ON MAKING WINGMODIFICATIONS

Part II

THESPORTPLANEBUILDER

Bv Antoni iTimyi BingelisEAA Designee Program Adi-mor

8509 Greenflint LaneAustin, Texas 78759

Du,JRING MORE THAN three-quarters of a centuryof amazing aviation achievements and developments,literally hundreds of unusual aircraft designs passedour way. Many a one was heralded as the ultimate safe,stall and spin proof airplane. And yet today, 1981, wefind that no valid design systems or methods exist forthe prediction of stall and spin characteristics for a newdesign, much less for a production aircraft.

Sure, designers have access to plenty of theoreticalformulas and even empirical means for the formulationof wing and tail configurations and the like, but inreality, none of them can predict stall and spin charac-teristics for a new design.

There have been stall-proof and spin-proof airplanes,yes indeed! However, many of them were odd-lookingand commercially unsaleable. Nevertheless, whatever itwas that endowed them with their unique flight char-acteristics could not be proven nor put into aerody-namic's magic lamp for reliable retrieval, at wil l , bylight plane designers . . . it's not that nobody had evertried.

Ever hear of the Tail Damping Power Factor (TDPF)criterion? NASA's forerunner, the National AdvisoryCommittee for Aeronautics (NACA), during the late1940's did attempt to provide the general aviation com-munity with design criterion which would ensure satis-factory spin recovery.

This design criterion was based entirely on the pre-mise that a satisfactory spin recovery would be assured

by applying the proper geometry to the tail surfacedesign.

Industry and light aircraft designers alike followedthis TDPF criterion for years, as a matter of standarddesign practice, before they became aware that it wasinaccurate and seriously deficient for the prediction ofspin behavior.

Unfortunately, at least to my way of thinking, muchof the emphasis has always been on the prediction ofspin behavior and not on stall avoidance . . . that is,making stall entry difficult and stall recovery easy. Butthe emphasis may finally be changing for the good.

A bottom view ol the added-on glove modification (or thedrooped leading edge. NASA's modification results in a flatunder surface.

Note the sharp discontinuity of the leading-edge droop ofthis particular configuration. Fairing in the discontinuity re-sults in an unfavorable unrecoverable spin tendency.

In 1973 NASA generated a limited project to re-evaluate that controversial (TDPF) tail design criterionand to search out its short-comings.

After many repeated experiments and flight tests,NASA concluded that the use of TDPF criterion alone,as a means for prediction of stall and spin behavior, isneither adequate nor necessary. In fact, some researchersconsider it downright inaccurate and seriously lacking.

Be that as it may, this stall/spin work put NASAon a new track. Although the element of wing designwas not included in the original criterion as a factor,data newly gathered by NASA shows it indeed to be amajor factor in determining the stall and spin recoverycharacteristics of an aircraft.

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

At or near the point of stall, most aircraft, particu-larly those with unswept wings, develop poor damping-in-roll characteristics and a tendency to autorotate (spin) .This objectionable damping- in-rol l tendency often de-grades to rapid rolling and yawing actions which maybe difficult for the pilot to control. At low traffic alti-tudes, as in making that final turn, the resulting lackof controllability could become a causative factor in afatal stall/spin accident.

NASA's ongoing experiments proved that slightchanges in the geometry of the wing leading edge canresult in improved stall and spin recovery characteris-tics, even in cases where, previously, the original tailconfiguration exhibited unrecoverable spin behavior.

The conclusion was unmistakable. Wing design doeshave a substantial effect on an aircraft's spin charac-teristics and any design criterion for good spin recoverycharacteristics must obviously include that element.

One conclusion led to another. Leading edge modifica-tions have a profound effect on the spin characteristicsof a wing. For example, all leading edge droop configura-tions tested consistently demonstrated an improved stallbehavior. (My own experience with the modified TurnerT-40 drooped leading edge bears this out.)

Without a doubt there is a great potential for ex-perimenters and light aircraft designers to develop stallproofing in an airplane through aerodynamic design.Emulating NASA's technique for experimental wingmodification may be one more step in this direction.

All of the test modifications NASA made were ef-fected by using the add-on "glove" or cuff method de-scribed in the previous article. By using these contour-changing added-on gloves, a variety of leading edge con-figurations could be tested on a single basic wing. Hereare some other findings NASA gathered from the wingmodification experiments.

A most revealing view of the modified Sundowner sporting a segmented leading-edge droop. Note the wooltufts 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-SpanDrooped Leading-Edge Configuration

The basic wing had an abrupt roll-off and a posi-tive break for the stall. On the other hand, when modi-fied to a full-span drooped configuration it consistentlydisplayed a stall behavior that was improved and slightlymore docile than that of the basic wing. However, NASA'stest pilot indicated that he found it necessary to makebrisk anticipatory rudder and aileron inputs to preventa rapid roll-off at the stall. Apparently, the airplanewould roll-off at the stall regardless of the pilot's bestefforts.

NASA's experience with the pre-stall characteris-tics of their full-span add-on drooped leading edge isdifferent from my own with the Turner T-40's built-indrooped leading edge design.

During the course of this writing I talked with thepresent owner of the Turner to refresh my original im-pressions of its flight behavior. It was as I rememberedit.

