Research in Supersonic Flight and
the Breaking of the Sound Barrier

by John D. Anderson, Jr.

> "We call the speed range just below and just above the sonic
> speed-Mach number nearly equal to 1-the transonic range. Dryden [Hugh Dryden,
> well-known fluid dynamicist and past administrator of the National Advisory
> Committee for Aeronautics] and I invented the word 'transonic' We had found
> that a word was needed to denote the critical speed range of which we were
> talking. We could not agree whether it should be written with one s or
> two. Dryden was logical and wanted two s's. I thought it wasn't necessary
> always to be logical in aeronautics, so I wrote it with one s. I introduced
> the term in this form in a report to the Air Force. I am not sure whether
> the general who read it knew what it meant, but his answer contained the
> word, so it seemed to be officially accepted. . . . I will remember this
> period (about 1941) when designers were rather frantic because of the unexpected
> difficulties of transonic flight. They thought the troubles indicated a
> failure in aerodynamic theory. "1

The morning of Tuesday, October 14, 1947, dawned bright and beautiful
over the Muroc Dry Lake, a large expanse of flat, hard lake bed in the
Mojave Desert in California. Beginning at 6:00 a.m., teams of engineers
and technicians at the Muroc Army Air Field readied a small rocket-powered
airplane for flight. Painted orange, and resembling a 50-caliber machine
gun bullet mated to a pair of straight, stubby wings, they carefully installed
the Bell X-1 research vehicle in the bomb bay of a four-engine B-29 bomber
of World War II vintage. At 10:00 a.m., the B-29 with its soon-to-be historic
cargo took off and climbed to an altitude of 20,000 feet. As it passed
through 5,000 feet, Captain Charles E. (Chuck) Yeager, a veteran P-51 pilot
from the European theater during World War II, struggled into the cockpit
of the X-1. This morning Yeager was in pain from two broken ribs incurred
during a horseback riding accident the previous weekend. However, not wishing
to disrupt the events of the day, Yeager informed no one at Muroc about
his condition, except his close friend Captain Jack Ridley, who helped
him to squeeze into the X-1 cockpit. At 10:26 a.m., at a speed of 250 miles
per hour, the brightly painted X-1 dropped free from the bomb bay of the
B-29. Yeager fired his Reaction Motors XLR-11 rocket engine and, powered
by 6,000 pounds of thrust, the sleek airplane accelerated and climbed rapidly.
Trailing an exhaust jet of shock diamonds from the four convergent-divergent
rocket nozzles of the engine, the X-1 soon approached Mach 0.85, the speed
beyond which there existed no wind tunnel data on the problems of transonic
flight in 1947. Entering this unknown regime, Yeager momentarily shut down
two of the four rocket chambers, and carefully tested the controls of the
X-1 as the Mach meter in the cockpit registered 0.95 and increased still.
Small invisible shockwaves danced back and forth over the top surface of
the wings. At an altitude of 40,000 feet, the X-1 finally started to level
off, and Yeager fired one of the two shutdown rocket chambers. The Mach
meter moved smoothly through 0.98, 0.99, to 1.02. Here, the meter hesitated
then jumped to



---

1. Theodore von Kármán, Aerodynamics
(Ithaca, NY: Cornell University Press, 1954), p. 116.

59

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60 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

1.06. A stronger bow shockwave now formed in the air ahead of the needlelike
nose of the X-1 as Yeager reached a velocity of 700 miles per hour, Mach
1.06, at 43,000 feet. The flight was smooth; there was no violent buffeting
of the airplane and no loss of control as feared by some engineers. At
this moment, Chuck Yeager became the first pilot to fly faster than the
speed of sound, and the small but beautiful Bell X-1, became the first
successful supersonic airplane in the history of flight.2

|Photo of The Bell X-1     
|--------------------------
|The Bell X-1. (NASA photo)



---

2. This description of the first supersonic flight
is excerpted from John D. Anderson, Jr., Modern Compressible Flow: With
Historical Perspective (New York, NY. McGraw-Hill Book Co., 1990 2d
ed.), pp. 2-4. For a general reference, from Chuck Yeager's point of view,
see General Chuck Yeager and Leo Janos, Yeager.- An Autobiography
(New York, NY: Bantam Press, 1985). For a definitive history of the circumstances
leading tip to and surrounding the development and flight testing of the
Bell X-1, see Richard P. Hallion, Supersonic Flight (New York, NY.
Macmillan, 1972).



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 61

As the sonic boom from the X-1 propagated across the California desert,
this flight became the most significant milestone in aviation since the
Wright brothers' epochal first flight at Kill Devil Hills fourty-four years
earlier. But in the history of human intellectual accomplishment, this
flight was even more significant; it represented the culmination of 260
years of research into the mysteries of high-speed gas dynamics and aerodynamics.
In particular, it represented the fruition of twenty-three years of insightful
research in high speed aerodynamics carried out by the National Advisory
Committee for Aerodynamics (NACA) research that represented one of the
most important stories in the history of aeronautical engineering. The
purpose of this chapter is to tell this story. The contribution by the
NACA to the Bell X-1 was much more technical than it was administrative.
Therefore, this chapter will highlight the history of that technology.

The NACA's work on high-speed aerodynamics described in this chapter
is also one of the early examples in the history of aerodynamics where
engineering
science played a deciding role. Beginning in 1919, the NACA embarked
on a systematic intellectual quest to obtain the knowledge required
to eventually design proper high-speed airfoil shapes. Historian
James R. Hansen, in his chapter on the NACA low-drag engine cowling, in
the present book, asks the following question about the cowling work: Was
it science, or was it engineering? He comes to the conclusion that it was
somewhere in between that it was an example of engineering science in
action at the NACA. In arriving at this conclusion, Hansen draws from the
thoughts in Walter Vincenti's book, What Engineers Know and How They
Know It, where Vincenti clearly makes the following distinction between
science and engineering: science is the quest for new knowledge for the
sake of enhancing understanding, and engineering is a self-standing body
of knowledge (separate from science) for the sake of designing artifacts.
For the purpose of the present chapter, I suggest this definition of engineering
science: Engineering science is the search for new scientific knowledge
for the explicit purpose of (1) Providing a qualitative understanding which
allows the more efficient design of an engineering artifact, and/or (2)
Providing a quantitative (predictive) technique, based on science, for
the more efficient design of an engineering artifact. In this chapter
we will see that NACA researchers in the 1920s and 1930s were working hard
to discover the scientific secrets of high-speed aerodynamics just so they
could properly design airfoils for highspeed flight truly engineering
science in action. Also, within the general framework of the historical
evolution of aerodynamic thought over the centuries, the NACA's highspeed
research program is among the earliest examples of engineering science,
although that label had not yet been coined at the time.

The Prehistory of High-Speed Flight: Point and
Counterpoint

Most golfers know the following rule of thumb: When you see a flash
of lightning in the distance, start counting at a normal rate  one,
two, three.... For every count of five before you hear the thunder, the
lightning bolt struck a mile away. Clearly, sound travels through air at
a definite speed, much slower than the speed of light. The standard sea
level speed of sound is 1, 117 feet per second in five seconds a sound
wave will travel 5,585 feet, slightly more than a mile. This is the basis
for the golfer's "count of five" rule of thumb.

The speed of sound is one of the most important quantities in aerodynamics;
it is the dividing line between subsonic flight (speeds less than that
of sound) and supersonic flight (speeds greater than that of sound). The
Mach number is the ratio of the speed of a gas to the speed of sound in
that gas. If the Mach number is 0.5, the gas flow velocity is one-half
the speed of sound; a Mach number of 2.0 means that the flow velocity is
twice



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62 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

that of sound. The physics of a subsonic flow is totally different from
that of a supersonic flow a contrast as striking as that between day and
night. This is why the first supersonic flight of the X-1 was so dramatic,
and why the precise value of the speed of sound is so important in aerodynamics.

Knowledge of the speed of sound is not a product of twentieth century
science. Precisely 260 years before the first supersonic flight of the
X-1, Isaac Newton published the first calculation of the speed of sound
in air. At that time it was clearly appreciated that sound propagated through
air at some finite velocity. Newton knew that artillery tests had already
indicated that the speed of sound was approximately 1,140 feet per second.
The seventeenth century artillery men were preceding the modern golfer's
experience; the tests were performed by standing a known large distance
away from a cannon, and noting the time delay between the light flash from
the muzzle and the sound of the discharge. In Proposition 50, Book II of
his Principia (1687), Newton calculated a value of 979 feet per
second for the speed of sound in air fifteen percent lower than the existing
artillery data. Undaunted, Newton followed a now familiar ploy of theoreticians;
he proceeded to explain away the difference by the existence of solid dust
particles and water vapor in the atmosphere. However, in reality Newton
had made the incorrect assumption in his analysis that the air temperature
inside a sound wave was constant (an isothermal process), which caused
him to underpredict the speed of sound. This misconception was corrected
more than a century later by the famous French mathematician, Pierre Simon
Marquis de Laplace, who properly assumed that a sound wave is adiabatic
(no heat loss), not isothermal.3 Therefore,
by the time of the demise of Napoleon, the process and equation for the
speed of sound in a gas was fully understood.

