with an area-rule fuselage went supersonic without difficulty. The modifications proved 

successful, and the Air Force accepted its first production F-102A 

on June 24, 1955. The F-102A showed that a delta-wing aircraft 

could fulfill the promise hinted at in the XF-92A. These planes were 

soon followed by Convair’s F-106 interceptor and B-58 medium 

bomber, which were also semi-tailless delta-wing aircraft. Other U.S. 

semi-tailless delta-wing aircraft were Lockheed’s Blackbird family—the A-12, YF-12, M-21, SR-71, and D-21, as well as North 

American Aviation’s XB-70 Valkyrie. France undertook the development of a family of semi-tailless delta-wing aircraft, beginning 

with the Dassault MD 550 Mirage I test aircraft, the Mirage II and 

Mirage III fighters, and the Mirage IV strategic bomber (the same 

sequence of a test aircraft, two fighters, and a bomber that Convair 

had followed). Another French delta aircraft, the Nord 1500-02 

Griffon II, was designed to be a ramjet-powered fighter similar to 

the original XP-92 concept and, flown by Andre Turcat, successfully 

demonstrated the concept. British researchers extensively developed deltas, beginning with the Avro Type 707 research aircraft, 

which served as a flying mockup of the Vulcan delta-wing strategic 

bomber. The Boulton Paul P.111 and P.120 served as transonic deltawing research aircraft. The Fairey Delta 2 extended this research 

to supersonic speeds and, in modified form, influenced the “ogee” 

wing planform of the Anglo-French Concorde SST, one of only two 

commercial deltas, which introduced airline passengers to Mach 2 

flight. The Soviet Tu-144 also featured a delta wing, of the so-called 

“double delta” planform. Sweden introduced a remarkably advanced 

multipurpose fighter, the delta-wing Saab 35 Draken, following this 

with the JA-37 Viggen and the JAS-39 Gripen. With the end of the 

Cold War, several air forces now fly the Eurofighter, a combination 

of a classic semi-tailless delta planform with small canards. Finally, 

on the horizon are several proposed delta-shaped Unmanned Aerial 

Vehicles (UAVs), which eliminate the vertical tail. Thus, across a 
Probing the Sky

266

span of six decades, the descendants of the XF-92A fly, marking it as 

one of the most successful of the NACA’s configuration demonstrator X-planes. 

Regarding these aircraft, see Bill Gunston, Early Supersonic Fighters 

of the West (New York: Charles Scribner’s Sons, 1975); Marcelle 

Size Knaack, Post–World War II Fighters 1945–1973 (Washington, 

DC: Office of Air Force History, 1986); and John W.R. Taylor, 

Jane’s Pocket Book of Research and Experimental Aircraft (New York: 

Collier Books, 1976). Regarding Whitcomb and his “Area Rule,” 

see Richard T. Whitcomb, “A Study of the Zero-Lift Drag-Rise 

Characteristics of Wing-Body Combinations Near the Speed of 

Sound,” NACA RM L52H08 (September, 3, 1952); Richard P. 

Hallion, “Richard Whitcomb’s Triple Play,” Air Force Magazine

93, no. 2 (February 2010): 68–72; and Jeremy Kinney, “Richard 

Whitcomb and the Quest for Aerodynamic Efficiency,” in Richard 

P. Hallion, ed., NASA’s Contributions to Aeronautics, vol. 1: 

Aerodynamics, Structures, Propulsion, Controls, NASA SP-2010-570-

Vol 1 (Washington, DC: NASA, 2010), pp. 89–133.

268

The third D-558-1 in flight with scattered clouds in the distance and the desert below. The 

Skystreak did not have the impact the XS-1 had on aviation technology, but it did contribute to 

the understanding of transonic flight. (NASA)
269

Round One: A Reflection

The United States research-airplane program has been a fruitful one 

which resulted in a systematic and orderly approach to extending 

man’s flight capabilities in both the atmosphere and space.

—Kenneth S. Kleinknecht1

In the final years of the Second World War, knowledgeable authorities recognized that aviation was poised on the cusp of a revolution. The piston-powered 

propeller airplane was in decline, the jet was in the ascendant, and the rocket 

and missile were an uncertain emerging technology. Aircraft were already entering the transonic region, and the ballistic missile had already traversed it and 

was even nudging the hypersonic. The next clear challenge in piloted, winged 

flight was the achievement of supersonic flight. 

But supersonic flight required a reliable knowledge base, and at this pivotal 

moment in aeronautical history, the NACA and other Federal institutions 

lacked the ability to produce it using conventional means. The standard tool 

of aeronautical research—the wind tunnel—was unable to cope with the distorted flows caused by shock impingement at transonic velocities. Theory was 

contradictory. Flight experience was replete with examples of aircraft that had 

already crashed and pilots who had already died in the grips of “compressibility.” It was in this environment that the military services, the NACA, and 

the industry formed a robust partnership to seek a new kind of research tool, 

one that would use the sky itself as a laboratory—the transonic and supersonic 

research airplane. 

The decade following the initial research flights of the X-1 and D-558-1 was 

a dynamic period in the history of aviation, arguably exceeded only by the first 

decade after the invention of the airplane by the Wright brothers in 1903. It 

marked a great divide between what came before and what came after. When 

the decade began, jet aircraft represented a new and unreliable technology and 

Mach 1 seemed to many an unattainable goal. By decade’s end, supersonic 

flight was routine, humanity had placed small satellites in orbit, and space 

now beckoned. In the same fashion that aviation progressed in the first years 

after the Wrights, researchers, designers, engineers, and pilots were feeling their 
Probing the Sky

270

way into the supersonic era. Not surprisingly, aircraft of both eras had severe 

shortcomings in control, handling, stability, and performance. 

Years after the First World War, an authentic reproduction of a World War I 

Sopwith Camel was built and test-flown to see how well the plane flew. By 

then, designers, engineers, and pilots well understood how an airplane should 

fly and what handling qualities it should have. The description of the Camel’s 

flight characteristics was telling: 

Once in the air...the pilot is faced with almost total control disharmony. The Camel is mildly unstable in pitch and considerably 

unstable in yaw, and both elevator and rudder are extremely light 

and sensitive...the ailerons are in direct and quite awe-inspiring 

contrast. The Camel...has four enormous...barn doors [for 

ailerons] which require an equally enormous force to be moved 

quickly. And when you have moved them, the wing section is so 

degraded...that the roll response is very slow indeed.... At the 

same time, aileron drag is quite staggering.2

While the wording of NACA research memorandums relating to the Round 

One research aircraft was not as “colorful,” one need only compare this description of the Sopwith Camel to the shortcomings of the X-planes of the early transonic and supersonic era to understand that both were flying into the unknown:

D-558-1: Landing gear and brake problems, vulnerability of the 

control cables to damage, an unreliable capsule escape system, 

nose-heavy above Mach 0.80, wing heaviness at high Mach numbers, rolling-yawing oscillation, and elevator vibrations approaching the maximum load limit. 

D-558-2: Long and slow takeoff using the original jet engine 

and JATO rockets, pitch-up during maneuvers leading to loss of 

control, poor high-speed stability resulting in uncontrolled rolls, an 

unreliable capsule escape system, and oscillations during landing. 

X-3: Unreliable and underpowered jet engines, tire failures, 

very high takeoff and landing speeds, failure to reach design performance, flawed control system, and dangerous combined oscillations in flight. The most serious issue was inertial coupling. 

Walker encountered forces during the inertial coupling flight 

that reached the fuselage’s maximum-load limit. Had they been 

slightly greater, both plane and pilot would have been lost. 

X-4: Severe stability and control issues, flight control problems, 

jet engine problems, and poor handling at speeds near Mach 1.
Round One: A Reflection

271

X-5: Complexity and weight of the wing-sweep mechanism, 

undesirable lateral-stability and control characteristics, violent 

stall/spin characteristics, and slow recovery. The aircraft was so 

unstable under some conditions that one pilot doubted he could 

keep it flying right side up.

XF-92A: Overly sensitive longitudinal and lateral controls 

combined with a lag in the response, unreliable and underpowered engines, poor directional stability, poor low-speed control 

during approach and landing, and poor brakes.3

Indeed, somewhat ironically, having shortcomings proved in some respects 

to offer an advantage. They allowed the NACA, the Air Force, the Navy, and 

contractors to anticipate and correct deficiencies before operational aircraft 

were built using their configurations. 

It is well worthwhile, then, to review these aircraft from the point of view 

of what they contributed: 

D-558-1: With a largely similar configuration to the more famous 

XS-1, this aircraft complemented the results achieved with the 

X-1. The rocket-powered X-1 provided data at speeds above Mach 

1, and the jet-powered D-558-1 covered the high transonic speed 

range. As a result, the X-1 did not have to make low-speed flights; 

this accelerated the overall collection of data. Additionally, this 

aircraft was the first to test vortex generators, which improved 

wing aerodynamics at transonic speed and became standard on 

jet aircraft. 

D-558-2: Research with this pioneering swept-wing aircraft showed how to avoid the stability problems inherent in 

these wings, in particular that the horizontal stabilizers should 

be mounted low on the fuselage. When this aircraft was being 

designed, all operational U.S. and British jet aircraft had straight 

wings. Swept wings introduced the potential for much higher 

performance. The D-558-2 was also used in external stores testing, 

which allowed combat aircraft to fly at high speeds while carrying 

streamlined weapons and fuel tanks. Information derived from 

the D-558-2 program made all swept-wing aircraft that came after 

it safer and certainly more controllable.

X-3: In terms of performance, this was the biggest disappointment among the early X-planes. It never came close to reaching 

its design speed of Mach 2 due to its underpowered jet engines. 

But it did show that low-aspect-ratio wings were practical, and 
Probing the Sky

272

such wings were subsequently used on aircraft such as the F-104, 

F-5/T-38, and X-15. The X-3’s most valuable contribution was to 

reveal inertial coupling to be anything but theoretical. The fully 

instrumented X-3 greatly enhanced the NACA’s understanding 

of the phenomenon and was passed to industry as well. 

X-4: The semi-tailless swept wing was the only configuration 

tested in the 1950s that proved impractical. It would be another 

three decades before computer fly-by-wire systems would make 

the semi-tailless swept-wing configuration a workable reality, in 

the first generation of stealth aircraft and in many other aircraft 

and remotely piloted systems that have followed. 

X-5: The aircraft pioneered variable-sweep wings, but at a 

terrible price. While its wing pivot concept was impractical, it 

did validate the concept of variable sweep, encouraging further 

development that spawned the fixed outboard pivot point, used 

on subsequent aircraft such as the F-111, MiG-23, B-1, F-14, 

and Tu-160. 

XF-92A: This accidental research airplane had more than its 

share of poor flying characteristics, control system flaws, design 

shortcomings, and engine deficiencies. Unloved by its pilots, it 

nevertheless demonstrated the value of the delta-wing configuration, which became one of the iconic planforms of the supersonic—and indeed hypersonic—era. The XF-92A directly led to 

a notable family of delta successors, and from this sprang as well 

(though not solely because of it) Richard Whitcomb’s concept of 

area ruling. 

The X-series family was generally hailed for its contributions to aeronautical science but was not without some critics. For example, Clarence L. “Kelly” 

Johnson, head of the famed Lockheed “Skunk Works,” dismissed the X-planes 

as drawing resources and technical effort that could best be focused on the 

development of practical aircraft designs. But in retrospect it is hard to imagine, given the utter unknowns of supersonic flight, that any wind tunnel or 

rocket-boosted model tests could alone have produced the volume of data 

needed to determine which designs would work and which would not. The 

X-planes also provided practical experience operating exotic aircraft with unfamiliar designs and as-yet-unidentified problems. Had there been no X-planes, 

designers would have had to try out different configurations on prototype 

production aircraft. This would have been a waste of time and money and 

would have interfered with the building of U.S. air power. And despite his 

public dismissal, Kelly Johnson himself quietly took full advantage of NACA 
Round One: A Reflection

273

research when he designed the Lockheed F-104. That task would have taken 

considerably longer and cost much more than it did had the NACA not undertaken X-plane research.4

This book has traced the development and significance of seven of those 

research airplanes. Each had its own distinctive “personality,” and each contributed to aeronautical advancement in its own way. While each program had its 

own trials, tribulations, and difficulties, in great measure all worked together 

to advance the cause of flight.

Beyond these “Round One” airplanes was “Round Two”: the hypersonic 

X-15 that took piloted, winged flight across the divide from the atmosphere 

into space. If more glamorous, more exciting, and better known than its predecessors, it was nevertheless largely dependent for its success upon the structure, 

organizational culture, and expertise that the military services, the NACA (and 

NASA thereafter), and the industry had developed during the earlier Round 

One effort.

As the Round One aircraft wound down and Round Two began its own gestation, a shocking event occurred that dramatically reshaped not only American 

but global perspectives on air and space: the Soviet Union’s launch of Sputnik, 

the world’s first artificial satellite, on October 4, 1957. The significance of 

Sputnik’s launch became even more apparent in ensuing weeks and was the 

impetus for a permanent U.S. space program and a new agency to run it.5

 Less 

than a week after Sputnik I was launched, the American Rocket Society called 

for the establishment of an “Astronautical Research and Development Agency” 

that would be responsible for nonmilitary space activities. The Rocket and 

Satellite Research Panel of the National Academy of Sciences made a similar 

suggestion on November 21, 1957. 

President Dwight D. Eisenhower responded with several actions. These 

included announcing the establishment of the position of the Special Assistant 

to the President for Science and Technology, and he named Massachusetts 

Institute of Technology (MIT) president James R. Killian, Jr., to the post. 

Soon after, the President’s Scientific Advisory Committee (PSAC) was established, with Killian named its chairman. Killian’s first assignment in his new 

position was to review the alternatives for a U.S. space program. His report, 

titled “Memorandum on Organizational Alternatives for Space Research and 

Development,” was completed on December 30, 1957, and offered two options. 

The first was for the DOD to establish a central space laboratory that could 

undertake basic space research as well as military-related work. This laboratory 

might also have the authority to sponsor research in civilian institutions.6

 His 

second was using the NACA as an alternative to a military-sponsored central 

space laboratory, an idea first proposed by James McCormack, a vice president of MIT, and James B. Fisk, of Bell Telephone Laboratories.7

 In Killian’s 
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274

proposal, space activities would be split between the DOD and “some other 

agency or agencies external to the DOD” that “might engage in basic research.”8

Killian continued, “One obvious way of doing this would be to encourage 

the NACA to extend its space research and to provide it with the necessary 

funding to do so.” 

In February 1958, Eisenhower asked Killian to study options for organizing the U.S. space effort. Killian appointed a PSAC panel headed by Nobel 

laureate Edward Purcell that included the University of California’s Herbert 

York, NACA chairman James H. “Jimmy” Doolittle, and Polaroid’s Edwin 

Land. The Purcell committee members were favorably disposed to accepting 

Killian’s recommendation regarding the NACA, particularly since other contenders (including the National Science Foundation and the Atomic Energy 

Commission) had various shortcomings. Accordingly, in early March, the committee recommended a reorganized NACA as the organization best prepared 

to manage civilian space activity. Draft legislation was soon written, leading 

to the establishment of the National Aeronautics and Space Administration, 

the follow-on agency to the NACA, and began operations on October 1, 

1958. The “new-old” Agency absorbed the NACA’s existing laboratory structure—the Langley, Lewis, Ames, and Edwards facilities—joining them to the 

Development Operations Division of the Army Ballistic Missile Agency (subsequently the Marshall Space Flight Center) and partnering with the California 

Institute of Technology’s Jet Propulsion Laboratory (JPL).

Each of the Centers brought different skills, traditions, cultures, and 

experiences to the new Agency. These had been shaped in the course of the 

transonic-supersonic revolution, which had made the NACA the only organization realistically qualified to take America into space. On October 5, 1958, 

4 days after opening its doors, NASA Administrator T. Keith Glennan said, 

“All right. Let’s get on with it!”9

 Thus began the Mercury program, first to put 

an American astronaut into orbit.

Mercury was where the legacy of the Round One research programs paid 

off. In contrast to the academic research culture of Ames, Langley, and Lewis, 

the High-Speed Flight Station was more operationally oriented. This was due 

to its focus on flying research aircraft, the lack of wind tunnels or similar 

ground research facilities, and the close ties it maintained with the military 

services. The experiences of Round One airplanes, and particularly the HighSpeed Flight Station pilots, ground crews, and researchers, became the basis 

for planning Mercury.10

From December 1946 onward, the High-Speed Flight Research Station 

personnel had been flying piloted rockets, with hundreds of successful flights 

to the facility’s credit. No one else on Earth, in the United States, Europe, or 

the U.S.S.R., had such experience. The Round One pilots—Air Force, Navy, 
Round One: A Reflection

275

NACA, Marine, or contractor—were the first to see the sky turn black and 

see Earth’s curvature, its hallmark as a planet. Their flights reached speeds and 

altitudes not exceeded until the first Soviet and American space missions. They 

wore the first partial pressure suits and then, later, full pressure suits anticipating the spacesuits of the 1960s in order to survive cockpit depressurization at 

the altitudes at which they flew. Their ground crews developed and mastered 

the procedures for checking out the aircraft; maintaining the temperamental 

rocket engines; and handling liquid oxygen, pure hydrogen peroxide, alcohol/

water mixtures, anhydrous ammonia, and the other dangerous chemicals used 

in the engines. High-Speed Flight Station engineers had done the first test 

work with the reaction controls necessary to stabilize an orbiting spacecraft 

operating in a vacuum. Indeed, the very design developed for testing on the 

X-1B became the basis for the Mercury spacecraft’s thrusters. And it was the 

High-Speed Flight Research Station’s control room that the Mercury project 

copied when casting about for a way to monitor the spacecraft.

