i

A History of Suction-Type

Laminar-Flow Control

 with Emphasis on

Flight Research

MONOGRAPHS IN AEROSPACE HISTORY #13

by

Albert L. Braslow
ii

A History of Suction-Type

Laminar-Flow Control

 with Empahsis on

Flight Research

by

Albert L. Braslow

NASA History Division

Office of Policy and Plans

NASA Headquarters

Washington, DC 20546

Monographs in

Aerospace History

Number 13

1999
iii

Table of Contents

Foreword.........................................................................................................................iv

Preface .............................................................................................................................v

Laminar-Flow Control Concepts and Scope of Monograph ...........................................1

Early Research on Suction-Type Laminar-Flow Control

Research from the 1930s through the War Years ......................................................3

Research from after World War II to the Mid-1960s ................................................5

Post X-21 Research on Suction-Type Laminar-Flow Control

Hiatus in Research ..................................................................................................13

Resumption of Research .........................................................................................13

Research from the Mid-1970s to the Mid-1990s ....................................................16

Natural Laminar Flow (NLF) on Swept Wings: F-111/TACT and F-14 .....................17

Noise: Boeing 757...................................................................................................18

Insect Contamination: JetStar .................................................................................20

Leading-Edge Flight Test (LEFT) Program: JetStar...............................................21

Surface Disturbances: JetStar .................................................................................26

Atmospheric Ice Particles: Boeing 747s and JetStar ..............................................28

Hybrid Laminar-Flow Control (HLFC): Boeing 757 .............................................29

Supersonic Laminar-Flow Control: F-16XL ..........................................................32

Status of Laminar-Flow Control Technology in the Mid-1990s ...................................34

Glossary.........................................................................................................................35

Documents

Document 1 –Aeronautics Panel, AACB, R&D Review, Report of the

Subpanel on Aeronautical Energy Conservation/Fuels .........................38

Document 2 –Report of Review Group on X-21A Laminar Flow

Control Program .....................................................................................46

Document 3 –Langley Research Center Announcement: Establishment

of Laminar Flow Control Working Group..............................................61

Document 4 –Intercenter Agreement for Laminar Flow Control

Leading Edge Glove Flights, LaRC and DFRC .....................................62

Document 5 –Flight Report, NLF-144, of AFTI/F-111 Aircraft with

the TACT Wing Modified by a Natural Laminar Flow Glove ...............66

Document 6 –Flight Record, F-16XL Supersonic Laminar Flow

Control Aircraft ......................................................................................71

Index ..............................................................................................................................76

About the Author ...........................................................................................................78
iv

Foreword

Laminar-flow control is an area of aeronautical research that has a long history

at NASA’s Langley Research Center, Dryden Flight Research Center, their

predecessor organizations, and elsewhere. In this monograph, Albert L.

Braslow, who spent much of his career at Langley working with this research,

presents a history of that portion of laminar-flow technology known as active

laminar-flow control, which employs suction of a small quantity of air through

airplane surfaces. This important technique offers the potential for significant

reduction in drag and, thereby, for large increases in range or reductions in fuel

usage for aircraft. For transport aircraft, the reductions in fuel consumed as a

result of laminar-flow control may equal 30 percent of present consumption.

Given such potential, it is obvious that active laminar-flow control with suction

is an important technology. In this study, Al covers the early history of the

subject and brings the story all the way to the mid-1990s with an emphasis on

flight research, much of which has occurred here at Dryden. This is an important monograph that not only encapsulates a lot of history in a brief compass but

also does so in language that is accessible to non-technical readers. NASA is

publishing it in a format that will enable it to reach the wide audience the

subject deserves.

Kevin L. Petersen

Director, Dryden Flight Research Center

February 18, 1999
v

Preface

This monograph is the result of a contract with the NASA Dryden History

Office to write a brief history of laminar-flow-control research with an emphasis

on flight research, especially that done at what is today the Dryden Flight

Research Center (DFRC). I approached the writing of this history from the

perspective of an engineer who had spent much of his career working on laminar-flow-control research and writing about the results in technical publications.

I found out that writing history is quite a bit different from technical writing, but

I hope that what I have written will explain laminar-flow control to the nontechnical reader while at the same time providing historical background to the

interested technical reader.

After completion of the final draft of this technical history in October 1998, I

was made aware of NASA TP-1998-208705, October 1998, by Ronald D.

Joslin, entitled Overview of Laminar Flow Control. Although some overlap

exists between this publication and my own, as would be expected from the two

titles, Joslin’s intent was quite different from mine. He provides an extensive

technical summary for engineers, scientists and technical managers of the

content of many key papers without much evaluation of the significance of

specific results over the years.

I would like to express my gratitude to the following DFRC personnel: David

Fisher, Lisa Bjarke, and Daniel Banks for reading the initial draft; Jim Zeitz for

reworking the figures; and Stephen Lighthill for doing the layout. My special

thanks go to J.D. (Dill) Hunley, DFRC historian, who patiently guided this

technical author through the vagaries of historical composition.

Albert L. Braslow

Newport News, Virginia

19 February 1999
vi

A

History of

SuctionType

LaminarFlow

Control

Laminar-Flow Control Concepts and

Scope of Monograph

This monograph presents a history of

suction-type laminar-flow-control research in the National Advisory Committee for Aeronautics and its successor

organization, the National Aeronautics

and Space Administration, plus selected

other organizations, with an emphasis on

flight research. Laminar-flow control is a

technology that offers the potential for

improvements in aircraft fuel usage,

range or endurance that far exceed any

known single aeronautical technology.

For transport-type airplanes, e.g., the fuel

burned might be decreased a phenomenal

30 percent. Fuel reduction will not only

help conserve the earth’s limited supply

of petroleum but will also reduce engine

emissions and, therefore, air pollution. In

addition, lower fuel usage will reduce the

operating costs of commercial airplanes

at least eight percent, depending upon the

cost of the fuel and, therefore, will curtail

ticket prices for air travel. Laminar-flow

control is also the only aeronautical

technology that offers the capability of

designing a transport airplane that can fly

nonstop without refueling from anywhere

in the world to anywhere else in the world

or that can remain aloft without refueling

for approximately 24 hours. These

enormous performance improvements

that are potentially available for commercial or military applications, therefore,

have made the concept the “pot of gold at

the end of the rainbow” for aeronautical

researchers.

A brief review of some of the fundamentals involved will improve an understanding of this technological history.

When a solid surface moves through a

fluid (such as the air), frictional forces

drag along a thin layer of the fluid

adjacent to the surface due to the viscosity (stickiness) of the fluid. A distinguished theoretician, Ludwig Prandtl,

showed in 1904 how the flow around a

solid body can be divided into two

regions for analysis—this thin layer of

fluid adjacent to the surface, called the

boundary layer, where fluid friction plays

an essential part, and the remaining region

outside the boundary layer where friction

may be neglected. The boundary layer

generally exists in one of two states:

laminar, where fluid elements remain in

well-ordered nonintersecting layers

(laminae), and turbulent, where fluid

elements totally mix. The frictional force

between the fluid and the surface, known

as viscous drag, is much larger in a

turbulent boundary layer than in a laminar

one because of momentum losses associated with the mixing action. The energy

required to overcome this frictional force

on an airplane is a substantial part of the

total energy required to move the airplane

through the air. In the case of a transport

airplane flying at subsonic speeds, for

example, approximately one-half of the

energy (fuel) required to maintain level

flight in cruise results from the necessity

to overcome the skin friction of the

boundary layer, which is mostly turbulent

on current transport-size airplanes.

The state of the boundary layer, in the

absence of disturbing influences, is

directly related to the speed of the surface

and the distance along the surface—first,

laminar and then changing to turbulent as

the speed or distance increases. Laminar

flow is difficult to attain and retain under

most conditions of practical interest, e.g.,

on the surfaces of large transport airplanes. Laminar flow is an inherently

unstable condition that is easily upset, and

transition to turbulent flow may occur

prematurely as a result of amplification of

disturbances emanating from various

sources. Two basic techniques are available to delay transition from laminar to

turbulent flow—passive and active.

Laminar flow can be obtained passively

over the forward part of airplane lifting

surfaces (wings and tails) that have

leading-edge sweep angles of less than

about 18 degrees by designing the surface

cross-sectional contour so that the local

pressure initially decreases over the

surface in the direction from the leading

edge towards the trailing edge. The

laminar flow obtained in this passive

manner is called natural laminar flow

(NLF). In the rearward region of well1
vii

designed wings, where the pressure must

increase with distance towards the trailing

edge (an adverse pressure gradient),1 active

laminar-flow control must be used. Even in

a favorable pressure gradient, active

laminar-flow control is required to attain

laminar flow to large distances from the

leading edge.

The principal types of active laminarflow control are surface cooling (in air) and

removal of a small amount of the boundarylayer air by suction through porous materials, multiple narrow surface slots, or small

perforations. For highly swept wings that

are usually required for flight at high

subsonic and supersonic speeds, only

suction can control sweep-induced

crossflow disturbances that promote

boundary-layer transition from laminar to

turbulent flow. The use of suction has

become the general method of choice for

active laminar-flow control and has become

known as LFC. A combination of LFC (in

regions where pressure gradients due to the

sweep introduce large destabilizing

crossflow disturbances) and NLF (in

regions with low crossflow) is an approach

to simplifying the application of LFC and is

known as hybrid LFC (HLFC). Although

the potential performance gains due to

HLFC are somewhat lower than those

obtainable with LFC, the gains are still very

large.

At this point, a brief description of a

parameter of fundamental importance is

necessary for the non-technical reader. This

parameter is called Reynolds number and

was named after Osborne Reynolds who, in

1888, was the first to show visually the

transition from laminar to turbulent flow

and the complete mixing of the fluid

elements in turbulent flow. Reynolds

number is non-dimensional and is equal to

the product of the velocity of a body

passing through a fluid (v), the density of

the fluid ( ) and a representative length (l)

divided by the fluid viscosity (μ) or v ρ 1 /μ.

Engineers select various representative

lengths (1) in the formulation of the

Reynolds number for different purposes. For

example, non-dimensionalized aerodynamic

forces acting on a body moving through air

vary with the value of the Reynolds number

based on the body length. This phenomenon

is called “scale effect” and is important in the

determination of the non-dimensional

aerodynamic forces acting on a full-size (fullscale) airplane or airplane component from

data measured on a small wind-tunnel model.

When engineers select the distance from the

component’s leading edge to the end of

laminar flow as the representative length, the

resultant length Reynolds number (or transition Reynolds number) is a measure of the

distance from the leading edge to the end of

the laminar flow. For any value of transition

Reynolds number, then, that has been experimentally determined, the distance to the end

of laminar flow on any size airplane component can be calculated for any stream-flow

velocity, density, and viscosity from the

above Reynolds number formulation. The

attainable value of transition Reynolds

number, as previously indicated, is dependent

upon the component’s geometrical shape (the

primary controller of the variation of surface

pressure), various disturbances, and the type

and magnitude of laminar-flow control used.

This monograph will review the history

of the development of LFC and HLFC with

emphasis on experimentation, especially

flight research. A sufficient number of

activities up to 1965, when a 10-year hiatus in

U.S. experimental LFC research began, will

illustrate the early progress as well as the

principal problems that inhibited the attainment of laminar flow in flight with either

passive or active laminar-flow control.

Discussion of a resurgence of research on

LFC in 1975 will concentrate on the flightresearch portion of an American program

defined to solve the technological problems

uncovered during the previous research.

Included will be a discussion of the significance of aircraft size on the applicability of

passive or active control.

1 A decreasing pressure in the direction towards the trailing edge is called a favorable pressure gradient and an increasing

pressure is called an adverse pressure gradient.

2
viii

Early Research on Suction-Type

Laminar-Flow Control

Research from the 1930s through the

War Years

The earliest known experimental

work on LFC for aircraft was done in the

late 1930s and the 1940s, primarily in

wind tunnels.2 In 1939, research engineers

at the Langley Memorial Aeronautical

Laboratory of the National Advisory

Committee for Aeronautics (NACA) in

Hampton, Virginia, tested the effect on

boundary-layer transition of suction

through slots in the surfaces of windtunnel models. These tests provided the

first aerodynamic criteria on the design of

multiple suction slots and obtained

laminar flow up to a length Reynolds

number of 7 million, a phenomenally

large value at that time. The first LFC

flight experiments ever made followed

these favorable results in 1941. Researchers installed seventeen suction slots

between 20 and 60 percent of the chord3

of a test panel (glove)4 on a wing of a B18 airplane (Figure 1). Maximum airplane

speed and constraint in the length of the

Figure 1. B-18

airplane with test

glove for natural

laminar flow and

later for active

laminar-flow

control. (NASA

photo L-25336)

2 Three citations that provide extensive bibliographies on both passive and active control of the laminar boundary layer are:

Dennis M. Bushnell and Mary H. Tuttle, Survey and Bibliography on Attainment of Laminar Flow Control in Air Using

Pressure Gradient and Suction (Washington, DC: NASA RP-1035, September 1979); Charles E. Jobe, A Bibliography of

AFFDL/FXM Reports on Laminar Flow Control ( U.S. Air Force: AFFDL-TM-76-26-FXM, March 1976); and Mary H.

