THE ULTIMATE AIRFOIL FOR SPORT AIRCRAFT By Michael C. Myal (EAA 7978) 28763 Cunningham Warren, Michigan 48092
SELECTION OF AND recommendations for light aircraft airfoils have been the subject of many articles in
SPORT AVIATION over the past twenty some years. A recent phone call to Jack Cox at EAA Headquarters confirmed that an update on new developments might prove interesting to fellow EAA members. In looking back over past issues, it is a reflection of our maturity that we literally have progressed from yesterday's ". . . most any airfoil constructed satisfactorily will fly well..." to today's penultimate answer " . . . based on your design parameters for cruise, climb and landing, the computer program developed . . ." i————————
The author of "Megatrends", John Naisbitt, observes that our technological inventions become accepted and commonplace through first use as novelties or toys. There is much truth to that conclusion when we look back at barnstorming before the advent of commercial flying, introductions of horseless carriages, radio, television and project ourselves beyond today's 48k home computer to year 2001.
Low Speed/High Lift Stuff (Vso = 19.75 V W/S* C L max The lowest possible flying speed of an aircraft equates to the need for an airfoil with a high maximum lift coefficient, CL max or a large wing area or the appropriate combination of the two. Early airfoils were thin and highly cambered, producing good lift but excessive drag. Structural and speed requirements evolved thick airfoils with less camber. Flaps (plain, split, Zap, slotted, Krueger, Fowler, blown, etc.) were then invented to increase the lift of thick shapes to those of the early days. Lift coefficients of the plain vanilla airfoil grew from about 1.5 with no flaps to 3.2 and better with these high-lift devices, ultimately making it possible for the Jet Age to come to your home town. Today, there are a number of high-lift, unflapped airfoils available to the amateur designer, whereas in the 30's the 23015 was about the only choice if high cruise speed and low drag were a specification. High Speed/Low Drag Stuff Once the principle of metal construction was accepted in lieu of fabric and flying wires, research was focused on the drag of the wing. Metal cantilevers of corrugated aluminum soon gave way to even more efficient stressed skin surfaces. Rivets became invisible; sanding fillers were one solution. Conditions of airstream flow described as
laminar and turbulent were discovered which explained differences in interaction between the wing and the supporting air. Attention was diverted to a new problem dealing with Mach numbers and the sound barrier. Work on subsonic laminar flow problems essentially stopped while the challenges of the Jet Age were being answered. 32 AUGUST 1983
FIGURE 1 — The shape of United States airfoil development over some 40 years. Compare Eagle I to the Jacobs (P-51) section.
In a very real sense sport aviation is part of this natural progression from uniqueness to utility. The commercial aircraft designs of tomorrow (beginning with the Lear Fan) will owe much of their parentage to the fun ships of today. The link from sport flying to seat cents per mile is nowhere more evident than in the history of sailplane airfoil development. It was the European sailplane builders who prompted fresh approaches to the study of subsonic lift and drag and the use of analytic methods. The scientific community responded and the name of Wortmann soon became known in the winner's circle. Modern airfoil concepts have their roots in these endeavors. The Computer
In his SPORT AVIATION article of June 1978, Dr. Robert T. Jones chronicled the achievements of a number of airfoil pioneers. Each of these men also contributed much knowledge to the solid foundation which is the basis for airfoil design today. We are at the point of learning
where theory and experience are shadows of each other; the theory can predict results while experience further verifies the theory. So where does the computer come in? It is merely the "number cruncher" which does all the complex and repetitive computations in a precise, accurate, speed of light manner, in accordance with human instructions based on the sum total of established theory and experience. This tool is as good as its program of instruction and, in the case of airfoil design, it is better than anything else around! The Program
For those of you who are interested yet wonder what a computer program looks like, I have included a few lines of instruction from the NASA program written in
Fortran. The airfoil design program in use today totals some 2600 lines of such program statements. Use of this program is not the beginning nor the end of the solution . . . for intelligence must be applied in the form of specific instructions to the program. Airfoil Engineering Here is where experience enters the picture! Imagine as you begin the design of a new airfoil at a computer console the unfolding pattern of pressure distribution lines, as recalled by you from some existing airfoil plot. Now begin to adjust these lines via keyboard commands to maintain laminar flow along the chord to the maximum extent possible, considering local airflow velocities. Meanwhile, also vary the angle of attack while making adjustments to minimize separation. As you change and manipulate these variables, the computer program maintains a continuing process of defining the upper and lower surfaces of an airfoil meeting your parameters. The individual's skills in aeronautical engineering, fluid dynamics and computer programming come together intuitively to produce a product which in all respects surpasses the former cut and try wind tunnel methods! Since the majority of amateur designers/homebuilders do not have the financial resources or ready access to a wind tunnel, it is no big loss to realize that this airfoil design process is also out of reach for the majority of us. However . . .!