In many respects, the NACA’s human contribution to the X-series program 

was its most significant. The transonic and supersonic revolution was, like history itself, the working of men and women over time. Those of the NACA who 

had advanced flight through the speed of sound were, to a great degree, the 

men and women who now advanced flight into space.11 Robert Gilruth, who 

pioneered the interim “wing flow” method of transonic research and who had 

favored a thin wing for the XS-1, became Director of the Space Task Group, 

responsible for America’s first piloted space program, Project Mercury, and later 

Director of the Manned Spacecraft Center, now the NASA Lyndon B. Johnson 

Space Center. Walter C. Williams, who had gone to Muroc from Langley to 

head the Muroc Flight Test Unit, remaining there to lead the High-Speed 

Flight Station into the NASA era, became Operations Director for Project 

Mercury, and later NASA’s Chief Engineer, a singular distinction. Hartley 

Soulé, the NACA’s Research Airplane Projects Leader, supervised establishment 

of its tracking network. Gerald Truszynski, who had instrumented and tracked 

the XS-1 at Pinecastle and Muroc, remaining through the Round One era at 

Edwards as chief of instrumentation at the HSFS, eventually became NASA 

Associate Administrator for Tracking and Data Acquisition in the Apollo and 

Shuttle eras.

Fittingly, one of their colleagues, a young research pilot who flew the X-1, 

the X-5, and later (in the early years of NASA) the X-15, went on to fly into 

space and become the first human to set foot on another celestial body: Neil 

A. Armstrong. He was humanity’s link from the surface of Earth through the 

atmosphere and into space and from the subsonic to the transonic, supersonic, 

hypersonic, and onwards to escape velocity and footprints on other worlds.12
Probing the Sky

276

Progress always extracts a price, and flight has always extracted a particularly 

high tribute from those who have sought to advance its frontiers. The early 

Round One X-series was no different: between 1948 and 1957, accidents and 

incidents related to the Round One program claimed nine airplanes—seven 

research airplanes and two launch aircraft—and the lives of five aircrew (two in 

airplanes discussed in this work). It is to them—pilots Howard Lilly, Raymond 

Popson, Jean Ziegler, Milburn Apt, and aircrewman Frank Wolko—that this 

book is dedicated.13
Round One: A Reflection

277

Endnotes

1. Kenneth S. Kleinknecht, “The Rocket Research Aircraft,” in Eugene 

M. Emme, ed., The History of Rocket Technology: Essays on Research, 

Development, and Utility (Detroit: Wayne State University Press, 

1964), p. 210.

2. Brian Lecomber, “Flying the Sopwith Camel and Fokker Triplane,” 

Flight International (April 8, 1978), quoted in Robert P. Harper, Jr., 

and George E. Cooper, “Handling Qualities and Pilot Evaluation,” 

1984 Wright Brothers Lectureship in Aeronautics (New York: 

American Institute of Aeronautics and Astronautics, 1984), p. 7.

3. Hallion and Gorn, On the Frontier, pp. 56–59.

4. Miller, The X-Planes X-1 to X-45, pp. 285–289. Years later, it should 

be noted, Johnson himself sought to acquire “X-series” status for his 

Model CL-1200 Lancer, a proposed derivative of the F-104, which 

did in fact receive the designation X-27, though it was never actually 

built and flown.

5. Robert A. Divine, The Sputnik Challenge: Eisenhower’s Response to 

the Soviet Satellite (New York: Oxford University Press, 1993), pp. 

7–9; Roger D. Launius, “An Unintended Consequence of the IGY: 

Eisenhower, Sputnik, and the Founding of NASA,” AIAA Paper No. 

2008-860 (2008).

6. J.R. Killian, Jr., “Memorandum on Organizational Alternatives for 

Space Research,” accessed on July 5, 2011, http://history.nasa.gov/

sputnik/iv1.html.

7. Brian R. Page, “The Creation of NASA,” Journal of the British 

Interplanetary Society 32 (October 1979): 449–450.

8. Killian, “Memorandum on Organizational Alternatives for Space 

Research.”

9. Loyd S. Swenson, Jr., James M. Grimwood, and Charles C. 

Alexander, This New Ocean: A History of Project Mercury, NASA 

SP-4201 (Washington DC: NASA, 1966), p. 109.

10. William M. Bland, Jr., “Project Mercury,” in Emme, ed., The History 

of Rocket Technology, pp. 212–240.

11. As related in Sylvia Doughty Fries, NASA Engineers and the Age of 

Apollo, NASA SP-4104 (Washington, DC: NASA, 1992).

12. As noted in Hallion, Supersonic Flight, pp. 203–204.

13. The aircraft were the Bell X-1-3, X-1A, and X-1D; the Bell X-2 #1 

and X-2 #2; the Bell X-5 #2; the Douglas D-558-1 #2; and two 

Boeing EB-50 launch aircraft (one lost with the ground explosion 

of the X-1-3 and one scrapped after the in-flight explosion and loss 
Probing the Sky

278

of the X-2 #2). The aircrew were Jean Ziegler, Frank Wolko, and 

Milburn Apt (both X-2s); Ray Popson (X-5 #2); and Tick Lilly 

(D-558-1 #2). Additionally, Bell pilot Joseph Cannon was seriously injured in the explosion of the X-1-3, fortunately surviving to 

fly again.

Probing the Sky

280

Technical Specifications for the Round One Aircraft

APPENDIX

Aircraft Type: Bell XS-11 Douglas D-558-1 Douglas D-558-2 Douglas X-3 Northrop X-4 Bell X-5 Convair XF-92A Jet only All rocket

Engine Number, Manufacturer, TypeStatic Thrust (lb)

1 RMI XLR-116,000

1 GE TG-1805,000

1 West. J343,000

1 RMIXLR-86,000

2 West. J344,850 ea. 

2 West.J301,600 ea.

1 Allison J354,900

1 Allison J355,600

Propellants (gallons and type of propellant)2

311 LOX 293 DEA 230 kerosene 260 gasoline 345 LOX 378 DEA 970 kerosene 240 gasoline 340 kerosene 560 kerosene

Weights

T.O./Launch (lb) 12,250 10,105 10,572 15,787 21,900 7,820 9,960 15,560

Landing (lb) 7,000 7,711 7,914 9,421 17,500 6,452 7,850 11,808

Height (ft) 10.85 12.14 12.67 12.8 12.52 14.8 12.2 17.75

Length (ft) 30.90 35.71 42.0 66.75 23.25 33.6 42.80
Technical Specifications for the Round One Aircraft

281

Aircraft Type: Bell XS-11 Douglas D-558-1 Douglas D-558-2 Douglas X-3 Northrop X-4 Bell X-5 Convair XF-92A Jet only All rocket

Wing

Airfoil Section NACA 65-108 NACA 65-110 NACA 63-010 root to NACA 63-012 tip Douglas Modified Hexagon3 NACA 0010-64 NACA 64(10)A011 to 64(08)A008.28 NACA 65(06)-006.5

Span (ft) 28.0 25.0 25 22.69 26.1 31.9 max 20.0 min 31.33

Wing Area (sq ft) 130 150.7 175.0 166.50 200 184.3 max 166.9 min 425

Root Chord (ft) 6.18 6.21 MAC4 9.04 10.58 10.25 5.61 min MAC 10.05 max MAC 27.13 Tip Chord (ft) 3.09 5.1 4.11 4.67 0.0

Flap Area (sq ft) 11.6 n/a 12.58 17.22 16.7 15.9 n/a

Aileron Area (sq ft total) 6.30 7.94 9.8 8.08 17.2 [elev.] 7.24 76.19 [elev.]

Horizontal Tail

Airfoil Section 65-006 65-008 63-010 hexagon none 65A-006 none

Area (sq ft) 26 35.98 39.9 n/a

43.24

n/a 31.5 n/a

Elevator Area (sq ft total) 5.2 8.6 9.4 17.2 [elev.] 6.9 76.19 [elev.]

Vertical Tail

Area (sq ft) 25.6 25.68 36.6 23.73 16.0 29.5 75.35

Rudder Area (sq ft) 5.2 7.92 6.15 5.44 4.1 4.7 11.50

Perf.

Mach No. atAltitude, ft(mph)

1.45 35,000 (960)

0.832 sea level(632)

0.825 20,000(585)

2.0162,000(1,290)

1.2130,000(822)

0.9230,000(625)

1.0330,000(700)

1.0130,000(686)

1. Specification for XS-1 no. 1 (46-062) at time of first supersonic flight, October 14, 1947. 2. LOX: Liquid Oxygen; DEA: Diluted Ethyl Alcohol. 3. Douglas, not an NACA, airfoil section. 4. MAC: Mean Aerodynamic Chord.
282

Acknowledgments

Writing a book as lengthy and complicated as a history of the early X-planes 

could not have been done without the help of many people. They include 

Tony Springer at NASA Headquarters, who provided funding for this and 

earlier projects; Christian Gelzer, the project supervisor; and Peter W. Merlin, 

who tracked down half-century-old weekly progress reports on the X-planes 

in the Dryden archives. Thanks to Karl A. Bender and Freddy Lockarno, at 

the Dryden Flight Research Center’s Technical Library, for tracking down 

NACA Research Memorandums describing the data collected and problems 

experienced with the early X-planes. Thanks as well to Albion Bowers for his 

explanation of rocket science. Sarah Merlin receives my thanks for the first 

round of copyediting. Thanks also to Dr. Richard P. Hallion, who added significantly to the text, and to the copy editors at NASA Headquarters. 

Finally, I wish to recognize the courage of the test and research pilots, be 

they NACA, contractor, or military, who flew the X-planes. They were the 

explorers of an undiscovered territory. Like the Wright brothers, they had to 

find their way without guideposts. The body of experience that had been built 

up over the previous decades was of no use in understanding the demands of 

transonic and supersonic flight. The engineers, wind tunnel researchers, and 

pilots were on their own. But in their efforts to overcome the unknowns, they 

were unknowingly setting the stage not simply for supersonic flight, but for a 

far greater leap. The procedures, safety rules, and experience gained in breaking 

the sound barrier were later used when the NACA became NASA, the agency 

whose new role was to explore space. 

Curtis Peebles
283

Selected Bibliography

I. Reports and Memorandums, by Type

General

Beeler, De E., and George Gerard. “Wake Measurements Behind a Wing 

Section of a Fighter Airplane in Fast Dives.” NACA TN No. 1190, 1947.

Briggs, L.J., and H.L. Dryden. “Pressure Distribution over Air Foils at High 

Speeds.” NACA Report No. 255, 1926.

Briggs, L.J., and H.L. Dryden. “Aerodynamic Characteristics of Twenty-Four 

Airfoils at High Speeds.” NACA Report No. 319, 1929.

Briggs, L.J., and H.L. Dryden. “Aerodynamic Characteristics of Circular-Arc 

Airfoils at High Speeds.” NACA Report No. 365, 1931.

Briggs, L.J., G.F. Hull, and H.L. Dryden. “Aerodynamic Characteristics of 

Airfoils at High Speeds.” NACA Report No. 207, 1924.

Buckingham, Edgar. “Jet Propulsion for Airplanes.” NACA Report No. 159, 

1923.

Caldwell, F.W., and E.N. Fales. “Wind Tunnel Studies in Aerodynamics 

Phenomena at High Speeds.” NACA Report No. 83, 1920.

Chief, Bureau of Aeronautics, United States Navy. “Transfer of Aircraft to the 

NACA Under Public Law 266 of the 81st Congress.” December 27, 1950.

Chief of Naval Operations, United States Navy. “Transfer of Aircraft to the 

NACA Under Public Law 266 of the 81st Congress.” January 5, 1951.

Day, Richard E. Coupling Dynamics in Aircraft: A Historical Perspective. NASA 

SP-532. Edwards AFB, CA: NASA Dryden Flight Research Center, 1997.
Probing the Sky

284

Harper, Robert P., Jr., and George E. Cooper. “Handling Qualities and Pilot 

Evaluation.” Wright Brothers Lectureship in Aeronautics. New York: 

American Institute of Aeronautics and Astronautics, 1984.

Jones, Robert T. “Wing Plan Form for High-Speed Flight.” Collected Works of 

Robert T. Jones. NACA Report No. 863, in NASA TM X-3334. Washington, 

DC: NASA, 1976.

Loftin, Laurence K., Jr. Quest for Performance: The Evolution of Modern Aircraft. 

NASA SP-468. Washington, DC: NASA, 1985.

Marschak, Thomas A. The Role of Project Histories in the Study of R&D, Report 

P-2850. Santa Monica: The RAND Corporation, January 1964.

McAvoy, William H., Oscar W. Schey, and Alfred W. Young. “The Effect on 

Airplane Performance of the Factors That Must Be Considered in Applying 

Low-Drag Cowling to Radial Engines.” NACA Report No. 414, 1931.

Moss, Sanford A. Superchargers for Aviation. New York: National Aeronautics 

Council, Inc., 1942.

National Advisory Committee for Aeronautics, High-Speed Flight Station, 

“Research Aircraft Projects Panel 1952 Annual Report.”

National Advisory Committee for Aeronautics, High-Speed Flight Station, 

“Research Aircraft Projects Panel 1953 Annual Report.”

Parks, James H. “Experimental Evidence of Sustained Coupled Longitudinal 

and Lateral Oscillations from a Rocket-Propelled Model of a 35° Swept 

Wing Airplane Configuration.” NACA RM L54D15, 1954.

Phillips, William H. “Effects of Steady Rolling on Longitudinal and Directional 

Stability.” NACA TN No. 1627, 1948.

Polhamus, Edward C., and Thomas A. Toll. “Research Related to Variable 

Sweep Aircraft Development.” NASA TM 83121, 1981.

Saltzman, Edwin J., and Theodore G. Ayers. Selected Examples of NACA/

NASA Supersonic Flight Research. NASA SP-513. Edwards AFB, CA: NASA 

Dryden Flight Research Center, 1995.
Selected Bibliography

285

Shortal, Joseph Adams. A New Dimension—Wallops Island Flight Test Range: 

The First Fifteen Years. NASA RP-1028. Washington, DC: NASA, 1978.

Stack, John, W.F. Lindsey, and R.E. Littell, “The Compressibility Burble and 

the Effect of Compressibility on Pressures and Forces Acting on an Airfoil.” 

NACA Report No. 646, 1938.

Stillwell, Wendell H. “Results of Measurements Made During the Approach 

and Landing of Seven High-Speed Research Airplanes.” NACA RM 

H54K24, 1955.

Strass, H. Kurt, and Edison M. Fields. “Flight Investigation of the Effect 

of Thickening the Aileron Trailing Edges on Control Effectiveness for 

Sweptback Tapered Wings Having Sharp- and Round-Nose Sections.” 

NACA RM L9L19, 1950.

Thompson, Milton O., with J.D. Hunley. Flight Research: Problems Encountered 

and What They Should Teach Us. NASA SP-2000-4522. Washington, DC: 

NASA, 2000.

Weick, Fred E. “Drag and Cooling with Various Forms of Cowling for a 

‘Whirlwind’ Engine in a Cabin Fuselage.” NACA TN No. 301, 1928.

Whitcomb, Richard T. “A Study of the Zero-Lift Drag-Rise Characteristics 

of Wing-Body Combinations Near the Speed of Sound.” NACA RM 

L52H08, 1952.

Williams, Walter C., and A. Scott Crossfield. “Handling Qualities of HighSpeed Airplanes.” NACA RM L52A08, 1952.

Wood, Donald H. “Tests of Nacelle-Propeller Combinations in Various 

Positions with Reference to Wings. Part I. Thick Wing–NACA Cowled 

Nacelle–Tractor Propeller.” NASA Technical Report No. 415, 1933.

Williams, Walter C. Letter to NACA Headquarters. “Choice of Color for 

Research Aircraft at Edwards.” NACA HSFRS, December 3, 1951.

Bell XS-1 (X-1)

Angle, Ellwyn E., and Euclid C. Holleman. “Determination of Longitudinal 

Stability of the Bell X-1 Airplane from Transient Responses at Mach 
Probing the Sky

286

Numbers up to 1.12 at Lift Coefficients of 0.3 and 0.6.” NACA RM 

L50I06a, 1950.

Beeler, De E., and John P. Mayer. “Measurements of the Wing and Tail Loads 

During the Acceptance Tests of Bell XS-1 Research Airplane.” NACA RM 

L7L12, 1948.

Carman, L. Robert, and John R. Carden. “Lift and Drag Coefficients for the 

Bell X-1 Airplane (8-Percent-Thick Wing) in Power-Off Transonic Flight.” 

NACA RM L51E08, 1951.

Drake, Hubert M., and John R. Carden. “Elevator-Stabilizer Effectiveness 

and Trim of the X-1 Airplane to a Mach Number of 1.06.” NACA RM 

L50G20, 1950.

Drake, Hubert M., Harold R. Goodman, and Herbert H. Hoover. “Preliminary 

Results of NACA Transonic Flights of the XS-1 Airplane with 10-PercentThick Wing and 8-Percent-Thick Horizontal Tail.” NACA RM L8I29, 

1948.