This is not to say that the precise value of the speed of sound was
totally agreed upon. The debate lasted well into the twentieth century.
Indeed, although this event is little known today, the NACA was an arbiter
in setting the standard sea level speed of sound. On October 12, 1943,
twenty-seven distinguished U.S. leaders in aerodynamics walked through
the doorway of NACA Headquarters at 1500 New Hampshire Avenue in Washington,
DC. They were attending a meeting of the Committee on Aerodynamics, one
of the various adjunct committees set up by the main NACA. Among the experts
present were Hugh L. Dryden from the Bureau of Standards, and John Stack,
whose career as an aerodynamicist at the NACA Langley Memorial Laboratory
was on a meteoric rise. Also present was Theodore von Kármán,
director of the Guggenheim Aeronautical Laboratories at Cal Tech, who represented
an intellectual pipeline to the seminal aerodynamic research by Ludwig
Prandtl at Göttingen University in Germany, where von Kármán
had been Prandtl's Ph.D. student before World War I. After subcommittee
reports on progress in helicopter aerodynamics, and recent aerodynamic
problems in wing flutter and vibration, the matter of speed of sound was
brought up as new business by John Stack, who stated that "the problem
of establishing a standard speed of sound was raised by an aircraft manufacturer."4

Stack reported that the Committee's laboratory staff had surveyed the
available information on specific heats of air thermodynamic information
that goes into the calculation of the speed of sound which led to a calculated
value of the speed of sound of



---

3. Pierre Simon Marquis de Laplace, "Sur la vitesse
do son dans I'aire et dan I'eau," Annales de Chimie et de Physique,
1816.

4. Minutes of the Meeting of Committee on Aerodynamics,
October 12, 1943, p. 9. Found by the author in the John Stack files at
the NASA Langley Research Center Archives, Langley Research Center, Hampton,
VA. Originally marked with security classification Confidential, the minutes
have since been declassified. The Langley Archives are kept by Richard
T. Layman, who was exceptionally helpful to the author during the course
of research for this chapter.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 63



|Photo of John Stack holding a model of the X-1                                                                                                                                                                                                                       
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|John Stack, Langley Research Center scientist, was
Presented the Collier Trophy in 1947, awarded for his conception of transonic
research airplanes. His research contributed to the X-1 breaking the sound
barrier on October 14, 1947. (NASA Photo No. LMAL 48991).

1,116.2 feet per second. Measured values gave weighted means of 1,116.8
to 1,116.16 feet per second. Dryden noted that the specific heats were
"not necessarily the same for all conditions" and suggested that the Committee
select 1,117 feet per second as a round figure for a standard value of
the speed of sound for sea level conditions for aeronautical usage. The
outcome of this discussion appeared in the meeting minutes: "After further
discussion it was agreed that the recommendation of a standard value for
the speed of sound would be left for Dr. Dryden and Mr. Stack to work out
jointly." Today, the accepted standard speed of sound depends on which
"standard atmosphere" table you look at, ranging from a value of 1,116.4
feet per second in the 1959 ARDC Model atmosphere to 1,116.9 feet per second
in the 1954 ICAO Model atmosphere. However, for engineering purposes this
is splitting hairs, and Dryden's suggestion of a round value of 1,117 feet
per second is still used today for many engineering calculations. Here
is a little-known example of how the NACA played a role in the fundamentals
of high-speed compressible aerodynamics even to the mundane extent of
providing to industry a "standard" value of the speed of sound.

On October 14,1947, as the Bell X-1 nudged closer to Mach one, a region
of the aerodynamic flow over the wing became locally supersonic. This is
because the airflow increases its velocity while moving over the top of
the wing, and hence there is always a region of the flow over the wing
where the local velocity is larger than the velocity of the airplane itself.
As the X-1 accelerated through Mach 0.87, a pocket of locally supersonic
flow formed over the top of the wing. This supersonic pocket was terminated
on the downstream end by a shockwave oriented almost perpendicular to the
flow called a normal



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64 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER



|Schematic diagram of transonic flow over an airfoil                                                                                                                                                                                                                                                
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Schematic of transonic flow over an airfoil. (a) Freestream
flow slightIy below the speed of sound, typically
a subsonic freestream Mach number from about 0.8 to 0.999. (b) Freestream
flow Slightly above the speed of sound typically a supersonic freestream
Mach number from 1.0 to about 1.2.

shock (as shown above). This shock formation was the culprit which made
flight through Mach one such a harrowing concern at that time. Finally,
when the X-1 accelerated through Mach one to supersonic speeds, another
shock wave formed a short distance in front of the nose; this shock, called
the bow shock, was curved and more oblique to the flow (As shown above).
Shock waves are extremely thin regions much thinner than the thickness
of this page across which dramatic and almost discontinuous increases
in pressure and temperature occur. Shock waves are a fact of life in the
aerodynamic flow over transonic and supersonic airplanes.

Knowledge of shock waves is not unique to the twentieth century; their
existence was recognized in the early nineteenth century. The German mathematician
G. F. Bernhard Riemann first attempted to calculate shock properties in
1858, but he neglected an essential physical feature and hence obtained
incorrect results.5Twelve years later,
William John Rankine, a noted engineering professor at the University of
Glasgow, correctly derived the



---

5. A shock wave is, in thermodynamic language,
an irreversible process, caused by viscosity and thermal conduction effects
inside the shock wave. A measure of the amount of irreversibility is a
thermodynamic variable called entropy, which from the Second Law of Thermodynamics
always increases in any process involving such irreversibilities. The entropy
of a gas always increases as it passes through a shock wave. Unfortunately,
Riemann made the incorrect assumption that the entropy remained constant
across a shock.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 65

proper equations for the flow across a normal shock wave. Not cognizant
of Rankine's work, the French ballistician Pierre Hugoniot rediscovered
the normal shock wave equations in 1887. To the present day, the governing
equations for flow across a shock wave are called the Rankine-Hugoniot
equations, in honor of these two men.6
This work was expanded to include oblique shock waves by the famous German
aerodynamicist, Ludwig Prandtl and his student Theodor Meyer at Göttingen
University in 1908.7 Hence, only five
years after the first flight by the Wright brothers, the necessary theory
for the calculation of shock wave properties in a supersonic flow was in
hand, albeit considered a purely academic subject at that time.

The nineteenth century was also a time of experimental work on supersonic
flow. Perhaps the most important event was the proof that shock waves were
not just a figment of the imagination they really existed in nature. This
proof was given by the physicistphysician-philosopher Ernst Mach in 1887.
Mach, while a professor of physics at the University of Prague, took the
first photographs of shock waves on a body moving at supersonic speeds.
Shock waves are normally invisible to the naked eye. But Mach devised a
special optical arrangement (called a shadowgraph) by which he could see
and photograph shock waves. In 1887, he presented a paper to the Academy
of Sciences in Vienna where he showed a photograph of a bullet moving at
supersonic speeds. Using his shadowgraph system, the bow shock and trailing
edge shock were made visible (as shown below). This historic photograph
allowed scientists, for the first time in history, to actually see a shock
wave. The experimental study of shock waves was off and running.

|Photograph of a bullet in supersonic flight, published by Ernst Mach in 1887 
|-----------------------------------------------------------------------------
|Photograph of a bullet in supersonic flight, published
by Ernst Mach in 1887.



---

6. John D. Anderson, Jr., Modern Compressible
Flow: With Historical Perspective, (New York, NY. McGraw Hill, 1990),
pp. 92-95.

7. Ibid., pp 140- 43.



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66 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

This prehistory of supersonic flight, both theoretical and experimental,
was carried out by basic researchers who were interested in the subject
on an academic basis only. The true practical value of this work did not
come to fruition until the advent of supersonic flight in the 1940s. However,
this is an excellent example of the value of basic research on problems
that appear only purely academic at the time. In the 1940s, when basic
supersonic flow theory and fundamental understanding of shock waves was
suddenly needed due to the advent of highspeed airplanes and rockets, it
was there quietly residing and sleeping in a few dusty books and archive
journal articles in the library.

In light of our earlier discussion of engineering science, was this
early work on shock waves engineering science? Emphatically no! The researchers
involved in this work were after scientific knowledge, and just that. There
was no force behind these researchers driving them to design any related
engineering artifacts at the time.8

Compressibility Problems: The First Inklings
(1918-1923)

Airplane aerodynamics, from the time of the Wright Flyer to the beginning
of World War II, assumed that changes in air density were negligible as
the air flowed over the airplane. This assumption, called incompressible
flow, was reasonable for the 350 mph or slower flight speeds of airplanes
during that era. Theoretically, it was a tremendous advantage to assume
constant density, and physically the low-speed aerodynamic flows usually
exhibited smooth variations with no sudden changes or surprises. All this
changed when flight speeds began to sneak up close to the speed of sound.
Aerodynamic theory had to account for changes in the air density in the
flow field around the airplane, and physically the flow field sometimes
acted erratically, and frequently surprised and greatly challenged aerodynamicists.
Aerodynanticists in the 1930s simply threw these phenomena into one pot
and called them generically "compressibility problems."

