ON MAKING WING MODIFICATIONS

Part II

THE SPORTPLANE BUILDER Bv Antoni iTimyi Bingelis EAA Designee Program Adi-mor

8509 Greenflint Lane Austin, Texas 78759

Du,

J RING MORE THAN three-quarters of a century of amazing aviation achievements and developments, literally hundreds of unusual aircraft designs passed our way. Many a one was heralded as the ultimate safe, stall and spin proof airplane. And yet today, 1981, we find that no valid design systems or methods exist for the prediction of stall and spin characteristics for a new design, much less for a production aircraft. Sure, designers have access to plenty of theoretical

by applying the proper geometry to the tail surface design. Industry and light aircraft designers alike followed this TDPF criterion for years, as a matter of standard design practice, before they became aware that it was inaccurate and seriously deficient for the prediction of spin behavior. Unfortunately, at least to my way of thinking, much

formulas and even empirical means for the formulation of wing and tail configurations and the like, but in reality, none of them can predict stall and spin characteristics for a new design. There have been stall-proof and spin-proof airplanes, yes indeed! However, many of them were odd-looking and commercially unsaleable. Nevertheless, whatever it was that endowed them with their unique flight characteristics could not be proven nor put into aerodynamic's magic lamp for reliable retrieval, at will, by light plane designers . . . it's not that nobody had ever tried. Ever hear of the Tail Damping Power Factor (TDPF) criterion? NASA's forerunner, the National Advisory Committee for Aeronautics (NACA), during the late 1940's did attempt to provide the general aviation community with design criterion which would ensure satisfactory spin recovery. This design criterion was based entirely on the premise that a satisfactory spin recovery would be assured

spin behavior and not on stall avoidance . . . that is, making stall entry difficult and stall recovery easy. But the emphasis may finally be changing for the good.

of the emphasis has always been on the prediction of

Note the sharp discontinuity of the leading-edge droop of this particular configuration. Fairing in the discontinuity results in an unfavorable unrecoverable spin tendency.

In 1973 NASA generated a limited project to reevaluate that controversial (TDPF) tail design criterion and to search out its short-comings. After many repeated experiments and flight tests, NASA concluded that the use of TDPF criterion alone,

A bottom view ol the added-on glove modification (or the

drooped leading edge. NASA's modification results in a flat under surface.

as a means for prediction of stall and spin behavior, is neither adequate nor necessary. In fact, some researchers consider it downright inaccurate and seriously lacking. Be that as it may, this stall/spin work put NASA on a new track. Although the element of wing design was not included in the original criterion as a factor, data newly gathered by NASA shows it indeed to be a major factor in determining the stall and spin recovery characteristics of an aircraft. SPORT AVIATION 19

At or near the point of stall, most aircraft, particularly those with unswept wings, develop poor dampingin-roll characteristics and a tendency to autorotate (spin). This objectionable damping-in-roll tendency often degrades to rapid rolling and yawing actions which may be difficult for the pilot to control. At low traffic altitudes, as in making that final turn, the resulting lack of controllability could become a causative factor in a

fatal stall/spin accident. NASA's ongoing experiments proved that slight

changes in the geometry of the wing leading edge can result in improved stall and spin recovery characteristics, even in cases where, previously, the original tail configuration exhibited unrecoverable spin behavior. The conclusion was unmistakable. Wing design does

have a substantial effect on an aircraft's spin characteristics and any design criterion for good spin recovery characteristics must obviously include that element. One conclusion led to another. Leading edge modifications have a profound effect on the spin characteristics

of a wing. For example, all leading edge droop configurations tested consistently demonstrated an improved stall behavior. (My own experience with the modified Turner T-40 drooped leading edge bears this out.)

Without a doubt there is a great potential for experimenters and light aircraft designers to develop stall proofing in an airplane through aerodynamic design. Emulating NASA's technique for experimental wing modification may be one more step in this direction.

All of the test modifications NASA made were effected by using the add-on "glove" or cuff method described in the previous article. By using these contour-

This view of the modified Yankee shows the outer outboard leading-edge droop modification.

changing added-on gloves, a variety of leading edge configurations could be tested on a single basic wing. Here

are some other findings NASA gathered from the wing modification experiments.