Drake, Hubert M., Milton D. McLaughlin, and Harold R. Goodman. “Results 

Obtained During Accelerated Transonic Flight Tests of the Bell XS-1 

Airplane in Flights to a Mach Number of 0.92.” NACA RM L8A05a, 1948.

Goodman, Harold R., and Hubert M. Drake. “Results Obtained During 

Extension of U.S. Air Force Transonic-Flight Tests of XS-1 Airplane.” 

NACA RM L8I28, 1948.

Goodman, Harold R., and Roxanah B. Yancey. “The Static-Pressure Error of 

Wing and Fuselage Airspeed Installations of the X-1 Airplanes in Transonic 

Flight.” NACA RM L9G22, 1949.

Hamlin, Benson. The Design Conception of Supersonic Flight. Amherst, NY: The 

Amherst Museum/Niagara Frontier Aviation and Space Museum, 1996.

Mattson, Axel T. “Force and Longitudinal Control Characteristics of a 1/16-

Scale Model of the Bell XS-1 Transonic Research Airplane at High Mach 

Numbers.” NACA RM L7A03, 1947.

United States Air Force. Air Force Supersonic Research Airplane XS-1 Report 

No. 1. Wright Field, OH: USAF Air Materiel Command, January 9, 1948.
Selected Bibliography

287

Williams, Walter C., and De E. Beeler. “Results of Preliminary Flight Tests of 

the XS-1 Airplane (8-Percent Wing) to a Mach Number of 1.25.” NACA 

RM L8A23a, 1948.

Williams, Walter C., Charles M. Forsyth, and Beverly P. Brown. “General 

Handling Qualities Results Obtained During Acceptance Flight Tests of 

the Bell X-1 Airplane.” NACA RM L8A09, 1948.

Young, James O. Meeting the Challenge of Supersonic Flight. Edwards AFB, CA: 

Air Force Flight Test Center, 1997.

Bell X-1A

Drake, Hubert M., and Wendell H. Stillwell. “Behavior of the Bell X-1A 

Research Airplane During Exploratory Flights at Mach Numbers Near 2.0 

and at Extreme Altitude.” NACA RM H55G25, 1955.

Powell, J.L. “X-1A Airplane Contract W33-038-ac-20062, Flight Test Progress 

Report No. 15, Period from 9 December through 20 December 1953.” Bell 

Aircraft Corporation Report No. 58-980-019, February 3, 1954.

Bell X-2

Stiffler, Ronald. The Bell X-2 Rocket Research Aircraft: The Flight Test Program. 

Edwards AFB, CA: Air Force Flight Test Center, August 12, 1957.

Douglas X-3 Stiletto

Day, Richard E., and Jack Fischel. “Stability and Control Characteristics 

Obtained During Demonstration of the Douglas X-3 Research Airplane.” 

NACA RM H55E16, 1955.

Douglas Aircraft Company, Flight Testing Division. X-3 test reports: 

“Flight Report No. 7,” 1953.

“Flight Report No. 8,” 1953. 

“Flight Report No. 23,” 1953.

Douglas Aircraft Company, “Model X-3 Mock-up Conference Airplane 

Descriptions and Illustrations.” Santa Monica, CA: Douglas Aircraft 

Company, December 6, 1948. 
Probing the Sky

288

Fischel, Jack, Euclid C. Holleman, and Robert A. Tremant. “Flight Investigation 

of the Transonic Longitudinal and Lateral Handling Qualities of the 

Douglas X-3 Research Airplane.” NACA RM H57I05, 1957. 

Fleming, Frank N. “Evolution of the Configuration of the X-3 Supersonic 

Research Aircraft.” Santa Monica, CA: Douglas Aircraft Company, August 

1, 1949.

Keener, Earl R., Norman J. McLeod, and Norman V. Taillon. “Effect of 

Leading-Edge-Flap Deflections on the Wing Loads, Load Distribution, and 

Flap Hinge Moments of the Douglas X-3 Research Airplane at Transonic 

Speeds.” NACA RM H58D29, 1958.

Marcy, William L. “High-Speed Loads Measured on the Douglas X-3 Research 

Airplane.” NACA RM H57L08, 1958.

Marcy, William L., Harriet J. Stephenson, and Thomas V. Cooney. “Analysis of 

the Vertical-Tail Loads Measured During a Flight Investigation at Transonic 

Speeds of the Douglas X-3 Airplane.” NACA RM H56H08, 1956. 

NACA HSFRS, X-3 Progress Report Memorandums for Research Airplane 

Projects Leader: 

“Progress Report for the X-3 Research Airplane for the Period November 

1 to November 30, 1953.”

“Progress Report for the X-3 Research Airplane for the Period July 1 to 

July 31, 1954.”

“Progress Report for the X-3 Research Airplane for the Period August 1 to 

August 31, 1954.”

“Progress Report for the X-3 Research Airplane for the Period September 

1 to September 30, 1954.”

“Progress Report for the X-3 Research Airplane for the Period October 1 

to October 31, 1954.”

NACA HSFRS, X-3 Status Reports for Chief of Research: 

“X-3 Status Report for Period 14 September to 18 October 1952.”
Selected Bibliography

289

“X-3 Status Report for Week Ending 25 October 1952.” 

“X-3 Weekly Status Report Week Ending 25 April 1953.” 

“X-3 Weekly Status Report Week Ending 2 May 1953.”

“X-3 Weekly Status Report Week Ending 20 June 1953.”

“X-3 Weekly Status Report Week Ending 18 July 1953.”

“X-3 Weekly Status Report Week Ending 25 July 1953.”

“X-3 Weekly Status Report Week Ending 1 August 1953.”

“X-3 Weekly Status Report for Week Ending 1 November 1953.” 

“X-3 Weekly Status Report for Week Ending 15 November 1953.”

NACA HSFRS Memorandum for Chief of Research. “X-3 Airplane Visit of 

Harold F. Kleckner of the Douglas Company to Langley on October 18, 

1951.”

NACA HSFRS Memorandum for Chief of Research. “Proposed Modifications 

of X-3 Airplane to Rocket Power and Air Launch.” October 24, 1952.

NACA HSFS. “Flight Experience with Two High-Speed Airplanes Having 

Violent Lateral-Longitudinal Coupling in Aileron Rolls,” NACA RM 

H55A13, 1955.

Stephenson, Harriet J. “Flight Measurements of Horizontal-Tail Loads on the 

Douglas X-3 Research Airplane.” NACA RM H56A23, 1956.

Northrop X-4 Bantam

Drake, Hubert M. “Stability and Control Data Obtained from First Flight of 

X-4 Airplane.” NACA RM L9A31, 1949.

Matthews, James T., Jr. “Results Obtained During Flights 1 to 6 of the Northrop 

X-4 Airplane (A.F. no. 46-677).” NACA RM L9K22, 1950.
Probing the Sky

290

NACA HSFRS. X-4 Progress Memorandums for NACA Research Airplanes 

Project Leader: 

“Acceptance of the X-4 Airplane.” n.d. 

“Progress Report on Acceptance Tests of X-4 Airplanes from January 1 to 

January 14, 1949.” 

“X-4, January 14 to June 10, 1949.”

“X-4, June 17 through July 1, 1949.”

“X-4, June 20, 1949.”

“X-4, July 16 through July 29, 1949.”

“X-4 Airplanes July 30 through August 12, 1949.”

“X-4 Airplanes July 31 through August 12, 1949.”

“X-4, August 27 to September 14, 1949.”

“X-4 Airplanes October 8 to October 21, 1949.”

“X-4, November 19 to December 2, 1949.”

“X-4, December 3 to December 16, 1949.”

“X-4, December 31, 1949, to January 1, 1950.”

“X-4, January 27 to February 24, 1950.”

“X-4, February 24 to March 24, 1950.”

“X-4, May 5 to May 19, 1950.”

“X-4, July 15 to July 28, 1950.”

“X-4, August 26 to September 8, 1950.”

“X-4, September 8 to September 22, 1950.”
Selected Bibliography

291

“X-4, September 22 to October 6, 1950.”

“X-4, October 6 to October 20, 1950.”

“X-4, November 3 to November 17, 1950.”

“X-4, November 17 to December 1, 1950.”

“X-4, December 15 to December 29, 1950.”

“X-4, May 18 to June 1, 1951.”

“X-4, June 16 to June 29, 1951.”

“X-4, June 30 to July 13, 1951.” 

“X-4, July 14 to July 27, 1951.”

“X-4, July 28 to August 10, 1951.”

“X-4, August 11 to August 25, 1951.”

“X-4, September 22 to October 5, 1951.”

“X-4, October 6 to October 19, 1951.”

“X-4, October 20 to November 2, 1951.” 

“X-4, November 3 to November 16, 1951.”

“X-4, January 26 to February 8, 1952.”

“X-4, February 9 to February 22, 1952.”

“X-4, March 8 to March 21, 1952.”

“X-4, March 22 to April 4, 1952.”

“X-4, April 5 to April 18, 1952.”

“X-4, April 19 to May 2, 1952.”
Probing the Sky

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“X-4, May 17 to May 30, 1952.”

“X-4, May 31 to June 13, 1952.”

“X-4, June 28 to July 11, 1952.”

“X-4, July 12 to July 25, 1952.”

“X-4, August 9 to August 22, 1952.”

“X-4, September 5 to October 1, 1952.”

“X-4, October 1 to November 1, 1952.”

“X-4, November 1 to December 1, 1952.” 

“X-4, January 1 to January 31, 1953.” 

“X-4, February 1 to February 28, 1953.”

“X-4, March 1 to March 31, 1953.”

“X-4, April 1 to April 30, 1953.”

“X-4, May 1 to May 31, 1953.”

“X-4, July 1 to July 31, 1953.”

“X-4, August 1 to August 31, 1953.”

“X-4, September 1 to September 30, 1953.”

Sadoff, Melvin, and A. Scott Crossfield. “A Flight Evaluation of the Stability 

and Control of the X-4 Swept-Wing Semitailless Airplane.” NACA RM 

H54G16, 1954.

Sadoff, Melvin, and Thomas R. Sisk. “Summary Report of Results Obtained 

During Demonstration Tests of the Northrop X-4 Airplanes.” NACA RM 

A50I01, 1950.
Selected Bibliography

293

Valentine, George M. “Stability and Control Data Obtained from Fourth and 

Fifth Flights of the Northrop X-4 Airplane (A.F. No. 46-676).” NACA RM 

L9G25a, 1949.

Williams, Walter C. “Results Obtained from Second Flight of X-4 Airplane 

(A.F. No. 46-676).” NACA RM L9F21, 1949.

Williams, Walter C. “Results Obtained from Third Flight of Northrop X-4 

Airplane (A.F. No. 46-676).” NACA RM L9G20a, 1949.

Bell X-5

Banner, Richard D., Robert D. Reed, and William L. Marcy. “Wing-Load 

Measurements of the Bell X-5 Research Airplanes at a Sweep Angle of 

58.7°.” NACA RM H55A11, 1955.

Bell Aircraft Corporation. “Bell X-5 Ready for Flight Tests.” June 14, 1951. 

Bellman, Donald R. “Lift and Drag Characteristics of the Bell X-5 Research 

Airplane at 59° Sweepback for Mach Numbers from 0.60 to 1.03.” NACA 

RM L53A09c, 1953. 

Briggs, Donald W. “Flight Determination of the Buffeting Characteristics of the 

Bell X-5 Research Airplane at 58.7° Sweepback.” NACA RM L54C17, 1954.

Campbell, John P., and Hubert M. Drake. “Investigation of Stability and 

Control Characteristics of an Airplane Model with Skewed Wing in the 

Langley Free-Flight Laboratory.” NACA TN No. 1208, 1947.

Childs, Joan M. “Flight Measurements of the Stability Characteristics of the 

Bell X-5 Research Airplane in Sideslips at 59° Sweepback.” NACA RM 

L52K13b, 1953.

Donlan, Charles J., and William C. Sleeman. “Low-Speed Wind-Tunnel 

Investigation of the Longitudinal Stability Characteristics of a Model 

Equipped with a Variable-Sweep Wing.” NACA RM L9B18, 1949.

Finch, Thomas W., and Donald W. Biggs. “Preliminary Results of Stability 

and Control Investigation of the Bell X-5 Research Airplane.” NACA RM 

L52K18b, 1953.
Probing the Sky

294

Green, Warren E. The Bell X-5 Research Airplane. Wright-Patterson AFB, OH: 

Wright Air Development Center, March 1954.

Kress, Robert W. “Variable-Sweep Wing Design.” AIAA Paper No. 83-1051, 

1983.

Martin, James A. “Longitudinal Flight Characteristics of the Bell X-5 Research 

Airplane at 59° Sweepback with Modified Wing Root.” NACA RM L53E28, 

1953.

NACA HSFRS, X-5 Flight Research Summaries:

“Investigation of Flight at Speeds Below Minimum Drag, Flight No. 60.” 

January 27, 1953. 

“Investigation of Flight at Speeds Below Minimum Drag, Flight No. 61.” 

January 29, 1953. 

“Investigation of Flight at Speeds Below Minimum Drag, Flight No. 63.” 

February 6, 1953. 

NACA HSFRS, X-5 Progress Memorandums for Chief of Research:

“Progress Report for the X-5 Research Airplane for the Period May 3 to 

May 16, 1952.”

“Progress Report for the X-5 Research Airplane for the Period July 12 to 

July 25, 1952.”

“Progress Report for the X-5-1 Research Airplane for the Period July 26 to 

August 8, 1952.”

“Progress Report for the X-5-1 Research Airplane for the Period October 

1 to November 1, 1952.”

“Progress Report for the X-5-1 Research Airplane for the Period July 1 to 

July 31, 1955.”

“Progress Report of Engineering and Instrumentation for the X-5 Airplane 

During the Period August 25 to September 7, 1951.”
Selected Bibliography

295

“Progress Report of Engineering and Instrumentation for the X-5 Airplane 

During the Period December 1 to December 14, 1951.”

“Progress Report of Engineering and Instrumentation for the X-5 Airplane 

During the Period December 15 to December 28, 1951.”

“Progress Report of Engineering and Instrumentation for the X-5 Airplane 

During the Period December 29, 1951, to January 11, 1952.”

NASA Langley Research Center. “Summary of NACA/NASA Variable-Sweep 

Research and Development Leading to the F-111 (TFX).” NASA LRC 

Working Paper LWP-285, 1966.

Nugent, Jack. “Lift and Drag of the Bell X-5 Research Airplane in the 45° 

Sweptback Configuration at Transonic Speeds.” NACA RM H56E02, 

1956.

Perry, Robert L. Innovation and Military Requirements: A Comparative Study. 

RAND Report RM-5182PR. Santa Monica, CA: The RAND Corporation, 

1967.

Reed, Robert D. “Flight Measurements of Horizontal-Tail Loads on the Bell 

X-5 Research Airplane at a Sweep Angle of 58.7°.” NACA RM H55E20a, 

1955.

Rogers, John T., and Angel H. Dunn. “Preliminary Results of HorizontalTail Load Measurements of the Bell X-5 Research Airplane.” NACA RM 

L52G14, 1952.

United States Air Force. “Report of Major Aircraft Accident—X-5, 50-1839A.” 

Edwards AFB: AFFTC, October 27, 1953.

Videan, Edward N. “Flight Measurements of the Dynamic Lateral and 

Longitudinal Stability of the Bell X-5 Research Airplane at 58.7° Sweepback.” 

NACA RM H55H10, 1955.

Douglas D-558-1 Skystreak

Angle, Ellwyn E., and Euclid C. Holleman. “Longitudinal Frequency-Response 

Characteristics of the D-558-I Airplane as Determined from Experimental 

Transient-Response Histories to a Mach Number of 0.90.” NACA RM 

L51K28, 1952.
Probing the Sky

296

Barlow, William H., and Howard C. Lilly. “Stability Results Obtained with 

Douglas D-558-1 Airplane BuAero No. 37971 in Flights up to a Mach 

Number of 0.89.” NACA RM L8K03, 1949.

Beeler, De E., Donald R. Bellman, and John H. Griffith. “Flight Determination 

of the Effects of Wing Vortex Generators on the Aerodynamic Characteristics 

of the Douglas D-558-I Airplane.” NACA RM L51A23, 1951.

Bellman, Donald R. Letter to NACA Headquarters. “Looseness of Stabilizer 

Mountings for the D-558-I Airplane.” November 5, 1952.

Bellman, Donald R. Letter to NACA Headquarters. “Reply to Request for 

Information on D-558-I Vertical-Tail Load Investigation.” January 12, 1953.

Douglas Aircraft Company. “Skystreak World’s Speed Records.” El Segundo, 

CA: DAC, September 19, 1947.

Heinemann, Edward H. “The Development of the Navy-Douglas Model 

D-558 Research Project.” El Segundo, CA: Douglas Aircraft Company, 

November 17, 1947.

Hyatt, 1st Lt. Abraham, United States Marine Corps Reserve. “Proposed HighSpeed Research Airplane.” USN Bureau of Aeronautics Memorandum 

AER-E-225-AH. September 22, 1944. 

Keener, Earl R., and Mary Pierce. “Tabulated Pressure Coefficients and 

Aerodynamic Characteristics in Flight on the Wing of the Douglas D-558-I 

Airplane for a 1g Stall, a Speed Run to a Mach Number of 0.90, and a 

Wind-Up Turn at a Mach Number of 0.86.” NACA RM L50J10, 1950.