Ironically, the first inklings of compressibility problems occurred
during the age of the strut-and-wire biplanes, with flight velocities about
as far away from the speed of sound as you can get. It had to do with an
airplane part, namely the propeller. Although typical flight speeds of
World War I airplanes were less than 125 miles per hour, the tip speeds
of propellers, because of their combined rotational and translational motion
through the air, were quite large, sometimes exceeding the speed of sound.
This fact was appreciated by aeronautical engineers at the time. This drove
the British Advisory Committee for Aeronautics to show some interest in
compressible flow theory. In 1918 and 1919, G. H. Bryan, working for the
Committee at the Royal Aeronautical Establishment, carried out a theoretical
analysis of subsonic and supersonic flows over a circular cylinder (a simple
geometric shape chosen for convenience). He was able to show that in a
subsonic flow the effect of compressibility was to displace adjacent streamlines
farther apart. His analysis was cumbersome and complex a harbinger of
things to come and provided little data of value. But it was evidence
of the concern felt by the British over the effects of compressibility
on propeller performance.9

At the same time, Frank Caldwell and Elisha Fales of the propeller branch
of the Army Air Service Engineering Division at McCook Field in Dayton,
Ohio, took a purely experimental



---

8. Report for the Year 1909-10, Advisory
Committee for Aeronautics, England, p. 5.

9. G.H. Bryan, "The Effect of Compressibility
on Streamline Motions," R & M No. 555, Technical Report of the Advisory
Committee for Aeronautics, Vol. I, Dec. 1918; G .H. Bryan, "The Effect
of Compressibility on Streamline Motions, Part II," R & M No. 640,
Advisory Committee for Aeronautics, April 1919.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 67



|plot of lift coefficient, Ky, versus velocity in miles per hour. Lift drops sharply at 350mph for this wing.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|The first data to show the adverse compressibility
effects of high-speed flow over an airfoil. Caldwell and Fales, NACA TR
83, 1920. This is a plot of lift coefficient, Ky, versus velocity in miles
per hour: The definition used for Ky at that time differed from the modern
definition of  lift coefficient (usually denoted by CL
today) by a factor of two, i.e., CL = 2 Ky. The large drop in
Ky seen at the right of the graph is the adverse effect of compressibility.
(The ramp in Ky seen at the left of the graph was not explained by Caldwell
and Fales; it is the present author's educated guess that the ramp was
a low Reynolds number effect, because of the small size of the airfoil
models used, namely a one inch chord.)

approach to the problem. (This was the beginning of a blurred dichotomy
between British and American research on compressibility effects. Over
the next two decades, the major experimental contributions to understanding
compressibility effects were to be made in the United States, principally
by the NACA, and the major theoretical contributions were to be made in
England.) In 1918, Caldwell and Fales designed and built the first high-speed
wind tunnel in the United States purely to investigate the problems associated
with propellers. The tunnel velocity range was from 25 to a stunning 465
miles per hour. It had a length of almost nineteen feet, and the test section
was fourteen inches in diameter. This was a big and powerful machine for
its day. Six different airfoils, with thickness ratios (ratio of maximum
thickness to the chord length) from 0.08 to 0.2, were tested. At the higher
speeds, the results showed "a decreased lift coefficient and an increased
drag coefficient, so that the lift-drag ratio is enormously decreased."
Moreover, the airspeed at which these dramatic departures took place was
noted as the "critical speed."10 Because
of its historical significance, some of their data is shown above, reproduced
directly from NACA TR 83. Here, the lift coefficient for the airfoil at
eight-degree



---

10. The critical Mach number is precisely defined
as that freestream Mach number at which sonic flow is first encountered
on the surface of a body. The large drag rise due to compressibility effects
normally occurs at a freestream Mach number slightly above the critical
Mach number; this is called the drag-divergence Mach number. In reality,
Caldwell and Fales had reached and exceeded the drag-divergence Mach number
in their experiments. But their introduction of the word "critical" in
conjunction with this speed was eventually the inspiration for its rise
in later coining the term "critical Mach number."



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68 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

angles of attack is plotted versus airstream velocity. Note the dramatic
drop in lift coefficient at the "critical speed" of 350 miles per hour
the compressibility effect. This plot, and ones like it for other angles
of attack that were published in NACA TR 83, are the first published data
in the history of aerodynamics to show the adverse effects of compressibility.
Although Caldwell and Fales made an error in the reduction of their data
(an understandable error associated with the inexperience of dealing with
compressible flow conditions at the early date of 1919) which caused their
reported lift and drag coefficients to be about ten percent too low at
the higher speeds, this did not compromise the dramatic and important discovery
of the large increase in drag and decrease in lift when the airfoil sections
were tested above the "critical speed." Moreover, they were the first to
show that the "critical speed" for thin airfoils was higher than that for
thick airfoils, and hence by making the airfoil section thinner, the adverse
compressibility effects can be delayed to higher Mach numbers. This was
an important finding, and one which would have a lasting impact on high-speed
vehicle design.11

It is noteworthy that the fledgling NACA was the government agency which
published the results of Caldwell and Fales. 12
The NACA was carrying out its duty as stated in Public Law 271, which created
the Committee in 1915, namely "to supervise and direct the scientific study
of the problems of flight, with a view to their practical solution, and
to determine the problems which should be experimentally attacked, and
to discuss their solution and their application to practical questions."
Publishing the Caldwell and Fales work is in the latter category the NACA
was already earmarking compressibility effects as a problem "which should
be experimentally attacked."

In the chronology of events, the British were next to examine the effects
of compressibility on propellers. In 1923, G. P. Douglas and R. McK. Wood,
two aerodynamicists at the Royal Aeronautical Establishment, tested model
propellers at high rotational speeds in the seven-foot low-speed wind tunnel
(100 miles per hour airstream) at the National Physical Laboratory in London.13
They also carried out flight tests on a DeHaviland D.H. 9A biplane. Their
data were the global measurements of the thrust and torque generated by
the whole propeller, so the details of the compressibility effects affecting
the airfoil sections at the tip of the propeller were somewhat obscured.
However, one of their conclusions anticipated the adverse effects of compressibility,
namely that "higher tip speeds than at present used will probably involve
a serious loss of efficiency."



---

11. This author, upon Studying Caldwell and
Fales detailed data reduction, has found that, although they recognized
that the density of the airflow changed inside the wind tunnel at the higher
speeds, their accounting for this in calculating their lift and drag coefficients
from their measured lift and drag forces was done incorrectly. They thought
they had worked their data reduction so that "density does not enter into
the calculation." Rather, they expressed their lift and drag coefficients
in terms of the impact pressure the difference between total and static
pressure. This is why they said that "density does not enter into the calculation."
But they incorrectly and rather naively used the incompressible Bernoulli
equation to replace the velocity-squared term in the definition of lift
coefficient with the impact pressure. This resulted in about a ten percent
error in the values of their reported lift and drag coefficients at high
speeds. For more details, see John D. Anderson, Jr., The History of
Aerodynamics, and its Impact on Flying Machines (New York, NY: Cambridge
University Press, 1997).

12. F. W. Caldwell, and E. Fates, "Wind Tunnel
Studies in Aerodynamic Phenomera at High Speed." NACA TR 83,1920.

13. G.P. Douglas and R. McK. Wood, "The Effects
of Tip Speed on Airscrew Performance. Air Experimental Investigation of
the Performance of an Airscrew Over a Range of Speeds of Revolution from
'Model' Speeds tip to Tip Speeds in Excess of the Velocity of Sound in
Air," R & M No. 884, Advisory Committee for Aeronautics, 1923.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 69

The Compressibility BurbleNACA's Seminal
Research, 1924-1929

Meanwhile, the NACA was forging ahead. During the 1920s, the Committee
sponsored a series of fundamental experiments in high-speed aerodynamics
at the Bureau of Standards with Lyman J. Briggs and Dr. Hugh L. Dryden.
Hugh Dryden was a fresh, young Ph.D. graduate from Johns Hopkins University
in physics; he had received his Ph.D. in 1919 at the age of twenty. (Dryden
much later was to become the Director of Research for the NACA front 1947
to 1958.) This work progressed in three stages, each one documented in
a separate NACA Technical Report, and covered the period from 1924 to 1929.
As before, the primary motivation for this research was to understand the
compressibility effects at the tips of propellers.

The first stage simply confirmed the trends already observed by Caldwell
and Fales four years earlier. Briggs and Dryden, with the help of Lt. Col.
G. F. Hull of the Army Ordnance Department, jury-rigged a highspeed wind
tunnel by connecting a vertical standpipe thirty inches in diameter and
thirty feet high to a large centrifugal compressor at the Lynn Works of
the General Electric Company in Massachusetts. At the other end of the
pipe was a cylindrical orifice that served as a nozzle 12.24 inches in
diameter. With this device "air speeds approaching the speed of sound were
obtained."14 Unlike Caldwell and Fales,
Briggs and Dryden used the proper equations for compressible flow to calculate
the air velocity Although not yet in the standard textbooks, these equations
were known by Dryden as a result of his Ph.D. studies in physics. (The
first engineering textbook in English to focus on compressible flow was
not published until 1947.)15 Rectangular
planform models, with a span of 17.2 inches and a chord length of three
inches, were placed in the high-speed airstream, and lift, drag and center-of-pressure
were measured. The results supported the earlier trends observed by Caldwell
and Fales. In particular, Briggs found:16

1. Lift coefficient for a fixed angle of attack decreases very rapidly as
the speed increases.
2. The drag coefficient increases rapidly.
3. The center-of-pressure moves back towards the trailing edge.
4. The "critical speed" at which these occur decreases as the angle of attack
is increased and the airfoil thickness is increased.

In 1924, the culmination of this work, as well as that which went
before, was the waving of a red flag compressibility effects were nasty
and they markedly degraded airfoil performance. But nobody had any fundamental
understanding of the physical features of the flow field which were causing
these adverse effects. This was not to come for another decade.

Briggs and Dryden made an important step towards this fundamental understanding
in the second stage of their work. Because the Lynn Works compressor was
no longer available to them, Briggs and Dryden moved their experimental
activity to the Army's Edgewood Arsenal, where they constructed another
high-speed wind tunnel, this one much smaller, with an airstream only two
inches in diameter. However, by careful design of the small airfoil models,
two pressure taps could be placed in each model. Seven identical



---

14. L. J. Briggs; G.F. Hull; and Hugh L Dryden,
"Aerodynamic Characteristics of Airfoils at High Speeds," NACA TR 207,
1924.

