By P. KENNETH PIERPONT NASA Langley Research Center

Wings of Change RESEARCH PROGRAM

During the past few years NASA has responded to urgings from the aircraft industry and Congress to refocus its efforts toward aeronautical research. It has done so by recent substantial increases in research funding, and by reallocating some ad- ditional research personnel to aeronautics. With these resources NASA has begun several new programs, such as that in airfoil aerodynamics.

Today's NASA Airfoil Research Program re- presents a substantial investment in new technology and calls for a close coupling of two powerful tools- the electronic computer and the wind tunnel. The computer helps analyze and design airfoils, and then the wind tunnel helps experimentally verify selected cases. The approach has not changed greatly from former years but has infinitely more power today. The modern electronic computer, with its speed of computation and its flexibility has enabled theoreticians to tackle problems so complex they were not even considered in the days of airfoil research under the National Advisory Committee for Aeronautics (NACA). Then, the mechanical com- puters available were little more than glorified adding machines. At best, theoretical computations were slow and laborious.

By contrast, the wind tunnel-actually an analog computer-has not changed that milch. But in- strumentation, data acquisition systems, and data reduction methods have progressed rapidly. Today's airfoil research makes extensive use of the computer to make preliminary sorting of many details of

aerodynamics to reduce to more manageable proportions the formidable task of obtaining by wind tunnel test the large amount of detailed data re- quired by the airplane designer.

Organized airfoil research within NACA, NASA's esteemed predecessor, had its beginnings in the early 1920s soon after the formation of the Langley Memorial Aeronautical Laboratory. Until about 1950, when airfoil development was stopped because of priorities for research in other areas, NACA continuously .and vigorously explored the various avenues for airfoil improvement. It gained preeminence in this field with its 4-digit and 5-digit airfoils, the 1-series high-speed airfoils, and the laminar-flow airfoils of which the 6-series are, perhaps, the best known. For example, the NACA 241 2 flies on several of today's light aircraft. NACA 230xx airfoils, or small modifications thereof, lift general aviation's higher performance twins, such as the Cessna Citation and the Beech King Air. The NACA 1-series high-speed airfoils still supply the patterns for many propellers. The 6-series laminar- flow airfoils, or derivatives, were used on most high- speed military aircraft of the 1940s. and since then have been used almost continuously on many other aircraft including most of today's large commercial transports.

NACA had as its underlying philosophy to systematize airfoil design and prediction of their aerodynamic characteristics. This approach gave the aircraft designer ready access to a large bank of

Reprinted from Astronautics & Aeronautics,, October 1975. Copyright 1975 by the American Institute of Aeronautics and Astronautics

and reprinted by permission of the copyright owner.

aerodynamic performance information for selecting an airfoil best suited to his particular airplane and its mission. NACA investigators of the early 1920s first sought to correlate the data available on the large number of airfoils and wing sections already tested. Lack of any real progress prompted them to begin their own systematic investigation of the ef- fects of variables. They made their first break- through by correlating the aerodynamic charac-

scaling techniques, constituted a major advance in airfoil technology.

The new Low Turbulence Pressure Tunnel built in 1939-40 and at first known as the Two-Dimensional Tunnel (TDT) was the mainstay of the experimental research on these new airfoils. In it researchers could test over a very large range of unit Reynolds numbers up to nearly 15 million per foot at low speeds with a high quality airstream. Much of the later research

After a 1 byear lapse bridged on l y by Whitcomb's individual triumph with the super-

critical wing, NASA has picked up the NACA banner to lead the way to semiautomatic

airfoil design

teristics of a consistent family of airfoils on the basis of the amount and position of maximum camber and the maximum thickness of the airfoil shape. Now the airplane designer could choose an airfoil confident in its expected performance on his airplane. Selected examples of these NACA 4-digit airfoils were tested in the Variable Density Wind Tunnel and the results described in the well-known "78 Related Airfoils" report.

Extensions of this research into high speeds for propellers in the early 1930s resulted first in the modified 4-digit airfoils and later in the NACA 1- series of high-speed airfoils. At about the same time, the investigators were developing a new camber line family to increase the maximum lift. The 230 camber line is the best known of this family. Combining these new camber lines with the standard NACA thickness distribution created the NACA 5-digit airfoil family, for example, NACA 230 12.

