Some Considerationsin the Designof Light PlaneWings

A. A. Blackstrom

PART IAirfoil Selection

Among the amateur airplanedesigners a great deal of empha-sis is placed on the selection ofan airfoil. To those few peoplewho have done extensive testwork on actual airplanes usingnormal construction methods ithas become very apparent that,unless the airfoil selected isbuilt as accurately as the testmodels, the basic airfoil select-ed is of little consequence.Figure 1 shows the NACA 4416airfoil used on a SchweizerTG-3A sailplane.1 The wingstructure used gives a smoothersurface than that of the normalfabric covered light plane wing.Note that this airfoil has a lowerminimum drag than that obtain-ed on the NACA 632-615 of theRJ-5 sailplane. The RJ-5 winghad a smoothed surface to 70%chord and fabric covered aftsection. This figure also illus-trates the drag of several lightplane wings of normal construe-

FSC. 2^——tion and the effect of cleaningup a Cessna 170-B wing. Itshould also be noted thatNACA's Standard Roughness da-ta does not offer sufficient dragincrease increments to accountfor the difference found betweennormal construction wings andwind tunnel models.

Based on the above considera-tions it becomes apparent thatalmost any reasonable airfoil se-lected can give good perform-ance if well built. That wingscan be built to match the wind

tunnel data is illustrated in Fig-ure 1. Figure 2 shows the con-struction details of this wingas used on the TG-3A. Theriblets are of special importanceas they help eliminate the breakof the fabric where it meets thesolid leading edge structure. Itshould also be remembered thatthe leading edge waviness wasreduced in the method describedby Swartzberg in the September1956, Experimenter* Of course,ideal wings may be built bysmoothing a completely skinnedwing. If a wing has a metalskin, buckling which occurs inflight must be eliminated forthe smoothing to have an ap-preciable effect.

If you are not after ultimateperformance you may not wantto expend much effort on yourwing surfaces and may be look-ing for a method of selecting agood section for standard con-struction accuracy. In a systemwhich I refer to as eyeball se-lection, first decide whether youwant a low speed, short fieldairplane, or one with good cruis-ing and top speed.

If you want good, low-speedperformance then look for anairfoil which has a large lead-ing edge radius and a high cam-ber. If the point of maximumcamber is well forward it willhelp reduce the pitching mo-ments and consequently reducethe tail load required. Sincethis type of airplane will needto be flown near the stall toreach its maximum perform-ance, a wing section with a gen-tle fall off to the curve of liftcoefficient vs. angle of attackshould be chosen. The types oflift coefficient vs. angle of at-tack curves are shown in Figure3. The sections which show agentle fall off in the curve arestalling as the result of turbu-lent flow separating from thetrailing edge and moving slowlyforward3. This action cannotbe eliminated by geometricmeans. The sections which show

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a sharp break in the lift curvesare stalling due to the separationof laminar flow at the leadingedge. This can be partly elimin-ated by a larger leading edgeradius. The above criterionwould indicate the use of an air-foil of the NACA 6400 series orsimilar section. While talkingof low speed airplanes it shouldbe mentioned that the best lowspeed device ever invented is alow wing loading.

If you are designing a ship forhigh speed flight look for a thinsection with low camber. Thistype of section will tend to havea bad stall which may have tofce cured in the wing planformdesign or by using twist. Stallcharacteristics are not quite asimportant in this type of shipas in the slower ships, as it willspend most of its flying time athigh speed. This criterion wouldindicate the use of an airfoil ofthe NACA 14 or 00 series of 9or \2°7c thickness. Rememberthis however, no airplane willbe fast if it is not clean, regard-

less of the airfoil used. As anexample of this Tailwind4 has awing drag coefficient of aproxi-mately 0.0063 and an over-alldrag coefficient of 0.025. Thisshows that on this relativelyclean airplane the wing drag isonly approximately one fourththe total airplane drag.

