6

Some Considerations

in the Design of Light Plane Wings A. A. Blackstrom

PARTI Airfoil Selection Among the amateur airplane

designers a great deal of emphasis is placed on the selection of

an airfoil. To those few people who have done extensive test work on actual airplanes using normal construction methods it has become very apparent that, unless the airfoil selected is built as accurately as the test

models, the basic airfoil selected

is

of

little

consequence.

Figure 1 shows the NACA 4416 airfoil used on a Schweizer TG-3A

sailplane.

1

The wing

structure used gives a smoother surface than that of the normal fabric covered light plane wing.

Note that this airfoil has a lower minimum drag than that obtained on the NACA 632-615 of the

RJ-5 sailplane. The RJ-5 wing had a smoothed surface to 70% chord and fabric covered aft

section. This figure also illustrates the drag of several light plane wings of normal construe-

FSC. 2 ^—— tion and the effect of cleaning up a Cessna 170-B wing. It should also be noted that NACA's Standard Roughness data does not offer sufficient drag

increase increments to account for the difference found between normal construction wings and wind tunnel models.

Based on the above considerations it becomes apparent that almost any reasonable airfoil selected can give good performance if well built. That wings can be built to match the wind

tunnel data is illustrated in Figure 1. Figure 2 shows the construction details of this wing as used on the TG-3A. The

riblets are of special importance as they help eliminate the break

of the fabric where it meets the solid leading edge structure. It should also be remembered that the leading edge waviness was reduced in the method described by Swartzberg in the September 1956, Experimenter* Of course, ideal wings may be built by smoothing a completely skinned

wing.

If a wing has a metal

skin, buckling which occurs in

flight must be eliminated for the smoothing to have an appreciable effect.

If you are not after ultimate performance you may not want to expend much effort on your wing surfaces and may be looking for a method of selecting a good section for standard construction accuracy. In a system which I refer to as eyeball selection, first decide whether you want a low speed, short field airplane, or one with good cruising and top speed. If you want good, low-speed performance then look for an airfoil which has a large leading edge radius and a high camber. If the point of maximum camber is well forward it will help reduce the pitching moments and consequently reduce the tail load required. Since this type of airplane will need to be flown near the stall to reach its maximum performance, a wing section with a gentle fall off to the curve of lift coefficient vs. angle of attack should be chosen. The types of lift coefficient vs. angle of attack curves are shown in Figure 3. The sections which show a gentle fall off in the curve are stalling as the result of turbulent flow separating from the trailing edge and moving slowly forward 3 . This action cannot be eliminated by geometric means. The sections which show

7

a sharp break in the lift curves are stalling due to the separation of laminar flow at the leading edge. This can be partly eliminated by a larger leading edge radius. The above criterion would indicate the use of an airfoil of the NACA 6400 series or

similar section. While talking of low speed airplanes it should be mentioned that the best low

speed device ever invented is a

low wing loading. If you are designing a ship for high speed flight look for a thin section with low camber. This type of section will tend to have

a bad stall which may have to

fce cured in the wing planform design or by using twist. Stall characteristics are not quite as important in this type of ship as in the slower ships, as it will spend most of its flying time at high speed. This criterion would indicate the use of an airfoil of the NACA 14 or 00 series of 9 or \2°7c thickness. Remember

this however, no airplane will

be fast if it is not clean, regard-

less of the airfoil used. As an example of this Tailwind4 has a wing drag coefficient of aproximately 0.0063 and an over-all drag coefficient of 0.025. This

shows that on this relatively

clean airplane the wing drag is only approximately one fourth the total airplane drag. When checking wind tunnel information for wing sections be

sure to use data which was run

at approximately the same Reynolds Number as the one that you will have on your airplane. This information should be in-

cluded on all airfoil test curves under the symbol of R or RN. To calculate the approximate RN of your wing use the equation: RN = V X L x 10,000 where V = flight speed in MPH and L, = chord length in feet. The above series of remarks should serve to illustrate that, if the time spent in analyzing the fine points of the myriads of acceptable airfoils was used in a good construction system, the seemingly poorer sections could out perform the best. If these

good construction systems are

not used then a generalized method of selecting an airfoil for the type of airplane to be built

will be as good as an elaborate evaluation of the data. PART II

Wing Planform The wing planform is generally dictated by the type of operation for which the airplane is designed. A low speed, short field airplane needs to be of

light weight and therefore, external bracing is normally used in conjunction with a rectangu-

lar planform. In an airplane designed for good cruising performance, weight is secondary

to low drag. This class of airplane will generally have a cantilever wing of tapered plan-

form. When the type of wing planform has been decided upon, an investigation of the stall properties associated with the planforms should be made. In Figure 4 are illustrated approximate stall patterns for untwisted

wings of four different planforms*. These show that the

8

perienced severe tip stalling7 or used slots near the tips8. rectangular and straight leading

edge planforms give a stall progressing from the root, and leaving the ailerons in a region where they can still operate with reasonable effect. The wing with a swept back leading edge has a stall which starts near the tip and reduces aileron effectiveness early in the stall range. It should be stated here that poor design and workmanship can seriously affect the stall characteristics. Such things as skin laps and protuberances at the leading edge tend to make the wing stall earlier than expected. As an example of this point, the Navion which normally uses stall strips to make the wing stall in the root section, did not require the stall strips when the wing was smoothed6. The use of stall strips to cure tip stalling problems is a very poor solution. Stall strips increase the stall speed by making the best section of the wing stall early. A much better solution to the problem is to increase the leading edge radius near the tips (Figure 5). This method has been used successfully on airplanes which previously ex-

The shape of the wing tip is a factor which should be governed by the intended use of the airplane. In Figure 6 the characteristics of several wing tips are presented9. It is shown that the best tip for low speed is

the square tip (No. 1). The best over-all tip is No. 5. The

rounded tip (No. 2) is acceptable only for airplanes which will fly at very low lift coefficients, such as the midget racers. Wing

tip plates 10 as shown in Figure 7 do little for the aspect ratio

and their drag should reduce the

top speed.