An important theoretical breakthrough about 1930 permitted calculating the inviscid flow about an arbitrary airfoil for the first time.2 The new tool, coupled with new results on turbulence, transition, and boundary layer growth from the tests in the Low Turbulence Tunnel (LTT) now permitted researchers to mathematically design airfoils with prescribed pressure gradients in order to obtain long runs of laminar flow at design conditions. During the late 1930s and the 1940s NACA designed. tested. and reported the laminar flow airfoils. This family, the result of arduous hand calculations and special

October 1975

results of the NACA airfoil programs went into the book Theo y of Wing Sections.

The end of this productive era came about 1950 because of the more immediate and pressing business of developing the technology for supersonic aircraft. Even at the close new ideas for developing more maximum lift were under preliminary in- vestigation. The "stop" decision shut down the specialized wind tunnels and test apparatus which were dismantled, or converted to other uses. Only the Low Turbulence Pressure Tunnel, finally shut down in 1964, remained in condition for reactivation.

For almost 15 years no systematic airfoil research went on in this country. The resurgence of interest came largely through the persistent, dedicated ef- forts of Richard T. Whitcomb and his research in the Langley 8-foot transonic pressure tunnel, which resulted in the now-famous NASA supercritical airfoil breakthrough in 1965.4 Also, the excellent research elsewhere by men like Pearcys working on the "peaky" airfoils in Great Britain, and F. X. Wortmann6 and his high-performance sailplane airfoils in Germany served to call attention to the need for a new airfoil thrust.

During the years since the first supercritical airfoil, a number of strides in airfoil research have been made (T-1). Airfoil research has now gathered substantial momentum. Formulation of the NASA Airfoil Research Program in the spring of 1973 established a set of technical objectives. The ob- jectives take three distinct directions. Those aimed

21

T-1 MAJOR NASA AIRFOIL MILESTONES

Supercritical airfoil breakthrough Decision to reactivate LRC airfoil facilities Series-1 supercritical airfoilg defined Flight demonstrations of supercritical airfoils

6 x 9-inch T.T. began operations Lockheed multi-element airfoil analysis program

NYU transonic inviscid airfgil program G A(W)-1 low speed airfoil developed Airfoil program integrated High RN tests of supercritical airfoils Rotorcraft airfoil design criteria LRC incompressible and compressible design programs

NYU transonic viscous airfqil program delivered

LTPT recertified to full design pressure 6 x 28-inch T.T. calibrated and operational 8 x 24-inch Cryo. test sectiqn constructed Low speed airfoil family defined First flight GA(W)-1 airfoil . . New design rotorcraft airfoils tested - -

Series-2 supercritical airfoil family initiated

toward software seek the development of mathemat- ical flow models and computational codes, thus per- taining to basic 2-D flow phenomena, such as shock boundary layer interactions; those with which to calculate or predict the complete flow about an arbitrary airfoil in both low-speed and supercritical viscous flows; those used for the direct design of airfoil shapes to meet specific criteria, for instance, the inverse airfoil problem; and those which will ultimately optimize an airfoil for operation over a complete flight regime. Basic experiments and se- lected verification tests play a vital part in this effort.

Those objectives directed toward hardware call for developing test facilities and experimental . tech- niques. They include upgrading existing facilities and acquiring new wind tunnels, developing ap- paratus and test methods to yield higher quality data, and devising new ways of interpreting the test numbers. Searching for better test hardware leads to some combined experimental-theoretical research for answering such sticky questions as "What are the wall boundary effects, and how can they be eliminated?"

P. KENNETH PIERPONT heads the Airfoil Research Section of the Transonic Aerodynamics Branch of the Subsonic-Transonic Aerody- namics Div. He is also program manager for the Langley Airfoil Research Program. He has been a research scientist with NACA and NASA since 1942. Since 1970 he has revitalized the Low Turbulence Pressure Tunnel and developed new transonic airfoil test facilities. He received a BSE from the Univ. of Mich., and MSEfrom UVa.

Objectives pointed toward applications primarily concern the vehicle on which the airfoil will fly. Applications represent the ultimate goal of the whole program. This part of the program includes all the work of design and test of new supercritical and low- speed airfoils, helicopter rotor blade sections, and airfoils for special applications such as RPVs, STOL aircraft, and extremely large aircraft.

Both the Langley and the Ames Research Centers cooperate closely on the airfoil program. Langley is the lead center, and therefore supports most of the major research grants and contracts for both the development of the theoretical tools to supplement its in-house theoretical investigations, and acquisition of supplen~ental or unique experimental

F-1 AIRFOIL OPTIMIZATION-MINIMIZING PRESSURE DRAG At M = 0.8, zero lift. Cp' i s critical pressure coefficient at M = 1, Cdp, pressure drag coefficient.