15. Hans W. Liepmann, and Allen E. Puckett,
Introduction
to Aerodynamics of a Compressible Fluid (New York, NY. John Wiley and
Sons, 1947).

16. L. J. Briggs and Hugh L. Dryden, "Pressure
Distribution Over Airfoils at High Speeds," NACA TR 255, 1926.



---

70 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

models were used, each one with different locations of the pressure
taps. A total of thirteen pressure tap locations, seven on the upper surface
and six on the lower surface, were employed (for the reader who is counting,
the seventh model had only one tap).

With this technique, Briggs and Dryden measured the pressure distributions
over the airfoil at Mach numbers from 0.5 to 1.08. The results were dramatic!
Beyond the "critical speed," the pressure distributions over the top of
the airfoil exhibited a sudden pressure ,jump at about one-third to one-half
the distance from the leading edge, followed by a rather long plateau towards
the trailing edge. Such a pressure plateau was familiar it was very similar
to that which exists over the top surface of an airfoil in low-speed flow
when the airfoil stalls at high angle of attack. And it was well known
that airfoil stall was caused by the separation of the flow off the top
surface of the airfoil. Briggs and Dryden put two-and-two together, and
concluded that the adverse effects of compressibility were caused by flow
separation over the top surface, even though the airfoil was at low (even
zero) angle of attack. To substantiate this, they conducted oil flow tests,
wherein a visible, pigmented oil was painted on the model surface, and
the model was placed in the high-speed airstream. During the tests, the
tell-tale flow separation line formed on the oil pattern. Clearly, beyond
the "critical speed," flow separation was occurring on the top surface
of the airfoil. The next question was: Why? What was causing the flow to
separate? The answer to this question still lay eight years in the future.

Was this work of Briggs and Dryden engineering science? Emphatically
yes! Their experiments were designed to obtain basic scientific information
about the physics of the high-speed flow over an airfoil, but always for
the purpose of learning how to design better airfoil shapes for high-speed
flight.

The third stage of the work by Briggs and Dryden was utilitarian, and
was in keeping with the stated duty of the NACA to work on the problems
of flight "with a view to their practical solution." Towards the end of
the 1920s, they carried out a large number of detailed measurements of
the aerodynamic properties for 24 different airfoils at Mach numbers from
0.5 to 1.08. The airfoils chosen were those conventionally used by the
Army and the Navy for propellers, consisting of the standard family of
British-designed RAF airfoils, and the American-designed Clark Y family.
These data provided the first definitive measurements on standard
series of airfoils showing compressibility effects. 17

It should be noted that theoretical solutions of high-speed compressibility
effects in a subsonic flow were virtually non-existent during the 1920s.
The only major contribution was that by the famous British aerodynamicist
Herman Glauert, who rigorously derived a correction to be applied to the
low-speed, incompressible lift coefficient in order to correct it for compressibility
effects.18 This was the first of a
series of theoretical rules labeled "compressibility corrections." Because
it was known that Ludwig Prandtl in Germany had also derived the same rule
a few years earlier, but had not published it, Glauert's result has come
down through the decades as the Prandtl-Glauert Rule. However, such
compressibility corrections are applicable to the variation of lift coefficient
with speed below the "critical speed," and hence have no way of predicting
the lift coefficient in the "compressibility burble."

Throughout this, the primary motivation for all the above work on compressibility
effects was for application to airplane propellers. But the focus was about
to change, and change dramatically.



---

17. L. J. Briggs and Hugh L. Dryden, "Aerodynamic
Characteristics of Twenty-Four Airfoils at High Speeds," NACA TR 319,1929.

18. H. Glauert, "The Effect of Compressibility
on the Lift of an Airfoil, "Journal of the Royal Society 118 (1927):
113. Also published as R & M No. 1135, Advisory Committee for Aeronautics,
September 1927.



---

FROM ENGINEERING SCIENCE TO BIG SCIENCE 71

John Stack and the NACA Compressible Flow
ResearchA Breakthrough

In July 1928, a young New Englander, born and raised in Lowell, Massachusetts,
began his career with the NACA Langley Memorial Aeronautical Laboratory.
Having just graduated from the Massachusetts Institute of Technology with
a B.S. degree in aeronautical engineering, John Stack was assigned to the
Variable Density Tunnel, the premier wind tunnel in the world at that time.
Stack was absolutely dedicated to aeronautical engineering. While in high
school, he earned money so that he could take a few hours of flight instruction
in a Canuck biplane. He helped out with the maintenance of a Boeing biplane
owned by one of his part-time employers. Before he went to college, he
had made up his mind to be an aeronautical engineer. However, his father,
a carpenter who was also very successful in real estate, wanted his son
to study architecture at MIT. Instead, when Stack entered MIT, he enrolled
in aeronautical engineering, keeping it a secret from his father for the
first year, but with the understanding approval of his mother. Much later,
Stack commented: "Then when Dad heard about it, it was too late to protest."19

When John Stack first walked into the Langley laboratory that July of
1928, a year's worth of design work had already been done on Langley's
first high-speed tunnel, and the facility was already operational with
an open throat test section.20 Success
had been achieved by the work of Briggs and Dryden, and the growing importance
of high-speed research was perceived by some visionaries. Because of this
perception, Joseph S. Ames, President of Johns Hopkins University and the
new Chairman of the NACA, in 1927 gave priority to high-speed wind tunnels
and research.21 Eastman Jacobs, who
had joined the NACA in 1925 after receiving his B.S. degree in mechanical
engineering from the University of California, Berkeley, was the chief
designer of the open-throat eleven-inch High Speed Tunnel. (Jacobs would
later earn air international reputation for his work on the famous NACA
airfoil sections in the 1930s, and for his conception of, and pioneering
research on, the NACA laminar flow airfoils just before the beginning of
World War II.) An innovative aspect of the eleven-inch High Speed Tunnel
was that it was driven from the twenty atmosphere pressure tank of the
Langley Variable Density Tunnel. For a change in models in the Variable
Density Tunnel, the twenty atmosphere tank which encased the entire tunnel
was blown down to one atmosphere; this represented a wasted energy source
which the Langley engineers ingeniously realized could be tapped for the
eleven-inch High-Speed Tunnel. The 5,200 cubic foot capacity of the high
pressure tank allowed about one minute of operation for the tunnel. John
Stack was given the responsibility for improving the High-Speed Tunnel
by designing a closed throat. This improved facility, shown on the next
page, was operational by 1932. It was his participation in the design and
development of the eleven-inch High-Speed Tunnel that launched John Stack
on his life-long career in high-speed aerodynamics.

While Stack was working on the High-Speed Tunnel, an event occurred
in England which made a great impression on him, and which would rapidly
refocus the NACA high-speed research program. On Sunday, September 13,
1931, a beautiful, highly streamlined Supermarine S.6B flashed through
the clear early afternoon sky at Calshot, near Portsmouth along the southern
English coast. Flown by Flt. Lt. John N. Boothman, this exquisite racing



---

19. Lou Davis, "No Time for Soft Talk," National
Aeronautics, January 1963, pp. 9-12. 'This is an interesting biographical
article written about Stack at the time of his receiving the 1962 Wright
Memorial Trophy Award from the National Aeronautic Association.

20. James R. Hansen, Engineer in Charge:
A History of the Langley Aeronautical Laboratory, 1917-1958 (Washington,
DC: NASA SP-4305,1987), p. 446.

21. Donald D. Corliss, Wind Tunnels of NASA
(Washington, DC: NASA SP-440,1981).



---

72 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER



|Patent Diagram The 11-inch High-Speed Tunnel at NACA Langley; closed-throat modification in 1932.
|-------------------------------------------------------------------------------------------------
|The 11-inch High-Speed Tunnel at NACA Langley; closed-throat
modification in 1932.               

FROM ENGINEERING SCIENCE TO BIG SCIENCE 73

airplane averaged a speed of 340.1 mph around a long, seven-lap course,
winning the coveted Schneider Trophy permanently for Britain. Later that
month, on September 29, Flt. Lt. George H. Stainforth set the world's speed
record of 401.5 mph in the same S.6B. Looking at this figure, it does not
take an aerodynamic expert to appreciate that by 1931 the concept of streamlining
in order to reduce drag had taken root. The Supermarine S.6B simply looked
like it could fly at 400 miles per hour-at Mach 0.53, over half the speed
of sound. Suddenly, the aeronautical engineer's concern over compressibility
effects on propeller tips, an important but tolerable situation, became
an absolutely major concern over compressibility effects on the airplane
itself, a problem of showstopper proportions.

Such concern was beginning to dawn on the aircraft industry itself In
1936, Lockheed's Kelly Johnson began early design studies for the P-38,
which was the first airplane to encounter major, and sometimes fatal, compressibility
effects. By the mid-1930s, the aircraft industry was wading into uncharted
water, and the NACA's high-speed research program became absolutely vital
to the future progress of high-speed airplane design.

|Drawing of the Supermarine S.6B, the airplane used by the British to win the Schneider Trophy, 1931.
|----------------------------------------------------------------------------------------------------
|The Supermarine S.6B, the airplane used by the British
to win the Schneider Trophy, 1931.           