Tuttle and Dal V. Maddalon, Laminar Flow Control (1976-1991) – A Comprehensive, Annotated Bibliography (Washington,

DC: NASA TM 107749, March 1993). Significant references, primarily of summary natures, that were published since these

are included in subsequent footnotes. A sparse number of technical sources already included in the bibliographies are also

repeated in subsequent notes to assist readers in locating pertinent technical information discussed in the narrative.

3 Chord is the length of the surface from the leading edge to the trailing edge.

4 A glove is a special section of an airplane’s lifting surface, usually overlaying the basic wing structure, that is designed

specifically for research purposes.

3
ix

wing glove, however, limited achievement

of a length Reynolds number for transition to a value lower than that achieved in

the wind tunnel.

Experimentation in NACA on LFC

ceased during the years of World War II in

order to develop natural laminar-flow

airfoils, the so-called NACA 6- and 7-

series airfoils, under the leadership of

Eastman N. Jacobs, Ira H. Abbott, and

Albert E. von Doenhoff at the Langley

Memorial Aeronautical Laboratory.5

Significant progress in furthering the

understanding of the boundary-layer

transition process, however, continued to

be made in the U.S.A., both analytically

and experimentally, principally at the

National Bureau of Standards by G.B.

Schubauer, H.K. Skramstad, P.S.

Klebanoff, K.P. Tidstrom, and Hugh L.

Dryden.6 Development of the laminarflow airfoils was made possible by the

introduction into service of the LowTurbulence Pressure Tunnel (LTPT) at the

LaRC with an exceptionally low airstream-turbulence level.7

 The author and

Frank Visconti measured natural laminar

flow in the LTPT up to length Reynolds

numbers on the order of 16 million.8

Researchers in Great Britain obtained

significant flight experience in the mid1940s on natural laminar-flow airfoils

with wing gloves on the British King

Cobra and Hurricane military fighters.9

Large extents of laminar flow were

obtained, but only after considerable

effort to attain wave-free and smooth

surfaces. Although attainment of large

regions of laminar flow was not possible

in daily operations, aircraft designers used

laminar-flow type airfoils with large

regions of favorable pressure gradient on

new aircraft intended for high-subsonicspeed flight because of their superior

high-speed aerodynamic characteristics,

e.g., the North American P-51 Mustang.

In Germany and Switzerland, efforts

to develop LFC technology with suction

were under way during the war. The

Germans emphasized the analysis of

laminar stability with continuous suction

rather than discrete suction through slots.

Walter Tollmien and Hermann Schlichting

discovered theoretically that the boundary

layer resulting from continuous suction is

very stable to small two-dimensional type

disturbances (named after them as

Tollmien-Schlichting waves)10 and that

5 In a later reorganization, the Langley Memorial Aeronautical Laboratory was renamed the Langley Research Center

(LaRC), and that name will be used hereafter to avoid possible confusion. An interim name for the Laboratory from 1948

to 1958 was the Langley Aeronautical Laboratory.

6 Dryden later became the Director of the NACA and then the first Deputy Administrator of the National Aeronautics

and Space Administration (NASA).

7 A low level of high-frequency airstream turbulence, a condition approximating that in the atmosphere, is required to

obtain natural laminar flow. This turbulence, of extreme importance to NLF, contrasts with occasional low-frequency

turbulence in the atmosphere, known as gusts. Gusts affect an aircraft through changes in the relative angle of the aircraft

with respect to the direction of flight (angle of attack).

8 Albert L. Braslow and Fioravante Visconti, Investigation of Boundary-Layer Reynolds Number for Transition on an

65(215)—114 Airfoil in the Langley Two-Dimensional Low-Turbulence Pressure Tunnel (Washington, DC: NACA TN

1704, October, 1948).

9 See, for example: W.E. Gray and P.W.J. Fullam, Comparison of Flight and Tunnel Measurements of Transition on a

Highly Finished Wing (King Cobra) (RAE Report Aero 2383, 1945); F. Smith and D. Higton, Flight Tests on King Cobra

FZ. 440 to Investigate the Practical Requirements for the Achievement of Low Profile Drag Coefficients on a “Low

Drag” Aerofoil (British A.R.C., R and M 2375, 1950); R.H. Plascoff, Profile Drag Measurements on Hurricane II z.

3687 Fitted with Low-Drag Section Wings (RAE Report Aero 2153, 1946).

10 Examples of two-dimensional type disturbances are stream turbulence, noise, and surface irregularities having large

ratios of width (perpendicular to the stream flow direction) to height, like spanwise surface steps due to mismatches in

structural panels.

4
x

the quantity of air that must be removed

to achieve this marked stabilizing effect is

extremely small. German researchers

derived methods for calculating the

boundary-layer characteristics and drag

reductions resulting from continuous

suction. The Germans also wanted to

validate their findings experimentally but

were unable to produce a permeable

surface suitable for continuous suction

with the necessary degree of smoothness.

Alternatives were tried, i.e., suction

through a perforated plate and suction

through multiple slots. Suction through

perforated plates failed due to excessive

disturbances emanating from the edges of

the holes. Suction through multiple slots

permitted attainment of extensive regions

of laminar flow up to a length Reynolds

number of 3.2 million. In Switzerland,

Werner Pfenninger was also investigating

the use of multiple suction slots. He

obtained full-chord laminar flow on both

surfaces of an airfoil but only up to a

maximum chord (length) Reynolds

number of 2.3 million. He attributed the

limitation in the maximum attainable

Reynolds number for laminar flow with

LFC to increased airstream turbulence in

the wind tunnel. From more recent results,

he and other researchers agree that

increased disturbances from small irregularities in the slot contours could have

contributed.

Research from after World War II to

the Mid-1960s

Release of the German LFC reports

on continuous suction after the war

generated renewed interest in both the

United States and the United Kingdom.11

The NACA initiated a series of windtunnel tests at the LaRC in 1946, which

culminated in the attainment of full-chord

laminar flow on both surfaces of an airfoil

with continuous suction through a porous

bronze surface. The author, Dale Burrows,

and Frank Visconti obtained full-chord

laminar flow to a length Reynolds number

of about 24 million, which was limited

only by buckling of the low-strength

porous-bronze skin.12 Neal Tetervin

performed theoretical calculations indicating that the experimental suction rates

were consistent with values predicted

from the then-available stability theory to

the largest chord Reynolds number tested.

These wind-tunnel results, therefore,

provided the first experimental verification of the theoretical indication that the

attainment of full-chord laminar flow with

continuous suction would not be prevented by further increases in Reynolds

number, i.e., further increases in airplane

size or speed (at least subsonically).13

Because porous bronze, however, was

obviously unsuitable for application to

aircraft (low strength and large weight)

and no suitable material was available,

work on the simulation of continuous

suction with multiple slots was reactivated by the NACA. In the late 1940s,

NACA researchers investigated in the

LaRC LTPT an NACA design,14 and Dr.

Werner Pfenninger, who had come to the

Northrop Corporation from Zurich,

Switzerland, investigated a U.S. Air

11 A team of experts from the allied countries, including Eastman N. Jacobs of the NACA, gathered these reports in

Germany soon after the end of hostilities.

12 This was the author’s indoctrination into active laminar-flow control research, which followed previous involvement

in the development of the NACA natural-laminar-flow airfoils.

13 Albert L. Braslow, Dale L. Burrows, Neal Tetervin, and Fioravante Visconte, Experimental and Theoretical Studies of

Area Suction for the Control of the Laminar Boundary Layer on an NACA 64A010 Airfoil (Washington, DC: NACA

Report 1025, 30 March 1951).

14 Dale L. Burrows and Milton A. Schwartzberg, Experimental Investigation of an NACA 64A010 Airfoil Section with

41 Suction Slots on Each Surface for Control of Laminar Boundary Layer (Washington, DC: NACA TN 2644, 1952).

5
xi

Force-sponsored design.15 In the first case,

the researchers obtained full-chord

laminar flow up to a Reynolds number of

about 10 million (greatly exceeding that

obtained previously in Germany and

Switzerland), but the slot arrangement had

been designed for a considerably larger

Reynolds number of 25 million. In the

second case, Dr. Pfenninger obtained fullchord laminar flow up to a Reynolds

number of 16-17 million for a model

designed for 20 million. In both of these

cases with slots, as well as during the

previous continuous-suction tests, an

overriding problem in attainment of

laminar flow was an increased sensitivity

of laminar flow to discrete three-dimensional type surface disturbances16 or slot

irregularities as wind-tunnel Reynolds

number was increased. This occurred in

spite of the theory, which indicated that

suction increased the stability of the

laminar boundary layer with respect to

two-dimensional type disturbances. More

on this subject will be included later in the

monograph.

After the war, the first work the

British did on LFC was to extend the

German analytical research on continuous

suction. In 1948, Cambridge University

experimented on a flat plate in the floor of

a wind tunnel. This was followed in 1951

by flight tests on an Anson aircraft of

continuous suction from 10- to 65-percent

chord in a flat pressure distribution.17

Researchers obtained experimental

suction rates very close to theoretical

values for a zero pressure gradient up to a

length Reynolds number of 3 million and

good agreement with theory in the

measured boundary-layer profiles.18 The

experiments indicated adverse effects of

roughness.

The British Royal Aircraft Establishment (RAE) tested a porous surface on a

Vampire aircraft19 starting in 1953

(Figure 2). Researchers initially employed

a rolled metallic cloth for the surface, but

roughness picked up in the mesh caused

premature transition from laminar to

turbulent flow. With the use of special

procedures to provide very smooth

surfaces back to 25 percent of chord, fullchord laminar flow was established at a

length Reynolds number of 29 million.

With candidates not yet available for a

practical porous surface, attention was

diverted to simulation of a porous surface

with a perforated metal sheet. From 1954

to 1957, the RAE investigated various

arrangements of hole size, spacing and

orientation, as did John Goldsmith at the

Norair Division of the Northrop Corporation in the United States. Some worked

and some did not because of differences

in disturbances generated by the suction

flow through the different hole arrangements.20

15 Werner Pfenninger, Experiments With a 15%-Thick Slotted Laminar Suction Wing Model in the NACA, Langley Field,

Low Turbulence Wind Tunnel (U.S. Air Force Tech. Rep. 5982, April 1953).

16 Three-dimensional type surface disturbances are those with width to height ratios near a value of one.

17 M.R. Head, The Boundary Layer with Distributed Suction (British A.R.C., R.&M. No. 2783, 1955).

18 A boundary-layer profile is the shape of the variation of a boundary-layer characteristic like local velocity or temperature with height above the surface.

19 M.R. Head, D. Johnson, and M. Coxon, Flight Experiments on Boundary-Layer Control for Low Drag (British

A.R.C., R.&M. No. 3025, March 1955).

20 Significant sources are: John Goldsmith, Critical Laminar Suction Parameters for Suction Into an Isolated Hole or a

Single Row of Holes (Northrop Corp., Norair Division Report NAI-57-529, BLC-95, February 1957); N. Gregory and

W.S. Walker, Experiments on the Use of Suction Through Perforated Strips for Maintaining Laminar Flow: Transition

and Drag Measurements (British A.R.C., R.&M. No. 3083, 1958). Northrop Corp., Norair Division reports cited in this

monograph and others related to its laminar-flow research can be found in the files of Albert L. Braslow located in the

Langley Historical Archives (LHA) at the Langley Research Center, Hampton, VA.

6
xii

From 1951 to 1955, the British firm

Handley Page tested, in wind tunnels and

in flight on a Vampire trainer, the concept

of suction strips whereby researchers

hoped to eliminate the structural difficulties associated with fully distributed

suction or with the need for precise slots.

Tests included both porous strips and

perforated strips with single and multiple

rows of holes. The best of the perforated

configurations consisted of staggered

multiple rows of holes. Tests of porous

sintered-bronze strips in both the wind

tunnel and flight were troubled by great

difficulty in ensuring sufficiently smooth

joints between the strips and the solid

surface. The joints introduced large

enough two-dimensional type disturbances to cause premature transition. With

the final perforation configuration,

researchers obtained repeatable laminar

flow to 80 percent of the chord on the

Vampire trainer wing, equivalent to a

length Reynolds number of 15 million. An

inability to obtain laminar flow in the last

20 percent of chord was attributed to the

effects of a forward sweep of the wing

trailing edge.

Figure 2. Vampire

active laminar-flowcontrol flight

experiments.

7
xiii

Previously, in 1951, the RAE had

been unable to obtain the design extent of

laminar flow on a natural laminar-flow

airfoil employed in a sweptback wing on

an AW52 airplane. This led to a series of

tests of sweptback surfaces of various

aircraft during which visual records of

boundary-layer transition were obtained.

For sufficiently large leading-edge

sweepback, transition occurred very close

to the leading edge. Subsequent tests,

using a flow-visualization technique,

showed closely-spaced striations in the

flow on the surface, indicating strongly

that transition took place on swept

surfaces as a result of formation of

streamwise vortices in the laminar boundary layer.21 Dr. Pfenninger’s boundarylayer research group at the Norair Division of the Northrop Corporation in the

1950s provided a method of analyzing the

cross-flow instability due to sweep. It also

obtained experimental data showing that

the cross-flow instability could be controlled by reasonable amounts of suction

initiated sufficiently close to the wing

leading edge.22

The Northrop group in the 1950s and

early 1960s made many other major

contributions to the development of the

LFC technology. Under a series of Air

Force contracts, the group performed

rather extensive investigations in several

areas of concern. Although some work

was done on suction through holes, the

principal efforts were on suction through

slots. In addition to the improved understanding of laminar-flow stability and

control on swept wings, the Northrop

researchers developed criteria in the areas

of multiple-slot design, internal-flow

metering, and duct design plus techniques

for alleviating the adverse effects of

external and internal acoustic disturbances. In addition, Northrop conducted

analytical investigations of structural

design methods and construction techniques. These were supported by a limited

effort on construction and test of smallscale structural samples. The results,

however, were insufficient to provide

transport manufacturers with confidence

that LFC wings for future transports could

be manufactured to the required close

tolerances for LFC with acceptable cost

and weight penalties.23 An area receiving

analytical attention only was that of the

suction pumping system. Although the

suction pumping system is of significant

importance to overall aircraft performance, analyses indicated that no radically new mechanical developments were

required to provide the necessary suction.