A most revealing view of the modified Sundowner sporting a segmented leading-edge droop. Note the wool tufts taped to the wing and fuselage surfaces to aid in evaluating flight characteristics of the modified wing. 20 MARCH 1981

Flight Characteristics of a Full-Span Drooped Leading-Edge Configuration

The basic wing had an abrupt roll-off and a positive break for the stall. On the other hand, when modified to a full-span drooped configuration it consistently displayed a stall behavior that was improved and slightly more docile than that of the basic wing. However, NASA's test pilot indicated that he found it necessary to make brisk anticipatory rudder and aileron inputs to prevent a rapid roll-off at the stall. Apparently, the airplane would roll-off at the stall regardless of the pilot's best efforts. NASA's experience with the pre-stall characteristics of their full-span add-on drooped leading edge is different from my own with the Turner T-40's built-in drooped leading edge design. During the course of this writing I talked with the present owner of the Turner to refresh my original impressions of its flight behavior. It was as I remembered it. The airplane can be slowed to what should be its approximate stalling speed with little evidence of an approaching stall condition other than perhaps the smallest momentary tremble imaginable in the aircraft. The ailerons remain effective throughout all flight regimes up to and including the fully stalled condition. You realize you are thoroughly stalled only when you note that the rate of climb needle (vertical speed indicator) is indicating approximately 1,000 to 1,400 ft./minute descent. Power-on or power-off does not seem to alter the aircraft's control effectiveness. When in a deep stall, power-off condition, the application of full power will slow the rate of descent but the aircraft will remain "behind the power curve" and will not fly out of the stall . . . at least at higher altitudes much above 4,000 feet. Release the back pressure, however, as in a normal stall recovery procedure and the stalled condition is eliminated. During the stalled condition it is possible to make nice aileron turns while maintaining equally effective stabilator control throughout. Perhaps the difference in behavior that NASA experienced serves to lend emphasis to their original finding that wing design is an important factor in establishing the stall/spin characteristics of an airplane. The NASA full-span drooped leading-edge modification differs from that built into the Turner, and undoubtedly accounts for the differing flight characteristics at stall. Of course, the configurations of both aircraft differ too. At any rate, the performance characteristics are similar in some respects, but in other respects, apparently, inconclusive. Inconclusive, that is, because nobody would predict that similar modifications on other aircraft would produce identical results. As for the spin characteristics of the full-span drooped leading-edge wing, NASA found that their airplane in this configuration would spin fiat regardless of the prospin control input used to get it into the spin. The airplane would readily enter into the flat spin without

Full_span leading-edge droop

Test aircraft A modified Grumman American A A - I Yankee

A —i

Outboard leading-edge droop

-0.57 b/2—4—036b/2-b/2 HGURE 2) BOSIC ajrfoil

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NACA

Leading edge droop •

64

modification*

A—I

-415

Flat underside Section A - A

Segmented leading-edge droop

Modified outboard leading-edge droop

special encouragement from the pilot's control movements and reach a steady-state condition after the fourth

or fifth turn. After this steady spin state is reached, the spin becomes unrecoverable and requires the use of a spin-recovery parachute. However, although the full-span drooped leading-edge NASA tested had unrecoverable spin tendencies in fully "locked-in" spins, this configuration and all others tested would recover from a one-turn spin within one additional turn, using normal spin recovery control input.

REPRESENTATIVE WING

MODIFICATIONS FLIGHT TESTED BY NASA SPORT AVIATION 21

A familiar configuration well known by most EAA'ers. Not generally known is that NASA and a number of prestigious universities known for their aerodynamic research are as equally familiar with the design and flight

characteristics of the VariEze.

To my knowledge, the Turner has not been spun, as a parachute suitable to wear in its snug cockpit is not a part of its standard accommodations. Nobody should ever attempt to spin an aircraft, particularly a new untested one, without the proper preparations and equipment. Flight Characteristics of the Segmented Leading-Edge Droop Modification

Not only does this configuration look wrong (odd),

apparently NASA found little benefit in pursuing its testing in any great degree. The configuration was accomplished by attaching

the leading edge glove in a discontinuous drooped arrangement to the basic wing. The arrangement had sharp discontinuities at the juncture of the two airfoil sections as depicted in Figure 3. The segmented leading edge was tested with varied gap arrangements and each appeared to give similar results. As with the full-span drooped modification, the segmented installation also had improved stall characteristics over that of the basic wing. But it, too, exhibited a tendency to readily enter into spins which, when allowed to reach their "locked in" (steady) state, would become unrecoverable, requiring deployment of the spin recovery parachute. 22 MARCH 1981

Flight Characteristics of a Drooped Outboard Leading-Edge Configuration

NASA's wing modification with an abrupt chord extension and drooped leading edge on only the outboard portion of the wing also demonstrated an improved lateral stability, over that of the basic wing, well beyond the normal stall angle of attack. This modification eliminated the roll-off characteristic of the basic wing and exhibited no roll-off tendency whatsoever. It would go, instead, into a wings-level sink rate flight condition with the control stick full aft. (These flight characteristics of the outboard wing modification sound very much like those demonstrated by the Turner T-40's full span drooped leading edge configuration.) With the drooped outboard wing modification installed, the test airplane exhibited a marked hesitancy in entering the first turn of a spin and would recover quickly regardless of the corrective control techniques used. Actually, a simple relaxation of the pressure on any of the controls resulted in an immediate recovery. The airplane, when forcibly induced to spin, would do so very steeply and would require 2 to 3 turns to reach a stabilized state. In an inspired modification of their outboard droop modification, the NASA folks tried fairing the inboard area of the discontinuity between the baseline wing and the drooped leading edge glove. (See Figure 4) Aesthetically, it looked better but what a surprise for the experimenters! The tapered fairing did not affect the im-