---

74 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

Stack became acutely aware of this new compressibility challenge. In
1933, he published in NACA TR 463 the first data to come from the newly
modified, closed-throat High-Speed Tunnel. Although the airfoils were propeller
sections, Stack wrote in the introduction, obviously referring to the Schneider
Trophy racer:

> A knowledge of the compressibility phenomenon is essential,
> however because the tips speeds of propellers now in use are commonly in
> the neighborhood of the velocity of sound. Further, the speeds that have
> been attained by racing airplanes are as high as half the velocity of sound.
> Even at ordinary airplane speeds the effects of compressibility should
> not be disregarded if accurate measurements are desired .22

For the most part, Stack's data in 1933 served to confirm the trends
observed earlier. For example, Stack's measurements of the variation of
drag coefficient with Mach number for a ten percent thick Clark Y airfoil
are shown below; the large drag rise at high

|Graphs of the first compressibility data published by John Stack. From NACA TR 463, 1933                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|The first compressibility data published by John Stack.
From NACA TR 463, 1933. The three graphs are, from left to right, the variations
of lift coefficient, drag coefficient, and moment coefficient, respectively,
versus the ratio of  the freestream velocity to the speed of sound
(the Mach number). The test model was a 3C1D airfoil, shown at the top
of the figure. The adverse effects of compressibility are seen in the precipitous
decease in lift coefficient and dramatic increase in drag coefficient as
the Mach number is increased.



---

22. John Stack, "The N.A.C.A. High-Speed Wind
Tunnel and Tests of Six Propeller Sections," NACA TR 463, 1933. At about
the time of World War I, aerodynamicists were familiar with the fact that
an airfoil stalled at high angle of attack because the flow, separated
from the top surface. The resulting drastic loss of lift was given the
term "lift burble." Hence, after Briggs and Dryden had shown that the drastic
loss of lift at high speeds, beyond the "critical speed," was also due
to flow separation, it was natural to call this effect the "coumpressibility
burble." This terminology, coined by the NASA in 1933, pervaded the high-speed
aerodynamic literature throughout the 1930s.



---

FROM ENGINEERING SCIENCE TO BIG SCIENCE 75

speeds is clearly evident. He also confirmed that the onset of the adverse
compressibility effects occur at lower Mach numbers as either or both the
airfoil thickness and angle of attack increase. One of his conclusions
reflected on the theoretical Prandtl-Glauert compressibility correction
mentioned earlier. From his measurements, Stack concluded: "These results
indicate that the limited theory available may be applied with sufficient
accuracy for most practical purposes only for speeds below the compressibility
burble." This conclusion presaged almost forty years of a theoretical void.
The aerodynamic equations applicable to the transonic flight regime, Mach
numbers between about 0.8 and 1.2 are non-linear partial differential equations
that defied solution until the 1970s. And even then the solution was by
brute force-numerical solutions using the power of the newly developed
discipline of computational fluid dynamics carried out on high-speed digital
supercomputers.

By the way, the term "compressibility burble" was coined by Stack in
the same NACA Technical Report. He wrote:

> The lift coefficients increase as the speed is increased,
> slowly as the speed is increased over the lower portion of the range, then
> more rapidly as speeds above half the velocity of sound are exceeded, and
> finally at higher speeds, depending on the airfoil section and the angle
> of attack, the flow breaks down as shown by a drop in the lift coefficient.This
> breakdown of the flow, hereinafter called the compressibility burble, occurs
> at lower speeds as the lift is increased by changing the angle of attack
> of the model.23

> The effects of compressibility have commonly been neglected
> because until the relatively recent development of the last Schneider trophy
> aircraft the speeds have been low as compared with the velocity of sound
> and the consequent local pressures over the surfaces of high-speed airplanes
> have differed but slightly from atmospheric pressure. At the present time,
> however, the speeds associated with the fastest airplanes approach 60 percent
> of the velocity of sound, and the induced velocities over their exposed
> surfaces lead to local pressures that differ appreciably from the pressure
> of the atmosphere. When this condition exists, air can no longer be regarded
> as an incompressible medium. The effects of compressibility on the aerodynamic
> characteristics of airfoils have been under investigation by the N.A.C.A.
> in the highspeed wind tunnel, and it is the purpose of this paper to examine
> the possibility of further increases in speeds in the light of this relatively
> recent research.



---

23. Ibid.

24. John Stack, "Effects of Compressibility
on High Speed Flight," Journal of the Aeronautical Sciences 1(January
1934): 40-43.



---

76 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

and Dryden at the Bureau of Standards, and now by its own carefully
conducted experiments at Langley, the NACA had been able to identify the
first two aspect's of the basic nature of compressibility effects, namely
that (1) above a certain "critical speed," the lift decreased dramatically
and the drag skyrocketed almost beyond comprehension, and (2) this behavior
was caused by sudden and precipitous flow separation over the top surface
of the wing or airfoil. There remained one question, the most important
of all Why?

John Stack and the NACA were responsible for the answer to this question
a breakthrough that occurred in 1934. By this time, Stack had a new instrument
with which to work a schlieren photographic system, an optical arrangement
that made density gradients in the flow visible. One of nature's mechanisms
for producing very strong density gradients is a shock wave; hence a shock
wave ought to be visible in a schlieren photograph. Stack's boss, Eastman
Jacobs, was familiar with such optical systems through his hobby of astronomy;
it was in keeping with Jacob's innovative mind to suggest to Stack that
the use of a schlieren system might make visible some of the unknown features
of the compressible flow field over an airfoil, and might shed some light
on the nature of the compressibility burble. It did just that, and more!

With the 11-inch tunnel running above the "critical speed" for an NACA
0012 symmetric airfoil mounted in the test section, and with the aid of
the schlieren system, Stack and Jacobs observed for the first time in the
history of aerodynamics a shock wave in the flow over the top surface of
the airfoil. The shockwave was like that sketched in the figure below.
It became immediately clear to these two experimentalists that the separated
flow over the top surface of the airfoil, and the resulting compressibility
burble with all its adverse consequences, was caused by the presence of
a shock wave. The nature of this flow is sketched below, and it clearly
shows that the shock wave interacts with the thin, friction-dominated boundary
layer adjacent to the surface of the airfoil. This causes the boundary
layer to separate in the region where the shock impinges on the surface.
A massive region of separated flow trails downstream, greatly increasing
the drag and decreasing the lift. One of the pioneering schlieren pictures
of the flow over the NACA 0012 airfoil taken by Stack in 1934 is shown
on the page 73.25 The quality is poor
by present-day standards, but it is certainly sufficient for identifying
the phenomena. This is a historic photograph in the annals of the history
of aerodynamics one which led to the final understanding of the physical
nature of the compressibility burble. This was a breakthrough of enormous
intellectual and practical importance. And it was totally due to the work
of two innovative and highly intelligent aerodynamicists at the NACA Langley
Laboratory, John Stack and Eastman Jacobs, operating under the umbrella
of an inspired creative atmosphere associated with the NACA in general,

|Schematic of shock-induced separated flow. the source of the compressibility burble.
|------------------------------------------------------------------------------------
|Schematic of shock-induced separated flow the
source of the compressibility burble.



---

25. Found by the author in the John Stack Files,
NASA Langley Historical Archives.



---

FROM ENGINEERING SCIENCE TO BIG SCIENCE 77

|An early schlieren photograph of the shock pattern on an NACA 0012 airfoil in a freestream above the critical speed.                                                                                                                                                                                                                                                                                                                           
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|An early schlieren photograph of the shock pattern
on an NACA 0012 airfoil in a freestream above the "critical speed". From
the first group of schlieren Photographs of the compressibility burble
taken by John Stack, 1934. In this photograph the nature of the flow Pattern
causing the compressibility burble was seen for the first time. From the
John Stack papers in the NASA Langley Archives. Courtesy of Richard Layman,
Archivist.



---

78 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

and the foresight of Joseph Ames and George Lewis at NACA Headquarters
in Washington who placed priority on the NACA high-speed research program
at a time when most airplanes of the day were lumbering along at 200 mph
or slower.

Was this work by Stack and Jacobs engineering science? Absolutely yes!
It provided the fundamental physical understanding of the root source of
compressibility problems. This understanding was mainly qualitative at
the time, but it allowed designers of high-speed airfoils to make more
intelligent decisions about proper airfoil shapes-it helped to make the
uncharted waters more navigable.

As with many new discoveries in science and technology, there are always
those skeptical at first. One of those was Theodore Theodorsen, the best
theoretical aerodynamicist in the NACA at the time, with a worldwide reputation
for his pioneering papers on airfoil theory. John Becker, who joined the
NACA in 1936 and who went on to become one of the most respected high-speed
acrodynamicists at Langley, tells the following anecdote about Theodorsen's
reaction to the schlieren photographs taken by Stack and Jacobs. It is
repeated here because it reflects just how much a radical departure from
the expected norm the results were.

> The first tests were made on a circular cylinder about 1/2
> inch in diameter and the results were spectacular in spite of the poor
> quality of the optics. Shockwaves and attendant flow separations were seen
> for the first time starting at subsonic stream speeds of about 0.6 times
> the speed of sound. Visitors from all over the Laboratory, from Engineer-in-Charge
> H .J. E. Reid on down, came to view the phenomena. Langley's ranking theorist,
> Theodore Theodorsen, viewed the results skeptically, proclaiming that since
> the stream flow was subsonic, what appeared to be shockwaves was an 'optical
> illusion,' an error in judgement which he was never allowed to forget.26

An interesting confluence of events occurred in 1935 that allowed
the NACA in a timely fashion to inform the international research community
of this intellectual breakthrough in understanding compressibility effects
and the compressibility burble. One was the exisitence of the data itself
fresh, exciting, and revolutionary. The other was the scheduling of the
fifth Volta conference in Italy.27
Since 1931, the Royal Academy of Science in Rome had been conducting a
series of important conferences sponsored by the Alessandro Volta Foundation.
The first conference dealt with nuclear physics, and then rotated between
the sciences and the humanities on alternate years. The second Volta conference
had the title "Europe" and in 1933 the third conference was on the subject
of immunology.