Northrop, in a USAF-sponsored

program at Muroc Dry Lake (known both

before and after this period as Rogers Dry

Lake) in California, also reactivated flight

research on LFC in the United States with

the use of a glove on an F-94 aircraft.

Muroc is today the site of the Edwards Air

Force Base and the Dryden Flight Research Center (DFRC). Northrop investigated three different slot arrangements on

a modified NACA laminar-flow airfoil

(Figure 3). Essentially full-chord laminar

flow was attained on the wing’s upper

21 W.E. Gray, The Effect of Wing Sweep on Laminar Flow (RAE TM Aero. 255, 1952).

22 W. Pfenninger, L. Gross, and J.W. Bacon, Jr., Experiments on a 30 Degree Swept, 12 Percent Thick, Symmetrical,

Laminar Suction Wing in the 5-Foot by 7-Foot Michigan Tunnel (Northrop Corp., Norair Division Report NAI-57-317,

BLC-93, February 1957).

23 Structural design of airplanes requires consideration of manufacturing procedures, capabilities, limitations, and

available materials as well as compatibility with in-service inspection, maintenance, and repair while providing a high

degree of reliability and minimization of cost and weight. Airplane weight not only directly affects an airplane’s performance but also its total life-cycle economics through its effect on construction costs, operating costs, and perhaps

maintenance costs. The incorporation of laminar-flow control by suction imposes unique structural requirements in that

smooth, substantially wave-free external surfaces are mandatory. Any associated additional weight or cost must not

dissipate the advantages of LFC to a degree that the manufacturer or user would judge the remaining advantages insufficient to warrant the increased complexities or risk.

8
xiv

surface at Reynolds numbers over 30

million, the highest attained on a lifting

wing. When the F-94 aircraft speed was

increased to the point where the local

Mach number24 on the airfoil surface

exceeded about 1.09, a new potential

problem appeared. Full-chord laminar

flow was lost with the slot configuration

tested. This was probably due to the steep

pressure rise through the shock waves that

formed. Other data since that time,

however, have shown that laminar flow

can be maintained through some shock

waves with a properly designed slot

configuration. Another most important

point is that for the F-94 glove tests, the

airfoils were exceptionally well made

with minimum waves and were maintained in a very smooth condition; even

so, very small amounts of surface roughness, for example from local manufacturing irregularities or from bug impacts,

caused wedges of turbulent flow behind

each individual source of turbulence.25

Figure 3. F-94

active laminar-flowcontrol flight

experiments.

24 Mach number is a measure of airplane speed in terms of the ratio of the airplane speed to the speed of sound at the

flight altitude. Airplane speeds up to the speed of sound are termed subsonic, above the speed of sound, supersonic, with

the supersonic speeds greater than approximately Mach 5 (or 5 times the speed of sound) referred to as hypersonic. The

region between about Mach 0.85 and 1.15 is termed transonic. Because of the cross-sectional curvature of lifting surfaces

like wings, local Mach numbers of the air above the wing exceed the airplane Mach number.

25 W. Pfenninger, E.E. Groth, R.C. Whites, B.H. Carmichael, and J.M. Atkinson, Note About Low Drag Suction Experiments in Flight on a Wing Glove of a F94-A Airplane (Northrop Corp., Norair Division Report NAI-54-849, BLC-69,

December 1954).

9
xv

By this time, there was a better

understanding that the use of increased air

density in some wind tunnels, to more

closely approximate full-scale flight

values of Reynolds number, aggravated

the surface roughness problem in the wind

tunnel as compared with flight.26 Nevertheless, the vast NACA experience in the

development of laminar-flow airfoils in

the late 1930s and early 1940s, the British

flight tests of natural laminar-flow airfoils

on the King Cobra and Hurricane airplanes in the mid-1940s, and the NACA

and other previously mentioned tests of

laminar-flow control through porous

surfaces and slots in the late 1940s

convinced the NACA that the inability to

manufacture and maintain sufficiently

wave-free and smooth surfaces was the

principal impediment to the attainment of

extensive regions of laminar flow for

most airplane missions then conceived.

The primary focus of the NACA (at least

until its transformation in 1958 into the

National Aeronautics and Space Administration [NASA]) was on the business of

advancing the understanding of aeronautical phenomena and not on solving manufacturing or operational problems, which

it considered to be the province of the

manufacturer and user. The NACA,

therefore, turned its attention away from

LFC per se and concentrated its laminarflow activities on expanding the understanding of the quantitative effects of

surface roughness on transition, with and

without suction. Based on these NACA

data and pertinent data from numerous

other researchers, a correlation was

developed with which the permissible

three-dimensional type surface-roughness

height can be estimated within reasonable

accuracy.27

NASA became aware in 1960 of a

renewed U. S. Air Force (USAF) interest

in active laminar-flow control through a

visit of Philip P. Antonatos of the USAF

Wright Air Development Division

(WADD) to the author, who was then

head of the General Aerodynamics

Branch of the LaRC Full-Scale Research

Division.28 Contemplated Air Force

missions at that time included a highaltitude subsonic aircraft of long range or

endurance, an ideal match with laminarflow control. Laminar flow was required

to obtain the long range or endurance and

high altitude alleviated the adverse effects

of surface protuberances. Any special

operational procedures needed to maintain

the required surface smoothness in the

presence of material erosion and corrosion and to cope with weather effects,29

aircraft noise, and accumulation of dirt

and insects could only be evaluated

through actual flight experience. WADD

also considered it important to provide an

impressive flight demonstration of

improved airplane performance to be

better able to advocate the advantages of

the contemplated new aircraft.

WADD proposed use of two WB-66D

airplanes based on minimum cost, high

degree of safety, and short development

time. The Northrop Corporation, under

sponsorship of the Air Force (with a

monetary contribution from the Federal

Aviation Administration),30 later modified

26 The method of increasing the Reynolds number on small models in wind tunnels involves increasing the air density

through an increase in air pressure (higher unit Reynolds number, i.e., Reynolds number based on a unit length). The

minimum size of a three-dimensional type disturbance that will cause transition is smaller on a small model in an

airstream of higher density than that required to cause transition on a full-size airplane at altitude (and, therefore, lower

density) at the same relative distance from the leading edge.

27 Albert E. von Doenhoff and Albert L. Braslow, “The Effects of Distributed Surface Roughness on Laminar Flow,” in

Boundary-Layer and Flow Control - Its Principles and Application, Vol. 2, edited by G. V. Lachmann (Oxford, London,

New York, Paris: Pergamon Press, 1961), pp. 657-681.

28 ALB files, LHA, notebook on Norair and LRC Memos re X-21: memo for LaRC Associate Director, 17 June 1960.

29 Weather effects include the effects of icing, precipitation, clouds, and low-frequency atmospheric turbulence.

10
xvi

Figure 4. One of

two X-21 active

laminar-flow-control

airplanes.

these airplanes with slotted suction wings

and designated them as experimental

aircraft X-21A and X-21B (Figure 4).

Beginning with the first developmentengineering review of the X-21A in

January 1963, the author acted as a NASA

technical consultant to the Air Force.31

Northrop began flight research in

April of 1963 at Edwards Air Force Base.

Several problems arose early in the

project that consumed significant periods

for their solution. Principal among these

was the old surface smoothness and

fairness problem32 and an unexpected

severity of a spanwise contamination

problem. With respect to the smoothness

and fairness problem, in spite of a concerted effort to design and build the

slotted LFC wings for the two airplanes to

the close tolerances required, the resulting

hardware was not good enough.

Discontinuities in spanwise wing splices

were large enough to cause premature

transition. Putty, used to fair out these

discontinuities, chipped during flight with

resulting roughness large enough to

trigger transition.

The combination of X-21 wing

geometry, flight altitudes, and Mach

numbers was such that local turbulence at

the attachment line, e.g., from the fuselage or induced by insect accumulation,

caused turbulent flow over much of the

wing span (spanwise contamination).33 At

about the same time, British flight tests of

a swept slotted-suction wing mounted

vertically on the fuselage of a Lancaster

bomber indicated similar results (Figure

5).34 Although flight experimentation and

small-scale wind-tunnel tests by the

British had previously indicated the

existence of the spanwise-contamination

problem, its significance had gone

unrecognized. With the large-scale X-21

flight tests and further wind-tunnel tests,

Northrop developed methods for avoidance of spanwise contamination. The

phenomenon is now understood but

30 ALB files, LHA, notebook on Norair and LRC Memos re X-21: memo for LaRC Associate Director, 10 December

1963.

31 See ALB files, LHA, notebook on Norair and LRC Memos re X-21: memo to Air Force Aeronautical Systems

Division from Charles J. Donlan, Acting LaRC Director, dated 2 January 1963, and for other memos and program

reviews.

32 Surface smoothness is a measure of surface discontinuities like protuberances or steps. Surface fairness is a measure

of the degree of waviness of surface contour (shape).

33 On a sweptback wing, the line at which the airflow divides to the upper and lower surfaces is called the attachment

line. If the boundary layer at the attachment line becomes turbulent for any reason and if certain combinations of wing

sweep, wing leading-edge radius, and flight conditions exist, the turbulence spreads outward along the attachment line

and contaminates (makes turbulent) the boundary layer on both wing surfaces outboard of the initial turbulence.

34 R.R. Landeryou and P.G. Porter, Further Tests of a Laminar Flow Swept Wing with Boundary Layer Control by

Suction (College of Aeronautics, Cranfield, England, Report Aero. No. 192, May 1966).

11
xvii

Figure 5. Swept,

suction-type

laminar-flowcontrol wing

mounted vertically

on Lancaster

bomber.

requires careful attention in the design of

large LFC aircraft.35

Another problem that was uncovered

during the X-21 flight tests was associated

with ice crystals in the atmosphere.

Researchers noted that when the X-21

flew in or near visible cirrus clouds,

laminar flow was lost but that upon

emergence from the ice crystals, laminar

flow was immediately regained. G.R. Hall

at Northrop developed a theory to indicate

when laminar flow would be lost as a

function of atmospheric particle size and

concentration.36 Little statistical information, however, was available on the size

and quantity of ice particles present in the

atmosphere as a function of altitude,

season of the year, and geographic

location. Therefore, the practical significance of atmospheric ice particles on the

amount of time laminar flow might be lost

on operational aircraft was not known.

By October of 1965, attainment of

“service experience comparable to an

operational aircraft,” one of the program’s

principal objectives, had not even been

initiated because of the effort absorbed by

the previous problems. To proceed with

this initiative, the advisors to the Air

Force recommended that a major wing

modification would be needed before

meaningful data on service maintenance

could be obtained.37 This, unfortunately,

was never done because of various

considerations at high levels of the Air

Force, probably predominantly the

resource needs of hostilities in Vietnam.

Much extremely valuable information,

however, was obtained during the X-21

flight program, supported by wind-tunnel

and analytical studies. At the end of the

program,38 flights attained laminar flow

on a fairly large airplane over 95 percent

of the area intended for laminarization.

Unfortunately, top management in government and industry remembered the

difficulties and time required to reach this

point more than they did the accomplishment.

35 W. Pfenninger, Laminar Flow Control-Laminarization (AGARD Special Course on Concepts for Drag Reduction,

AGARD Report No. 654, June 1977).

36 G.R. Hall, “On the Mechanics of Transition Produced by Particles Passing Through an Initially Laminar Boundary

Layer and the Estimated Effect on the LFC Performance of the X-21 Aircraft” (Northrop Corp., October 1964).

37 ALB files, LHA, folder labeled X-21 Tech Reviews: USAF Aeronautical Systems Division X-21 DAG Review

Agenda and Attendees with Report of Review Group on X-21A Laminar Flow Control Program, 8 November 1965.

38 Special Section, “Laminar Flow Control Prospects,” Astronautics and Aeronautics 4, no. 7 (July 1966): 30-62. This

section contains articles by several different authors. On X-21, see also document 2 at the end of this monograph.

12
xviii

Post X-21 Research on Suction-Type

Laminar-Flow Control

Hiatus in Research

With the cessation of military support,

a general hiatus in the development of

active laminar-flow control technology

ensued in the United States from the mid1960s to the mid-1970s. Other interest

was lacking because of two principal

reasons: 1) a lack of a contemplated need

for very long-range missions for commercial aircraft for which the benefits of

active laminar-flow control were a

necessity and 2) the fact that the price of

jet fuel was then so low that the estimated

fuel-cost savings for commercial transports with ranges of interest was almost

offset by estimated increases in manufacturing and maintenance costs. Researchers

did perform significant analytical work

and conceptual studies during this period,

however.