Keener, Earl R., James R. Peel, and Julia B. Woodbridge. “Tabulated Pressure 

Coefficients and Aerodynamic Characteristics Measured in Flight on the 

Wing of the Douglas D-558-I Airplane Throughout the Normal-Force 

Range at Mach Numbers of 0.67, 0.74, 0.78, and 0.82.” NACA RM 

L50L12a, 1951.

NACA, “NACA Aircraft Accident Investigation Report Douglas D-558-1 

Airplane, BuNo 37971.” Muroc, CA: NACA Muroc Flight Test Unit. May 

3, 1948.
Selected Bibliography

297

Sadoff, Melvin, William S. Roden, and John M. Eggleston. “Flight Investigation 

of the Longitudinal Stability and Control Characteristics of the Douglas 

D-558-I Airplane BuAero No. 37972 at Mach Numbers up to 0.89.” 

NACA RM L51D18, 1951.

Soulé, Hartley A. Letter to NACA Headquarters. “Request for Comments 

on Abandonment of D-558-I Vertical-Tail Load Investigation.” December 

19, 1952.

Thompson, Jim Rogers, William S. Roden, and John M. Eggleston. “Flight 

Investigation of the Aileron Characteristics of the Douglas D-558-I Airplane 

BuAero No. 37972 at Mach Numbers Between 0.6 and 0.89.” NACA RM 

L50D20, 1950.

Williams, Walter C. “Flight Measurement of the Stability Characteristics of the 

D-558-1 Airplane BuAero No. 37971 in Sideslips.” NACA RM L8E14a, 1949.

Williams, Walter C. “Limited Measurements of Static Longitudinal Stability 

in Flight of Douglas D-558-1 Airplane, BuAero No. 37971.” NACA RM 

L8E14, 1948.

Douglas D-558-2 Skyrocket

Ankenbruck, Herman O., and Chester H. Wolowicz. “Lateral Motions 

Encountered with the Douglas D-558-II All-Rocket Research Airplane During 

Exploratory Flights to a Mach Number of 2.0.” NACA RM 54I27, 1954.

Ankenbruck, Herman O., and Theodore E. Dahlen. “Some Measurements of 

Flying Qualities of a Douglas D-558-II Research Airplane During Flights 

to Supersonic Speeds.” NACA RM L53A06, 1953.

Brunn, Cyril D., and Wendell H. Stillwell. “Mach Number Measurements and 

Calibrations During Flights at High Speeds and at High Altitudes Including 

Data for the D-558-II Research Airplane.” NACA RM H55J18, 1956.

Dahlen, Theodore E. “Maximum Altitude and Maximum Mach Number 

Obtained with the Modified Douglas D-558-II Research Airplane During 

Demonstration Flights.” NACA RM L53B24, 1953.

Fischel, Jack. “Effects of Wing Slats and Inboard Fences on the Longitudinal 

Stability Characteristics of the Douglas D-558-II Research Airplane in 
Probing the Sky

298

Accelerated Maneuvers at Subsonic and Transonic Speeds.” NACA RM 

L53L16, 1954.

Fischel, Jack, and Cyril D. Brunn. “Longitudinal Stability Characteristics in 

Accelerated Maneuvers at Subsonic and Transonic Speeds of the Douglas 

D-558-II Research Airplane Equipped with a Leading Edge Wing ChordExtension.” NACA RM H54H16, 1954.

Fischel, Jack, and Donald Reisert. “Effects of Several Wing Modifications 

on the Subsonic and Transonic Longitudinal Handling Qualities of the 

Douglas D-558-II Research Airplane.” NACA RM H56C30, 1956.

Fischel, Jack, and Jack Nugent. “Flight Determination of the Longitudinal 

Stability in Accelerated Maneuvers at Transonic Speeds for the Douglas 

D-558-II Research Aircraft Including the Effects of an Outboard Wing 

Fence.” NACA RM L53A16, 1953.

Fischel, Jack, Robert W. Darville, and Donald Reisert. “Effects of WingMounted External Stores on the Longitudinal and Lateral Handling 

Qualities of the Douglas D-558-II Research Airplane.” NACA RM 

H57H12, 1957.

Sjoberg, S.A., James R. Peele, and John H. Griffith. “Flight Measurements 

with the Douglas D-558-II (BuAero No. 37974) Research Airplane: Static 

Longitudinal Stability and Control Characteristics at Mach Numbers up 

to 0.87.” NACA RM L50K13, 1951.

Sjoberg, S.A. “Preliminary Measurements of the Dynamic Lateral Stability 

Characteristics of the Douglas D-558-II (BuAero No. 37974) Airplane.” 

NACA RM L9G18, 1949.

Consolidated Vultee XF-92A Dart

Holleman, Euclid C. “Flight Measurements of the Lateral Response 

Characteristics of the Convair XF-92A Delta-Wing Airplane.” NACA RM 

H55E26, 1955.

Holleman, Euclid C., John H. Evans, and William C. Triplett. “Preliminary 

Flight Measurements of the Dynamic Longitudinal Stability Characteristics 

of the Convair XF-92A Delta-Wing Airplane.” NACA RM L53E14, 1953.
Selected Bibliography

299

Redd, Joseph W., Jr., Maj. Charles E. Yeager, and Lt. Col. Frank K. Everest. 

“Performance Flight Tests of the XF-92A Airplane, USAF S/N 46-682, with 

a J33-A-29 Power Plant.” AFFTC Technical Report No. 53-11. Edwards 

AFB, CA: Air Force Flight Test Center, 1953.

Sisk, Thomas R., and Duane O. Muhleman. “Lateral Stability and Control 

Characteristics of the Convair XF-92A Delta-Wing Airplane as Measured 

in Flight.” NACA RM H55A17, 1955.

Sisk, Thomas R., and John M. Mooney. “Preliminary Measurements of Static 

Longitudinal Stability and Trim for the XF-92A Delta-Wing Research 

Airplane in Subsonic and Transonic Flight.” NACA RM L53B06, 1953.

II. Autobiographies, Biographies, and Memoirs

Bridgeman, William, and Jacqueline Hazard. The Lonely Sky. New York: Henry 

Holt and Company, Inc., 1955.

Cebeci, Tuncer. Legacy of a Gentle Genius: The Life of A.M.O. Smith. Long 

Beach, CA: Horizons Publishing, Inc., 1999.

Connor, Margaret. Hans von Ohain: Elegance in Flight. Reston, VA: American 

Institute of Aeronautics and Astronautics, 2001.

Crossfield, A. Scott, and Clay Blair, Jr. Always Another Dawn: The Story of a 

Rocket Test Pilot. New York: Arno Press, 1972.

Everest, Lt. Col. Frank K., as told to John Guenther. The Fastest Man Alive. 

New York: E.P. Dutton, 1958.

Glennan, T. Keith, edited by J.D. Hunley and in association with Roger D. 

Launius. The Birth of NASA: The Diary of T. Keith Glennan. NASA SP-4105. 

Washington, DC: NASA, 1993. 

Golley, John (in association with Frank Whittle and with technical assistance 

from Bill Gunston). Whittle: The True Story. Washington, DC: Smithsonian 

Institution Press, 1987. 

Grierson, John. Jet Flight. London: Samson Low, Marston & Co., Ltd., 1944.
Probing the Sky

300

Heinemann, Edward H., and Rosario Rausa. Ed Heinemann: Combat Aircraft 

Designer. Annapolis, MD: Naval Institute Press, 1980. 

Hoover, Robert A. “Bob,” with Mark Shaw. Forever Flying. New York: Orion, 1996.

Johnston, A.M. “Tex,” and Charles Barton. Tex Johnston: Jet-Age Test Pilot. 

Washington, DC: Smithsonian Institution Press, 1991.

Lundgren, William R. Across the High Frontier: The Story of a Test Pilot—Major 

Charles E. Yeager, USAF. New York: William Morrow, 1955.

Meyer, Corwin H. Corky Meyer’s Flight Journal: A Test Pilot’s Tales of Dodging 

Disasters—Just in Time. North Branch, MN: Specialty Press, 2006.

Nahum, Andrew. Frank Whittle: Invention of the Jet. Duxford, U.K.: Icon 

Books, 2004.

Peebles, Curtis, ed. The Spoken Word: Recollections of Dryden History: The Early 

Years. SP-2003-4530. Washington, DC: NASA, 2003.

Phillips, W. Hewitt. Journey in Aeronautical Research: A Career at NASA Langley 

Research Center. Washington, DC: NASA, 1998.

Späte, Wolfgang. Top Secret Bird: The Luftwaffe’s Me 163 Comet [sic]. Missoula, 

MT: Pictorial Histories Publishing Company, 1989.

Stoliker, Fred, Bob Hoey, and Johnny Armstrong. Flight Testing at Edwards: 

Flight Test Engineers’ Stories 1946–1975. Edwards AFB, CA: Flight Test 

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Whittle, Sir Frank. Jet: The Story of a Pioneer. New York: Philosophical Library, 

1954.

Yeager, Chuck, and Leo Janos. Yeager: An Autobiography. New York: Bantam, 

1985.

Young, James O., ed. The Men of Mach One: A Supersonic Symposium. Edwards 

AFB, CA: Air Force Flight Test Center, 1990.

Ziegler, Mano. Rocket Fighter. Garden City, NY: Doubleday and Co., Inc., 

1963.
Selected Bibliography

301

III. Miscellaneous Sources

Abzug, Malcolm J., and E. Eugene Larrabee. Airplane Stability and Control: 

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Cambridge University Press, 1997.

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Bancarella Aeronautica, n.d.

Aronstein, David C., and Albert C. Piccirillo. Have Blue and the F-117: Evolution 

of the “Stealth Fighter.” Reston, VA: American Institute of Aeronautics and 

Astronautics, 1997.

Ball, John, Jr. Edwards: Flight Test Center of the USAF. New York: Duell, Sloan, 

and Pearce, 1962.

Becker, John V. The High-Speed Frontier: Case Histories of Four NACA Programs, 

1920–1950. NASA SP-445. Washington, DC: NASA, 1980.

Boyne, Walter J., and Donald S. Lopez, eds. The Jet Age: Forty Years of Jet 

Aviation. Washington, DC: National Air and Space Museum in association 

with the Smithsonian Institution Press, 1979.

Bradley, Robert E. “The Birth of the Delta Wing.” American Aviation Historical 

Society (winter 2003).

Brown, Capt. Eric, RN. “An Ill-Fated ‘Swallow’...but a Harbinger of Summer.” 

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Divine, Robert A. The Sputnik Challenge: Eisenhower’s Response to the Soviet 

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Emme, Eugene M., ed. The History of Rocket Technology: Essays on Research, 

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Fries, Sylvia Doughty. NASA Engineers and the Age of Apollo. NASA SP-4104. 

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Gray, George W. Frontiers of Flight: The Story of NACA Research. New York: 

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Gorn, Michael H. Expanding the Envelope: Flight Research at NACA and NASA.

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Hallion, Richard P. Supersonic Flight: Breaking the Sound Barrier and Beyond—

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Gelzer. Unconventional, Contrary, and Ugly: The Lunar Landing Research 

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Program. Reston, VA: AIAA, 2008.

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Triumph, and Tragedy. Newark, U.K.: Red Kite/Wing Leader Publishers, 

2012.

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Boston: Harvard Business School, 1950.

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Tradition of Excellence. Atlanta, GA: International Gas Turbine Institute of 

the American Society of Mechanical Engineers, 1999.

Smith, G. Geoffrey. Gas Turbines and Jet Propulsion for Aircraft. New York: 

Aircraft Books Inc., 1946.

Swenson, Loyd S., Jr., James M. Grimwood, and Charles C. Alexander. This 

New Ocean: A History of Project Mercury. NASA SP-4201. Washington, 

DC: NASA, 1966.

Taylor, John R. Jane’s Pocket Book of Research and Experimental Aircraft. New 

York: Collier Books, 1976.

Thompson, Milton O., and Curtis Peebles. Flying Without Wings: NASA Lifting 

Bodies and the Birth of the Space Shuttle. Washington, DC: Smithsonian 

Institution Press, 1999.

Winter, Frank H. “‘Black Betsy’: The 6000C4 Rocket Engine, 1945–-1989.” 

Paper presented at the 23rd Symposium on the History of Astronautics, 

40th International Astronautical Congress of the International Astronautical 

Federation. Malaga, Spain: October 1989.

Witkin, Richard. The Challenge of the Sputniks. New York: Doubleday Headlines 

Publications, 1958.
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305

Young, James O. Meeting the Challenge of Supersonic Flight. Edwards AFB, CA: 

Air Force Flight Test Center History Office, 1997.

307

Index

Numbers in bold indicate pages with illustrations and figures

A

A-12 program, 140, 264–66n31

AD-1 research aircraft, 208

aerial refueling and X-5 formation test flights, 

221–24, 239n29, 239n33

aerodynamics

flow separation and, 2–3

speed and rules for, v

supersonic flight and, 34

transformation of technology, tools, and 

procedures for supersonic era, v, 3–4

Aerojet rocket engine, 10

ailerons

D-558-1 data collection and research 

results, 54–56, 59–61

D-558-1 lateral stability and aileroneffectiveness studies, 69–70, 73

D-558-2 lateral stability and aileroneffectiveness studies, 112

X-5 aileron overbalance and effectiveness, 

234

aircraft

carrier landings and design of, 207–8, 235

changes in design of, 198

designation system for numbering, 

37–38n32, 38n34

design of for jet propulsion, 8, 37n24

evolutionary process of development of, v, 

3–4

first flight, v, 1

military role in aircraft design, 35–36n12

See also research aircraft/X-series research 

aircraft

aircraft carriers, 207–8, 235

airflow

calculation of as inviscid fluid, 1

compressibility effects and swept-wing 

configurations, 17, 39–40n43

compressibility effects at transonic and 

supersonic speeds, 3, 6–7

flow separation, concept of, 2

flow separation, discovery of, 2

flow separation, research on, 2–3

potential flows, 17

research on supersonic flight, 1–2

X-3 design and compressibility effects, 

137–38

airfoils

circular-arc airfoils, 3

D-558-2 wing design, 87

drag and design of, 2–3

high-speed flight and design of, 14–15

research on high-speed aerodynamics, 

2–3, 4

Air Force Academy, U.S., 195

Air Force/Army Air Forces (AAF)/Army Air 

Corps, U.S.

aerial refueling operations, 239n33

aircraft designation system, 37–38n32, 

38n34

airfoil research by, 2–3

communication between NACA and, 230
Probing the Sky

308

fighter aircraft design competition, 243–44

German rocket plane development, 

concerns about, 6

Mach 2 sustained speeds, aircraft for, 135, 

160n2

research aircraft designation system, 

37–38n32

rocket-powered research aircraft study, 

10–12

transonic research aircraft, role in 

development of, 9–12

turbojet-powered research aircraft study, 

10–12

variable-sweep aircraft used by, 236

X-3 flights, 146–49

X-5 flights, 213–16, 226–30

X-5 program role, 212, 238n10

XF-92A flights, 246, 249–54, 262–63n10

XF-92A program role, 243–47

XS-1 program publicity, 26–27

XS-1 program role, 12, 14–15, 16, 25–31, 

34, 72

XS-1 wing design, 20

Air Force Flight Test Museum, 195

Air Force Museum, U.S., 157, 195, 232, 257

Air Force Test Center, vi

air launching and mother ships

advantages of, 124

aircraft available for air launching, 15

chase planes, 100, 116, 227

D-558-1 air-launching recommendation, 53

D-558-2 air launch and P2B-1S engine 

failure, 118

D-558-2 air launching, 95–101, 109, 110, 

115–17, 124

D-558-2 air launching, loading into launch 

aircraft for, 117

D-558-2 air launching, modifications for, 84

drop-body testing, 19

rocket-powered aircraft launching from, 11

value of air launching, 53

X-3 all-rocket/air-launch configuration, 

140–41

X-15 air launching, 140

XS-1 launching from, 15, 25, 27, 32, 33, 

53, 124

See also B-29 Superfortress

Allison J33 engines, 247, 248, 250–51, 254, 

262–63n10

Allison J35 engines, 210, 211–12

altitude

D-558-2 altitude-record flight, 101, 102

Mach numbers, air temperature, and, 78n12

stability and control and, 56

American F-100A aircraft, 38n35

American Rocket Society, 273

Ames-Dryden AD-1 research aircraft, 208

Ames Research Center/Ames Aeronautical 

Laboratory

creation of, 7

mission and purpose of, 7

Model 7002 wind tunnel testing, 248

research culture at, 274

space research by, 274

XS-1 test site tour, 27

Ankenbruck, Herman O., 113–14, 116

Antelope Valley College, 133n63

Apollo 11 Command Module, 81–82n51

Apollo program, 275

Apt, Milburn, 77n7, 276, 277–78n13

area rule design, 87, 259, 261, 264–

66n31, 272

Armstrong, Neil, 118, 232, 275

Armstrong Flight Research Center (Dryden 

Flight Research Center), vi, 74, 197

Armstrong Whitworth AW 52 aircraft, 196–97

Army Ballistic Missile Agency (Marshall Space 

Flight Center), 274

Arnold, H.H. “Hap,” 6, 7

Ascani, Fred, 29
Index

309

Ashkenas, Irving, 169

Atomic Energy Commission, 274

Avro Type 707 aircraft, 264–66n31

B

B-1/B-1B strategic bomber, 236, 272

B-2A Spirit stealth bomber, 196–97

B-24 aircraft, 86

B-29 Superfortress

aerial refueling formation test flights, 221, 

223–24

D-558 launch plane role of, iv, 13, 95, 

96–101, 110

drop-body testing launch plane role, 19

loading aircraft for air launching, 117

P2B-1S engine failure and near tragedy, 118

P2B-1S Superfortress, 96–101, 110, 

115–17, 130n30

XS-1 launch plane role, 15, 25, 27, 32, 33, 

53, 96, 124

XS-1 transport to Muroc attached to, 24

B-36 bombers, 140, 198

B-52 bombers, 140

B-58 bombers, 264–66n31

Barnes, Florence “Pancho,” 33

Becker, John V., 11–12, 74

Beech Bonanza aircraft, 198

Beeler, De E., 21

Bell, Lawrence D. “Larry”