Area of attached flow

EFFECT OF OUTER WING MODIFICATION WITH SHARP EDGED CUFFS AT HIGH ANGLES OF ATTACK

FIGURE 5.

Note: In last month's column it was unintentionally implied that NASA attaches their leading

edge cuss with duct tape only. In fact, rivets, metal tabs, etc., fix them firmly in place. proved stall characteristics normally associated with drooped leading edges, but it did totally eliminate the improved spin resistant characteristics of the original outboard drooped leading-edge configuration. Getting rid of the abrupt airfoil discontinuity apparently changes the wing section into one which will spin flatly and cannot be arrested with the use of normal recovery control inputs by the pilot. NASA and the VariEze

A considerable amount of the experimentation and testing of the outboard drooped leading edge arrangement by NASA revolved around the famous little homebuilt design . . . the VariEze. NASA found, from free flight wind t u n n e l tests of a large scale model of the VariEze, that the outboard drooped leading edge cuffs, when installed, would eliminate or minimize the wing

tip stall which caused the test model to have a wingrock motion in the wind tunnel free-flight tests. This information was passed to the VariEze's designer. Burt R u t a n . He found NASA's conclusions to be intriguing and quite significant as a similar wingrock behavior was. under some aft CG flight conditions,

being encountered in the VariEzes in real life.

Burt, w i t h the helpful cooperation of NASA's Joe

Chambers and others, ran his own tuft tests and experiments These ultimately led to a simple wing modifica-

tion consisting of a retro-fitted drooped leading-edge for the outboard portions of the swept wing. The results were as NASA had a n t i c i p a t e d . The outboard drooped leading-edge modification completely eliminated wing-rock and departures (a roll-off tendency). The VariEze is now unique among aircraft designs as it has a proven stall and spin resistance far superior to that found in conventional aircraft. And Finally . . . Some Specifics and Some Generalities

• Wing modification affects stall/spin recovery behavior and can make an airplane have satisfactory or unsatisfactory stall/spin characteristics . . . NASA has had its share of both experiences. • All NASA drooped leading-edge modifications resulted in an improved stall behavior over that of the original baseline wing. • Test aircraft that had been modified to the full span drooped leading-edge configuration, showed improved stall performance, but had poor spin resistance

and a propensity to enter into moderately flat unrecoverable spins. • The most satisfactory stall/spin recovery characteristics were obtained with the outboard leading-edge

droop. There is no doubt about it. the outboard leading

edge modification delays tip stall. Could it be that the abrupt discontinuity at the juncture of the two airfoils SPORT AVIATION 23

(sharp-edged cuffs) acts as a vortex generator providing a vortex over the wing that delays the stall to a point well beyond the angle of attack normally obtained with the modified airfoil? NASA is not sure that this is what happens but . . . it does work. (I would not be surprised to see more homebuilts in the future with add-on, outer wing, drooped leading-edge modifications. Here is something to think about. All configurations, even those that had unrecoverable spin tendencies in a fully developed spin, would recover from one-turn spins within one additional turn using normal spin recovery control input. A good thought to retain, then, is that the best insurance for a positive spin recovery is not to allow the aircraft to go past one turn before initiating spin recovery. However, if the spin is kickedoff close to the ground as in making the final turn to the runway this knowledge would be of small consolation. Somebody at NASA said it. "You cannot generalize

on the results obtained in any given test. It is more accurate to describe what happened at that time . . . under those particular conditions . . . with that particular pilot flying that airplane (period)."

And A Final Final Note . . .

NASA is now so intrigued with the canard configuration and its inherent stall/spin characteristics that they intend to conduct additional tests with their very own full-size VariEze. That VariEze, even now, may be undergoing extensive wind tunnel testing to establish a baseline design which may soon lead to aircraft designs having an inherent stall proofing. The VariEze tests were scheduled to begin late in 1980 and may even be underway.

Built to the exact aerodynamic and static scale, these models might well be midget clones of the big birds NASA uses for the piloted tests. Wind tunnel test models and radio control test models constitute an important part of the wing modification experiments and tests.

If you wondered how they attached the spin chute now you know. The airplane is the Yankee.

24 MARCH 1981