Resumption of Research

In 1973, Gerald Kayten, who was

Director of the Transportation Experiment

Program Office in the Office of Aeronautics and Space Technology at NASA

Headquarters, phoned the author with a

request that he prepare a “white paper” on

potential technology advances that might

reduce the use of fuel by commercial air

transports. The request was in response to

increased prices and increasingly insecure

sources of petroleum-based fuel resulting

from the oil embargo imposed by the

Organization of Petroleum Exporting

Countries in 1973. NASA, at that time,

was pursuing technological improvements

in various aircraft disciplinary areas

(identified and evaluated in the Advanced

Transport Technology Systems and

Design Studies)39 to reduce aircraft noise

and pollution, to improve economics, and

to reduce terminal-area delays. The

resultant “white paper,” printed December

20th of 1973,40 recommended that the

technological advances identified for

these purposes be pursued with an increased emphasis on their potential for

fuel reduction. It also identified additional

possibilities in the aeronautical disciplines

for fuel conservation. Principal among

these, with by far the largest potential for

fuel conservation of any discipline, was

drag reduction through active laminarflow control. Kayten, in a telephone

conversation with the author on 14

January 1974,41 called the paper “damn

good,” and he strongly urged that we get

going quickly. He indicated, however, that

the reception by others at Headquarters

was nothing more than lukewarm. The

same was true among LaRC researchers

in management positions who believed

that the problems previously evident in

the laminar-flow research were so severe

as to render the technology impractical

and that any further efforts would only

detract from the resources available for

other research endeavors.

Because of this continued adverse

reaction from many in positions of

authority, start of a significant program on

active laminar-flow control was continually deferred. Leaders of various groups

during the next couple of years, however,

initiated tasks to identify and recommend

Research and Technology (R&T) activities that would be required to develop

potential fuel-conservation technologies.

The following are examples of the studies

that resulted. In March of 1974, the

American Institute of Aeronautics and

Astronautics (AIAA) assembled a group

of 91 of its members in a workshop

conference. The objective was “to review

39 These studies were made under the Advanced Technology Transport (ATT) Program at LaRC under the direction of

Thomas A. Toll.

40 Albert L Braslow and Allen H. Whitehead, Jr., Aeronautical Fuel Conservation Possibilities for Advanced Subsonic

Transports (Washington, DC: NASA TM X-71927, 20 December 1973).

41 ALB files, LHA, chronological notebook on Advanced Technology Transport Office (later called Advanced Transport

Technology Office and later changed in emphasis to Aircraft Energy Efficiency Project Office): note dated 1-14-74.

13
xix

and discuss the technological aspects of

aircraft fuel conservation methods and to

recommend the initiation of those measures having the best prospects for shortterm and long-term impact.” One of the

resultant conclusions was that advances in

associated technologies since the 1960s

warranted a reevaluation of the application of laminar-flow control in the design

of future long-range transport aircraft.42 In

November of 1974, the Aeronautics Panel

of the DOD/NASA Aeronautics and

Astronautics Coordinating Board established a new subpanel on Aeronautical

Energy Conservation/Fuels, cochaired by

A. Braslow, NASA/LaRC and A. Eaffy,

USAF/Pentagon.43 The task was to

“review the on-going NASA and DOD

programs and recommend increased

activities in fuel-conservation technologies where deficiencies were noted.” The

subpanel supported further research on

LFC, including flight-testing.44 Also

recommended was the need for systemtechnology studies with fuel conservation

as a primary criterion so that the application of the various technological advances

could both separately and by interaction

produce further significant fuel savings.45

In 1975, NASA sponsored a Task Force of

engineers from within NASA, the Department of Transportation (DOT), Federal

Aviation Administration (FAA), and

Department of Defense (DOD) to examine the technological needs and opportunities for achievement of more fuelefficient transport aircraft and recommend

to NASA an extensive technological

development program. The Task Force

published its recommendations on 9

September 197546 and the Langley

Director, Edgar M. Cortright, immediately

established a Laminar-Flow-Control

Working Group, chaired by the author, “to

define a program of required R&T

activities.”47 After definition of detailed

plans and a process of evaluation, advocacy, and approval by NASA management, the U.S. Office of Management and

Budget (OMB) and the U.S. Congress, the

Task Force’s recommendations evolved

into the NASA Aircraft Energy Efficiency

(ACEE) Program. The Office of Aeronautics and Space Technology (OAST) at

NASA Headquarters managed the program.

The ACEE Project Office was established at the LaRC48 to define, implement

and manage three of six Program elements. The three elements were Composite Structures, Energy Efficient Transport

(subdivided into Advanced Aerodynamics

and Active Controls), and Laminar-Flow

Control.49 The acceptance of active

42 “Aircraft Fuel Conservation: An AIAA View” (Proceedings of a Workshop Conference, Reston, VA, 13-15 March,

edited by Jerry Grey, 30 June 1974).

43 ALB files, LHA, folder labeled Aeronautics Panel, AACB, Energy Conservation/Fuels: Minutes of Special Meeting,

NASA/DOD Aeronautics Panel, AACB, 11 November 1974, and Memorandum to Members of the Aeronautics Panel,

AACB, 25 November 1974.

44 ALB files, LHA, folder labeled Aeronautics Panel, AACB, Energy Conservation/Fuels: Report of the Subpanel on

Aeronautical Energy Conservation/Fuels, Aeronautics Panel, AACB, R&D Review, 5 December 1974, sect. 4.1.2. See

document 1 at the end of this monograph.

45 Ibid., sect. 3.8.

46 NASA Task Force for Aircraft Fuel Conservation Technology (Washington, D.C.: NASA TM X-74295, 9 September

1975).

47 See document number 3 at the end of this monograph.

48 ALB files, LHA, Project Plan, Aircraft Energy Efficiency Program, Langley Research Center, L860-001-0, May

1976. Inserted is a page summarizing some key events.

49 Ralph J. Muraca was Deputy Manager for LFC to Robert W. Leonard, ACEE Project Manager in the LaRC Projects

Group headed by Howard T. Wright. The author acted as Muraca’s assistant.

14
xx

laminar-flow control with suction (LFC)

as part of the NASA ACEE program was

based on the success of the previous

experimental programs in attaining

extensive regions of laminar flow on an

operational airplane and more recent

advances in materials and manufacturing

technology that might make LFC more

economically attractive. The principal

motivation was the potentially larger gain

in transport-aircraft performance resulting

from laminarization of the boundary layer

over wing and tail surfaces as compared

with all other technological disciplines.

Formulation of the approved program

received very extensive input and support

from the air-transport industry.50 An

important example was the active participation of people from the industry in an

LFC technology workshop held at the

Langley Research Center on 6 and 7

April 1976.51 Representatives of the

airlines, manufacturers of large aircraft

and aircraft engines, and individuals with

expertise in LFC from the industry and

government attended.52 Objectives were to

review the state of the art, identify and

discuss problems and concerns, and

determine what was necessary to bring

LFC to a state of readiness for application

to transport aircraft. The ACEE Project

Office relied heavily on the discussions.

A change in LFC emphasis from the

previous military application to the more

difficult one of commercial transports,

where manufacturing and operational

costs are more important, made the LFC

task even more challenging. The objective

of the LFC element was to provide

industry with sufficient information to

permit objective decisions on the feasibility of LFC for application to commercial

transports. It was expected that the

technology developed would be applicable to but not sufficient for very longrange or high-endurance military transports. The focus was on obtaining reliable

information regarding the ability to

provide and the cost of providing required

surface tolerances as well as on the ability

to maintain laminar flow in an airline

operational environment. Improvements

in computational ability for providing a

reliable design capability were also of

importance in the event practicality could

be established. Implementation of the

three project elements involved a major

change in Agency philosophy regarding

aeronautical research—a judicious

extension of the traditional NACA

research role to include demonstration of

technological maturity in order to stimulate the application of technology by

industry.

The ACEE/LFC project to bring

active LFC from an experimental status to

“technology readiness” for actual application required solutions to many difficult

technical problems and entailed a high

degree of risk—characteristics that

dictated reliance on government support.

A phased approach to require that

progress in each area be evaluated prior to

funding the next phase was accepted as a

means of controlling the large resource

commitments required and of alleviating

the concern about the risk factor. This

approach led to considerable heartburn in

the project office in its attempt to complete a successful overall development in

a timely fashion; a need to wait for

successful results on intermediate steps

was required before there could be

adequate advocacy for the inclusion of

subsequent phases in an annual government budget cycle. The project office

50 ALB files, LHA, notebook labeled Industry Comments: responses from industry top management to letter from

Robert E. Bower, LaRC Director for Aeronautics, requesting response to five specific questions regarding LFC; internal

ACEE Project Office memos on visits to industry to review detailed program proposals; and personal notes on trips to

industry.

51 ALB files, LHA, Workshop on Laminar Flow Control held at LaRC, compiled by Charles T. DiAiutolo, 6-7 April

1976.

52 General chairmen were Adelbert L. Nagel and Albert L. Braslow of LaRC.

15
xxi

adopted the following guidelines for the

LFC part of the program: “technology

readiness” should be validated by the

aircraft industry, and in particular, by

those companies involved in production

of long-range aircraft; the program should

be cognizant of technological advances in

other disciplines where those advances

would be of particular benefit to LFC or

where their application to future turbulent

jet transports appeared likely; and the

program should build on the existing data

base, in particular, the USAF X-21 flights

and associated programs previously

discussed.

Research from the Mid-1970s to the

Mid-1990s

For various reasons, the ACEE/LFC

project required flight research in the

following activities:

• Determination of the severity of the

adverse effects of surface contamination by insects on the extent of laminar

flow and the development and validation of an acceptable solution

• Evaluation of LFC surface and wing

structural concepts employing advanced materials and fabrication

techniques

• Development of improved aerodynamic

and acoustic design tools and establishment of optimized suction criteria

• Validation of airfoil and wing geometries optimized for LFC

• Validation of high-lift devices and

control surfaces compatible with LFC

• Demonstration of predicted achievement of laminar flow and validation of

acceptable economics in the manufacture and safe commercial operation of

LFC airplanes.

A few flight programs that investigated aerodynamic phenomena associated

with attainment of natural laminar flow

(NLF) provided information that was also

of importance to active laminar-flow

control at high subsonic speeds. These are

discussed in the following subsections

along with those that used LFC.

Figure 6. F-111/

TACT variablesweep transition

flight experiment.

(NASA photo ECN

3952)

16
xxii

Natural Laminar Flow (NLF) on Swept

Wings: F-111/TACT and F-14

Of principal significance in NLF

flight research done with an F-111 airplane and an F-14 airplane was quantification of the adverse effect of crossflow

instability due to wing sweep. Researchers installed supercritical, natural laminarflow airfoil gloves on an F-111 aircraft

(Figure 6), re-designated as the F-111/

TACT (Transonic Aircraft Technology)

airplane, and tested it in early 1980 at the

Dryden Flight Research Center (DFRC)53

through a range of sweep angles.54 These

results were limited by a restricted

spanwise extent of the gloves, an abbreviated test schedule (caused by the required

return to the Air Force of the borrowed

aircraft), and limited instrumentation.55

The results,56 however, provided the basis

for a follow-on program with another

variable-sweep aircraft (an F-14 on loan

to NASA from the Navy, Figure 7) that

enabled attainment of a much broader and

more accurate transition database. The F14 research began in 1984 at the DFRC

and was completed in 1987.57

Flush static-pressure orifices and

Figure 7. F-14

variable-sweep

transition flight

experiment. (NASA

photo)

53 From 1981 to 1994, Dryden was subordinated to the NASA Ames Research Center as the Ames-Dryden Flight

Research Facility, but to avoid confusion I will refer to it as DFRC throughout the narrative.

54 NASA flight-test participants were: Einar K. Enevoldson and Michael R. Swann, research pilots; Lawrence J. Caw

followed by Louis L. Steers, project managers; Ralph G. (Gene) Blizzard, aircraft crew chief; and Robert R. Meyer, Jr.,

followed by Louis L. Steers, DFRC principal investigators. For an example of a flight report on the F-111 with the NLF

gloves, see document 5 at the end of this monograph.

55 ALB files, LHA, folder labeled SASC 1980-81: memo on Natural Laminar Flow Flight Tests At DFRC On F-111

Aircraft, August 1980.

56 Boeing Commercial Airplane Company, Preliminary Design Department, F-111 Natural Laminar Flow Glove Flight Test

Data Analysis and Boundary Layer Stability Analysis (Washington, DC: NASA Contractor Report 166051, January 1984).

57 NASA flight-test participants were: Edward T. Schneider and C. Gordon Fullerton, research pilots; Jenny BaerRiedhart, project manager; Bill McCarty, aircraft crew chief; Harry Chiles, instrumentation engineer; Robert R. Meyer,

Jr., chief engineer; Marta R. Bohn-Meyer, operations engineer; Bianca M. Trujillo, DFRC principal investigator; and

Dennis W. Bartlett, LaRC principal investigator.

17
xxiii

surface arrays of hot films58 were distributed over gloves with a different airfoil

contour on each wing to determine local

wing pressures and transition locations.