XS-1 flight test program direction and goals, 

27, 29

XS-1 project and Collier Trophy, 42n83

Bell Aircraft Corporation

“black” aircraft program, involvement in 

first, 7

Niagara Falls facility, 213

XS-1 program publicity, 26–27

XS-1 program role, 12, 14–15, 22–23, 

25–31

XS-1 project and Collier Trophy, 42n83

XS-1 test site considerations, 16, 

38–39n41

Bell L-39 aircraft, 87

Bell P-39 Airacobra aircraft, 7, 9

Bell P-63 aircraft, 247

Bell X-1/XS-1 aircraft. See X-1/XS-1 

(Experimental Sonic-1) aircraft

Bell X-2/XS-2 aircraft. See X-2/XS-2 aircraft

Bell X-5 aircraft. See X-5 aircraft

Bell XP-59A jet, 7–8, 9, 11, 22, 37n24, 243

Bellman, Donald, 70–71

Big Bear Lake, 116

Biggs, Lyman J., 2–3

Boeing B-29 Superfortress. See B-29 

Superfortress

Boeing EB-50 aircraft, 277–78n13

Boeing supersonic transport (SST), 236

Boeing XB-47 jet bomber, 87

Boulton Paul P.111 aircraft, 264–66n31

Boulton Paul P.120 aircraft, 264–66n31

Boyd, Albert, 28, 29, 30, 31, 49, 214, 252–53

Bridgeman, William “Bill”

D-558-2 all-rocket/air-launch flights, 

96–101

D-558-2 altitude-record flight, 101

D-558-2 flights, 94–95, 109, 112–13, 

117, 126

D-558-2 speed-record flights, 99–100

X-3 flights, 142, 143–44, 145–46

Brown, Clinton, 20

BT-13 aircraft, 247

Buckingham, Edgar, 4, 5

buffeting

D-558-1 data collection and research 

results, 56–57, 60, 62–64, 66, 73

D-558-2 pitch-up characteristics and, 93

external stores research flights, 120, 123

SB2C Helldiver design, 7

X-4 data collection and research results, 180

X-5 handling characteristics, 225–26
Probing the Sky

310

XS-1 data collection and research results, 

27–28, 29, 33

Burstein, Adolph, 244

Busemann, Adolf, 39–40n43, 75–76

Bush, George H.W., 80n43

Bush, Vannevar, 6, 37n21

Butchart, Stanley P.

D-558-1 flights, 66–67, 70, 71

D-558-2 air launch and P2B-1S engine 

failure, 118

education of, 66–67

experience of, 66–67, 80n43

external stores research flights, 122

flight tests, 69

P2B-1S pilot role, 115–16

pilot-escape systems, opinion about, 77n7

X-4 flights, 191

X-5 flights, 221, 231–32

C

C-74 transport aircraft, 140

C-119 cargo aircraft, 213, 226

C-124 transport aircraft, 140

Caldwell, Frank W.

D-558-1 canopy design and crash helmets, 

79n18

propeller aerodynamics research, 2

speed record flights, 49–50, 78n12

California Institute of Technology, Jet Propulsion 

Laboratory, 274

California State Polytechnic College, 81–82n51

Campini engine and Secondo Campini, 36n13

Cannon, Joseph, 277–78n13

canopy

D-558-1 design, 48, 53, 79n18

D-558-2 design, 88

X-4 design and lock, 173, 183

XF-92A design, 247, 260

XS-1 design, 88

Carl, Marion E.

D-558-1 canopy design and crash helmets, 

79n18

D-558-2 flights, 114

Mach 2 flight, 114

speed record flights, 49–50, 78n12

Carolina Aviation Museum, 81–82n51

Champine, Robert A. “Bob”

D-558-1 flights, 59, 59, 62

D-558-2 flights, 90

D-558-2 pitch-up characteristics, 91–92

transfer of, 130n38

Chance Vought F7U/F7U-3 Cutlass program, 

64, 160n7, 195

chase planes, 100, 116, 227

circular-arc airfoils, 3

Clousing, Lawrence, 27

cockpit design, D-558-1 design, 53, 70

Collier Trophy, 42n83, 76

Comet I jetliner, 200n4

Concorde SST, 264–66n31

Consolidated-Vultee (Convair) F-106 aircraft, 

264–66n31

Consolidated-Vultee (Convair) XF-92A aircraft. 

See XF-92A aircraft

Consolidated-Vultee (Convair) YF-102/F-102 

aircraft, 259–61, 259, 260, 261, 264–66n31

control surfaces

painting of D-558-1 control surfaces, 58

supersonic flight and effectiveness of, 1

XS-1 horizontal stabilizer, elevator, and tail 

design, 38n35

See also specific surfaces

Cooper, George, 194

cowling design and location, 4

Cox, L.K., 180, 181

Cox, R.B., 58

Crawford, Alden R., 20

Crimson Test Tube, 48. See also D-558-1 

Skystreak
Index

311

Crossfield, A. Scott

D-558-1 flights, 64, 65, 70, 71

D-558-2 flights, 92, 97, 102–8, 109, 

111–18, 115, 121, 125–26, 130n38

D-558-2 pitch-up characteristics, 92, 

102–8, 125

dirt road named after, 254, 255, 263n18

education of, 64

experience of, 64, 67, 126

external stores research flights, 120

first flight jinx, 254, 255

hydrogen peroxide accident, 131–32n54

inertial coupling research missions, 155

Mach 2 flight, 112–17

North American Aviation job, 117–18, 126, 

132n55

pilot-escape systems, opinion about, 77n7

X-4 contributions to aviation research, 195

X-4 flight data reports, 186–88

X-4 flights, 185–88, 189, 190, 193, 194, 

205n70

X-5 aircraft, value of, 235

X-5 flights, 217, 218, 231

X-5 handling characteristics, 230, 231

X-15 flights, 118, 126

XF-92A flights, 254–56, 255, 258

XF-92A handling characteristics, 258

XS-1 flights, 126

Crossfield Pike, 254, 255, 263n18

Crowley, John

swept-wing testing, approval for, 18–19

XS-1 specifications for research 

requirements, 21

XS-1 test site considerations, 16

Curtiss SB2C Helldiver aircraft, 7, 9

D

D-21 aircraft, 264–66n31

D-558 program

development of, decisions leading to, 8–12

development program for, 12–13

engines for aircraft, 12

flight-test program direction, plan, and 

goals, 27, 85

funding for, 44

models of, 9

personnel involved in program, iv, 13

phase 1 aircraft and goals, 12–13, 27, 85

phase 2 aircraft and goals, 12–13, 27, 85

phase 3 aircraft and goals, 12, 27, 85

research flight preparations, iv, 13

specifications and preliminary design 

development, 12

D-558-1 Skystreak

accident and death of Lilly, 51–53, 57, 74, 

277–78n13

air-launching recommendation, 53

canopy design, 48, 53, 79n18

cockpit design, 53, 70

contributions to aviation research, 71–75, 

124, 268, 269, 271

control surfaces, painting of, 58

data collection and research results, 13, 

50–51, 53–57, 67–71, 73–74, 81n44

data collection at level flight, requirement 

for, 48

design of, 44, 45–48, 68, 72–73, 85–86, 

139, 199, 268

design of, approval for, 13–14

design team, 45

designation and numbering of, 77n2

display of aircraft at end of program, 

81–82n51

dive tests, 57, 67–68, 73

elevator vibrations and replacement of 

elevators, 64–65

fates of aircraft and perceived importance, 

81–82n51

final flights and end of program, 71

flight safety recommendations, 53

flight-test program direction, plan, and 

goals, 27, 71–74
Probing the Sky

312

flight tests, 48–51, 53–71, 66

frequency-response data, collection of, 

68–69

fuel system and capacity, 47, 58, 280

funding for, 44

fuselage design and materials, 45–46, 

86, 87

instrumentation for data collection, 50–51, 

61–62

landing gear configuration and landing gear 

problems, 47, 48, 51

modifications to, 57–58

NACA, transfer to, 59, 130n30

NACA research flights, 58–69, 59

nickname for, 48

paint schemes, 47–48, 51, 58, 72, 

78–79n15, 250

personnel involved in program, 44

phase 1 aircraft designation, 12–13

pilot-escape system, 47, 77n7

pilots, dangers faced by, 64

pressure-distribution research, 62–63, 73

shortcomings of, 270

smallest aircraft around largest jet 

engine, 46

specifications, technical, 280–81

speed brakes, 78–79n15, 211

speed capabilities of, 45, 68, 71–74

speed record flights, 48–50, 78n12

stability, control, and handling 

characteristics, 270

stabilizer and tail design and materials, 

38n35, 46, 67–69, 70–71, 138, 281

stall/spin behavior, 62

structural design and materials, 46–47

tail-loads studies, 69, 70–71, 73–74

TG-180 engines issues, 50, 51–53, 58–59, 

62, 74

third aircraft, 44, 68, 72, 75

tip-tank study, 71, 73

transonic flight with, 27

vortex generator research flights, 62–63, 

73, 80n37

wing design, 46–47, 281

XS-1 compared to, 45

D-558-2 Skyrocket

air launching, loading into launch aircraft 

for, 117

air launching and P2B-1S engine failure, 118

air launching of, 95–101, 109, 110, 

115–17, 124

air launching of, modifications for, 84, 

94–96, 110

all-rocket/air-launch configuration, 

95–101, 123

altitude-record flight, 101, 102

canopy design, 88

contributions to aviation research, 

124–27, 271

data collection and research results, 

121, 127

data collection from D-558-1 aircraft and 

design of, 13

design of, 73, 85–88, 139, 199

display of aircraft at end of program, 119, 

133n63

dive tests, 89

elevator design, 88

engine test firings, 116

external stores research flights, 119–23, 

119, 127, 132n60, 271

final flights and end of program, 118–19, 

123–24, 133n63

first flights, 88–91

flight-test program direction, plan, and 

goals, 27, 85–86, 111, 124–27

flight tests, 88–124, 100, 121, 129n16, 

132n59

fuel systems, turbopump, and capacities, 

86, 87, 88, 89, 95, 97, 128n4, 280

fuselage design, 86, 87

inertial coupling behavior of model, 152
Index

313

instrumentation for data collection, 87, 126

JATO-boosted flights, 89, 89, 94–95

jet power for and configuration 

specifications, 85–86, 87, 88, 90, 95, 96, 

98, 101–2, 280–81

jet-rocket system research flights, 101–9, 

102, 124–27

landing gear, 87, 136

launch areas for, 115–16

Mach 2 flight, 110, 112–17

NACA, transfer to, 89, 96, 97, 101, 109, 

130n30

NACA research flights, 89–94, 101–24

number of research flights, 157

performance potential, 95, 99–100, 124, 

128n4

personnel involved in program, 121

phase 2 aircraft designation, 12–13

pilot-escape system, 47, 77n7

pitch-up characteristics, 91–94, 97, 

102–9, 125–26, 125, 127, 133n64, 157

research flight preparations, 13

rocket power for and configuration 

specifications, 12, 85–86, 87, 89–90, 95, 

96, 97, 101–2, 114–15, 114, 280–81

shortcomings of, 270

specifications, technical, 280–81

speed capabilities and speed records, 84, 

99–100, 102, 109, 111, 112–17

stability, control, and handling 

characteristics, 90–94, 97, 99–101, 

102–12, 125–26, 127, 270

stabilizer and tail design, 88, 108–9, 

128n5, 138, 281

stall/spin behavior, 62, 92

supersonic flight with, 27

survival of aircraft at end of program, 

126–27

swept-wing design, 73, 85–88, 124–26, 

125, 137–38, 281

tail-loads studies, 73–74, 127

takeoff runs, length of, 88–89, 94–95

wing fences, 88, 97, 102–6, 103, 108, 256

wing-load data collection flights, 122

wing modification to add sawtooth leading 

edge, 106–8, 107

wing-strain-gauge calibration, 91

Dassault MD 550 Mirage I aircraft, 264–66n31

Dassault MD 550 Mirage II aircraft, 264–66n31

Dassault MD 550 Mirage III aircraft, 

264–66n31

Dassault MD 550 Mirage IV aircraft, 

264–66n31

data collection systems and methodologies

automatic computing equipment for data 

analysis, 204–5n69

backlogs of data processing, 175

“computers” for data analysis, 24, 91

D-558-1 data collection at level flight, 

requirement for, 48

D-558-1 data collection and research 

results, 13, 50–51, 53–57, 67–71, 

73–74, 81n44

D-558-1 instrumentation, 50–51, 61–62

D-558-2 data collection flights and 

research results, 121, 127

D-558-2 instrumentation, 87, 126

D-558-2 wing-strain-gauge calibration, 91

frequency-response data, collection of, 

68–69

instrumentation for data collection, 

standard, 171

Reeves Electronic Analog Computer 

(REAC), 252

stability data collection methods, 252

transformation of technology, tools, and 

procedures for supersonic era, v

women “computers” for data analysis, 24, 

175, 204–5n69

X-3 data collection and research results, 

147–50
Probing the Sky

314

X-5 data collection and research results, 

214, 215–26, 232–34

XF-92A data collection and research 

results, 251–57

XS-1 data collection and research results, 

54, 56, 57, 73–74

Davidson, Milton, 8–9

Davis, Frank W., 244

Day, Richard E. “Dick,” 154, 155–56

Daytona Beach, 16

DC-3 aircraft, 4

de Havilland, Geoffrey, Jr., 168–69

de Havilland DH 100 Vampire jet fighter, 168

de Havilland DH 106 jet airliner, 168, 200n4

de Havilland DH 108 Swallow aircraft, 168–69, 

170, 196–97

Defense, Department of (DOD), 273–74

delta-wing aircraft

advantages and value of, 247, 253, 

258–61

birth of, 243

German designs, 244–45

improvement recommendations, 253–54

model testing of concept, 248

popularity of design, 243

wing fences, 255, 256, 257

XF-92A design, 244–45, 247, 281

Derry, John, 168

design theory and transformation of technology, 

tools, and procedures for supersonic era, 

v, 3–4

Diehl, Walter Stuart, 9, 11

digital fly-by-wire flight control systems, 

196–98, 272

dive maneuvers

D-558-1 dive tests, 57, 67–68, 73

D-558-2 dive tests, 89

loss of aircraft during high-speed dives, 

6–7, 9

X-3 dive tests, 145–46

X-4 dives and dive brakes, 176, 179–80, 

181, 188–89, 191, 193, 196, 203n53

X-5 dive tests, 219–20

DM-1 glider, 244–45

Donaldson, E.M., 49

Donlan, Charles J., 208, 236

Donovan, Robert C., 45

Doolittle, James H. “Jimmy,” 274

Douglas Aircraft Corporation

D-558 program role, 12–14, 44

Model 558 High-Speed Test Aircraft. See

D-558-1 Skystreak; D-558-2 Skyrocket; 

D-558 program

Douglas C-54 Skymaster transport, 15

Douglas C-74 transport aircraft, 140

Douglas C-124 transport aircraft, 140

Douglas X-3 aircraft. See X-3 Stiletto

drag

airfoil design and, 2–3

area rule and fuselage design, 87, 259, 

261, 264–66n31, 272

external stores research flights, 119–23, 

119, 127, 132n60, 271

flow separation and, 2–3

high fineness ratio and, 45–46, 138

Mach 2 flight preparations, 114–15

research on, 72

swept wings and, 17–20, 207

X-3 design and, 136, 137–38

X-4 lift-over-drag studies, 190–92, 195

X-5 handling characteristics, 225

Drake, Hubert, 156

drop-body testing, 18–20

Dryden, Hugh L.