This was followed by the subject "The Dramatic Theater" in 1934. During
this period, the influence of Italian aeronautics was gaining momentum,
led by General Arturo Crocco, an aeronautical engineer who had become interested
in ramjet engines in 1931, and therefore was well aware of the potential
impact of compressible flow theory and experiment on future aviation. This
led to the choice of the topic of the fifth Volta conference "High Velocities
in Aviation." Participation was by invitation only, and the select list
included all the leading aerodynamicists at that time. Because of his reputation
in the design and testing of the famous NACA four-digit airfoil series,
and the fact that he was the Section Head of the NACA Variable Density
Tunnel which had put the NACA on the international aerodynamic map in the
1920s, Eastman Jacobs received an invitation. He took the opportunity to
present a paper on the new NACA compressibility research.



---

26. John V Becker, The High-Speed Frontier:
Case Histories of Four NACA Programs, 1920-1950 (Washington, DC: NASA
SP-445, 1980), p. 16.

27. Anderson, Modern Compressible Flow,
pp. 282-84.



---

FROM ENGENEERING SCIENCE TO BIG SCIENCE 79

Hence, during the period between September 30 and October 6, 1935, the
major figures in the development of high-speed aerodynamics of the 1930s
(with the exception of John Stack) gathered inside an impressive Renaissance
building in Rome that served as the city hall during the Holy Roman Empire,
and discussed flight at high subsonic, supersonic, and even hypersonic
speeds. The fifth Volta Conference was to become the springboard for new
thought on the development of high-speed flight.

In the midst of all this discussion was Eastniann Jacobs representing
the NACA. Jacobs' paper, entitled "Methods Employed in America for the
Experimental Investigation of Aerodynamic Phenomena at High Speeds," was
both tutorial and informative.28 He
took the opportunity to derive and present the basic equations for compressible
flow assuming no friction and no thermal conduction. Then he described
the NACA High-Speed Tunnel, the schlieren system, and the airfoil experiments
carried out in the tunnel. Then came the blockbuster. He showed, for the
first time in a technical meeting, some of the schlieren pictures taken
at Langley. One of these was the photograph shown on page 73. Conscious
of the NACA:s penchant for perfection, especially in its publications,
Jacobs apologized for the quality of the photographs, a very modest gesture
considering their technical (and historical) importance: "Unfortunately
the photographs were injured by the presence of bent celluloid windows
forming the tunnel walls through which the light passed. The pictures nevertheless
give fundamental information in regard to the nature of the flow associated
with the compressibility burble."29
With this, the NACA high-speed research program was not only on the map,
it was leading the pack.

By this time, Stack had a newer, larger facility the 24-inch High Speed
Tunnel equipped with an improved schlieren system, The basic testing of
compressibility effects on flows over airfoils continued in this facility.
In 1938, Stack published the most definitive document yet on the nature
of high-speed compressible flow over airfoils, including many detailed
surface pressure measurements.30 With
this, the NACA continued to be the undisputed leader in the study of the
effects of compressibility and the consequences of the compressibility
burble.

The atmosphere at the Langley Laboratory during the 1930s allowed engineering
science to flourish, although the laboratory never explicitly adopted this
as a priority. lt just happened when it needed to happen. The culture among
its engineers was one, of inquiry and free exchange of information; thoughts
were readily shared on an inter-personal basis. Moreover, Langley had engineers
who were adept at building new facilities, especially new wind tunnels.
It was natural that a high-speed wind tunnel was built at Langely providing
a unique facility for Langley engineers to unlock the secrets of high-speed
aerodynamics. And the fact that the NACA had money, even during the depression
years, allowed such wind tunnels to be first-class facilities. All this,
in combination with first-class engineers and scientists, made Langley
the leading research institution in high-speed compressibility effects
during the 1930s.

Jacobs' paper at the fifth Volta conference represented in some sense
a celebration of the second phase of the NACA research on high-speed flight.
The First phase was the embryonic wind tunnel compressibility work of the
1920s, clearly oriented towards applications to propellers.



---

28. Eastman Jacobs, "Methods Employed in America
for the Experimental Investigation of Aerodynamic Phenomena at High Speeds,"
NACA Misc. Paper No. 42, March 1936. A copy of this paper. which is the
printed version of Jacobs' presentation at the fifth Volta conference,
is available in the Technical Documents Section, Mathematics, Engineering
and Physical Sciences Library, University of Maryland, College Park.

29. Ibid.

30. John Stack, W.F. Lindsey, and Robert E.
Littell, "The Compressibility Burble and the Effect of Compressibility
on Pressures and Forces Acting on an Airfoil," NACA TR 646,1938.



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80 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OFTHE SOUND BARRIER

The second phase was the refocusing of this high-speed wind tunnel research
on the airplane itself, complemented by a new initiative the design and
development of an actual research airplane.

The High-Speed Research Airplane: An NACA Idea



|Sketch of high-Speed plane by John Stack 1933                 
|--------------------------------------------------------------
|Hypothetical high-speed airplane conceived by John
Stack, 1933



---

31. Stack, "Effects of Compressibility on High
Speed Flight," pp. 40-43.

32. Hansen, Engineer in Charge, p. 256.



---

FROM ENGINEERING SCIENCE TO BIG SCIENCE 81



|Graph and sketch hand-drawn by John Stack, 1933. Speed vs Horse Power                                                                                                                                                                                                                       
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Graph and sketch hand-drawn by John Stack, 1933. The
effect of compressibility on the power required for a hypothetical airplane.This
sketch was subsequently sent to the October 1933 Committee Meeting of the
NACA in Washington. From the John Stack papers at the NASA Langley Archives.



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82 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OFTHE SOUND BARRIER

The High-Speed Research Airplane-For Real

The state of high-speed aerodynamics in 1939 can be illustrated by one
graph, shown schematically in the figure below. Here, the variation of
drag coefficient for an airplane is shown as a function of free stream
Mach number. On the subsonic side, below Mach one, wind tunnel data indicated
the familiar rapid increase in drag coefficient as Mach one is approached.
On the supersonic side, ballisticians had known for years, supported by
the results of linearized supersonic theory developed by Jakob Ackeret
in Germany since 1928, how the drag coefficient behaved above Mach one.33
Of course, all airplanes at that time were on the subsonic side of the
curve shown in the figure below. John Stack nicely summarized the situation
in 1938:

> The development of the knowledge of compressible-flow phenomena,
> particularly as related to aeronautical applications, has been attended
> by considerable difficulty. The complicated nature of the phenomena has
> resulted in little theoretical progress, and, in general, recourse to experiment
> has been necessary. Until recently the most important experimental results
> have been obtained in connection with the science of ballistics, but this
> information has been of little value in aeronautical problems because the
> range of speeds for which most ballistic experiments have been made extends
> from the speed of sound upward; whereas the important region in aeronautics
> at the present time extends from the speed of sound downward. 34

In essence, the flight regime Just below and just beyond the speed
of sound was unknown a transonic gap, as shown schematically below.

|Graph of the subsonic and supersonic variations of drag coefficient for an airfoil,                                                                                                                               
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Schematic of the subsonic and supersonic variations
of drag coefficient for an airfoil, illustrating the position of the transonic
regime for which virtually no information was available in the 1930s and
1940s.



---

33. Anderson, Modern Compressible Flow,
pp. 270-73.

34. Stack; Lindsey; and Littell, "Compressibility
Burble and the Effect of Compressibility on Pressures and Forces Acting
on an Airfoil."



---

FROM ENGINEERING SCIENCE TO BIG SCIENCE 83

The general aeronautics community was suddenly awakened to the realities
of the unknown flight regime in November 1941, when Lockheed test pilot
Ralph Virden could not pull the new, high-performance P-38 out of a high-speed
dive, and crashed. Virden was the first human fatality due to adverse compressibility
effects, and the P-38, shown below, was the first airplane to Suffer from
these effects. The P-38 exceeded its critical Mach number in an operational
dive, and penetrated well into the regime of the compressibility burble
at its terminal dive speed, as shown by the bar chart on page 80 .35
The problem encountered by Virden, and many other P-38 pilots at that time,
was that beyond a certain speed in a dive, the elevator controls suddenly
felt as if they were locked. And to make things worse, the tail suddenly
produced more lift, pulling the P-38 into an even

|Drawing: side, top and front views of Lockheed P-38                            
|-------------------------------------------------------------------------------
|Lockheed P-38, the first airplane to encounter severe
compressibility problems.



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35. This chart is taken from the figure on page
78 of the article by R. L. Foss, "From Propellers to Jets in Fighter Aircraft
Design," in Jay D. Pinson, ed., Diamond Jubilee of Powered Flight: The
Evolution of Aircrafeet Design(New York, NY: American Institute of
Aeronautics and Astronautics, 1978), pp. 51-64.