Data from these sources and associated

flight parameters were telemetered to the

ground and monitored in real time by the

flight-research engineer. Figure 8 presents

results from the F-111 and F-14 sweptwing flight research along with results

from low-speed wind-tunnel research in

the LaRC Low-Turbulence Pressure

Tunnel (previously mentioned in the Early

Research section) and the Ames Research

Center 12-Foot Tunnel. The results are

presented as the variation of the maximum transition Reynolds number with

wing leading-edge sweep. The research

engineers, after careful consideration of

the differences in accuracy of the various

data, have judged that the extent of

laminar flow (a direct function of the

transition Reynolds number) is unaffected

by wing sweep up to a value of about 18

degrees. At higher sweep angles, the

extent of laminar flow is appreciably

reduced by crossflow disturbances. The

F-14 transition data also provided sufficient detailed information to improve the

understanding of the combined effects of

wing cross-sectional shape, wing sweep,

and boundary-layer suction (even though

suction was not used on the F-14) on the

growth of two-dimensional and crossflow

disturbances.59 This improved understanding permits a significant increase in

maximum transition Reynolds number

through the use of suction in only the

leading-edge region of swept wings in

combination with an extent of favorable

pressure gradient aft of the suction, a

concept called hybrid laminar-flow

control (HLFC), to be discussed later.

Noise: Boeing 757

Under a NASA contract, the Boeing

Company performed flight research in

1985 on the wing of a 757 aircraft (Figure

9) to determine the possible effects of the

acoustic environment on boundary-layer

transition. Because of a lack of sufficient

data on the acoustic environment associated with wing-mounted high-bypassratio turbofan engines, a concern about

Figure 8. Maximum

transition Reynolds

number as a

function of wing

sweep.

58 The hot-film sensors consisted of nickel-film elements deposited on a substrate of polyimide film with an installed

thickness of less than 0.007 inch. Electric current is passed through the nickel elements and circuitry maintains a constant

element temperature. The changes in current required to maintain the temperature constant are measured when changes

in boundary-layer condition cause changes in cooling of the elements. The difference in cooling between a laminar and

turbulent boundary layer and the fluctuating variations during the transition process from laminar to turbulent can then

be measured and the transition location determined.

59 R.D. Wagner, D.V. Maddalon, D.W. Bartlett, F.S. Collier, Jr., and A.L. Braslow, “Laminar Flow Flight Experiments,”

from Transonic Symposium: Theory, Application, and Experiment held at Langley Research Center (Washington, DC:

NASA CP 3020, 1988).

18
xxiv

potential adverse effects of engine noise

led to a belief that the engines needed to

be located in an aft position on the

fuselage. This location has a potentially

severe adverse impact on performance

and LFC fuel savings. Boeing replaced a

leading-edge slat just outboard of the

wing-mounted starboard engine with a

10-foot span smooth NLF glove swept

back 21 degrees. Seventeen microphones

were distributed over the upper and lower

surfaces to measure the overall sound

pressure levels, and hot films were used to

measure the position of transition from

laminar to turbulent flow. The starboard

engine was throttled from maximum

continuous thrust to idle at altitudes of 25

to 45 thousand feet and cruise speeds of

Mach 0.63 to 0.83.

Although this flight research was not

expected to provide answers on noise

effects for all combinations of pertinent

parameters, it did provide important

indications. The most important was that

engine noise does not appear to have a

significant effect on crossflow disturbances so that if the growth of crossflow

disturbances in the leading edge is

controlled by suction, large extents of

laminar flow should be possible even in

the presence of engine noise. If, however,

in an HLFC application, the growth of

two-dimensional type disturbances is

comparable to or greater than the growth

Figure 9. 757

transport noise

experiments.

19
xxv

of crossflow disturbances, engine noise

might be a more significant factor. The

results were unable to validate theoretical

predictions of the magnitude of noise

levels at high altitudes and subsonic

cruise speeds.60

Insect Contamination: JetStar

A major concern regarding the

dependability of laminar flow in flight

involved the possibility (most thought,

probability) that the remains of insect

impacts on component leading edges

during flight at low altitudes during

takeoff or landing would be large enough

to cause transition of the boundary layer

from laminar to turbulent during cruise

flight. As a first step, the LaRC measured

the insect remains that had accumulated

on the leading edges of several jet airplanes based at the Center. The Langley

researchers calculated that the insect

remains were high enough to cause

transition, even at altitudes as high as

40,000 feet.61 (Remember that an increase

in altitude alleviates the adverse effect of

surface roughness in that the minimum

height of roughness that will induce

transition increases as altitude increases.)

An observation, however, had been made

previously by Handley Page in England

where flight tests of a Victor jet indicated

that insect remains eroded to one-half

their height after a high-altitude cruise

flight. The Langley researchers, therefore,

deemed it necessary to investigate further

the possible favorable erosion but, if

erosion was determined to be insufficient

to alleviate premature transition at cruise

altitudes, to develop and validate an

acceptable solution to the insect contamination problem.

Researchers at the DFRC and the

LaRC used a JetStar airplane at Dryden

(Figure 10) in 1977 to investigate the

insect-contamination problem.62 With

Figure 10. JetStar

aircraft and research team for

investigation of

insect contamination . Left to right:

back row —

Thomas C.

McMurtry, test pilot;

Kenneth Linn,

instrumentation

technician; Robert S. Baron,

project manager;

Donald L. Mallick,

test pilot; Walter

Vendolski, aircraft

mechanic; John B.

Peterson, Jr., LaRC

principal investigator; front row —

Albert L. Braslow,

LaRC; James A.

Wilson, aircraft

crew chief; William

D. Mersereau, flight

operations; David

F. Fisher, DFRC

principal investigator. (Private photo

provided by author)

60 Boeing Commercial Airplane Company, Flight Survey of the 757 Flight Noise Field and Its Effect on Laminar

Boundary Layer Transition, Vol. 3: Extended Data Analysis (Washington, DC: NASA CR178419, May, 1988).

61 The calculations were based on von Doenhoff and Braslow, “The Effects of Distributed Surface Roughness on

Laminar Flow,” pp. 657-681, cited in footnote 27.

62 Dave Fisher was principal investigator at DFRC, and Jack Peterson formulated the program under the direction of the

author at the LaRC.

20
xxvi

contract support of the aircraft manufacturer, the Lockheed-Georgia Aircraft

Company, they modified the left outboard

leading-edge flap. Five different types of

superslick and hydrophobic surfaces were

installed in the hope that impacted insects

would not adhere to them. In addition,

researchers installed a leading-edge

washing system and instrumentation to

determine the position of boundary-layer

transition. Dryden research pilots first

flew the airplane with an inactive washer

system on numerous airline-type takeoffs

from large commercial airports. They flew

at transport cruise altitudes and then

landed at DFRC for post-flight inspection.

These early tests indicated that insects

were able to live in an airport noise and

pollution environment and accumulated

on the leading edge. The insects thus

collected did not erode enough to avoid

premature transition at cruise altitudes. It

is probable that insect impacts at the

much higher transport takeoff speed, as

compared with the slow takeoff speed of

the previously mentioned Victor airplane,

initially compresses the insects to a

greater degree where further erosion does

not take place. None of the superslick and

hydrophobic surfaces tested showed any

significant advantages in alleviating

adherence of insects. The need for an

active system to avoid insect accumulation, then, was apparent.63

Although researchers had considered

many concepts for such a system over the

years and had tested some, none had been

entirely satisfactory. The results of the

flight research using the leading-edge

washer system that had been installed on

the JetStar leading-edge flap showed that

a practical system was at hand. The tests

showed that keeping the surface wet while

encountering insects was effective in

preventing insect adherence to the wing

leading edge. After insect accumulation

was permitted to occur on a dry surface

spray could not wash the insect remains

off the leading edge (somewhat akin to

the inability of an automobile windshield

washer alone to remove bug accumulation

from the windshield). The pilots, named

in Figure 10, had flown the airplane with

the spray on at low altitudes over agriculture fields in an area with a high density

of flying insects in order to give the wetsurface concept a severe test.64 Supporting

analyses at LaRC also indicated an

acceptable weight penalty of a washer

system equal to less than one percent of

the gross weight of an LFC transport

airplane.

Leading-Edge Flight Test (LEFT)

Program: JetStar

Planning for a flight test program to

provide definitive information on the

effectiveness and reliability of LFC began

at LaRC soon after approval of the ACEE

Program. The Langley ACEE Project

Office expended considerable effort in

consideration of candidate flight vehicles.

Representatives of the airlines and

transport manufacturers strongly advocated the need for a test aircraft equal to

the size of a long-range transport (as

indicated in the question and answer

session of the 1976 LFC Workshop, cited

in footnote 51) to provide meaningful

results with respect to aerodynamic,

manufacturing, and operational considerations. Government managers applied

equally strong pressure towards the

selection of a smaller size for cost reasons. The Project Office eventually

formulated a satisfactory solution that

fulfilled both requirements. It decided to

restrict the tests to the leading-edge region

of a laminar-flow wing suitable for a

high-subsonic-speed transport airplane

because the most difficult technical and

design challenges that had to be overcome

63 David F. Fisher and John B. Peterson, Jr., “Flight Experience on the Need and Use of Inflight Leading Edge Washing

for a Laminar Flow Airfoil,” AIAA Aircraft Systems and Technology Conference, Los Angeles, CA (AIAA paper 78-

1512, 21-23 August 1978).

64 Details of these flight tests are included in ibid.

21
xxvii

before active laminar-flow control with

suction could be considered a viable

transport design option were (and still are)

embodied in this region. The external

surfaces at the leading edge must be

manufactured in an exceptionally smooth

condition (smoother than necessary at

more rearward locations) and must be

maintained in that condition while subject

to foreign-object damage, insect impingement, rain erosion, material corrosion,

icing, and other contaminants. In addition,

an insect-protection system, an anti-icing

system, a suction system, and perhaps a

purge system and/or a high-lift leadingedge flap must all be packaged into a

relatively small leading-edge box volume.

Most of these problems equally affect the

concept of hybrid LFC and the concept of

active laminar-flow control with suction

to more rearward positions.

The Project Office then selected the

same JetStar airplane that was previously

used for the initial insect-contamination

flight research. The test article would be

dimensionally about equivalent to the

leading-edge box of a DC-9-30 airplane, a

small commercial transport, where

solution of the packaging problems would

provide confidence for all larger HLFC

and LFC airplanes with suction to more

rearward positions. Its choice, however,

did not receive unanimous concurrence.

Dr. Pfenninger, who was then employed

by the LaRC, strongly objected to selection of an airplane with a leading-edge

sweep as high as the JetStar’s (33 degrees) because he expected greatly

increased difficulty in handling the large

crossflow disturbances that would be

introduced.65 The Project Office accepted

the risk, however, after extensive feasibility studies and technical evaluations of

several candidate aircraft.66

Selection of the most promising

approaches to satisfaction of LFC systems

requirements for both slotted-surface and

perforated-surface configurations was

based on several years of design, fabrication and ground testing activities.67 The

Douglas Aircraft Co. and the LockheedGeorgia Aircraft Co. were the major

contributors to this activity. Unfortunately, the Boeing Co. did not participate

initially because of a corporate decision to

concentrate its activities on the development of near-term transport aircraft.

Boeing became active in the laminar-flow

developments later. Inasmuch as no clearcut distinction existed at that time between multiple slots and continuous

suction through surface perforations made

with new manufacturing techniques

(although continuous suction had aerodynamic advantages), the Project Office

prudently decided to continue investigation of both methods for boundary-layer

suction. The Lockheed-Georgia Aircraft

Company installed a slotted configuration

on the left wing, and the Douglas Aircraft

Company installed a perforated configuration on the right wing. The leading-edge

sweep of both wing gloves was reduced

from the wing sweep of 33 degrees to 30

degrees to alleviate the crossflow instability problem somewhat. Figures 11-13

present illustrations of the airplane and

the leading-edge configurations.

The design of the slotted arrangement

represented a leading-edge region for a

future transport with laminar flow on both

surfaces in cruise flight and included

0.004-inch-wide suction slots (smaller

than the thickness of a sheet of tablet

65 ALB files, LHA, pocket-size “Memoranda” notebook: entry dated 2 September 1976.

66 ALB files, LHA, folder labeled LaRC Internal Memos on LFC dated 12/3/75 to 11/16/78: Memo to Distribution from

Ralph J. Muraca, Deputy Manager, LFC Element of ACEEPO on Feasibility Studies of Candidate Aircraft for LFC

Leading Edge Glove Flight — Request for Line Division Support, 16 November 1978.

67 Albert L. Braslow and Michael C. Fischer, “Design Considerations for Application of Laminar-Flow Control Systems

to Transport Aircraft,” presented at AGARD/FDP VKI Special Course on Aircraft Drag Prediction and Reduction at the

von Kármán Institute for Fluid Dynamics, Rhode St. Genese, Belgium on 20-23 May 1985, and at NASA Langley on 5-

8 August 1985, in AGARD Rept. 723, Aircraft Drag Prediction and Reduction (July 1985): 4-1 through 4-27.

22
xxviii

paper) cut in a titanium surface.68 The

slots that encompassed the wing stagnation line also served the dual purpose of

ejecting a freezing-point depressant fluid

film for anti-icing and for insect protection. During climb-out, these slots were

purged of fluid and they joined the other

suction slots for laminarization of the

boundary layer under cruise conditions.

The design of the perforated arrangement represented a leading-edge region

for a future transport with laminar flow on

the upper surface only—an approach that

can provide future transports with significant simplifying advantages at the expense of a somewhat higher drag. In the

design of future transports with uppersurface suction only, the adverse effect of

a loss in lower-surface laminarization will

not be as great as one might expect

because the skin friction is higher on the

upper surface due to higher local velociFigure 11a. JetStar

Test-bed aircraft for

the NASA LeadingEdge Flight Test

program.