When checking wind tunnelinformation for wing sections besure to use data which was runat approximately the same Rey-nolds Number as the one thatyou will have on your airplane.This information should be in-

cluded on all airfoil test curvesunder the symbol of R or RN.To calculate the approximateRN of your wing use the equa-tion:

RN = V X L x 10,000 whereV = flight speed in MPH andL, = chord length in feet.The above series of remarks

should serve to illustrate that,if the time spent in analyzingthe fine points of the myriads ofacceptable airfoils was used ina good construction system, theseemingly poorer sections couldout perform the best. If these

good construction systems arenot used then a generalized me-thod of selecting an airfoil forthe type of airplane to be builtwill be as good as an elaborateevaluation of the data.

PART IIWing Planform

The wing planform is general-ly dictated by the type of opera-tion for which the airplane isdesigned. A low speed, shortfield airplane needs to be oflight weight and therefore, ex-ternal bracing is normally usedin conjunction with a rectangu-lar planform. In an airplanedesigned for good cruising per-formance, weight is secondaryto low drag. This class of air-plane will generally have a can-tilever wing of tapered plan-form.

When the type of wing plan-form has been decided upon, aninvestigation of the stall pro-perties associated with the plan-forms should be made. In Fig-ure 4 are illustrated approxi-mate stall patterns for untwistedwings of four different plan-forms*. These show that the

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rectangular and straight leadingedge planforms give a stall pro-gressing from the root, and leav-ing the ailerons in a regionwhere they can still operatewith reasonable effect. The wingwith a swept back leading edgehas a stall which starts near thetip and reduces aileron effect-iveness early in the stall range.It should be stated here thatpoor design and workmanshipcan seriously affect the stallcharacteristics. Such things asskin laps and protuberances atthe leading edge tend to makethe wing stall earlier than ex-pected. As an example of thispoint, the Navion which nor-mally uses stall strips to makethe wing stall in the root sec-tion, did not require the stallstrips when the wing wassmoothed6. The use of stallstrips to cure tip stalling prob-lems is a very poor solution.Stall strips increase the stallspeed by making the best sec-tion of the wing stall early.A much better solution to theproblem is to increase the lead-ing edge radius near the tips(Figure 5). This method hasbeen used successfully on air-planes which previously ex-

perienced severe tip stalling7 orused slots near the tips8.

The shape of the wing tip isa factor which should be gov-erned by the intended use of theairplane. In Figure 6 the char-acteristics of several wing tipsare presented9. It is shown thatthe best tip for low speed isthe square tip (No. 1). Thebest over-all tip is No. 5. Therounded tip (No. 2) is acceptableonly for airplanes which willfly at very low lift coefficients,such as the midget racers. Wingtip plates 10 as shown in Figure7 do little for the aspect ratioand their drag should reduce thetop speed.

The aspect ratio (AR) of thewing should be determined bythe intended speed range of theairplane. Briefly, an airplanewhich must fly with low power,or have long range, should havea relatively high aspect ratio.Airplanes which are intended tofly at high speeds do not needa high aspect ratio. The rea-son for this is that the induceddrag (drag due to lift) equals;

At high lift coefficient (lowspeed) the induced drag can be-come quite high if a low aspectratio is used. At low lift co-efficient (high speed) the induc-ed drag will be small, comparedto the parasite drag of the air-plane, even with a low aspectratio. The low aspect ratio shipsare handicapped in climb per-

formance, but are not materiallyaffected in high speed flight.

In this paper I hope that Ihave emphasized sufficientlythat if the original choices ofairfoil sections and wing plan-forms are reasonable, the per-formance will be dependent up-on the workmanship. Theachievement of high perform-ance is dependent upon attentionto details such as those des-cribed above, as much as by thechoice of an airfoil or wing plan-form.

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1. Raspet, A. "Application ofSailplane A n a l y s i s to A i r-planes." Aeronautical Engi-neering Review. Aug. 1954.

2. Swartzberg, M. B. "Tech-nique of Smoothing LeadingEdges of Airplanes." Experi-menter. Sept. 1956.

3. Cornish, J. "Prevention ofTurbulent Separation by Suc-tion Through A Perforated Sur-face." Aerophysics Dept, ofMississippi State College. Re-search Rept. No. 7. Oct. 1953.