A Solution! Do you want an airfoil that has high lift and very low drag with a minimum amount of wing torsion (pitching moment)? Also, do you want an airfoil which will stall gently? Well, look no further because courtesy of NASA
FIGURE 2 — Overview of lift and drag characteristics for selected airfoils at available Reynolds Numbers.
SPORT AVIATION 33
6 f = 0°.
FIGURE 3 — The Eagle I (NLF(1)-0215F) section.
I'PPEB S U B F 4 C E »/C Z/C .001)17 .00°09 .01947 .0?00* .03027 .05527 .01.130 .05*69 .05201
.13*35 .17050 .30751 ,?*7?n .?S>>9* .33737 .3770?
.08175 .09019 .09761
.*'P64 .515?' .56?*7 .f 1010 .f 575? . 70*0"
x/C .00000 .002*5 .01099 .0259? .0*653 .07?*? .1032* .13854 .17788 .22073
.10389 .10887 .112*0 .11428 .11*27 .11219 .107P4
.36*68 .*1576 .46731 .51867 .56920
.79?Ct .S3??? .«»-°02
.057*6 .0*8** .039S3
.90193 .930*4 .95409 .972P5 .9»710 .99*- 5? l.COCPO
.03175 .02*28 .01737 .01082 .OC507 .00126 .00000
.7161* .766*5 .81565 .8*198 .90359 .•5386? .96588 .9850*
-.OC006 -.0070* -.01211 -.01656 -.0205? -.02399
|J | T R A 2 | | A D S Z | |FXPR| | PAN | [FLAp| | ENDEJ
-.03166 -.0333* -.03456 -.03531 -.03554 -.03519 -.03415 -.03225 -.02925 -.02441 -.01663 -.00705 .00167 .0080* .01155 .01198 .0099C .00655 .00323 .OOCSf .00000
FIGURE 4 — Eagle I coordinates. Note the high degree of coordinate accuracy generated by the computer in order to maintain laminar flow.
FIGURE 5 — Algorithm of the computer program, the path and operations to be followed during program execution.
research, you now can have all of the above! I predict this airfoil will become the decade's workhorse much as the Clark Y did its share for aviation when it
the aircraft was never built, the legacy of this work is free for all to use! The design parameters include a 15% thickness, a minimal pitching moment coefficient of -.05, and
was needed. In my estimation it's even better than the GA
series covered previously in SPORT AVIATION, but you be the final judge after examining the evidence. First, let's give it a "handle" we can all remember: EAGLE 1. We are already burdened with zip codes, SS #, aircraft N # . . . also because a "handle" sounds far more conversational and friendly than today's fancy airfoil naming conventions (or do you think NLF(1)-0215F is handier?). Eagle 1 was designed by Dan Somers, a research aerodynamicist at the NASA Langley Research Center, author of several technical papers, a sailplane pilot and aviation enthusiast. He has collaborated with Richard
Eppler on airfoil research and used the Eppler Airfoil Design and Analysis Program to develop Eagle 1. This airfoil was designed in 1979 to meet high performance specifications for a single engine project. Although 34 AUGUST 1983
a 25% chord simple flap. Up to 50% of the chord will sustain
laminar flow under smooth surface conditions! The most unusual feature of Eagle 1 is its "split personality". When dirty, this airfoil performs like the best of the GA series. It should be mentioned that the contour promotes, rather than forces, laminar flow. Indiscriminate changes to thickness by proportioning around the mean line will likely adversely affect laminar conditions. If the wing structure requires a different airfoil thickness, it is probably best to
For those EAA members who wish to learn more about
the excellent characteristics of Eagle 1, NASA Technical Paper 1865 is available by mail order from Aerospace Research Applications Center, P.O. Box 647, Indianapolis, IN 46223. I ordered a copy by phone on 317/264-4644. The
price was about $19.00 which was billed later.