Figure 11b. JetStar

aircraft and team

for Leading-Edge

Flight-Test Program. Left to right:

J. Blair Johnson,

aerodynamics

engineer; Gary

Carlson, aircraft

mechanic; Michael

C. Fischer, LaRC

principal investigator; James Wilson,

aircraft crew chief;

Donald L. Mallick,

test pilot; David F.

Fisher, DFRC

principal investigator; John P. Stack,

LaRC instrumentation technician;

Edward Nice,

aircraft mechanic;

Ron Young,

instrumentation

engineer; Earl

Adams, DFRC

instrumentation

technician; Robert

S. Baron, DFRC

project manager;

Russell Wilson,

aircraft inspector;

Richard D. Wagner,

LaRC project

manager; unidentified; Fitzhugh L.

Fulton, test pilot.

(NASA photo ECN

30203)

68 No leading-edge high-lift device was required for the transport aircraft conceptualized by Lockheed for this application of LFC.

23
xxix

Figure 13. LeadingEdge Flight-Test

program slotted

test article.

Figure 12. LeadingEdge Flight-Test

program perforated

test article.

ties. A relatively small extension in uppersurface laminarization, therefore, can be

used to significantly attenuate the increased drag of the lower surface.

The advantages of laminarization of

only the upper surface include several

features. Conventional access panels to

wing leading- and trailing-edge systems

and fuel tanks can be provided on the

wing lower surface for inspection and

maintenance purposes without disturbing

the laminar upper surface. Laminarized

surfaces in areas susceptible to foreignobject damage are eliminated. The wing

can be assembled from the lower surface

with the use of internal fasteners; this is

much preferable to concepts that use

external fasteners, where the fasteners

could induce external disturbances. The

initial manufacturing costs and the

maintenance costs are reduced. Uppersurface-only laminarization also will

permit deployment of a leading-edge

device for both high lift and shielding

from direct impacts of insects. Deployment, when needed, and retraction into

the lower surface, when not needed, will

be permitted because the need for stringent surface smoothness on the lower

surface will be eliminated. The test

arrangement used such a device with an

auxiliary nozzle to spray freezing-point

24
xxx

depressant fluid for anti-icing and to

provide conservatism in the elimination of

insect adherence.69 Finally, Douglas used

a system for reversing the flow of air

through the perforations on the test

arrangement to remove possible residual

fluid.

Use of electron-beam technology

made possible, for the first time, manufacture of holes of a small enough size and

spacing to avoid introduction of aerodynamic disturbances as large as those that

had previously caused premature transition in wind tunnels. The successful use

of laser “drilling” of holes followed later.

The perforations in the test arrangement

on the JetStar were 0.0025 inch in diameter (smaller than a human hair) and were

spaced 0.035 inch apart in a titanium skin

(over 4,000 holes per square foot of

surface area). Only very close inspection

would reveal a difference between a

perforated-wing surface and a solid one.

In general, instrumentation was

conventional but careful attention was

required to avoid any adverse interference

with the external or internal airflows. An

unconventional instrument called a

Knollenberg probe (a laser particle

spectrometer) was mounted atop a ventral

pylon on the fuselage upper surface to

measure the sizes and quantities of

atmospheric ice and water droplets. A

charging patch, mounted on the pylon

leading edge, provided a simple way to

detect the presence of atmospheric

particles and an impending loss of laminar

flow by responding to the electrostatic

charge developed when ice or water

droplets struck the aircraft surface. The

patch was investigated as a possible lowcost application to future laminar-flow

airplanes.

The Dryden Flight Research Center

again conducted the flight tests.70 After

initial tests to check out and adjust all

systems and instrumentation, the principal

effort focused on demonstration of the

ability to attain the design extent of

laminar flow under routine operational

conditions representative of LFC subsonic

commercial airplanes and on provision of

insight into maintenance requirements.

Simulated airline flights included ground

queuing, taxi, take off, climb to cruise

altitude, cruise for a sufficient time to

determine possible atmospheric effects on

laminar flow, descent, landing, and taxi.

Conditions representative of airline

operations included one to four operations

per day and flight in different geographical areas, seasons of the year, and

weather. Also, as in the case of commercial airline operations, the airplane

remained outdoors at all times while on

the ground and no protective measures

were taken to lessen the impact of adverse

weather or contamination on the test

articles. In order not to increase pilot

workload in the operation of LFC airplanes, the suction system was operated in

a hands-off mode (except for on-off

inputs).

All operational experience with the

LFC systems performance (for both

perforated and slotted configurations)

during the simulated-airline-service

flights was positive.71 Specifically, during

four years of flight testing from November 1983 to October 1987, no dispatch

delays were caused by LFC systems.

Laminar flow was obtained over the

69 After the early JetStar flight tests on the effectiveness of wetting the leading-edge surfaces for prevention of insect

adherence, analyses and wind-tunnel tests of live-insect impacts were made by both Lockheed and Douglas to develop

detailed arrangements of leading-edge-protection methods for their selected LFC configurations.

70 NASA Flight-test participants were: Donald L. Mallick and Fitzhugh L. Fulton, research pilots; Robert S. Baron,

project manager; Ronald Young, instrumentation engineer; David F. Fisher followed by M.C. Montoya, DFRC principal

investigators; and Michael C. Fischer, LaRC principal investigator. For background to the flight testing, see document 4

at the end of this monograph.

71 Dal V. Maddalon and Albert L. Braslow, Simulated-Airline-Service Flight Tests of Laminar-Flow Control with

Perforated-Surface Suction System (Washington, DC: NASA Technical Paper 2966, March 1990).

25
xxxi

leading-edge test regions as planned after

exposure to heat, cold, humidity, insects,

rain, freezing rain, snow, ice, and moderate turbulence. Removal of ground

accumulations of snow and ice was no

more difficult than the then-normal

procedures for transport aircraft. No

measurable degradation of the titanium

surfaces occurred. Surface cleaning

between flights was not necessary. Pilot

adjustment of suction-system operation

was unnecessary. The simple electrostatic

“charging patch” device appeared to offer

an inexpensive and reliable method of

detecting the presence of ice crystals in

flight (more about the atmospheric

particle problem later).

The emergence of electron-beam

perforated titanium as a wing surface that

met the severe aerodynamic, structural,

fabrication, and operational requirements

for practical aircraft applications was

considered to be a major advance in

laminar-flow control technology by the

principal government and industry

investigators. Fabrication of the slottedsurface test article resulted in a suction

surface that was only marginally acceptable, resulting in poorer performance.

Some further development of slottedsurface manufacturing techniques,

therefore, was (and is) still required. Also

needed is proof of satisfactory aerodynamic performance of the perforated

surface at larger values of length

Reynolds number, i.e., to distances greater

than the end of the leading-edge test

article. Nevertheless, the simulatedairline-service flights successfully demonstrated the overall practicality of baseline

designs for leading-edge LFC systems for

future commercial-transport aircraft, a

major step forward.

Surface Disturbances: JetStar

In 1986 and 1987, the LaRC LFC

Project Office, which had continued

research on LFC after termination of the

ACEE Project,72 took advantage of the

continued availability of the JetStar

airplane at the DFRC to further the

quantitative database on the effects of

two- and three-dimensional-type surface

roughness and on the effects of suction

variations.73 The most significant results

that were obtained concerned clarification

of the quantitative effects of crossflow

due to sweep on the roughness sizes that

would cause premature transition. As

indicated many times in this monograph,

the adverse effect of surface protuberances on the ability to maintain laminar

flow was the primary inhibiting factor to

the practicality of LFC. Although an

empirical method of determining the

quantitative effects of surface roughness

on transition had been developed much

earlier for unswept wings,74 some indications had later become available75 that

wing sweep (crossflow effects) might

72 Richard D. Wagner headed the LaRC LFC Project Office during the 1980s (at first, still under ACEE) and was followed

by F.S. Collier, Jr. The author, after his retirement from NASA in 1980, continued to provide significant input into the

planning, analysis and reporting of much of the experimental research and development activities through local aerospace

contractors. Dal V. Maddalon was technical monitor for these contracts. See ALB files, four folders labeled SASC (Systems

and Applied Sciences Corporation) and one folder labeled Analytical Services and Materials, Inc. (April, 1980 through Sept.,

1993).

73 Dal V. Maddalon, F.S. Collier, Jr., L.C. Montoya, and C.K. Land, “Transition Flight Experiments on a Swept Wing with

Suction” (AIAA paper 89-1893, 1989); Albert L. Braslow and Dal V. Maddalon, Flight Tests of Three-Dimensional Surface

Roughness in the High-Crossflow Region of a Swept Wing with Laminar-Flow Control (Washington, DC: NASA TM

109035, October 1993); Albert L. Braslow and Dal V. Maddalon, Flight Tests of Surface Roughness Representative of

Construction Rivets on a Swept Wing with Laminar-Flow Control (Washington, DC: NASA TM 109103, April 1994).

74 See von Doenhoff and Braslow, “The Effects of Distributed Surface Roughness on Laminar Flow,” pp. 657-681,

cited in footnote 27.

75 Dezso George-Falvy, “In Quest of the Laminar-Flow Airliner—Flight Experiments on a T-33 Jet Trainer,” 9th

Hungarian Aeronautical Science Conference, Budapest, Hungary (10-12 November 1988).

26
xxxii

exacerbate the roughness effects. Analysis

of the additional JetStar data76 indicated

that the adverse effect of crossflow

occurred for two- rather than threedimensional type roughness.77

Figure 14 plots a roughness Reynolds

number parameter against the ratio of

roughness width to height.78 The symbols

represent data for unswept wings with no

crossflow except for a group of three

identified for swept wings in high

crossflow.79 The vertical bracket indicates

a range of roughness data obtained on the

sweptback JetStar in a region of high

crossflow. The horizontal line represents

other roughness data obtained on the

JetStar in both low and high crossflow.

The important conclusions are: 1) for

practical engineering application, the

permissible height of three-dimensional

type roughness (ratios of roughness width

or diameter to height of approximately 0.5

to 5.0) located in a high crossflow region

is the same as that previously established

in zero crossflow; 2) only for more twodimensional type roughness (roughness

width to height ratios equal to or greater

than approximately 24) will high

crossflow decrease the permissible height

of roughness; and 3) for values of roughness width to height ratios equal to or

greater than approximately 30, development of a different criterion for permissible roughness height is required. Information of this kind is crucial for the

establishment of the manufacturing

tolerances and maintenance requirements

that must be met for surface smoothness.

Figure 14. Comparison of sweptwing surface

roughness data

with unswept-wing

von DoenhoffBraslow data

correlation.

76 From the second and third sources cited in footnote 73.

77 For any reader interested in a brief summary of the basic two- and three-dimensional roughness effects on laminar

flow without crossflow, the discussion on pages 2-4 of the second citation in footnote 73 is recommended.

78 From Figure 7 of the third source cited in footnote 73.

79 See von Doenhoff and Braslow, “The Effects of Distributed Surface Roughness on Laminar Flow,” pp. 657-681,

cited in footnote 27.

27
xxxiii

Atmospheric Ice Particles: Boeing 747s

and JetStar

As indicated in the section on the

post-World War II to mid-1960s period,

the practical significance of atmospheric

ice particles on the amount of time

laminar flow might be lost on operational

aircraft was not known because of a lack

of information on particle concentrations.

Unanalyzed cloud-encounter and particleconcentration data became available from

the NASA Lewis Research Center (LeRC)

Global Atmospheric Sampling Program

(GASP) in the late 1970s. From March

1975 to June 1979, NASA obtained data

with instruments placed aboard four 747

airliners on more than 3,000 routine

commercial flights that involved about

88,000 cloud-encounters.80

With the GASP data, Richard E.

Davis of the LaRC estimated average

cloud-cover statistics for several longdistance airline routes. He then made

conservative estimates of the probable

loss of laminar flow on these major airline

routes by assuming that all cloud encounters cause total loss of laminar flow, i.e.,

that the percentage loss of laminar flow

on a given flight is equal to the percentage

of time spent within clouds on that flight.

For further conservatism, he assumed that

pilots would make no attempt to avoid

flight through clouds. Figure 15 is an

example of the potential laminar-flow loss

on some of the major airline routes—Los

Angeles-Tokyo, New York-London, and

New York-Los Angeles. The figure also

includes a world average. These results

now make it apparent that cloud encounters during cruise of long-range commercial air transports are not frequent enough

to invalidate the large performance

improvements attainable through application of LFC.

Figure 15. Potential

laminar-flow loss

on some major

airline routes.

80 William H. Jasperson, Gregory D. Nastrom, Richard E. Davis, and James D. Holdeman, GASP Cloud- and ParticleEncounter Statistics, and Their Application to LFC Aircraft Studies, Vol. I: Analysis and Conclusions, and Vol. II:

Appendixes (Washington, DC: NASA Technical Memorandum 85835, October 1984).