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84 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER



|Bar chart showing the magnitude of how much the P-38 penetrated the compressibility regime.
|-------------------------------------------------------------------------------------------
|Bar chart showing the magnitude of how much the P-38
penetrated the compressibility regime.

steeper dive. This was called the "tuck-under" problem. It is important
to note that the NACA soon solved this problem, using its expertise in
compressibility effects. Although Lockheed consulted various aerodynamicists,
including Theodore Von Kármán at Caltech, it turned out that
John Stack at NACA Langley, with his accumulated experience in compressibility
effects, was the only one to properly diagnose the problem. The wing of
the P-38 lost lift when it encountered the compressibility burble. As a
result, the downwash angle of the flow behind the wing was reduced. This
in turn increased the effective angle of attack of the flow encountered
by the horizontal tail, increasing the lift on the tail, and pitching the
P-38 to a progressively steepening dive totally beyond the control of the
pilot. Stack's solution was to place a special flap under the wing, to
be employed only when these compressibility effects were encountered. The
flap was not a conventional dive flap intended to reduce the speed. Rather,
Stack's idea was to use the flap to maintain lift in the face of the compressibility
burble, hence eliminating the change in the downwash angle, and therefore
allowing the horizontal tail to function properly. This is a graphic example
of how, in the early days of high-speed flight, the NACA compressibility
research was found to be vital as real airplanes began to sneak up on Mach
one.36

Indeed, it was time for real airplanes to be used to probe the
mysteries of the unknown transonic gap. It was time for the high-speed
research airplane to become a reality. The earliest concrete proposal along
these lines was made by Ezra Kotcher, a senior instructor at the Army Air
Corps Engineering School at Wright Field (a forerunner of today's Air Force
Institute of Technology). Kotcher was a 1928 graduate of the University
of California,



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36. The "tuck-under" problem, and its technical
Solution, is described in John  D. Anderson, Jr., Introduction
to Flight (New York, NY. McGraw-Hill Book Co., 3rd ed., 1989), pp.
406-08.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 85

Berkeley, with a B.S. degree in mechanical engineering. The same year
that John Stack first walked through the gates of Langley as a junior aeronautical
engineer, Kotcher first walked into the Air Corps Wright Field, also as
a junior aeronautical engineer. These two engineers were contemporary with
each other, and both had a driving interest in high-speed aerodynamics.
The careers of these two people would come together for the development
of the Bell X-1 in the 1940s. Kotcher's proposal, drafted during the period
May-August 1939, was a response to Major General Henry H. "Hap" Arnold's
request for an investigation of advanced military aircraft in the future.
The proposal contained a plan for a high-speed flight research program.
Kotcher pointed out the unknown aspects of the transonic gap, and the problems
associated with the compressibility burble as elucidated by the NACA, and
concluded that the next important step was a full-scale flight research
program.37 The Army Air Corps did not
immediately respond to this proposal.

Meanwhile, back at Langley, the idea of a high-speed research airplane
was gaining momentum. By the time the United States entered World War II
in December 1941, John Stack had studied the behavior of the flow in wind
tunnels when the flow in the test section was near or at Mach one. He found
that when a model was mounted in the flow, the flow field in the test section
essentially broke down, and any aerodynamic measurements were worthless.
He concluded that the successful development of such transonic wind tunnels
was a problem of Herculean proportions, and was far into the future. In
order to learn about the aerodynamics of transonic flight, the only recourse
appeared to be a real airplane that would fly in that regime. Therefore,
during several visits by Dr. George Lewis, NACA's Director of Aeronautical
Research, Stack seized the opportunity to mention the idea. Lewis, who
liked Stack and appreciated the talent he brought to the NACA, was not
immediately partial to the idea of a research airplane. But in early 1942,
he left a crack in the door. In Hansen's words: "He left Stack with the
idea, however, that some low-priority, back-of-the-envelope estimates to
identify the most desirable design features of a transonic airplane could
not hurt anyone, providing they did not distract from more pressing business."38

Given Stack's driving personality, this was all that was needed. With
the blessing of the local management at Langley, Stack immediately formed
a small group of engineers, and started to work on the preliminary design
aspects of a transonic research airplane. By the summer of 1943, the group
had produced such a design. Its principal features are listed below. This
design established a mind-set for John Stack that guided NACA thinking
on the transonic research airplane for the next five years a mind-set
that was to clash with the later ideas coming from Kotcher and the Army.
The NACA design:

1. was a small turbo-jet powered airplane,
2. was to take off under its own power from the ground,
3. was to have a maximum speed of Mach one, but the main feature was to be
able to fly safety at high subsonic speeds,
4. was to contain a large payload of scientific instruments for measuring
the acrodynamic and flight dynamic behavior at near-sonic speeds, and
5. was to start its test program at the low end of the compressibility regime,
and progressively over time sneak up to Mach one in later flights.

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37. Kotcher's role in the development of the
high-speed research airplane is nicely presented by Hallion in Supersonic
Flight, starting with p.12, and continuing throughout the book. As
stated in note 1 above, Hallion's book is still today the most definitive
source on the circumstances leading to the Bell X-1.

38. Hansen, Engineer in Charge, p. 259.



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86 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER

The important goal was aerodynamic data at high subsonic speeds, not
necessarily to fly into the supersonic regime. These features became [almost]
a magna carta to Langley engineers, and to John Stack in particular.

The exigencies of wartime greatly accelerated research into high-speed
aerodynamics; compressibility problems now had the attention not only of
the NACA, but also of the Army and Navy as well. Stack, who had risen to
be Eastman Jacob's Assistant Section Chief of the Variable Density Tunnel
in 1935, and Head of the High-Speed Wind Tunnels in 1937, was made Chief
of the newly formed Compressibility Research Division in 1943.39
Stack now had his most influential position to date to push for the high-speed
research airplane.

The Bell X-1: Point and Counterpoint

Although the NACA had the compressibility knowledge and technology,
the Army and Navy had the money that would be necessary for the design
and building of a research airplane. So it was appropriate that the Bell
X-1 was conceived during a fateful visit by Robert J. Woods of Bell Aircraft
to the office of  Ezra Kotcher on 30 November 1944. Woods, who had
NACA ties because he had worked at Langley during 1928-1929 in the Variable
Density Tunnel, had joined with Lawrence D. Bell in 1935 to form the Bell
Aircraft Corporation in Buffalo, New York. That day in November, Woods
had dropped by Kotcher's office simply to chat. During the conversation,
Kotcher relayed the information that the Army, with the help of the NACA,
desired to build a special, non-military high-speed research airplane.
After detailing the Army's specifications for the aircraft, Kotcher asked
Woods if the Bell Corporation was interested in designing and building
the airplane. Woods said yes. The die was cast.40

When Kotcher had been talking with Woods, he was operating with some
authority. During 1944, Army and NACA engineers had been meeting to outline
the nature of a joint research airplane program. Moreover, by mid-1944,
Kotcher had received the Army's approval for the design and acquisition
of such an airplane. However, the Army's concept of the high-speed research
airplane was somewhat different than that of NASA. To understand this difference,
we have to examine two situations in existence at the time

The first situation was that of a common, public belief in the "sound
barrier." The myth of the sound barrier had its beginning in 1935, when
the British aerodynamicist W. F. Hilton was explaining to a newsman about
some of the high-speed experiments he was conducting at the National Physical
Laboratory. Pointing to a plot of airfoil drag, Hilton said: "See how the
resistance of a wing shoots up like a barrier against higher speed as we
approach the speed of sound." The next morning, the leading British newspapers
were misrepresenting Hilton's comment by referring to "the sound barrier."41
The idea of a physical barrier to flight that airplanes could never fly
faster than the speed of sound became widespread among the public. Furthermore,
even though most engineers knew differently, they still had uncertainty
in just how much the drag would increase in the transonic regime, and given
the low thrust levels of airplane powerplants at that time, the speed of
sound certainly loomed as a tremendous mountain to climb.



---

39. Official NASA biographical and job description
summary From the John Stack files, Langley Historical Archives.

40. Hallion, Supersonic Flight, p. 34.

41. W.F. Hilton, "British Aeronautical Research
Facilities," Journal of the Royal Aeronautical Society 70 (Centenary
Issue, 1966): 103-104.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 87

The second situation that colored the Army's thinking at that time was
local, namely that Kotcher was convinced that the research airplane must
be powered by a rocket engine rather than a turbojet. This stemmed from
his experience in 1943 as project officer on the proposed Northrop XP-79
rocket-propelled flying wing interceptor, as well as the knowledge within
the Army of' Germany's new rocket-propelled interceptor, the ME-163.

1. It should be rocket-powered.
2. It should attempt, early in its flight schedule, to fly supersonically-to
show everybody that the sound barrier could be broken.
3. Later in the design process, it was determined that it should be air-launched
rather than take off from the ground.

All of these were in conflict with the NACA's more careful and scientific
approach. However, the Army was paying for the X-1, and the Army's views
prevailed.

Although John Stack and the NACA did not agree with the Army's specifications,
they nevertheless provided as much technical data as possible throughout
the design of the X-1. Lacking appropriate wind tunnel data and theoretical
solutions for transonic aerodynamics, the NACA developed three stopgap
methods for the acquisition of transonic aerodynamic data. In 1944, Langley
carried out tests using the drop-body concept. Wings were mounted
on bomb-like missiles which were dropped from a B-29 at an altitude of
30,000 feet. The terminal velocities of these models sometimes reached
supersonic speeds. The data were limited, mainly consisting of estimates
of the drag, but NACA engineers considered it reliable enough to estimate
the power required for a transonic airplane. Also in 1944, Robert R. Gilruth,
Chief of the Flight Research Section, developed the wing-flow method,
wherein a model wing was mounted perpendicular at just the right location
on the wing of a P-51D. In a dive, the P-51 would pick up enough speed,
to about Mach 0.81, that locally supersonic flow would occur over its wing.
The small wing model mounted perpendicular on the P-51 wing would be totally
immersed in this supersonic flow region, providing a unique high-speed
flow environment for the model. Ultimately, these wing-flow tests provided
the NACA with the most systematic and continuous plots of transonic data
yet assembled.42 The third stopgap
method was rocket-model testing. Here, wing models were mounted
on rockets, which were fired from the NACA's facility at Wallops Island
on the coast of Virginia's Eastern Shore. The data from all these methods,
along with the existing core of compressibility data obtained by the NACA
over the past 20 years as described in the earlier sections of this chapter,
constituted the scientific and engineering base from which the Bell Aircraft
Corp. designed the X-1.