PROGRAM PROFILE I N P U T , O U T P U T . T APE 4 , T A P E S . T A P E 61 YF11211.BETAFt1211 DIMENSION IFU21 DIMENSION ANI7.7 AV< 7) OINENS1CN V U 4 ) , A » K E N I 2 0 > , A l ( A U 4 l , C A E < 2 l D I M E N S I O N B E ( 5 1 . illi " U < i l , T C 4 2 > ,'ERI5l,HU"m DIMENSION T N < 5 I . 5 > . C H I 5 . 2 . 1 4 1 , s u n , 2 . 1 4 > , S A O , 2,14i DIMENSION T S T I t l >P(121>,YP(121t,PUFFIlA),AGAMI14},IH21l, COMPION "Him.P 1TI121>>DSI122I>V 2JAB.JST.C«.ETA,A ,BOGEN,DARG,PURES(1)I.FUW(bO,7),RS(60l
COMMON /GP-1K/COK COMMON/PRAI./OIT. 1 »CPV*AlTIt CONHCN/PLTN/MP
,GAMMAI121.2I.AI ) H C A P C I GO 70 11 TPAlTRA2AlFAAGAHABS:STPKFNOE3!IGPf STPDFl 7MPLriAPlM TPF APPPCDCL
FIGURE 6 — A few lines of FORTRAN program statements from the program. Over 2600 lines of such instructions are needed.
Other Airfoils For a comparison let us review the features of some other airfoils:
Whitcomb/McGhee — Shown is the latest 17% NASA
turbulent-flow airfoil (LS(1)-0417) designed in 1981 for general aviation aircraft of conventional metal construction. It has very high lift and a somewhat abrupt stall. This airfoil represents a distinct improvement over the widely published GA series (GA(W)-2 shown) in that it achieves lower drag at climb lift coefficients while featuring a reduced pitching moment coefficient. The blunt trailing edge may present design and construction difficulties for hinged control surfaces.
Peterson/Chen — A NASA 16% general aviation airfoil (GA(PC)-l) of 1978 vintage, incorporating a flap for control of wing camber at climb and cruise at same deck angle (minimal fuselage drag). Non-laminar in concept, intended to be used with commercial quality metal construction. Stall is of trailing edge type and considered to be abrupt. Jacobs — Identified by R. T. Jones as a significant contributor to the understanding of laminar airfoil design, research which culminated in the famous P-51. Very low drag at high Reynolds numbers evidenced by the "bucket" shaped drag curve. The NACA 6 series generally is sensitive to surface roughness and stall abruptly. 23015 — Still the most developed and tested airfoil around having high lift, near zero pitching moment, fairly low drag and an abrupt stall characteristic. An excellent reference airfoil for understanding and predicting effects
of various flaps, slats, etc. on new airfoils. I flew a Minicab
in Canada years ago which was a baby in stalls because of the wing's engineered washout. I'm satisfied its stall problem can be eliminated through careful wing design. Davis — The mystery airfoil of World War II, used on
the Consolidated B-24 heavy bomber. Derived by unique mathematical formulae, the Davis airfoil claimed high load/range efficiency. The contour appears to resemble the NACA 4415 section. Included here as a curio. Wind tunnel data missing from normal references; does anyone have
this data to share for the historical record?
FIGURE 7 — Dan Somers, Designer of "Eagle I" (NLF(1)-0215F) and frequent lecturer at Oshkosh forums.
References 1. Somers, Dan M., "Design and Experimental Results For A Flapped Natural Laminar Flow Airfoil For General Aviation Applications", NASA Technical Paper 1865, NASA Langley Research Center 1981, (EAGLE 1). 2. McGhee, Robert J. and Beasley, William D., "Wind Tunnel Results For A Modified 17% Thick Low Speed Airfoil Section", NASA Technical Paper 1919, NASA Langley Research Center, 1981, (LS(1)-0417 Mod). 3. McGhee, Robert J., Beasley, William D. and Somers, Dan M., "Low Speed Aerodynamic Characteristics of a 13% Thick Airfoil Section Designed For General Aviation Applications", NASA TM X-72697 1875, (GA(W)2). 4. Barnwell, Richard W. et al, "Low Speed Aerodynamic Characteristics of a 16% Thick Variable Geometry Airfoil Designed For General Aviation Applications", NASA Technical Paper 1324, NASA Langley Research Center, December 1978, (GA(PC)-l). 5. Becar, Noel, "Selecting A Suitable Airfoil", SPORT AVIATION, EAA, June 1962, (NACA 63g 615). 6. Jones, Robert T., "Highlights From the History of Airfoil Development", SPORT A VIA TION, EAA, June 1978. 7. Abbott, Ira H. and Van Doenhoff, Albert E., "Theory of Wing Sections", Dover Publications, June 1958, (NACA 23015). 8. Davis, David R., "Fluid Foil", United States Patent 2,281,272 (filed May 9, 1938, granted April 28, 1942). 9. McCormick, Barnes W., "Aerodynamics, Aeronautics and Flight Mechanics", John Wiley and Sons, New York, 1979 (low speed airfoil data, winglets, etc. covered in this fresh textbook). NASA reports are available from: National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. SPORT AVIATION 35