airfoil research by, 2–3

D-558-2 modifications, support for, 95–96

Mach 2 flight, approval for, 113

photo of, 2

wing design and drag, Volta Congress 

presentation about, 39–40n43

XS-1 Mach 1 flight, confirmation of, 42n83
Index

315

Dryden Flight Research Center (Armstrong 

Flight Research Center), vi, 74, 197

ducted-fan engine designs, 5, 8. See also Jeep 

piston-engine/combustion-chamber engine

Durand, William F., 6, 7–8, 9

E

EB-50 aircraft, 277–78n13

Edwards, Glen, 172

Edwards Air Force Base/Muroc Army Air Field

aeronautical research and development 

at, vi

Air Force Flight Test Museum, 195

D-558-1 test site, 88–89, 89, 97

facilities at, 24

housing near, 24

landscape and terrain around, 24

renaming of, 96

space research at, 274

street names, 263n18

weather conditions at, 24

XP-59A program role, 7–8

XS-1 test site, 22–24

XS-1 test site considerations, 16, 27, 

38–39n41

Eisenhower, Dwight D., 113, 273–74

ejection seats. See pilot-escape systems/

ejection seats

elevators

D-558-1 data collection and research 

results, 54–55, 64–65, 67–69

D-558-2 design, 88

movable and flight control, 38n35

XS-1 stabilizer, elevator, and tail design, 

32–33, 38n35, 138

elevons

concept behind, 167

X-4 use of, 167, 172, 183, 193, 194, 196

XF-92A use of, 247

Elizabeth, Lake, 115

Ellis, Macon C., 20

endplates on wings, 57

engines

cowling design and location, 4

installation of and engine efficiency, 4

nacelle design and location, 4

See also jet and turbojet engines

Everest, Frank K. “Pete,” 98–99

F-100A aircraft, concerns about 

performance of, 153

X-3 flights, 147, 148–49

X-4 flights, 183

X-5 flights, 215

XF-92A flights, 249–51, 254

Explorer II balloon flight, 101

external stores research flights, 119–23, 119, 

127, 132n60, 271

F

F3D-2 Skyknight aircraft, 160n7

F3H-1 Demon aircraft, 160n7

F-5/T-38 aircraft, 159, 160n7, 272

F7U/F7U-3 Cutlass program, 64, 160n7, 195

F-8 aircraft, 197

F-14/F-14A Tomcat aircraft, 156, 236, 272

F-16 aircraft, 198

F-80 Shooting Star, 113, 221–23

F-86 aircraft

chase plane role of, 100, 227

F-86 all-moving tail, 68, 81n46

F-86A deployment for Korean War, 81n44, 

81n46

F-86A speed record, 78n12

F-86E stabilizer design, 38n35

handling characteristics, 249

high-speed research with, 74

performance of, 113

pitch-up characteristics and swept-wing 

design, 92

speed capabilities, 146

XP-86 aircraft, 28, 87

F-94 aircraft, 249
Probing the Sky

316

F-100/F-100A aircraft, 38n35, 113, 153, 

155–56, 158

F-101 Voodoo aircraft, 156

F-102/YF-102 aircraft, 259–61, 259, 260, 

261, 264–66n31

F-104 Starfighter aircraft

design and development of, 113, 158–59, 

272–73, 277n4

inertial coupling behavior, 156

success of design and use of by air forces, 

159

X-3 development and, 141, 158–59, 

161n10, 195

F-106 aircraft, 264–66n31

F-111/F-111A aircraft, 208, 236, 272

F-117A stealth fighter, 198

Fairchild C-119 cargo aircraft, 213, 226

Fairey Delta 2 aircraft, 264–66n31

Fales, Elisha N., 2

Fédération Aéronautique Internationale (FAI), 49

Ferri, Antonio, 75–76

Fields, Edison M., 203n50

fighter aircraft

design and development of, 37n24

design competition, 243–44

X-5 as basis for, 210, 235

Finch, Thomas W., 215–17

Fischel, Jack, 103–7, 154, 155–56

Fisk, James B., 273

FJ-1 aircraft, 247

Fleming, Frank N., 135, 136, 151

flight control systems

armor shielding for cables, 53, 138, 170

fly-by-wire systems, 196–98, 272

transformation of technology, tools, and 

procedures for supersonic era, v

XF-92A system, 245, 247, 249, 253, 257, 

259

fly-by-wire flight control systems, 196–98, 272

flying-wing aircraft, 167–68, 170, 171–74, 

196–97

Fokker trimotor aircraft, 4

formation test flights and aerial refueling, 

221–24, 239n29, 239n33

frequency-response data, collection of, 68–69

Frost, Richard, 25, 30, 32

fuel systems

armor shielding for hoses and tanks, 53, 

58, 138

D-558-1 fuel system and capacity, 47, 58, 

280

D-558-1 tip-tank study, 71, 73

D-558-2 fuel systems, turbopump, and 

capacities, 86, 87, 89, 95, 97, 128n4, 

280

fuel hoses, 58

wet wings and fuel tanks, 47

X-3 fuel system and capacity, 138, 280

X-4 fuel system and capacity, 170, 200n8, 

280

X-5 fuel-cell pressure gauge problems, 

213–14

X-5 fuel system and capacity, 212, 280

XF-92A fuel system and capacity, 246, 280

XS-1 fuel system, turbopump, and fuel 

capacity, 15–16, 21, 25–26, 41n63, 

128n4, 280

Full-Scale Tunnel, 245

fuselage

area rule design, 87, 259, 261, 264–

66n31, 272

D-558-1 design and materials, 45–46, 

86, 87

D-558-2 design, 86, 87

high fineness ratio, 45–46, 138

X-3 design, 136, 137, 138

XF-92A design, 242, 245, 247

G

General Electric “black” aircraft program, 7

General Electric I-A Whittle turbojet engines, 

7–8
Index

317

General Electric J85 engines, 160n7

General Electric TG-180 axial flow turbojet 

engine, 10, 46, 48, 50, 52–53, 58, 74, 85

Germany

aircraft development in, 6, 168

delta-wing aircraft, 244–45

jet engine research and development in, 5

rocket propulsion development in, 6, 10

swept-wing research and development in, 

20, 39–40n43, 85

Gilruth, Robert R.

contributions to X-plane and space 

programs, 275

photo of, 14

wing-flow technique, invention of by, 14, 18

XS-1 horizontal stabilizer, elevator, and tail 

design, 38n35

XS-1 wing design, 14–15

Glennan, T. Keith, 274

Gloster Meteor aircraft, 49

Goodlin, Chalmers H. “Slick”

experience of, 23

XS-1 flight tests, 23, 24, 27, 32, 42n83

XS-1 program publicity, 26–27

XS-1 supersonic flight by, 27, 28

Goodrich Corporation, 47

Gough, Melvin N.

photo of, 26

XS-1 flight-test program direction and 

goals, 25, 26

XS-1 test site tour, 27

Great Britain

jet engine research and development in, 5, 

7, 8, 9

sound barrier, first British aircraft to exceed, 

168

Greene, Carl, 11

Griffith, John H.

Chance Vought job, 130n38

D-558-1 flights, 59, 62–63, 64

D-558-2 pitch-up characteristics, 92, 93

X-4 flights, 183, 185

X-4 operational concerns, 178

Grumman F-14/F-14A Tomcat aircraft, 156, 

236, 272

Grumman XF10F-1 Jaguar fighter, 235, 236

H

Hallion, Richard P., 161n10, 236

Heinemann, Edward Henry “Ed”

D-558 program plan and goals, 12, 85

D-558-1 design role, 45, 46, 48

D-558-1 landing gear and tires, 47

Heinkel, Ernst, 5

Heinkel He 178 aircraft, 5

helmets and D-558-1 canopy design, 53, 

79n18

Hemphill, Thomas, 244

High-Speed Flight Station/High-Speed Flight 

Research Station

dress code at, 121

establishment of, 23–24, 23

facilities at, 24

housing near, 24

Mercury program role, 274–75

Muroc Flight Test Unit, 23–24, 23, 34, 50, 

275

name change of, 132n55

research culture at, 274–75

X-planes at, 199

High Speed Tunnel, 8-Foot, 76

High-Speed Wind Tunnel, 7-by-10-Foot, 138, 

206, 208

Hilton, W.F., 1

Hoover, Herbert, 42n83

Hoover, Robert A. “Bob,” 29, 32

Horton Ho 229 aircraft, 196–97

Hunsaker, Jerome, 20

Hyatt, Abraham, 12

hydrogen peroxide accident, 131–32n54
Probing the Sky

318

hypersonic flight

knowledge, dedication, expertise, and 

courage for development of, vi

transformation of technology, tools, and 

procedures for, v–vi, 269

hypoxia, 228–29

I

inertial coupling (roll coupling), 147, 150–56, 

157–59, 163n38, 233–34, 272

Irvine, Jack, 244

Italy, jet engine research and development in, 5

J

J33 engines, 247, 248, 250–51, 254, 

262–63n10

J34 engines

D-558-2 engines, 88, 95, 96, 98

P.1101 engine test program proposal, 210

performance of, 160n7

X-3 engines, 135, 139–40

J35 engines, 210, 211–12

J46 engine, 135, 139–41, 157, 160n7, 210

J47 engines, 210

J57 engines, 153

J58 engines, 140

J75 engines, 140

JA-37 Viggen aircraft, 264–66n31

Jacobs, Eastman N.

dispute between Theodorsen and, 19

Jeep engine–powered aircraft research, 

6, 7

turbojet engine and aircraft research, 

support for, 8, 37n24

wing design and drag, Volta Congress 

presentation about, 39–40n43

Jansen, George, 98

JAS-39 Gripen aircraft, 264–66n31

Jeep piston-engine/combustion-chamber 

engine, 6–7, 7, 8, 37n21

jet-assisted-takeoff (JATO) solid rocket 

boosters, 89, 89

jet engines

aircraft design for, 8, 37n24

applications for, complacency about, 4, 5, 

17

applications for and benefits of, reevaluation 

of, 5–6

axial-flow turbojet engines, 5

centrifugal-flow jet engine, 5

development of and aircraft development, 

139, 160n7

ducted-fan designs, 5, 8

early concepts of, 4–8, 36n13

gas turbine engines, importance of 

development of, 4

hybrid piston engine/jet concepts, 4–5

Jeep piston-engine/combustion-chamber 

engine, 6–7, 7, 8, 37n21

limitations of early engines, vi

limitations of research with, 11

materials for, 6

ramjets, 244, 246, 247

research aircraft with turbojet propulsion, 

study of, 9–12

transformation of aircraft development 

and, v–vi

See also specific engines

Jet Propulsion Laboratory, California Institute of 

Technology, 274

Johnson, Clarence L. “Kelly,” 272–73, 277n4

Johnson, Richard L., 185, 253

Johnson Space Center (Manned Spacecraft 

Center), 275

Jones, Robert T.

delta-wing research, 244

dispute between Theodorsen and, 19

swept-wing research paper, distribution of, 

19, 20
Index

319

swept-wing theoretical analysis and 

calculations, 17–18, 19–20, 39–40n43, 

72, 85

XS-1, swept-wing configuration for, 20–21

Jones, Walter P.

D-558-1 flights, 64, 65

D-558-2 flights, 97, 102

death of, 130n38, 192

education of, 64

Northrop test pilot role, 130n38, 192

X-4 flights, 186, 188, 190–91, 192

X-5 flights, 217, 218

K

Kent, Marion, 23

Killian, James R., Jr., 273–74

Kinkaid (mechanic), 131–32n54

Kleckner, Harold F., 152

Klein, P.B., 30

Korean War, 81n44, 81n46, 146

Kotcher, Ezra

Mach 0.999 study, 10–12, 42n83

turbojet-powered aircraft, limitations of 

research with, 11

XS-1 program role, 12, 42n83

Kuhn, Thomas, v

L

Land, Edwin, 274

landing gear

D-558-1 landing gear configuration and 

problems related to, 47, 48, 51

D-558-2 landing gear, 87, 136

wheel and tire size, 47

X-3 landing gear, 136

X-3 tire failures, 143, 145

X-4 landing gear lock and landing gear 

door issues, 170, 176, 177, 178, 179, 

180, 183

X-5 landing gear, 212

XF-92A landing gear, 247, 256

Langley Memorial Aeronautical Laboratory

adjustment for personnel transferring to 

Muroc for XS-1 program, 24

Full-Scale Tunnel, 245

High Speed Tunnel, 8-Foot, 76

High-Speed Wind Tunnel, 7-by-10-Foot, 

138, 206, 208

jet engine research by, 5–7

research culture at, 274

space research by, 274

wind tunnel testing breakthroughs, 74–76

XS-1 flight-test program direction and 

goals, 30

XS-1 program control, 16

XS-1 test site considerations, 16, 21, 22, 

25–26, 27, 40n52

Lewis, George W.

Langley jet engine research, support for, 6

transonic research aircraft, role in 

development of, 8, 9–10

XS-1 program role, 21

Lewis Research Center, 274

life-support systems, v

lift

swept wings and, 17–20, 207

X-4 lift-over-drag studies, 190–92, 195

X-5 handling characteristics, 225

Lilly, Howard C. “Tick,” 42n83, 49, 50–53, 56, 

57, 74, 170, 276, 277–78n13

Lippisch, Alexander, 244–45

Lippisch DM-1 glider, 244–45

Lockheed A-12 program, 140, 264–66n31

Lockheed Blackbird aircraft, 264–66n31

Lockheed F-104 Starfighter. See F-104 

Starfighter aircraft

Lockheed F-117A stealth fighter, 198

Lockheed P-38 Lightning aircraft, 6–7, 9, 57, 

86

Lockheed P-80/P-80R aircraft, 16, 27, 37n24, 

49, 247

Lockheed XF-90 aircraft, 160n7
Probing the Sky

320

Lockheed XST Have Blue stealth aircraft, 198

LR-8/XLR-8/XLR-11/RMI 6000C4 rocket 

engines

D-558-2 use of, 12, 85–86, 87, 89–90, 

95, 96, 97, 114–15, 114

nozzle extensions, 114–15

X-3 use of, 140, 157

X-15 use of, 140

XS-1 use of, 22–23

Lusk, Arthur, 169

Lyndon B. Johnson Space Center, 275

M

M-21 aircraft, 264–66n31

Mach 0.999 study, 10–12, 42n83

Mach 1 flight, 27–31, 32–34, 42n83, 

81–82n51, 88, 114

Mach 2 flight, 110, 112–17

Mach 2 sustained speed performance, 135, 

160n7

Mach numbers, altitude, and air temperatures, 

78n12

Manned Spacecraft Center (Johnson Space 

Center), 275

Marietta Field, 16

Marine Corps Air-Ground Museum, 81–82n51

Marschak, Thomas A., 139

Marshall Space Flight Center (Army Ballistic 

Missile Agency), 274

Martin, Bill, 249

Martin, John F., 88, 89, 128n6

materials

evolutionary process of aircraft development 

and changes in, v, 3–4

transformation of technology, tools, and 

procedures for supersonic era, v, 3–4

wood-and-fabric to all-metal aircraft, 

transition to, 4, 35–36n12

Mattson, Axel T., 33

Maxwell Air Force Base, 195

May, Eugene F. “Gene”

D-558-1 canopy design and crash helmets, 

79n18

D-558-1 flights, 48–49, 49, 57, 58–59

D-558-2 flights, 89, 95

Mayer, John, 33

McCormack, James, 273

McDonnell F3H-1 Demon aircraft, 160n7

McDonnell F-101 Voodoo aircraft, 156

McKay, John B. “Jack”

astronaut wings, qualification for, 126

D-558-1 flights, 71

D-558-2 air launch and P2B-1S engine 

failure, 118

D-558-2 flights, 118, 123, 126

education of, 71

experience of, 71, 126

external stores research flights, 120, 122

X-4 flights, 194, 205n70

X-5 flights, 231–32

X-15 flights, 126

XS-1 flights, 126

McTigue, John, 35n2, 157

MD 550 Mirage I aircraft, 264–66n31

MD 550 Mirage II aircraft, 264–66n31

MD 550 Mirage III aircraft, 264–66n31

MD 550 Mirage IV aircraft, 264–66n31

Mercury program, 274–75

Messerschmitt Me 163 Komet rocket fighter, 

39–40n43, 166, 167–68, 169, 209, 244

Messerschmitt Me 262 jet fighter, 39–40n43

Messerschmitt P.1101 aircraft, 209–10, 235

Meyer, Corwin H. “Corky,” 235

Meyer, J.C., 216

MiG-15 aircraft, 81n44, 81n46

MiG-19 aircraft, 38n35

MiG-23/27 aircraft, 236, 272

MiG-25 aircraft, 156

military services

aircraft design role of, 35–36n12
Index

321

aircraft designation system, 37–38n32, 

38n34

research aircraft, funding for, 8

transonic research aircraft, opinions about 

need for, 11

transonic research aircraft, role in 

development of, 9–12

missiles

nuclear weapon–armed rockets, attitudes 

toward practicality of, 37n21

surface-to-air missiles (SAMs), 262n7

V-2 missiles, 136

Model CL-1200 Lancer aircraft (X-27 aircraft), 

277n4

model testing

delta-wing concept testing, 248

drop-body testing, 18–20

inertial coupling behavior, 151–52

size of models for wind-tunnel testing, 

74–75

sting for mounting models, 75

XS-1 model, 32–33

See also wing-flow technique

Moise, Jack, 131–32n54

motorjet, 36n13

Munk, Max, 17, 39–40n43

Muroc Army Air Field. See Edwards Air Force 

Base/Muroc Army Air Field

Muroc Flight Test Unit, 23–24, 23, 34, 50, 275

Murray, Arthur A. “Kit,” 221, 227–29, 231, 253

museum-display aircraft, 81–82n51, 119, 

133n63, 157, 166, 195, 232, 257

MX-324 glider, 171

MX-656 project, 135, 160n2. See also X-3 

Stiletto

MX-810 project, 169. See also X-4 Bantam

N

N-9M aircraft, 196–97

nacelle design and location, 4

National Advisory Committee for Aeronautics 

(NACA)

airfoil research, support for, 3, 4

communication between Air Force and, 230

creation of, 3

duration of existence, 241n60

evolutionary process of aircraft 

development, role in, 3–4

focus of research on practical applications, 

3–4

human contributions of X-plane programs, 

275

laboratory/research center structure, 274

mission and purpose of, 3

name change of, 241n60

propulsion research by, 4

Research Aircraft Projects Panel, 140–41

space research by, 273–74

Special Committee on Jet Propulsion, 6

swept-wing concepts, failure of NACA to 

develop, 20–21, 72

variable-sweep wing studies, 208, 235–36, 

237n3

X-planes research plan, 71–72

National Aeronautics and Space Administration 

(NASA)

laboratory/research center structure, 274

mission and purpose of, 3, 241n60

NACA merger and creation of, 3, 241n60, 

274

space research by, 273–74

National Air and Space Museum, 81–82n51, 

119, 133n63

National Museum of Naval Aviation, 81–82n51

National Museum of the Air Force, 166

National Science Foundation, 274

Navy, U.S.