INITIAL ARBITRARY AIRFOIL

FINAL AIRFOIL

PERCENT CHORD, i l c x 1M

data. Langley also has responsibility for developing the principal wind tunnels for measuring the aerodynamic characteristics of the representative airfoil families. The Ames Research Center con- centrates on computational aerodynan~ics with limited in-house experiments to verify the theoretical and design computer codes it develops. Ames has as a primary goal developing "optimization" methods for designing high performance airfoils with a minimum of user intervention.

Several theoretical methods and computer codes are in hand for analyzing the flow about a given a i r f ~ i l , ' , ~ , ~ and for optimizing airfoils for a range of conditions. Those tools are not yet fully automated, so their use requires a highly skilled aerodynaniicist. Also, experiments must define the limits of ap- plicability of the design method or "fine tune" a particular airfoil. The theoretical analysis, basically consists of calculating pressure distributions around arbitrary airfoils including viscous effects. The inverse problem is to calculate an airfoil from a desired pressure distribution, or other criteria. With today's computers an experienced aerodynamicist can design a near optimum airfoil by an iterative,

Astronautics & Aeronautics

REYNOLDS NUMBER EFFECTS

REYNOLDS NUMBER, R x 10-6

A LOW SPEED, M % 0.2

- REYNOLDS NUMBER. R =

0 50 100

PERCENT CHORD, xlc x 100

B TRANSONIC, Mz0.8,

ANGLE OF AlTACK, a = 1 DEG

F-3 Langley air fo i l research facil i t ies include A tne 6 x 9-in. T ranson~c Tunnel. B Low Turbulence Pressure Tunnel, C 6 x 28-in T ranson~c Tunnel test section, and D the 8 x 24-in Cryogen~c Tunnel Test s e c t ~ o n

October 19 75 23

intuitive approach which produces high performance over a broad range of operating conditions.

The technique has proved its validity for several cases. as in the development of the GA(W)-1 airfoil by Whitcomb and his associate^.'^ This general aviation airfoil met all four requirements set for it:

Cruise drag approximately the same as for the airfoil it replaced.

Climb lift-to-drag ratio increase of 50% to improve en gine-out safety.

Unflapped airfoil maximum lift increase of 30%.

Stall behavior which would be soft and con- sistent. Whitcomb designed the airfoil entirely on the computer. but had to apply all the skills he had acquired in his supercritical research, and carefully evaluate all flight conditions at each step of the 17 iterations computed. No further changes were necessary after the airfoil had been committed to wind-tunnel verification.

Recently optimizing airfoils entirely on the computer has had some early success.' ,' As an example. a nonlifting airfoil (F-1) operating at transonic speeds in an inviscid flow has been op- timized on the computer to minimize the wave drag. At present the method can optimize only at a single point. Hence it cannot handle problems as complex as that of designing the GA(W)-1 airfoil.

The theoretical program has the remaining goals of rationalizing and automating design so that it requires much less aerodynamic skill and experience on the part of the designer, and defining the range of applicability and extending that range to cQver .nost possible applications. Alongside work on these goals effort continues to improve both the theory and the computational schemes so as to simplify the methods and reduce costs. Experiments document a limited number of nearly optimum airfoils which occupy key points in a matrix of advanced airfoils.

F-4 AIRFOIL REQUIREMENTS AND CAPABILITIES MAP

1w 1 r I r 24IN. CRYOGENIC

- g 70 C

= m

5 '9

WINDTUNNEL 2 0

> C

REOUIREMENTS

1 / I / I I I I I 1 4 6 8 1 .0 1.5

MACH NUMBER. M

Inadequate hardware-tunnels that cannot produce high enough Reynolds numbers-can give seriously misleading results. At low subsonic speeds they will not reach the true maximum lift of an airfoil (F-2A). A t supercritical speeds the pressure distribution and, by inference, the shock wave location, aerodynamic loads, and the total aerodynamic forces will differ from those that would be experienced in flight (F-2B). Use of inaccurate instrumentation or inadequate or improper testing techniques can result in different but equally disturbing data. Therefore, most of NASA's airfoil facilities have been, or are being equipped with variable-capacitance transducers.' These trans- ducers measure pressure to a quarter percent ac- curacy over a wide range of pressures. They are an absolute necessity when testing a model at speeds from low subsonic to transonic and from low to high Reynolds numbers in the same wind tunnel.