28
xxxiv

In addition, during the JetStar LEFT

program, the Dryden flight-test team

measured the size and concentration of

atmospheric particles encountered at the

same time they measured the degree of

laminar-flow degradation. With these

LEFT measurements, Davis at LaRC

provided some validation of the Hall

theory of laminar-flow loss as a function

of atmospheric particle size and concentration.81

Hybrid Laminar-Flow Control

(HLFC): Boeing 757

The hybrid laminar-flow control

concept integrates active laminar-flow

control with suction (LFC) and natural

laminar flow (NLF) and avoids the

objectionable characteristics of each. The

leading-edge sweep limitation of NLF is

overcome through application of suction

in the leading-edge box to control

crossflow and attachment-line instabilities

characteristic of swept wings. Wing

shaping for favorable pressure gradients

to suppress Tollmien-Schlichting instabilities and thus allow NLF over the wing

box region (the region between the two

wing structural spars) removes the need

for inspar LFC suction and greatly

reduces the system complexity and cost.82

HLFC offers the possibility of achieving

extensive laminar flow on commercial or

military transport aircraft with a system

Figure 16. Possibilities of laminar

flow on swept

wings.

81 See Hall, “On the Mechanics of Transition,” cited in footnote 36.

82 Examples of additional complexities associated with suction over the wing box include: manufacture of a structural

box of sufficient strength and light weight with slots or perforations over a much more extensive area of the wing skin;

extensive internal suction ducting that decreases the internal wing volume available for storage of airplane fuel; larger

suction pump(s) than otherwise needed; an increased difficulty in providing the required surface smoothness and fairness

for maintenance of laminar flow over inspection panels in slotted or perforated surfaces when laminarization of both

upper and lower surfaces is desired; and a need to avoid hazards due to possible leakage of fuel into the suction ducting.

These complexities, along with other special features, increase airplane weight and manufacturing costs as well as

maintenance costs.

29
xxxv

no more complex than that already proven

in the LEFT program on the NASA

JetStar.

The relative place of HLFC, LFC and

NLF in the wing-sweep-to-aircraft-size

spectrum is indicated in Figure 16. On a

grid of chord Reynolds number vs.

quarter-chord sweep are plotted various

items. The shaded area indicates the

approximate chordwise extent of natural

laminar flow attainable on a wing with

initially decreasing surface pressures in

the direction towards the trailing edge

(upper left plot). Ranges of wing chord

Reynolds number in cruise for four

commercial transport airplanes are

superimposed—for the Douglas DC-10,

Lockheed L-1011, Boeing-757 and

Douglas DC-9-80 airplanes. For each

airplane, the wing chord Reynolds

number decreases along the span from

root to tip because of a taper in the wing

planform.

The figure indicates that natural

laminar flow can be attained only on

regions of the wings near the wing tips.

As the wing-section chord increases

(increased Rc) due to either a location

nearer the wing root or an increase in

airplane size, the chordwise extent of

laminar flow decreases (due to increased

Tollmien-Schlichting instabilities). Also,

less laminar flow is attainable as the wing

sweep increases (due to increased

crossflow instabilities). The use of wall

suction, however, permits the maintenance of laminar flow to large chordwise

extents at both high sweep and large size

(high Reynolds number), as indicated by

the X-21 data point, but at the expense of

complexities due to the extensive suction

system. A combination of principles for

active laminar-flow control and natural

laminar flow—hybrid laminar-flow

control (HLFC)—greatly increases the

size of high subsonic-speed airplanes for

which large extents of laminar flow can

be obtained as compared with natural

laminar-flow airplanes. For example,

compare the chord Reynolds number for

Figure 17. Improvement in lift-to-drag

ratio due to laminar

concepts.

30
xxxvi

60-percent chord laminar flow with

HLFC on the upper surface with the chord

Reynolds number for natural laminar flow

(of a smaller relative extent) on Citation

III and Learjet airplanes, also plotted in

Figure 16.

Figure 17 plots the percentage

improvement of lift-to-drag ratio (L/D)83

for each of the three laminar-flow concepts as compared with a turbulent

airplane, plotted as a function of airplane

wing area. The figure shows a large

improvement in L/D for HLFC as compared with NLF. For the larger airplanes,

of course, appreciably larger benefits are

obtained with active laminar-flow control

with suction to positions farther aft. As in

the case of LFC farther aft, the concept of

hybrid laminar-flow control requires

smoothness of surface finish and contour

as well as protection from insect residue

and ice accumulation in the leading-edge

region. The systems developed in the

LEFT program for the leading-edge

region are equally applicable for the

hybrid laminar-flow control application.

Under a participatory arrangement

between the LaRC, the USAF, and the

Boeing Commercial Airplane Company,

Boeing flight tested the effectiveness of

hybrid laminar-flow control on a company-owned 757 airplane in 1990. Figure

18 shows an HLFC glove installed on a

large section of the left wing. The systems

in the leading-edge wing box are very

similar to those flight tested on the JetStar

airplane—a Krueger flap84 for insect

protection and high lift; a perforated

titanium suction surface; and suction to

the front spar with an ability to reverse

flow for purging. Rather than use ejection

of a freezing-point depressant, the design

encompassed thermal anti-icing, i.e.,

reversal of the airflow and expulsion of

heated air through the perforations in the

leading-edge region. Boeing pilots flew

the airplane at transport cruise Mach

numbers and altitudes.

The primary goal was to establish the

aerodynamics of HLFC at Reynolds

numbers associated with medium-size

transport airplanes to reduce industry

Figure 18. 757

subsonic hybrid

laminar flow control

flight experiment.

(NASA photo L-90-

9549)

83 L/D is a significant measure of aerodynamic performance.

84 “Krueger” designates a specific type of leading-edge high-lift device (flap) that retracts into the wing lower surface.

When used for an active laminar-flow control application, the flap also shields the wing from insect impacts during

takeoff and landing and when retracted under the leading edge for cruise, does not interfere with the upper-surface

laminar flow.

31
xxxvii

risks to acceptable levels. Results were

very encouraging. Transition location was

measured several feet past the end of

suction and with less suction than estimated. The Krueger leading-edge flap

proved effective as the insect shield.

Existing manufacturing technology

permitted construction of the leading-edge

box to laminar-flow surface-quality

requirements. All necessary systems

required for practical HLFC were successfully installed into a commercial

transport wing.85

Research engineers at the LaRC

calculated the benefits of the application

of hybrid laminar-flow control to a 300-

passenger long-range twin-engine subsonic transport.86 With what appear to be

reasonable assumptions of 50-percent

chord laminar flow on the wing upper

surface and 50-percent chord laminar flow

on both surfaces of the vertical and

horizontal tails, HLFC provides a 15-

percent reduction in block fuel from that

of a turbulent transport.87 Application of

HLFC to the engine nacelles has the

potential of at least an additional 1-

percent block-fuel reduction with laminar

flow to 40 percent of the nacelle length.88

Supersonic Laminar-Flow Control: F16XL

In the late 1980s, the Laminar-Flow

Control Project Office of the Langley

Research Center reactivated a longdormant consideration of LFC for commercial supersonic transports as part of a

NASA technology-development program

for high-speed civil transports. As is the

case for subsonic flight, potential benefits

of the application of LFC to supersonic

transports include increased range,

improved fuel economy, and reduced

airplane weight. Reduced fuel consumption will not only improve economics but

will also reduce a potential adverse

impact of engine emissions on the earth’s

ozone layer from flight of supersonic

airplanes at higher altitudes than those for

subsonic flight. Additional benefits of

reduced airplane weight at supersonic

speeds are a decrease in the magnitude of

sonic booms89 and a reduction in community noise during takeoff.90 Also, the lower

skin friction of laminar boundary layers as

compared with turbulent boundary layers

is of even more importance at supersonic

speeds than at subsonic speeds because

the associated aerodynamic heating of the

surface by the skin friction is an important

design consideration at supersonic

speeds.91 The Boeing Commercial Airplane Company and the Douglas Aircraft

Company of the McDonnell Douglas

Corporation,92 both under contract to the

LaRC LFC Project Office, first studied

needed aerodynamic modifications and

associated structural and systems requirements to arrive at a realistic assessment of

the net performance benefits of super85 A generally-available technical report on the HLFC flight tests has not been published.

86 Richard H. Petersen and Dal V. Maddalon, NASA Research on Viscous Drag Reduction (Washington, DC: NASA TM

84518, August 1982).

87 Block fuel is the fuel burned from airport gate to airport gate, excluding fuel burned due to any delays.

88 ALB files, LHA, P.K. Bhutiani, Donald F. Keck, Daniel J. Lahti, and Mike J. Stringas, “Investigating the Merits of a

Hybrid Laminar Flow Nacelle, The Leading Edge” (General Electric Company, GE Aircraft Engines, Spring 1993).

89 The magnitude of a sonic boom is proportional to the airplane lift which is proportional to the airplane weight at a

given cruise speed. If sonic-boom overpressures are reduced below a value of one pound per square foot, overland

supersonic cruise may become allowable.

90 Takeoff noise is reduced by a reduction in takeoff thrust requirements resulting from lower weight.

91 Reduced aerodynamic heating increases material options, enhances the potential for unused fuel as a heat sink for

airplane environmental control systems, and decreases the detectability of military aircraft.

32
xxxviii

sonic-LFC implementation. Although

promising conclusions were reached, the

studies indicated the need for additional

research and development specific to the

supersonic application. Recommendations

were made for supersonic flight research on

HLFC.93

After additional analyses, wind-tunnel

testing and exploratory flight research at the

DFRC on two prototype F-16XL airplanes

denoted as ship 1 and ship 2, DFRC also

flight researched a laser-perforated titanium

glove installed on the left wing of ship 2

(Figure 19).94 Under LFC Project Office

management, the Rockwell Corporation and

the Boeing Company manufactured and

installed the glove and the Boeing and

Douglas Companies supported DFRC with

the flight research and analysis. Specific

objectives were to determine the capability

of active LFC to obtain a large chordwise

extent of laminar flow on a highly-swept

wing at supersonic speeds and to provide

validated computational codes, design

methodology, and initial suction-system

design criteria for application to supersonic

transport aircraft. To make accurate measurements, the investigators installed an

extensive array of hot-film, pressure, and

temperature instrumentation and provided

real-time displays of the measurements.

They completed thirty-eight flights with

active boundary-layer suction and experienced very few problems with the suction

system.95 The laminar-flow data are

currently restricted in distribution.

Figure 19. Twoseat F-16XL

Supersonic Laminar-Flow-Control

flight research

aircraft with a

suction glove

installed on the left

wing. (NASA photo

EC96-43831-5 by

Jim Ross).

92 Now part of Boeing.

93 A.G. Powell, S. Agrawal, and T.R. Lacey, Feasibility and Benefits of Laminar Flow Control on Supersonic Cruise

Airplanes (Washington, DC: NASA Contractor Report 181817, July 1989); Boeing Commercial Airplane Company,

Application of Laminar Flow Control to Supersonic Transport Configurations (Washington, DC: NASA Contractor

Report 181917, July 1990).

94 NASA flight-test participants were: Dana Purifoy and Mark P. Stucky, research pilots; Marta R. Bohn-Meyer and

Carol A. Reukauf, project managers; Michael P. Harlow, aircraft crew chief; Lisa J. Bjarke, DFRC principal investigator;

and Michael C. Fischer, LaRC principal investigator.

95 See document number 6 at the end of this monograph for the flight log of the F-16XL number 2.

33
xxxix

Status of Laminar-Flow Control

Technology in the Mid-1990s

The status of laminar-flow control technology in the mid-1990s may be summarized as follows:

• Design methodology and related

enabling technologies are far advanced

beyond the X-21 levels.

• Improved manufacturing capabilities

now permit the general aviation industry to incorporate natural laminar flow

in some of its aircraft designs for chord

length Reynolds numbers less than 20

million, but active laminar-flow

control, required for larger aircraft and/

or aircraft with highly-swept wings, has

not yet been applied to any operational

aircraft.

• Although some additional structural

and aerodynamic developments are

required, the recent programs have

brought the promise of laminar flow for

moderately large and very large subsonic transport aircraft much closer to

fruition than ever before.

• Hybrid laminar-flow control simplifies

structure and systems and offers

potential for 10- to 20-percent improvement in fuel consumption for moderatesize subsonic aircraft.

• Hybrid LFC may be the first application of suction-type laminar-flow

control technology to large highsubsonic-speed transports because of its

less risky nature.

• Although much of what has been

learned about subsonic laminar-flow

control is applicable to supersonic

speeds, considerable additional work is

required before supersonic laminarflow control can be applied to operational aircraft.

34
35xl

Glossary

AACB

ACEE

AGARD

AIAA

Attachment line

Chord

DAG

DFRC

DOD

DOT

FAA

GASP

Glove

Hall

HLFC

Krueger flap

LaRC

LEFT

Length Reynolds number

Aeronautics and Astronautics Coordinating Board

Aircraft Energy Efficiency Program

Advisory Group for Aeronautical Research & Development, North

Atlantic Treaty Organization

American Institute of Aeronautics and Astronautics

On a sweptback wing, the line at which the airflow divides to the

upper and lower surfaces

The length of the surface from the leading to the trailing edge of an

airfoil

Division Advisory Group

Dryden Flight Research Center

Department of Defense

Department of Transportation

Federal Aviation Administration

Global Atmospheric Sampling Program

A special section of an airplane’s lifting surface, usually overlaying

the basic wing structure, that is designed specifically for research

purposes

Originator of a theory that indicates when laminar flow would be lost

as a function of atmospheric particle size and concentration

Hybrid Laminar-Flow Control

A specific type of leading-edge high-lift device (flap) that retracts

into the wing lower surface. When used for an active laminar-flow

control application, the flap also shields the wing from insect impacts

during takeoff and landing and when retracted under the leading edge

for cruise, does not interfere with the upper-surface laminar flow.