The aspect ratio (AR) of the

wing should be determined by

the intended speed range of the airplane. Briefly, an airplane

which must fly with low power,

or have long range, should have a relatively high aspect ratio.

Airplanes which are intended to

fly at high speeds do not need a high aspect ratio. The rea-

son for this is that the induced

drag (drag due to lift) equals;

At high lift coefficient (low speed) the induced drag can become quite high if a low aspect ratio is used. At low lift coefficient (high speed) the induced drag will be small, compared to the parasite drag of the airplane, even with a low aspect

ratio. The low aspect ratio ships are handicapped in climb per-

formance, but are not materially affected in high speed flight. In this paper I hope that I have emphasized sufficiently that if the original choices of airfoil sections and wing planforms are reasonable, the performance will be dependent upon the workmanship. The achievement of high performance is dependent upon attention to details such as those described above, as much as by the choice of an airfoil or wing planform.

9

A "PLAYBOY" and a HOUSEWIFE Helen Mace Standing on the ramp watch-

ing Hubby take off in our Super Cruiser was leaving me feeling a little blue, with an empty feel-

ing down deep; not for him, of

course, but after nearly three years of ownership of the swell

little

threeplace,

I'd

really

grown quite fond of the Cruiser.

As a flying housewife to a cropduster I've been exposed to, and soloed, a variety of planes; Luscombes, Interstates, PT's, BT's Waco Cabins etc. but none were as all around practical-wise as

the Cruiser and there he was taking off to trade it, and to trade it for what?

A homebuilt

single place no less! True, we had only taken one family cross country the past year, and I

hadn't been plugging away at

my flying much, plus being exposed to talk like "E.A.A.", "Homebuilts", "rugged", "fun flying", "economical", "interesting", "educational", plus being

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

I knew it my gentle turns were becoming steep ones, and for

the first time in 14 years of flying I felt I was finally in an airplane that invited to be tried.

For once, Hubby might be right. I would have to try it some more, although steep turns had always been my limit before and not many of those. I had only ridden through a barrelroll 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 in the Stitts to Pa's one and a half;

and amazing as it may seem, in that four hours I had accomplished snap rolls, snap and a

accused of not having flown a

"real" airplane, etc. etc. etc., I reluctantly gave in to the swap.

I had seen a "Flutterbug" or

two,

and

casually looked at

Trefethen's "Playboy", but not with an eye to ownership; so it

half of a split-S, loops, hammerwas with grudgingly renewed head stalls, (and a few accideninterest that I watched Hubby tal spins). bring the ex-Ken Smith C-90 The Playboy does quite nicely Playboy into our 1700' one way on the C-90, about 800 ft/min duster strip. climb, lots of right rudder here!) It was cute, at that! Pa said 118 mph at 2350, I get Well, I'd flown everything 121 (those men!), very close to slse he'd brought home, I guessed I could try this, so with Pa's 300' take off run with very little wind. It decelerates rapidly urging and my two boys looking

on I climbed in and, after some

m.p.h. but easily; you have to

her too steep", "glide her at

It has furnished a type of flying to my experience none of the previous planes I've flown could give, and fun!

off for the first take off. With "watch the torque", "don't climb 1. Raspet, A. "Application of

Sailplane A n a l y s i s to A i r-

planes." Aeronautical Engineering Review. Aug. 1954. 2. Swartzberg, M. B. "Technique of Smoothing Leading Edges of Airplanes." Experimenter. Sept. 1956. 3. Cornish, J. "Prevention of Turbulent Separation by Suction Through A Perforated Surface." Aerophysics Dept, of

6. Raspet,

A.

Aerophysics

Dept, of Miss. State College, Internal Report. 1955.

7. Raspet, A. and Parker. "The Low Drag Sailplane." Soaring. Nov. - Dec. 1954. 8. Sweeney, R. "Report on Learslar MK. II Conversion." Aviation Week. Sept. 10, 1956. 9. Raspet, A. "Control of the Boundary Layer on Sailplanes." Organisation Scientifique E t Technique Internationale Du Vol A Voile O.S.T.I.V. Pub. H. 1952. 10. Roemer, S. F. Aerodyna-

throttle off, lands smartly at 58

cockpit and taxi time, squared

80", etc. ringing in my ears, I

eased on the throttle. The acceleration was amazing, torque

not too bad, and I was airborne before I knew it. With a few hundred feet below, and the altimeter winding

up more than satisfactorily, I

decided to take stock of the thing. It sat quite nicely; those Mississippi State College. Reapparently inadequate wings search Rept. No. 7. Oct. 1953. were a real novelty, in their 4. Raspet, A. "Flight Measured Aerodynamics of Wittman's stubby little way. A couple of Tailwind." Experimenter. Oct. mic Drag. Published by the experimental wiggles of the con1956. author — 148 Busteed, Midland trols confirmed the really light 5. Dwinnell, V. Principles of Park, New Jersey. 1951, P. 81. action and ease the ship could Aerodynamics. McGraw - Hill be flown with. To a hundred Book Co., 1949.

keep awake on the roll out, but here again it's easily controlled.

I can honestly encourage any of the wives that have a chance to get in on these little ships, to please do, it's really marvelous, like husbands, they may seem overwhelming at first, but

once

you're

at the

they're quite docile!

controls

In fact, I think it's terrific, and I feel sorry for the girls who only get to fly Super Cruis-

ers. Flying the homebuilts is fun, they're rugged, fun, economical ———————! •