The agency has set out to get the dedicated wind tunnels needed for today's new technology airfoil research. Now, after about three years of refur- bishing and upgrading the research equipment and recertifying the pressure vessel, the 3-by-7.5-ft Low Turbulence Pressure Tunnel (F-3B) at Langley is again in full operation. The tunnel is used to measure the low speed aerodynamic characteristics of the new low-speed and supercritical airfoils, and to sort out and evaluate the Reynolds number ef- fects. For very-high-lift research the tunnel needs further upgrading.

Recently Langley has developed and brought the 6-by-28-in. transonic tunnel (F-3C) into full operation. In this facility we can independently evaluate transonic Mach number and Reynolds number effects up to about 12-16 million on supercritical airfoil candidates and rotorcraft blade sections. This tunnel is the mainstay of the transonic program.

A new two-dimensional test section (F-3D) for the Langley 1/3-m transonic cryogenic tunnel will supplement the 6-by-28-in. tunnel by extending the Reynolds number limits to 50 million or more.' It is now being installed. It will serve primarily as a Reynolds number "topping" facility. For example, very thick airfoils are highly sensitive to Reynolds number, so are prime candidates for testing in this wind tunnel. The cryogenic tunnel also can handle work on transonic fighter and some transport airfoils when full scale simulation is required (F-4). F-4 also displays the scope of the other tunnels.

The applications program brings all the developing theoretical and experimental technology into focus on supplying the needs of the user, the aircraft designer. Here, the NASA airfoil research prograni must ultimately prove its success. Airfoils see supercritical. low-speed, rotorcraft, and special

Astronautics &Aeronautics

F-5 AIRFOIL APPLICATIONS AND GAINS

I SUPERCRITICAL

NACA /

/-------- - NASA ---- c-----

I

M GAIN: EFFICIENCY

SPECIAL

NASA

. .

supercritical airfoils that perform well "on design." The design methods are long, require extensive

LIFT COEFFICIENT. CI GAIN - ENDURANCE

T-2 CURRENT AIRFOIL RESEARCH SUPPORT GRANTS AND CONTRACTS

human decision based upon broad experience, and the resulting airfoils need substantial refinement to perform adequately "off design."

Based on the growing fund of both experimental and theoretical expertise, the criteria for a second family of improved airfoils were formulated, and new 10% and 14% thick airfoils designed and tested in the Langley 8-ft transonic wind tunnel. The design variations for this second family cover a wide scope (F-6); some airfoils in addition to those marked in F- 6 will be verified experimentally. If all of these airfoil designs prove out. NASA will have reached one of its goals. With only a few test points, many airfoils can be designed with confidence that their performance

LOW SPEED

f - NASA .

LIFT COEFFICIENT. CI GAIN. EFFICIENCY. SAFETY

ROTORCRAFT

M GAIN. EFFICIENCY. PAYLOAD

application (E-51, and research must aim toward those end uses.

Whitcomb produced the first NASA shockless supercritical airfoil long before solutions to the basic transonic flow equations were known. Therefore, lacking quantified theoretical results he used the wind tunnel as an analog computer. He concentrated on understanding the aerodynamic effects of changing the shape of airfoils of approximately 10% thickness over a broad range of supercritical con- ditions. He developed equations defining a limited first family of supercritical airfoils. More im- portantly, design criteria for such airfoils began to emerge and are now quite well understood.' These criteria, together with the excellent transonic analysis codes now available, permit designing

Subject Investigator Organization

Computational : Navier-Stokes anal- Thompson Miss. St. U.

, ysis, multi-element airfoils

lmproved 2-D tran- Garabedian NYU sonic analysis methods

Transonic airfoil Carlson Tex. A. & M. design methods

Shock boundary layer Sichel U. of Mich. interactions

Low-speed separated Zumwalt U. of Wichita f lowlstal l analysis

Improved low-speed Goradia Lockheed-Ga. drag analysis

Viscous trailing edge Melnik Grumman interactions

Incompressible de- Ormsby U. of Illinois sign and analysis

Design and analysis Murman Flow Res Inc. methods for tran- sonic flow

Experimental : Flow measurements Wentz U. of Wichita with massive separation

Design and test of Wentz U. of Wichita G.A. airfoil flaps

Flight experiments Gregorek Ohio St. U. with GA(W)-2 airfoil

T-3 PROPOSED GENERAL AVIATION LOW-SPEED AIRFOIL DESIGN AND DEVELOPMENT SERVICE

Resource for information on new low-speed airfoils Analysis and assessment of airfoils specified by in- dustry 2-D design of low-speed airfoils optimized for specific requirements 2-D design and analysis of high-lift systems and lateral controls Technical assistance in obtaining tests of airfoils developed for specific applications Consultation services