4. Raspet, A. "Flight Measur-ed Aerodynamics of Wittman'sTailwind." Experimenter. Oct.1956.

5. Dwinnell, V. Principles ofAerodynamics. McGraw - HillBook Co., 1949.

6. Raspet, A. AerophysicsDept, of Miss. State College, In-ternal Report. 1955.

7. Raspet, A. and Parker."The Low Drag Sailplane."Soaring. Nov. - Dec. 1954.

8. Sweeney, R. "Report onLearslar MK. II Conversion."Aviation Week. Sept. 10, 1956.

9. Raspet, A. "Control of theBoundary Layer on Sailplanes."Organisation Scientifique E tTechnique Internationale DuVol A Voile O.S.T.I.V. Pub. H.1952.

10. Roemer, S. F. Aerodyna-mic Drag. Published by theauthor — 148 Busteed, MidlandPark, New Jersey. 1951, P. 81.

A"PLAYBOY"and aHOUSEWIFE

Helen MaceStanding on the ramp watch-

ing Hubby take off in our SuperCruiser was leaving me feeling alittle blue, with an empty feel-ing down deep; not for him, ofcourse, but after nearly threeyears of ownership of the swelllittle threeplace, I'd reallygrown quite fond of the Cruiser.As a flying housewife to a crop-duster I've been exposed to, andsoloed, a variety of planes; Lus-combes, Interstates, PT's, BT'sWaco Cabins etc. but none wereas all around practical-wise asthe Cruiser and there he wastaking off to trade it, and totrade it for what? A homebuiltsingle place no less! True, wehad only taken one family crosscountry the past year, and Ihadn't been plugging away atmy flying much, plus being ex-posed to talk like "E.A.A.","Homebuilts", "rugged", "funflying", "economical", "interest-ing", "educational", plus beingaccused of not having flown a"real" airplane, etc. etc. etc., Ireluctantly gave in to the swap.

I had seen a "Flutterbug" ortwo, and casually looked atTrefethen's "Playboy", but notwith an eye to ownership; so itwas with grudgingly renewedinterest that I watched Hubbybring the ex-Ken Smith C-90Playboy into our 1700' one wayduster strip.

It was cute, at that!Well, I'd flown everything

slse he'd brought home, I guess-ed I could try this, so with Pa'surging and my two boys lookingon I climbed in and, after somecockpit and taxi time, squaredoff for the first take off. With"watch the torque", "don't climbher too steep", "glide her at80", etc. ringing in my ears, Ieased on the throttle. The ac-celeration was amazing, torquenot too bad, and I was airbornebefore I knew it.

With a few hundred feet be-low, and the altimeter windingup more than satisfactorily, Idecided to take stock of thething. It sat quite nicely; thoseapparently inadequate wingswere a real novelty, in theirstubby little way. A couple ofexperimental wiggles of the con-trols confirmed the really lightaction and ease the ship couldbe flown with. To a hundred

pound gal this is important!By golly, it was fun! Before

I knew it my gentle turns werebecoming steep ones, and forthe first time in 14 years of fly-ing I felt I was finally in an air-plane that invited to be tried.

For once, Hubby might beright. I would have to try itsome more, although steep turnshad always been my limit beforeand not many of those. I hadonly ridden through a barrel-roll with Pa in the way of acro-batics, and that made me sick.

To make a long story short,I soon had four hours logged inthe Stitts to Pa's one and a half;and amazing as it may seem,in that four hours I had accom-plished snap rolls, snap and a

half of a split-S, loops, hammer-head stalls, (and a few acciden-tal spins).

The Playboy does quite nicelyon the C-90, about 800 ft/minclimb, lots of right rudder here!)Pa said 118 mph at 2350, I get121 (those men!), very close to300' take off run with very littlewind. It decelerates rapidlythrottle off, lands smartly at 58m.p.h. but easily; you have tokeep awake on the roll out, buthere again it's easily controlled.It has furnished a type of flyingto my experience none of theprevious planes I've flown couldgive, and fun!

I can honestly encourage anyof the wives that have a chanceto get in on these little ships,to please do, it's really marvel-ous, like husbands, they mayseem overwhelming at first, butonce you're at the controlsthey're quite docile!

In fact, I think it's terrific,and I feel sorry for the girlswho only get to fly Super Cruis-ers. Flying the homebuilts isfun, they're rugged, fun, eco-nomical ———————! •

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