aerial refueling operations, 239n33

aircraft designation system, 38n34

D-558 program funding from, 44
Probing the Sky

322

D-558 program role, 12–14, 72–73

transonic research aircraft, role in 

development of, 9–12

variable-sweep aircraft used by, 236

Nord 1500-02 Griffon II fighter, 244, 

264–66n31

North American Aviation, Crossfield role at, 

117–18, 126

North American Aviation XB-70 Valkyrie aircraft, 

264–66n31

North American FJ-1 aircraft, 247

North American X-15 aircraft. See X-15 aircraft

Northrop, John Knudsen “Jack,” 167

Northrop B-2A Spirit stealth bomber, 196

Northrop F-5/T-38 aircraft, 159, 160n7, 272

Northrop N-9M aircraft, 196–97

Northrop SM-62 Snark aircraft, 197

Northrop X-4 aircraft. See X-4 Bantam

Northrop XB-35 flying-wing bomber, 167, 171

Northrop XP-79B flying-wing fighter, 170, 171, 

196–97

Northrop YB-49 flying-wing bomber, 167, 172, 

196–97

Northrop YF-89D Scorpion aircraft, 192

nuclear weapons

A-bomb, Soviet development of, 37n21

nuclear weapon–armed rockets, attitudes 

toward practicality of, 37n21

Nugent, Jack, 103–4

O

oscillations

D-558-2 dynamic-lateral-stability 

characteristics, 90–91, 99, 101, 126

Dutch roll lateral-directional oscillation, 88

pilot-induced oscillations, 174

supersonic yaw, 126

X-4 characteristics, 180

P

P-38 Lightning aircraft, 6–7, 9, 57, 86

P-39 Airacobra aircraft, 7, 9

P-47 Thunderbolt aircraft, 7, 9

P-51 Mustang, viii, 18, 19, 20

P-63 aircraft, 247

P-80/P-80R aircraft, 16, 27, 37n24, 49, 247

P-84 aircraft, 27–28

P.13 fighter, 244

P.111 aircraft, 264–66n31

P.120 aircraft, 264–66n31

P.1101 aircraft, 209–10, 235

Panavia Tornado strike aircraft, 236

paradigm shift, v–vi

Phillips, William Hewitt “Bill,” 150–52, 156, 

163n38

pilot-escape systems/ejection seats

capsule systems, 47, 77n7

D-558-1 Skystreak, 47, 77n7

D-558-2 Skyrocket, 47, 77n7

human limits and design of, 47, 77n7

transformation of technology, tools, and 

procedures for supersonic era, v

X-2 aircraft, 47, 77n7

X-3 Stiletto, 153

X-4 Bantam, 173

X-5 aircraft, 212

XF-92A aircraft, 245–46, 247

Pilotless Aircraft Research Division (PARD), 152

pilots/test pilots

accidents and loss of, 77n7, 227–31, 

275–76, 277–78n13

age of when breaking the sound barrier, 2, 

35n2

altitudes and speeds experienced by, 

274–75

changes in D-558-2 program, 130n38

courage of, vi

dangers faced by, 64
Index

323

death of and naming of street after, 263n18

demands of test programs and operations 

on, 74

education of, 64, 65, 66–67, 71

experience and skills of, vi, 64, 66–67, 71, 

80n43, 88, 126

reluctance to fly aircraft, 128n6

voice of when breaking the sound barrier, 

2, 35n2

Pinecastle Army Air Field, 21, 22, 24, 26, 

40n52

pitch and pitch control

D-558-2 pitch-up characteristics, 

91–94, 97, 102–9, 125–26, 125, 127, 

133n64, 157

X-4 characteristics, 167, 170, 171–72, 

179, 186–87, 202n46

XS-1 aircraft, 32–33

Planes of Fame Museum, 133n63

point-defense mission, 246–47, 262n7

Popson, Raymond A. “Ray,” 227–31, 276, 

277–78n13

Pratt & Whitney engines

J58 engines, 140

J75 engines, 140

R-1535 radial piston engine, 6

President’s Scientific Advisory Committee 

(PSAC), 273–74

pressure-distribution research

D-558-1 flights, 62–63, 73

X-3 flights, 156

propellers

airfoil design and efficiency of, 2–3, 4

research on high-speed aerodynamics, 2–3

propulsion

NACA research on, 4

supersonic flight and, 34

transformation of technology, tools, and 

procedures for supersonic era, v–vi

Purcell, Edward, 274

R

R-1535 radial piston engine, 6

ramjets, 244, 246, 247

Reaction Motors Incorporated 6000C4 engine. 

See LR-8/XLR-8/XLR-11/RMI 6000C4 rocket 

engines

Reeves Electronic Analog Computer (REAC), 

252

refueling formation test flights and aerial 

refueling, 221–24, 239n29, 239n33

Reid, Henry

Jones swept-wing research paper, 

distribution of, 20

Langley jet engine research, support for, 

5–6

XS-1 flight-test program direction and 

goals, 25–26, 30

Republic P-47 Thunderbolt aircraft, 7, 9

research aircraft/X-series research aircraft

accidents with, 77n7, 227–31, 275–76, 

277–78n13

“black” aircraft program, first, 7–8

contributions to aviation research, vi, 

269–76

cost of research activities, 113

criticism of X-planes, 272–73

delays in research programs, 175

demands of test programs and operations, 

74

designation system for numbering, 

37–38n32, 38n34

engine test firings, 116

fates of and perceived importance, 

81–82n51

funding for, 8

human contributions of X-plane 

programs, 275

Jeep engine–powered aircraft, 6–7, 7, 8

jet engine development and aircraft 

development, 139, 160n7
Probing the Sky

324

museum-display aircraft, 81–82n51, 119, 

133n63, 157, 166, 195, 232, 257

NACA research plan, 71–72

opinions of military services about need 

for, 11

photo of, 199

public opinion about, 119

Round One aircraft, vi, 269–73, 274–76

Round Two aircraft, vi, 273–74

war and development of, 8

See also specific aircraft

Richardson, E.W., 180, 181

Ridley, Jackie L. “Jack,” 21, 29, 32, 33–34, 253

Ritland, Osmond J., 28

Robert J. Collier Trophy, 42n83, 76

rocket-powered aircraft

air-launching of, 11

contributions to aviation research, v–vi

German development of, 6, 10

limitations of research with, 11, 12

research aircraft using rocket motors, study 

of, 10–12

speed capabilities for research aircraft, 11

Stack, Robert, opposition to, 9, 11, 12, 

42n83

supersonic regime research with, vi

Rogers Dry Lake

conditions at, 22

flooding of, 51, 69, 102, 130n31, 

142–43, 175

XS-1 test site, 22–24

rolls and roll behavior

D-558-1 data collection and research 

results, 59–61

inertial coupling (roll coupling), 147, 

150–56, 157–59, 163n38, 233–34, 272

Root, L. Eugene “Gene,” 12, 45

Rosamond, 116

Round One aircraft, vi, 269–73, 274–76

Round Two aircraft, vi, 273–74

rudder

D-558-1 data collection and research 

results, 54–55

D-558-2 rudder lock, 99

X-4 rudder cable changes, 181, 182

X-4 rudder control, 171–74, 197

XF-92A rudder-pulse tests, 255

S

Saab 35 Draken aircraft, 264–66n31

Saab JA-37 Viggen aircraft, 264–66n31

Saab JAS-39 Gripen aircraft, 264–66n31

Sadoff, Melvin, 177–78, 179, 180, 187

safety procedures and transformation of 

technology, tools, and procedures for 

supersonic era, v, 3–4

Salina Field, 16

Samet, Arthur, 23

SB2C Helldiver aircraft, 7, 9

Schwartz, A.M., 169

semi-tailless aircraft, 167, 168, 195–98, 

200n4, 235. See also X-4 Bantam

Shannon, Ellis D. “Sam,” 248, 253

Sherman, Albert E., 5

Shick, Ralph H., 244

shock-stall, 14

shock waves

fuselage design to avoid shock waves near 

wings, 45–46

XS-1 horizontal stabilizer, elevator, and tail 

design, 38n35

Sleeman, William C., 208

SM-62 Snark aircraft, 197

Smith, A.M.O., 45, 46, 85

Smith, George F., 21–22, 25, 28–29

Smith, R.G., 45

Smith, Stan, 16, 38–39n41

Sopwith Camel, 270

Soulé, Hartley, 23, 27, 30–31, 70, 73, 152, 275

sound barrier

coining of phrase, 1
Index

325

conditions for pilots during flights to 

break, 32

first British aircraft to exceed, 168

mythology surrounding, 1–2, 11, 34, 35n2

opinions about breaking, 32, 135

research aircraft to collect data on, 9

XS-1 flight to break, 32–34, 42n83

South Base

D-558-1 aircraft at, 44

D-558-2 aircraft at, 84, 86, 110

hangar at, iv, 13

training facility role, 86

Spaatz, Carl, 29

space program, start of U.S., 273–75

Space Shuttle Program, 275

Space Task Group, 275

Sparks, Ralph, 181

Special Assistant to the President for Science 

and Technology, 273

speed

aerodynamic rules and increases in, v

compressibility effects during high-speed 

flight, 3, 6–7

D-558-1 speed capabilities, 45, 68, 71–74

D-558-2 speed capabilities and speed 

records, 84, 99–100, 102, 109, 111, 

112–17

evolutionary process of aircraft development 

and increase in, v, 3–4

F-86 speed capabilities, 146

structural heating and, 139

X-3 speed capabilities, 140, 146, 271

X-3 tire failures and, 143

XF-92A speed capabilities, 250, 251

speed records

D-558-1 flights, 48–50, 78n12

Donaldson record, 49

F-86A flights, 78n12

requirements for official record, 49 

speed-record flights, 99–100, 113

spin and spin control. See stall/spin behavior

Spirit of St. Louis, 81–82n51

Sputnik, 241n60, 273

SR-71 aircraft, 264–66n31

stability and control

altitude and, 56

D-558-1 data collection and research 

results, 53–57, 59–71, 73

D-558-1 handling characteristics, 270

D-558-1 lateral stability and aileroneffectiveness studies, 69–70, 73

D-558-2 dynamic-lateral-stability 

characteristics, 90–91, 99, 101, 126

D-558-2 handling characteristics, 90–94, 

97, 99–101, 102–12, 270

D-558-2 lateral stability and aileroneffectiveness studies, 112

D-558-2 pitch-up characteristics, 

91–94, 97, 102–9, 125–26, 125, 127, 

133n64, 157

data collection methods, 252

external stores research flights, 119–23, 

119, 127, 132n60, 271

inertial coupling (roll coupling), 147, 

150–56, 157–59, 163n38, 233–34, 272

semi-tailless aircraft, 167, 168, 195–98

Sopwith Camel, 270

tailless aircraft, 167–68

variable-sweep-wing aircraft, 208

X-3 handling characteristics, 142–46, 

148–50, 157–59, 270

X-3 inertial coupling research, 147, 151, 

152, 153–56

X-4 handling characteristics, 167, 171–74, 

176, 177–78, 179–80, 183–92, 193, 

194, 196–98, 270

X-5 handling characteristics, 215–26, 

235, 271

X-5 inertial coupling behavior, 233–34

XF-92A handling characteristics, 248–49, 

251–57, 258–59, 271

XS-1 data collection, 25, 26, 30–31
Probing the Sky

326

stabilizers

adjustable and flight control, 38n35

all-moving one-piece design, 138

D-558-1 data collection and research 

results, 54–57, 64, 67–69

D-558-1 stabilizer and tail design, 38n35, 

46, 67–69, 70–71, 138

D-558-2 stabilizer and tail design, 88, 

108–9, 128n5, 138

F-86 all-moving tail, 68, 81n46

X-3 stabilizer and tail design, 138

XS-1 stabilizer, elevator, and tail design, 

32–33, 38n35, 138

Stack, John

Collier Trophy award for slotted tunnel, 76

D-558 program role, 8–9, 9, 42n83, 71–72

D-558-1 swept-wing design, 85

photos of, 9, 10

rocket-powered aircraft, opposition to, 9, 

11, 12, 42n83

slotted tunnel development, 75–76

transonic research aircraft, development of, 

8–9, 10

turbojet-powered aircraft, commitment to 

acquire, 12

Vickers Swallow supersonic airliner role, 

236

XS-1 horizontal stabilizer, elevator, and tail 

design, 38n35

XS-1 program control, 16, 21

XS-1 project and Collier Trophy, 42n83

XS-1 specifications for research 

requirements, 15–16, 21–22

XS-1 test site considerations, 16

XS-1 wing design, 14–15

stall/spin behavior

D-558-1 characteristics, 62

D-558-2 characteristics, 62, 92

flying-wing aircraft, 171–72

sweeping wings during, 228

X-3 characteristics, 145

X-4 characteristics, 171–72, 177, 178, 183

X-5 characteristics, 217–18, 220–21, 

227–31, 229, 235

XF-92A characteristics, 249

Standards, U.S. Bureau of

airfoil research by, 2–3

jet engine research by, 4

Stanley, Robert, 25, 27–28

Strass, H. Kurt, 203n50

Strategic Air Command, 239n33

structures

NACA research on, 4

speed and structural heating, 139

transformation of technology, tools, and 

procedures for supersonic era, v

Su-17/22 aircraft, 236

Su-24 aircraft, 236

subsonic flight

evolutionary process of aircraft development 

for, v, 3–4

knowledge, dedication, expertise, and 

courage for development of, vi

validity of information and concepts in 

supersonic era, v, vi

supersonic flight

aircraft design for, 34, 191

dangers of and mystery surrounding, 1–2

reliability and validity of information and 

concepts for, vi

research aircraft development to study, 

71–72

routine nature of, 269–70

transformation of technology, tools, and 

procedures for supersonic era, v–vi, 3–4, 

269–70

wing-flow technique for research, viii

XS-1 flights, 26–27, 28–31

supersonic transport (SST), 236, 264–66n31

surface-to-air missiles (SAMs), 262n7

Swallow aircraft (de Havilland DH 108), 

168–69, 170, 196–97
Index

327

Swallow supersonic airliner, 235–36

Sweeney, William “Bill,” 12

swept wings

advantages of, 17–18, 207

carrier landings and, 207

concepts using, failure of NACA to develop, 

20–21, 72

D-558-2 design, 73, 85–88, 124–26, 125, 

137–38, 281

drop-body testing of concept, 18–20

German research and development of, 

20, 85

lack of compressibility effects with, 17, 

39–40n43

lift, drag, and, 17–20, 207

limitations of, 207

pitch-up characteristics, 91–94, 125–26, 

125, 127, 133n64, 157

sawtooth leading-edge extension, 

106–8, 107

stability and control and, 39–40n43

theoretical analysis and calculations, 

17–18, 19–20, 39–40n43, 72, 85

wing fences, 88, 97, 102–6, 103, 108, 256

wing-flow testing of concept, 18–20, 19, 

20

wing slats, 104–6, 107, 108

X-2 design, 27, 137–38

X-3 design, 136, 137–38, 137, 158, 281

X-3 design and compressibility effects, 

137–38

XS-1, swept-wing configuration for, 20–21, 

27

Symington, Stuart, 34

T

T-38/F-5 aircraft, 159, 160n7, 272

tails

all-moving slab tail, 138

all-moving tail and flight control, 38n35, 

68, 81n46

D-558-1 data collection and research 

results, 54–57, 64–65, 66, 67–69

D-558-1 stabilizer and tail design, 38n35, 

46, 67–69, 70–71, 138, 281

D-558-1 tail-loads studies, 69, 70–71, 

73–74

D-558-2 stabilizer and tail design, 88, 

108–9, 128n5, 138, 281

D-558-2 tail-loads studies, 73–74, 127

F-86 all-moving tail, 68, 81n46

failure of during high-speed dives, 6–7

inertial coupling behavior and design of, 

156

semi-tailless aircraft, 167, 168, 195–98, 

200n4

strengthening of tail structures, 7

tailless aircraft, 39–40n43, 167–68

wind tunnel testing and X-3 tail design, 138

X-3 stabilizer and tail design, 138, 281

X-4 semi-tailless design, 166, 167, 235, 281

X-4 tail, structural integrity of, 181, 182, 

185

X-5 tail design, 281

X-5 tail-loads studies, 73, 232–33

XF-92A design, 244, 247

XF-92A tail design, 281

XS-1 stabilizer, elevator, and tail design, 

32–33, 38n35, 138, 281

technology

changes in, 198

conservative attitudes toward, 37n21

process for technological and scientific 

breakthroughs, 39–40n43

transformation of for supersonic era, v–vi, 

3–4, 269–70

TG-180 axial flow turbojet engine, 10, 46, 48, 

50, 52–53, 58, 74, 85

Theodorsen, Theodore, 19, 20

Thiokol XLR-99 engines, 140

Thompson, Floyd, 14–15

Thompson Trophy air races, 23
Probing the Sky

328

time, breaking the sound barrier and reversal 

of, 2, 35n2

transonic flight

loitering in transonic regime, 45

research aircraft development to study, 

8–12, 71–74

research on, onset of, 2–4

thrust capacity for, 85–86

wing-flow technique for research, viii

Truszynski, Gerald “Gerry,” 21, 23, 50, 275

Tsien, Hsue-Shen, 17, 39–40n43

Tu-22M aircraft, 236

Tu-144 aircraft, 264–66n31

Tu-160 aircraft, 236, 272

Tucker, Charles, 170–71, 172, 173–75, 176, 

177, 178, 179, 180

Turcat, Andre, 264–66n31

Turner, Harold, Jr., 9

U

Ulmer leather gaskets, 126–27

University of the South, 257

unmanned aerial vehicles (UAVs), 264–66n31

V

V-2 missiles, 136

Van Every, Kermit

D-558-1 design role, 45

D-558-2 design, 125

D-558-2 design role, 86, 87–88

Vandenberg, Hoyt S., 42n83

variable-sweep wings

advantages and value of, 208, 235, 236

aircraft built using concept, 236

asymmetrical oblique wing sweeping, 208

carrier landings and, 207–8, 235

limitations of, 208

NACA/NASA studies of, 208, 235–36, 

237n3

stability, control, and handling 

characteristics, 208

Swallow supersonic airliner, 235–36

symmetrical wing sweeping, 208

wing-pivot system, 208, 272

X-1 model testing of concept, 206, 208

X-5 wing design, pivot point, and sweep 

angle, 210–11, 211, 222, 236, 

237–38n7, 281

See also X-5 aircraft

Vensel, Joseph “Joe,” 21, 23, 115

Vickers Swallow supersonic airliner, 235–36

Victorville, 116

Voigt, Waldemar, 209

von Kármán, Theodore, 39–40n43

von Ohain, Hans, 5

vortex generator research

D-558-1 flights, 62–63, 73, 80n37

XS-1 flight, 80n37

Vought XF8U-1 Crusader aircraft, 235

Voyles, James, 27

Vulcan strategic bomber, 264–66n31

Vultee BT-13 aircraft, 247

W

Walker, Joseph A. “Joe”