Langley Research Center

Leading-Edge Flight Test

When the representative length in the formulation of the Reynolds

number is chosen as the distance from the body’s leading edge to the

end of laminar flow, the resultant length Reynolds number can be

used as a measure of the length of laminar flow attained.
xli 36

LeRC

LFC

LTPT

NACA

NASA

NLF

OAST

RAE

Reynolds Number

Three-dimensional type surface

disturbances

Two-dimensional type disturbances

TACT

Tollmien-Schlichting instabilities

USAF

WADD

Lewis Research Center (now Glenn Research Center)

Laminar-flow control

[Langley] Low-Turbulence Pressure Tunnel

National Advisory Committee for Aeronautics

National Aeronautics and Space Administration

Natural laminar flow

Office of Aeronautics and Space Technology of NASA

Royal Aircraft Establishment

A non-dimensional value equal to the product of the velocity

of a body passing through a fluid, the density of the fluid, and

a representative length divided by the fluid viscosity.

Three-dimensional type surface disturbances are those with

width or diameter to height ratios near a value of one.

Examples of two-dimensional type disturbances are stream

turbulence, noise, and surface irregularities having large ratios

of width (perpendicular to the stream flow direction) to height,

like spanwise surface steps due to mismatches in structural

panels.

Transonic Aircraft Technology

Very small two-dimensional type disturbances that may induce

transition to turbulent flow—named after German aerodynamicists Walter Tollmien and Hermann Schlichting.

United States Air Force

Wright Air Development Division
xlii 37

Documents
xliii 38

Document 1—Aeronautics Panel, AACB, R&D Review, Report of the

Subpanel on Aeronautical Energy Conservation/Fuels.
xliv 39
xlv40
xlvi 41
xlvii 42
xlviii 43
xlix 44
45l
46li

Document 2—Report of Review Group on X-21A Laminar Flow Control

Program
47lii
liii 48
liv49
50lv
lvi 51
lvii 52
lviii 53
lix54
55lx
lxi 56
lxii 57
lxiii 58
lxiv 59
lxv60
lxvi 61

Document 3—Langley Research Center Announcement: Establishment of

Laminar Flow Control Working Group
lxvii 62

Document 4—Intercenter Agreement for Laminar Flow Control Leading

Edge Glove Flights, LaRC and DFRC
lxviii 63
lxix 64
lxx65
lxxi 66

Document 5—Flight Report, NLF-144, of AFTI/F-111 Aircraft with the

TACT Wing Modified by a Natural Laminar Flow Glove
lxxii 67
lxxiii 68
lxxiv 69
lxxv 70
lxxvi 71

Document 6—Flight Record, F-16XL Supersonic Laminar Flow Control

Aircraft
lxxvii 72
lxxviii 73
lxxix 74
lxxx75
lxxxi 76

Abbott, Ira H., 4

Adams, Earl, 23

Antonatos, Philip P., 10

Baer-Riedhart, Jenny, 17 n

Baron, Robert S., 20, 23, 25 n

Bartlett, Dennis W., 17 n

Bjarke, Lisa J., 33 n

Blizzard, Ralph G., 17 n

Bohn-Meyer, Marta R., 17 n, 33 n

Boundary layer

laminar, 1, 2, 3-10, 12, 15, 16, 18, 20, 23, 26, 29-

 30, 32, 33

turbulent, 1, 6-7, 9-12, 20, 25, 31-32

Bower, Robert E., 15 n

Braslow, Albert L., 5, 10-11, 13-14, 15 n, 20, 26 n

Burrows, Dale L., 5

Carlson, Gary, 23

Caw, Lawrence J., 17

Chiles, Harry, 21 n

Collier, F.S., Jr., 26 n

Cortright, Edgar M., 14

Davis, Richard E., 28-29

DiAiutolo, Charles T., 15 n

Disturbance inputs

insects, 11, 16, 20-21, 25, 26, 31-32

noise, 4 n, 8, 10, 13, 18-20, 32

stream turbulence, 4, 9-12, 26

surface irregularities, 6, 7, 9-10, 11, 20, 25, 26-27,

 31

weather, 10, 12, 23-25, 26, 28-29, 31

Disturbance types

attachment line, 11, 29

crossflow, 2, 8, 11, 17, 18-20, 22, 26,-27, 30

three-dimensional, 6, 10, 26-27

two-dimensional, 6, 7, 19-20, 26-27

Donlan, Charles J., 11 n

Dryden, Hugh L., 4

Eaffy, A., 14

Enevoldson, Einar K., 17 n

Fischer, Michael C., 23, 25 n, 33 n

Fisher, David F., 20, 23, 25 n

Flight-test and flight-research aircraft

747, 28-29

757, 18-19, 29-32

Anson, 6

AW52, 8

B-18, 3-4

Citation III, 29, 31

F-111, 16-18, 66-70

F-14, 17-18

F-16XL, 32-33, 71-75

F-94, 8-9

Hurricane, 4, 10

JetStar, 20-29

King Cobra, 4, 10

Lancaster, 11-12

Learjet, 29, 31

Vampire, 6-7

Victor, 20, 21

WB-66D, 10

X-21, 11-12, 16, 29, 34, 46-60

Fullerton, C. Gordon, 17 n

Fulton, Fitzhugh L., 23, 25 n

Goldsmith, John, 6

Hall, G.R., 12, 29

Harlow, Michael P., 33 n

Industry

Boeing, 18-19

Douglas, 22, 32-33

Handley Page, 7, 20

Lockheed-Georgia, 21-22, 23 n

Northrop,5-6, 8, 10-12

Rockwell, 33

Instrumentation

hot-film, 18-19, 33

Knollenberg probe, 25

Jacobs, Eastman N., 4, 5 n

Johnson, J. Blair, 23

Kayten, Gerald, 13

Klebanoff, P.S., 4

Laminar-flow control

active (LFC), 1-2, 5-6, 7, 8-9, 10-12, 3, 15-16, 22-

 29, 32-34, 46-60, 61-65, 71-75

combined active & passive, 2, 18, 29-32, 34

passive, , 1, 4, 8, 16-17, 19, 29-32, 66-70

Leonard, Robert W., 14 n

Linn, Kenneth, 20

Mach number, 9, 11, 19, 32

Maddalon, Dal V. , 26 n

Mallick, Donald L., 20, 23, 25 n

McCarty, Bill, 17 n

McMurtry, Thomas C., 20

Mersereau, William D., 20

Meyer, Robert R., Jr., 17 n

Montoya, M.C., 25 n

Muraca, Ralph J., 14 n, 22 n

Nagel, Adelbert L., 15 n

Natural Laminar Flow (NLF), 1, 4, 8, 16-17, 19, 29-32,

 66-70

Nice, Edward, 23

Organizations

Aeronautics and Astronautics Coordinating Board,

 14

Index
lxxxii 77

American Institute of Aeronautics and Astronautics,

 13

Department of Defense, 14

Department of Transportation, 14

Federal Aviation Administration, 10, 14

National Advisory Committee for Aeronautics,1, 3,

 10, 15

National Aeronautics and Space Administration, 1,

 10-11, 14, 28

Ames Research Center, 18

Dryden Flight Research Center, 8, 17, 20-29, 33

Langley Research Center, 3, 4, 10, 13, 15, 20-21, 31,

 32-33

Lewis Research Center, 28

Office of Aeronautics and Space Technology, 14

National Bureau of Standards, 4

Royal Aircraft Establishment (Great Britain), 6, 8

U.S. Air Force, 5-6, 8, 10-12, 31

Wright Air Development Division, 10

Perforation method

electron beam, 24-25

laser, 25

Peterson, John B. Jr., 20

Pfenninger, Werner, 5, 8, 22

Prandtl, Ludwig, 1

Programs

Aircraft Energy Efficiency (ACEE), 14-16, 21-22,

 26

Advanced Technology Transport, 13 n

Global Atmospheric Sampling Program, 28

Leading Edge Flight Test, 21-26, 29-31

Simulated airline flights, 21

Supersonic Laminar Flow, 32-33, 71-75

Pumping system, 8

Purifoy, Dana, 33 n

Reukauf, Carol A., 33 n

Reynolds number, 2, 3-4, 5, 6, 7-10, 18, 27, 30-31, 34

Reynolds, Osborne, 2

Schlichting, Hermann, 4, 29, 30

Schneider, Edward T., 17 n

Schubauer, G.B., 4

Skramstad, H.K., 4

Stack, John P., 23

Steers, Louis L., 17 n

Stucky, Mark P., 33 n

Suction type

continuous, 5-6, 8, 10, 24-26, 32-33

slotted, 5-6, 8-9, 22-23, 24-26

strips, 7

Swann, Michael R., 17 n

Tetervin, Neal, 5

Tidstrom, K.P., 4

Toll, Thomas A., 13 n

Tollmien, Walter, 4, 29, 30

Trujillo, Bianca M., 17 n

Vendolski, Walter, 20

Visconti, Fioravante, 4, 5

von Doenhoff, Albert E., 4

Wagner, Richard D., 23, 26 n

Wilson, James A., 20, 23

Wind tunnels

Ames 12-Foot Tunnel, 18

Langley Low-Turbulence Pressure Tunnel, 4, 18

Wright, Howard T., 14 n

Young, Ronald, 23, 26 n
lxxxiii 78

About the Author

Albert L. Braslow graduated from the Guggenheim School of Aeronautics, New York University, in 1942

(with a Bachelor of Aeronautical Engineering degree) and continued with post-graduate studies at the University of Virginia, UCLA, George Washington University. and Langley Research Center. During over 50 years

of research in fundamental and applied aircraft and space-vehicle aerodynamics at and for the Langley

Research Center of the National Aeronautics and Space Administration (previously the National Advisory

Committee for Aeronautics), he has published over 60 technical reports, book chapters, and encyclopedia

sections. Included in his research were activities at subsonic, transonic, supersonic, and hypersonic speeds in

the areas of wing design, boundary layers, stability and control, aircraft performance, propulsion aerodynamics, aerodynamic loads and heating, wind-tunnel techniques, advanced technology evaluation and integration,

and development and documentation of space-vehicle design criteria for structures. As an internationallyknown pioneering researcher on laminar-flow control and transition from laminar to turbulent flow, he has

lectured on these disciplines for the American Institute of Aeronautics and Astronautics, the [North Atlantic

Treaty Organization’s] Advisory Group for Aeronautical Research and Development, the University of Texas

(Austin), George Washington University, University of Tennessee Space Institute, University of Virginia,

University of California (Davis), the Continuing Education Program of the University of Kansas for eight

years, and the Lewis and Ames Research Centers of NASA.

Monographs in Aerospace History

This is the thirteenth publication in a new series of special studies prepared under the auspices of the NASA

History Program. The Monographs in Aerospace History series is designed to provide a wide variety of

investigations relative to the history of aeronautics and space. These publications are intended to be tightly

focused in terms of subject, relatively short in length, and reproduced in inexpensive format to allow timely

and broad dissemination to researchers in aerospace history. Suggestions for additional publications in the

Monographs in Aerospace History series are welcome and should be sent to Roger D. Launius, Chief

Historian, Code ZH, National Aeronautics and Space Administration, Washington, DC, 20546. Previous

publications in this series are:

Launius, Roger D. and Gillette, Aaron K. Compilers. Toward a History of the Space Shuttle: An Annotated

Bibliography. (Monographs in Aerospace History, Number 1, 1992)

Launius, Roger D. and Hunley, J. D. Compilers. An Annotated Bibliography of the Apollo Program. (Monographs in Aerospace History, Number 2, 1994)

Launius, Roger D. Apollo: A Retrospective Analysis. (Monographs in Aerospace History, Number 3, 1994)

Hansen, James R. Enchanted Rendezvous: John C. Houbolt and the Genesis of the Lunar-Orbit Rendezvous

Concept. (Monographs in Aerospace History, Number 4, 1995)

Gorn, Michael H. Hugh L. Dryden’s Career in Aviation and Space. (Monographs in Aerospace History, No. 5,

1996).

Powers, Sheryll Goecke. Women in Aeronautical Engineering at the Dryden Flight Research Center, 1946-1994.

(Monographs in Aerospace History, No. 6, 1997).

Portree, David S.F. and Trevino, Robert C. Compilers. Walking to Olympus: A Chronology of Extravehicular

Activity (EVA). (Monographs in Aerospace History, No. 7, 1997). 
lxxxiv 79

Logsdon, John M. Moderator. The Legislative Origins of the National

Aeronautics and Space Act of 1958: Proceedings of an Oral History Workshop.

(Monographs in Aerospace History, No. 8, 1998).

Rumerman, Judy A. Compiler. U.S. Human Spaceflights: A Record of

Achievement, 1961-1998. (Monographs in Aerospace History, No. 9, 1998).

Portree, David S.F. NASA’s Origins and the Dawn of the Space Age.

(Monographs in Aerospace History, No. 10, 1998).

Logsdon, John M. Together in Orbit: The Origins of International

Cooperation in the Space Station Program. (Monographs in Aerospace History,

No. 11, 1998).

Phillips, W. Hewitt. Journey in Aeronautical Research: A Career at NASA

Langley Research Center. (Monographs in Aerospace History, No. 12, 1998).