October 1975

F-6 NASA SUPERCRITICAL AIRFOIL FAMILY

: C

'01 / 0 BEING DESIGNED

TESTS COMPLETED

I I I I I I 0 2 4 6 8 1 0 1 2

ODlGN LIFT COEFFICIENT. CL drip

F-7 NASA LOW-SPEED AIRFOIL RESEARCH . 24 r

0 TESTSSCHEDVLED

TESTSCOMPLETED - OESlON LlFl COEFFICIENT. CI, -

will be as predicted. Not everyone will experience equal success. That must await improved analysis methods which will overcome present limit a t' lons. and further developnlent of a t least sen~iautoniatic computer codes.

The low-speed. or general aviation, airfoil

F-8 ROTORCRAFT AIRFOIL GAINS Fora gunship rotor velocity of 150 kt.

\

1.2 \

6 1.0

NEW AIRFOIL

IL LL .6 W ORIGINAL AIRFOIL

t .2

0 0.95 RADIUS

-.2I 1 I I I I .4 .5 .6 .7 .8 .9

MACH NUMBER, Mm

program began during the early stages of the Ad- vanced Technology Light Twin. or ATLIT airplane shown on the cover. With the highly successful GA(W)-1 airfoil designed for the ATLIT as the focal point. NASA has designed a modest family of airfoils for low speeds (F-7). Several of the airfoils have conlpleted test in the Langley low turbulence pressure tunnel, and others will enter test in the near future. In addition. experimental data are being obtained on representative airfoils with analytically designed Fowler flaps and simple trailing edge control surfaces. The 1395 thick GA(W)-2 has proved superior to the GA(W)-1 in some respects. and will begin flight-test evaluation this fall on a light single- engine aircraft. Once again testing the analytically predicted behavior against experiment will expand confidence in the computer codes.

The next phase in the low-speed airfoil program will focus on thin airfoils, which are sensitive to laminar separation near the thin leading edge, and consequently niav stall abruptly. and on thick airfoils, which are subject to excessive turbulent separation. Because over most airfoils a t high angles of attack near the stall flow separates on the upper surface. special effort is going into obtaining data with which to mathematically model the separated flow region." Finally the program will try to conlplete the last long lap to senliauton~atic design.

The rotorcraft airfoil program began on a dif- ferent footing than the others. because rotor structure and control constrain the blade elements to zero. or near-zero pitching moment coefficients. Also. the broad spectrum of angles of attack and Mach number the airfoil experiences in a single revolution niakes setting specific airfoil design criteria difficult. Consequently, research must first

BASIC AlRFOlL 3

ADVANCED AIRFOIL

Astronautics &Aeronautics

sort out, and understand which improved aerodynamic characteristics would benefit rotor performance. This has been done for several actual helicopters. Now we are trying to establish through conlputational parametric analysis which airfoil features offer the greatest promise for performance improvement.'

As an example of a rotor problem. a station near the tip of the rotor will suffer drag divergence through a portion of each rotation (F-8). A plot will show how much of the disk area operates above drag divergence and adds to the power needed (F-8). In a particular case, a new airfoil compared to the NACA 001 2 (F-8) in use reduced the disk area above diver- gence by 30% and the power needed by 10%.

The rotorcraft airfoil operates in a dynamic en- vironment. Mach number and attitude .of the airfoil continuously change through a single rotation. Little information exists concerning the change in dynamic airfoil characteristics with airfoil shape.

The rotorcraft airfoil program will establish a sound data base for several reference airfoils. Several airfoils with improved performance are being analytically evaluated and experimentally tested. Several more are being designed for test in the near future. And NASA plans to develop test hardware and techniques for comparing the effects of airfoil shape changes on its dynamic characteristics.

Several special appl ica t ions have un ique requirements. These include various types of remotely piloted vehicles, ranging from very-low- altitude drones flying at transonic Mach numbers to subsonic high altitude air data samplers uhich require low Reynolds number airfoils; con~petition aircraft, such as sailplanes with their laminar flow airfoils: STOL aircraft, which operate most of the time at high-lift coefficients; and very large cargo airplanes, which may require thick airfoils for cargo storage. Although NASA does not devote significant effort to any of these applications. it continues to develop the potential to do so.