astronaut wings, qualification for, 126

D-558-1 flights, 65–66

D-558-2 flights, 118, 126

education of, 65

experience of, 65, 126

formation test flights, 222–24

inertial coupling research missions, 153–55

X-3 flights, 149–50, 153–55, 156, 157

X-4 flights, 189, 190

X-5 accident panel testimony, 230

X-5 flights, 216, 217–20, 231

X-5 handling characteristics, 230

X-15 flights, 126

XS-1 flights, 126

Wallis, Bruce, 235–36

Weick, Fred, 4

Weil, Joe, 156
Index

329

Welch, George “Wheaties,” 153

Wendover Field, 16

Westinghouse engines

24C turbojet engines, 12, 85, 86, 87, 95

J30 engines, 170

J34 engines. See J34 engines

J46 engine, 135, 139–41, 157, 160n7, 210

jet engine development by, 160n7

Whitcomb, Richard T., 79–80n27, 259, 261, 

264–66n31, 272

Whittle, Frank, 5, 7

Whittle engine, 5, 7

Williams, Walter C.

aerial refueling formation test flights, 221

contributions to X-plane and space 

programs, 275

D-558-1 paint schemes, 51

D-558-1 research results, 53–57

D-558-2 program role, 115

housing for near Muroc, 24

inertial coupling research, 156

Mach 2 flight, approval for, 113

Muroc Flight Test Unit role, 21, 23–24, 23

painting of D-558-1 control surfaces, 58

X-4 equipment concerns, 181

X-4 flight data reports, 173–74, 186–87

XF-92A program role, 254

XS-1 flight-test program direction and 

goals, 25, 26, 27, 28, 41n63

XS-1 program publicity, 27

wind tunnels

choking in, 74–76, 124–25

Collier Trophy award for slotted tunnel, 76

Full-Scale Tunnel, 245

High Speed Tunnel, 8-Foot, 76

High-Speed Wind Tunnel, 7-by-10-Foot, 

138, 206, 208

limitations of research with, 1, 71–72, 

74–75, 269

Model 7002 wind tunnel testing, 248

size of models for testing in, 74–75

slotted tunnel development, 75–76

speed and aircraft design research in, v

sting for mounting models, 75

transformation of technology, tools, and 

procedures for supersonic era, v

X-3 tail design testing, 138

wing fences

delta-wing aircraft, 255, 256, 257

swept-wing aircraft, 88, 97, 102–6, 

103, 108

wing-flow technique

concept of, 18

development of, 14, 18

model use in adaptation of, viii

swept-wing concept testing, 18–20, 19, 20

transonic and supersonic airflow research 

with, viii

winglet, 57, 79–80n27

wings

D-558-1 data collection and research 

results, 54–57, 59–64, 67, 81n44

D-558-1 design and materials, 46–47, 281

D-558-2 wing-strain-gauge calibration, 91

endplates, 57

failure of during high-speed flight, 3

flaps on, 88

fuselage design to avoid shock waves near 

wings, 45–46

leading-edge slats, 88

reverse taper design, 88

strengthening of wing structures, 7

swept-wing aircraft, 256

trailing-edge thickness, 188–89, 203n50

wet wings and fuel tanks, 47

X-4 design and material, 170, 188–89, 

203n50, 281

X-4 wing flaps on trailing edge, 170

XS-1 wing design, 14–15, 20–21, 24, 27, 

46, 137–38, 281

See also delta-wing aircraft; swept wings; 

variable-sweep wings
Probing the Sky

330

wingtip vortices, 221–24

Wolf, Robert W., 9–10

Wolfe, J.E., 253

Wolko, Frank, 276, 277–78n13

Woods, Robert J,, 12, 209, 210

Woolams, Jack

death of, 23

XS-1 flight tests, 22, 32

XS-1 test site considerations, 16

Wright, Ray H., 75

Wright brothers, v, vi, 1, 269

Wright Field, 10–12

Wright Flyer, 81–82n51

Wright-Patterson Air Force Base, 195, 232, 257

X

X-1/XS-1 (Experimental Sonic-1) aircraft

accidents with, 22, 40n52, 277–78n13

air-launching from B-29 aircraft, 15, 25, 

27, 32, 33, 53, 124

altitude for flight tests, 15, 16, 27–28, 29, 

30–31

canopy design, 88

conference about results of program, 34

contributions to aviation research, 269, 271

control of program, 16, 28–30, 34

D-558-1 compared to, 45

data collection and research results, 54, 56, 

57, 73–74

design of, 14–15, 45, 139, 199

designation system for numbering, 

37–38n32

development of, decisions leading to, 8–12

display of aircraft at end of program, 

81–82n51

fate of and perceived importance, 

81–82n51

flight-test program direction, plan, and 

goals, 25–31, 41n63, 72, 74

flight tests, 21–24, 32–34, 42n83

flight test site for, 16, 21–24, 25–26, 27, 

40n52

fuel system, turbopump, and fuel capacity, 

15–16, 21, 25–26, 41n63, 128n4, 280

fueling and handling facilities for, 26

funding for, 28

inertial coupling behavior during falling 

bodies test, 151–52

Mach 1 flight, 32–34, 42n83, 81–82n51, 

88, 114

Mach 1 flight goal, 27–31

model testing, 32–33

NACA, transfer to, 50

negotiations to build, 12

paint schemes, 250

performance potential, 128n4

personnel involved in program, 21

pitch control, 32–33

publicity about program, 26–27, 34, 42n83

reports about program, 34, 42–43nn84–85

specifications, technical, 280–81

specifications for research requirements, 

15–16, 21–22

speed requirements for research, 15–16

stability and control data collection, 25, 26, 

30–31

stabilizer, elevator, and tail design, 32–33, 

38n35, 138, 281

supersonic flight capabilities of, 26–27, 

28–31

survival of aircraft at end of program, 

126–27

takeoff and landing of, 15–16, 21, 25, 27, 

41n63

transport to Muroc attached to B-29 

aircraft, 24

turbojet propulsion for, vi

vortex generator research flight, 80n37

weather conditions for testing, 22
Index

331

wing design, 14–15, 20–21, 24, 27, 46, 

137–38, 281

X-1A inertial coupling flight, 152, 157

X-2/XS-2 aircraft

loss of aircraft, 127, 277–78n13

pilot-escape system, 47, 77n7

wing design, 27, 137–38

X-3 Stiletto

Air Force flights, 146–49

all-rocket/air-launch configuration, 140–41

contract to develop, 135, 160n2

contributions to aviation research, 135, 

157–59, 271–72

data collection and research results, 

147–50

design of, 136–39, 137, 147, 151, 153, 

157–59, 199

display of aircraft at end of program, 

157, 257

dive tests, 145–46

drag and design of, 136, 137–38

engine-cooling arrangement, 136–37

engine failure during flight, 146

engines for, 136–37, 139–41, 157

F-104 Starfighter development and, 141, 

158–59, 161n10, 195, 272–73

final flights and end of program, 156–57

flight-test program direction, plan, and 

goals, 135–36, 141, 157

flight tests, 135–36, 142–50, 153–56

fuel system and capacity, 138, 280

fuselage design, 136, 137, 138

inertial coupling research, 147, 151, 152, 

153–56, 157–59, 272

instrumentation for data collection, 

147–48, 156

J34 engines for, 135, 139–40

J46 engines for, 135, 139–41, 157, 160n7

landing gear, 136

load feel control settings, 144

Mach 2 sustained speed performance, 135, 

160n7

mockup of, 134

NACA research flights, 135–36, 147–50

number of research flights, 157

paint schemes, 141, 150

performance potential, 134, 136, 139, 

141, 146

pilot-escape system, 153

pressure-distribution research, 156

safety modifications, 74

shortcomings of, 270

specifications, technical, 280–81

speed and structural heating, 139

speed capabilities, 140, 146, 271

stability, control, and handling 

characteristics, 142–46, 148–50, 

157–59, 270

stabilizer and tail design, 138, 281

stall/spin behavior, 145

taxi tests, 142

thrust requirements, 136, 139–41, 147

tire failures, 143, 145

transport of to lakebed for takeoff, 143, 158

wing design, 136, 137–38, 137, 158, 281

X-4 Bantam

aircraft for NACA, preference for, 181

canopy design and lock, 173, 183

contributions to aviation research, 

195–98, 272

design of, 166, 169–70, 169, 175, 182, 

198, 199, 209

display of aircraft at end of program, 166, 

195, 257

dive brakes, 176, 179–80, 181, 188–89, 

191, 193, 196, 203n53

elevons, 167, 172, 183, 193, 194, 196

engine malfunctions and replacements, 

175, 176, 180, 182, 188, 189–90, 

192–93
Probing the Sky

332

engines, access to, 177

engines for, 170

engines for, concerns about, 181

final flights and end of program, 192–95

flight status of both aircraft, 182

flight-test program direction, plan, and 

goals, 167, 169, 180–82

flight tests, 170–80, 182–94, 205n70

fuel-leak problems, 175–76, 182, 185, 189

fuel system and capacity, 170, 200n8, 280

instrumentation for data collection, 169–71, 

174–75, 176–77, 181, 183, 185, 193

landing gear lock and landing gear door 

issues, 170, 176, 177, 178, 179, 180, 183

lift-over-drag studies, 190–92, 195

NACA research flights, 185–94

pilot-escape systems, 173

rudder cable changes, 181, 182

rudder control, 171–74, 197

safety modifications, 74

semi-tailless design, 166, 167, 195–98, 

235, 281

shortcomings of, 270

specifications, technical, 280–81

stability, control, and handling 

characteristics, 167, 171–74, 176, 

177–78, 179–80, 183–92, 193, 194, 

196–98, 270

stability, control, and handling 

shortcomings, 186–88, 202n46

stall/spin behavior, 171–72, 177, 178, 183

tail, structural integrity of, 181, 182, 185

wing damage, 193–94

wing design and material, 170, 188–89, 

203n50, 281

wing flaps on trailing edge, 170

X-5 aircraft

accident and death of Popson, 227–31, 

277–78n13

aileron overbalance and effectiveness, 234

Air Force flights, 213–16, 226–30

contract to develop and build, 212

contributions to aviation research, 213, 

235–36, 272

data collection and research results, 214, 

215–26, 232–34

design of, 199, 209–13, 209, 211, 227, 

229, 231

display of aircraft at end of program, 232, 

257

dive tests, 219–20

engines for, 211–12

final flights and end of program, 231–34

flight characteristic differences between 

aircraft, 229

flight-test program direction, plan, and 

goals, 212–13

flight tests, 213–24, 226–34

formation test flights, 221–24, 239n29

fuel-cell pressure gauge problems, 213–14

fuel system and capacity, 212, 280

inertial coupling (roll coupling), 233–34

instrumentation for data collection, 214

jet fighter design based on, 210, 235

landing gear, 212

NACA, transfer to, 213

NACA research flights, 216–26, 231–34

number of research flights, 231

pilot-escape systems, 212

safety modifications, 74

shortcomings of, 271

specifications, technical, 212, 280–81

speed brakes, 211, 226

stability, control, and handling 

characteristics, 215–26, 235, 271

stall/spin behavior, 217–18, 220–21, 

227–31, 229, 235

symmetrical wing sweeping design, 208

tail design, 281

tail-loads studies, 73, 232–33

wing design, pivot point, and sweep angle, 

210–11, 211, 222, 236, 237–38n7, 281
Index

333

wing-root fillets, 219, 226

wing slats, 211, 223, 226

X-15 aircraft

air launching of, 140

contributions to aviation research, vi, 273

design and development of, 272

display of aircraft at end of program, 

81–82n51

engines for, 140

fate of and perceived importance, 

81–82n51

flight-test program direction, plan, and 

goals, 124

X-27 aircraft (Model CL-1200 Lancer aircraft), 

277n4

X-43A aircraft, 139

XB-35 flying-wing bomber, 167, 171

XB-47 jet bomber, 87

XB-52 bomber prototype, 198

XB-70 Valkyrie aircraft, 264–66n31

XF8U-1 Crusader aircraft, 235

XF10F-1 Jaguar fighter, 235, 236

XF-86 aircraft, 30

XF-90 aircraft, 160n7

XF-92A aircraft

Air Force flights, 246, 249–54, 262–63n10

birth of, 243–44

canopy design, 247, 260

cart for takeoff run, 246, 247

contract to develop, 247

contributions to aviation research, 256, 

258–61, 264–66n31, 272

data collection and research results, 

251–57

delta-wing design, 244–45, 247, 281

design of, 199, 242, 243–48, 245, 246, 

258, 260

display of aircraft at end of program, 257

elevons, 247

engine malfunctions and replacements, 

250–51, 256, 262–63n10

final flights and end of program, 256–57

flight control system, 245, 247, 249, 253, 

257, 259

flight-test program direction, plan, and 

goals, 254–55

flight tests, 248–57

flying mockup for proposed interceptor, 243

fuel system and capacity, 246, 280

funding for, 247

fuselage design, 242, 245, 247

improvement recommendations, 253–54

interceptor mission of, 246–47

jet power for, 247, 248, 250–51, 254, 

262–63n10

landing gear, 247, 256

Model 7002 designation, 244, 247–48

NACA, transfer to, 254

NACA research flights, 254–57, 255, 258

paint schemes and bare-metal finish, 242, 

246, 250, 250, 258

pilot-escape systems, 245–46, 247

propulsion system for, 244

ramjet power for XP-92, 244, 246, 247

refueling formation test flights, 221

rocket power for XP-92, 244, 246, 247

rudder-pulse tests, 255

shortcomings of, 271

specifications, technical, 280–81

speed capabilities, 250, 251

stability, control, and handling 

characteristics, 248–49, 251–57, 

258–59, 271

stall/spin behavior, 249

tail design, 244, 247, 281

taxi tests, 248–49

weapons aboard, 246

wing fences, 255, 256, 257

XP-92 design and prototypes, 243–48

XLR-8/XLR-11 rocket engines. See LR-8/

XLR-8/XLR-11/RMI 6000C4 rocket engines

XLR-99 engines, 140
Probing the Sky

334

XP-56 tailless fighter, 171

XP-59A jet, 7–8, 9, 11, 22, 37n24, 243

XP-79B flying-wing fighter, 170, 171, 196–97

XP-86 aircraft, 28, 87

XS-1 aircraft. See X-1/XS-1 (Experimental 

Sonic-1) aircraft

XS-3 aircraft

jet-powered aircraft design, 27

Mach 1 flight goal, 27

X-series research aircraft. See research 

aircraft/X-series research aircraft

XST Have Blue stealth aircraft, 198

Y

YB-49 flying-wing bomber, 167, 172, 196–97

YB-60 bombers, 140

Yeager, Charles E. “Chuck”

injury to falling from a horse, 33

X-1A inertial coupling flight, 152, 157

X-3 flights, 147, 147, 148–49

X-4 flights, 182

XF-92A flights, 249–51, 258

XF-92A handling characteristics, 258

XS-1 flight tests, 21, 29, 32–34

XS-1 Mach 1 flight, 32–34, 42n83, 

81–82n51, 88, 114

XS-1 project and Collier Trophy, 42n83

YF-12 aircraft, 264–66n31

YF-89D aircraft, 130n38

YF-102/F-102 aircraft, 259–61, 259, 260, 

261, 264–66n31

York, Herbert, 274

Z

Ziegler, Jean L. “Skip,” 213–14, 226, 276, 

277–78n13

Zisfein, Melvin B., 161n10