Finally, we note that the NACA was responsible for the instrumentation
that was housed inside the Bell X-1 This instrumentation and its location
on the X-1 is illustrated on page 84. This is an example of one of those
unseen aspects of technology upon which the acquisition of historic data
depends. It is fitting that the NACA excelled in both aspects of the X-1
concept the external configuration and the essential instruments mounted
inside for the acquisition of quantitative knowledge.



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42. Hansen, Engineer in Charge, p. 267.



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88 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER



|Schematic of the instrumentation mounted by the NACA in the Bell X-1.
|---------------------------------------------------------------------
|Schematic of the instrumentation mounted by the NACA
in the Bell X-1.

Breaking the Sound Barrier

We began this chapter by transporting ourselves back to October 14,
1947, and riding with Chuck Yeager as he flew the Bell X-1 through the
sound barrier, becoming the first human to fly faster than sound. The detailed
events of 1946 and 1947 that finally resulted in this flight the design,
construction and early flight testing program by Bell, and the Army's intense
preparations for the handling of the X-1 at Muroc are nicely related by
historians Richard P. Hallion and James O. Young.43
Nothing is served by repeating them here. Rather, we return to the purpose
of this chapter as stated in the introductory paragraphs. The first supersonic
flight of the Bell X-1 represented the culmination of 260 years of research
into the mysteries of high-speed aerodynamics. It was especially the fruition
of 23 years of insightful research in high-speed aerodynamics by the NACA
research that represents one of the most important stories in the history
of aeronautical engineering.



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43. Hallion, Supersonic Flight, James
O. Young, Supersonic Symposium: The Men of Mach I (Edwards Air Force
Base, CA: Air Force Flight Test Center History Office, September 1990),
pp. 1-89.



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FROM ENGINEERING SCIENCE TO BIG SCIENCE 89

On December 17, 1948, President Harry S. Truman presented the thirty-seven-year-old
Collier Trophyjointly to three men for "the greatest aeronautical achievement
since the original flight of the Wright Brothers' airplane."44
The Trophy, officially the Collier Trophy for the year 1947, was the highest
possible official recognition for the accomplishments embodied in the X-1.
The announcement page from the December 25, 1948 issue of Collier's
magazine is shown on page 86. Properly, John Stack was one of the three
men, recognized as the scientist, along with Lawrence D. Bell, the
manufacturer, and Captain Charles E. Yeager, the pilot. The citation to
Stack read: "for pioneering research to determine the physical laws affecting
supersonic flight and for his conception of transonic research airplanes."
A
major purpose of this chapter was to bring meaning to this citation
so much is hidden in these few words.45
Unseen in this photograph, but present in spirit, is the team of NACA researchers
who also worked towards determining the physical laws affecting supersonic
flight, and to conceptualize the transonic research airplane. In this sense,
the 1947 Collier Trophy was a "global" award to the entire NACA high-speed
research program.

The 1947 Collier Trophy was also a recognition of the role of engineering
science in the ultimate success of the Bell X-1. Note that in the award
John Stack is explicitly recognized as a scientist (not an engineer). This
is somewhat of a misnomer Stack was performing as an engineering scientist
in this activity, neither a pure scientist nor a pure engineer. The NACA
had provided all the elements that allowed this engineering science contribution
to occur.

At the time of this award, John Stack was Assistant Chief of Research
at NACA Langley. In 1952, he was made Assistant Director of Langley. By
that time he had been awarded his second Collier Trophy, the 1951 Trophy,
for the development of the Slotted-Throat Wind Tunnel. In 1961, three years
after the NACA was absorbed into the National Aeronautics and Space Administration,
Stack became Director of Aeronautical Research at NASA Headquarters in
Washington. Despairing of the de-emphasis of aeronautics in NASA, after
thirty-four years of government service with the NACA and NASA, Stack retired
in 1962 and became vice president for engineering for Republic Aircraft
Corporation in Long Island. When Republic was absorbed by Fairchild Hiller
in 1965, Stack was appointed a vice president of that company, retiring
in 1971. On June 18, 1972, Stack fell from a horse on his farm in Yorktown,
Virginia, and was injured fatally. He is buried in the churchyard cemetery
of Grace Episcopal Church in Yorktown, only a few miles away from NASA's
Langley Research Center. Today, F-15s from the nearby Langley Air Force
Base fly over the churchyard airplanes that can routinely fly at almost
three times the speed of sound, thanks to the legacy of John Stack and
the NACA high-speed research program.



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44. Collier's, December 25, 1948.

45. John Stack files, NASA Langley Archives.



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90 RESEARCH IN SUPERSONIC FLIGHT AND THE BREAKING OF THE SOUND BARRIER



|Picture of John Stack at his desk | Picture of Lawrence D. Bell standing in front of the X-1 rocketplane. | Picture of Captain Charles E. Yeager
|--------------------------------- | --------------------------------------------------------------------- | ------------------------------------

|SCIENTIST: John Stack, for the past 20 years government
research scientist with the National Advisory Committee for Aeronautics,
is the first of the three men who share the award of the Collier Trophy
for the achievement of human supersonic flight. It was because of Stack's
awareness of the absolute necessity for ever superior aircraft, and his
intensive study of the problems of supersonic flight that a workable program
for the construction of a research plane came into being. | MANUFACTURER: Lawrence D. Bell,  president
of Bell Aircraft Corporation, was awarded the contract by the Air Force
to design and build the plane evolved from Stack's scientific presentation
of supersonic flight. Bell has a reputation for taking on the unusual,
the unconventional and what some call the impossible. The ship he designed
and built was the Bell X-1 which, before delivery, was tested in 21 flights
at a speed slightly less than the speed of sound. | PILOT: Captain Charles E. Yeager, USAF, was chosen
from the nation's finest test-pilot talent as the man to fly the plane
pioneered by Stack and built by Bell. Deemed "a natural airman, if there
is such a thing," on October 14, 1947, Yeager became the first man to fly
faster than the speed of sound. It is for the combined achievement of these
three men in their successful penetration of the transonic barrier that
the Collier Trophy for 1947 has been awarded.
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|The Collier Trophy
For Flight Beyond the Speed of SoundBy FREDERICK R. NEELY

For bringing about the achievement of human supersonic Flight, John
Stack, Lawrence D. Bell and Captain Charles E. Yeager, USAF, win America's
highest aviation award
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|Graphic of Capitol Letter A inside box used to duplicate typsetting in Collier'sMERICA'S
highest aeronautical honor, the 37-year-old Collier Trophy, was presented
by President Truman at the White House Friday. December 17th, to the three
men adjudged most responsible for the attainment of human supersonic flight.
The trophy is awarded annually by a committee selected by the National
Aeronautic Association for "the greatest achievement in aviation in America.
the value of which has been demonstrated by actual use during the preceding
year." It will be shared equally for the ensuing year by:
John Stack: career government research scientist
of the National Advisory Committee for Aeronautics "for pioneering research
to determine the physical laws affecting supersonic flight and for his
conception of transonic research airplanes."
Lawrence D. Bell, president of Bell Aircraft
Corporation, "for the design and construction of the special research airplane
X-1."
Captain Charles E. Yeager. U.S. Air Force,
"who, with that airplane, on October 14, 1947, first achieved human flight
faster than sound."
To those three men goes the honor of playing
the major roles in an achievement which the Collier | Trophy committee termed "the greatest since the first
successful flight of the original Wright Brothers' airplane."
All three have been outstanding in their
contributions to the vitally important science of supersonic flight flight
that is faster than sound, the speed of which at sea level, with a temperature
of 59 degrees and in still air, is 761 miles an hour. However, at altitudes
ranging between 40.000 and 100.000 feet. the speed of sound is reached
at only 663 miles an hour. This is due to the fact that at such high altitudes
the temperature is almost constantly 67 degrees below zero and sound travels
more slowly in cold air. At just what altitude Capt. Yeager flew is as
much of a secret as the actual supersonic speed he attained.
The problem that confronted Stack, Bell
and Yeager was not so much that of flying faster than sound as it was successful
flying at speeds between 600 and 900 miles an hour the transonic range.
Aeronautical scientists were in grave doubt
as to just what took place when conventional aircraft entered the transonic
range in high-speed dives. They knew that both plane and pilot were kicked
around unmercifully for seconds that seemed like | centuries and that both were completely out of control.
Badly and naturally frightened, the pilots were unable to bring back detailed
scientific reports on the phenomenon, and they were usually unwilling to
repeat their flights.
Wind tunnel tests with small-scale models
revealed that the flow of air over a plane in the transonic range was partly
subsonic and partly supersonic. Because of this. the conventional planes
(usually fighter types) took on an extremely inconsistent and erratic behavior.
But the tunnel findings were not conclusive and since supersonic tunnels
large enough to mount a full-scale airplane are prohibitive in cost the
scientists concluded they needed a special research airplane equipped with
instruments capable of measuring and automatically recording all of the
forces acting upon an airplane in transonic flight.
This was where John Stack came in. It was
natural that be should have conducted the research phase for he had been
working on the fundamental problems of high-speed flight in the wind tunnels
and laboratories of the NACA at Langley Field,
Virginia, since 1929. shortly after he had
joined the government's great aeronautical research es-
Collier's for December 25. 1948
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

The first page of the Collier's magazine announcement of the winners
of the 1947 Collier Trophy, December 25,1948.



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