During the decade since the birth of the NASA supercritical airfoil, the number of research scientists within NASA working on airfoils has increased from about three to more than thirty. Annual funding for the program has increased from a few thousand dollars for materials and models to nearly three-quarters of a n~illion dollars for building new equipment and supporting many outside research scientists under grants and contracts (T-2).

NASA is now embarking on bringing the latest airfoil technology rapidly and directly to users who cannot design their own airfoils. NASA intends to make the latest and best airfoil analysis and design available to U.S. industry through a proposed yeneral aviation low-speed airfoil design and development service (T-3) run by a highly skilled

contractor. NASA will consider its efforts rewarded if the new airfoil technology being developed finds its way rapidly into the international marketplace aboard U.S.-manufactured aircraft.

References 1. Jacobs, Eastman N., Ward, Kenneth E., and

Pinkerton, Robert M., "The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel." NACA Report No. 460. 1933.

2. Theodorsen, T., "Theory of Wing Sections of Ar- bitrary Shape." NACA Report No. 41 1. 1931.

3. Abbott, Ira H., and von Doenhoff, Albert E., Theor?, of Wing Sections Including Summary of Airfoils, Dover Publications, 1949.

4. Whitcomb. Richard T., and Clark, Larry R.. "An Airfoil Shape for Efficient Flight at Supercritical Mach Numbers," NASA TM X-1109, May 1965.

5. Pearcey, H. H., "The Aerodynamic Design of Section Shapes for Swept Wings," Proceedings of the Second Interizational Congress in the Aeronautical Sciences. Vol. 3. Zurich, Sept. 12-16. 1960.

6. Althaus. D., Stuttgarter Profilkatalog I , Inst. of Aero. and Gasdynamic. Univ. of Stuttgart, 1972.

7. Stevens, W. A., Goradia. S. H., and Braden. J. A., "Mathen~atical Model for Two-Dimensional Multi- Component Airfoils in Viscous Flow," NASA CR-1843, 1971.

8. Bauer, Frances, Garabedian, Paul. Korn, David. and Jameson. Antony, "Supercritical Wing Sections 11." Lecture Notes in Economics and Mathematical Systems. Vol. 108 Control Theor?,, Springer-Verlag, 1975.

9. Barger, Raymond L., and Brooks, Cuyler W., Jr.. "A Streamline Curvature Method for Design of Supercritical ' and Subcritical Airfoils," NASA TN D-7770, Sept. 1974.

'

10. McGhee, Robert J., and Beasley, William J., "Low Speed Aerodynamic Characteristics of a 17-Percent Thick Airfoil Section Designed for General Aviation Ap- plications," NASA TN D-7428, Dec. 1973.

11. Hicks. Raymond M., Murman, Earll M.. and Vanderplaats, Garret N., "An Assessment of Airfoil Design by Numerical Optimization," NASA TM X-3092. July, 1972.

12. Hicks, Raymond M.. and Vanderplaats, Garret N., "Application of Numerical Optimization to the Design of Low Speed Airfoils," NASA TM X-3213. March, 1975.

13. Bynum, D. S., Ledford. R. L., and Smotherman, W. E.. "Wind Tunnel Pressure Measuring Techniques," AGARD-AG-145-70, 1970.

14. Ladson, Charles L., "Description and Calibration of the Langley 6- by 28-Inch Transonic Tunnel." Prospective NASA TN D.

15. Polhamus, E. C., Kilgore, R. A.. Adcock. J. B., and Ray. E. J., "The Langley Cryogenic High Reynolds Number Wind-Tunnel Program," Astronautics & Aeronautics, Oct., 1974, pp. 30-40.

16. Whitcomb, Richard T., "Review of NASA critical Airfoils," Ninth Congress of the International Council of the Aeronautical Sciences, Haifa. Aug. 25-30, 1974.

17. Seetharam, H. C., and Wentz, William H.. Jr., "Experimental Studies of Flow Separation and Stalling on a Two-Dimensional Airfoil at Low Speeds," NASA CR- 2560, July, 1975.

18. Bingham, Gene J., "An Analytical Evaluation of Airfoil Sections for Helicopter Rotor Applications," NASA TN D-7796,1975.

October 1975