The airplane can be slowed to what should be itsapproximate stalling speed with little evidence of anapproaching stall condition other than perhaps the small-est momentary tremble imaginable in the aircraft. Theailerons remain effective throughout all flight regimesup to and including the ful ly stalled condition. Yourealize you are thoroughly stalled only when you notethat the rate of climb needle (vertical speed indicator)is indicating approximately 1,000 to 1,400 ft./minutedescent. Power-on or power-off does not seem to alterthe aircraft's control effectiveness.

When in a deep stall, power-off condition, the ap-plication of full power will slow the rate of descent butthe aircraft will remain "behind the power curve" andwill not fly out of the stall . . . at least at higher alti-tudes much above 4,000 feet. Release the back pressure,however, as in a normal stall recovery procedure andthe stalled condition is eliminated. During the stalledcondition it is possible to make nice aileron turns whilemaintaining equally effective stabilator control through-out.

Perhaps the difference in behavior that NASA ex-perienced serves to lend emphasis to their original find-ing that wing design is an important factor in estab-lishing the stall/spin characteristics of an airplane.

The NASA full-span drooped leading-edge modifica-tion differs from that built into the Turner, and un-doubtedly accounts for the differing flight characteris-tics at stall. Of course, the configurations of both air-craft differ too. At any rate, the performance character-istics are similar in some respects, but in other respects,apparently, inconclusive. Inconclusive, that is, becausenobody would predict that similar modifications on otheraircraft would produce identical results.

As for the spin characteristics of the full-span droopedleading-edge wing, NASA found that their airplane inthis configuration would spin fiat regardless of the pro-spin control input used to get it into the spin. The air-plane would readily enter into the flat spin withoutspecial encouragement from the pilot's control move-ments and reach a steady-state condition after the fourthor fifth turn. After this steady spin state is reached,the spin becomes unrecoverable and requires the useof a spin-recovery parachute. However, although thefull-span drooped leading-edge NASA tested had unre-coverable spin tendencies in ful ly "locked-in" spins,this configuration and all others tested would recoverfrom a one-turn spin within one additional turn, usingnormal spin recovery control input.

Full_span leading-edge droop

Test aircraft -A modified Grumman American AA- I Yankee

A —i

Outboard leading-edge droop

-0.57 b/2—4—036b/2--b/2

A—I

HGURE 2) BOSIC ajrfoilNACA 64 -415

Leading edgedroop •

\modification*

Flat undersideSection A - A

Segmented leading-edge droop

Modified outboard leading-edge droop

REPRESENTATIVE WINGMODIFICATIONS FLIGHT -TESTED BY NASA

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

To my knowledge, the Turner has not been spun, asa parachute suitable to wear in its snug cockpit is nota part of its standard accommodations.

Nobody should ever attempt to spin an aircraft,particularly a new untested one, without the properpreparations and equipment.

Flight Characteristics of the SegmentedLeading-Edge Droop Modification

Not only does this configuration look wrong (odd),apparently NASA found little benefit in pursuing itstesting in any great degree.

The configuration was accomplished by attachingthe leading edge glove in a discontinuous drooped ar-rangement to the basic wing. The arrangement hadsharp discontinuities at the juncture of the two air-foil sections as depicted in Figure 3.

The segmented leading edge was tested with variedgap arrangements and each appeared to give similarresults.

As with the full-span drooped modification, the seg-mented installation also had improved stall character-istics over that of the basic wing. But it, too, exhibiteda tendency to readily enter into spins which, when al-lowed to reach their "locked in" (steady) state, wouldbecome unrecoverable, requiring deployment of thespin recovery parachute.22 MARCH 1981

Flight Characteristics of a Drooped OutboardLeading-Edge Configuration

NASA's wing modificat ion with an abrupt chordextension and drooped leading edge on only the outboardportion of the wing also demonstrated an improvedlateral stability, over that of the basic wing, well beyondthe normal stall angle of attack. This modification elimi-nated the roll-off characteristic of the basic wing andexhibited no roll-off tendency whatsoever. It would go,instead, into a wings-level sink rate flight conditionwith the control stick full aft. (These flight characteris-tics of the outboard wing modification sound very muchlike those demonstrated by the Turner T-40's full spandrooped leading edge configuration.)

With the drooped outboard wing modification in-stalled, the test airplane exhibited a marked hesitancyin entering the first turn of a spin and would recoverquickly regardless of the corrective control techniquesused. Actually, a simple relaxation of the pressure onany of the controls resulted in an immediate recovery.

The airplane, when forcibly induced to spin, woulddo so very steeply and would require 2 to 3 turns toreach a stabilized state.

In an inspired modification of their outboard droopmodification, the NASA folks tried fairing the inboardarea of the discontinuity between the baseline wing andthe drooped leading edge glove. (See Figure 4) Aestheti-cally, it looked better but what a surprise for the ex-perimenters! The tapered fairing did not affect the im-

Area ofattached

flow

EFFECT OF OUTER WING MODIFICATIONWITH SHARP EDGED CUFFS ATHIGH ANGLES OF ATTACK

FIGURE 5.

Note: In last month's column it was unintentionally implied that NASA attaches their leadingedge cuss with duct tape only. In fact, rivets, metal tabs, etc., fix them firmly in place.

proved stall characteristics normally associated wi thdrooped leading edges, but it did totally eliminate theimproved spin resistant characteristics of the originaloutboard drooped leading-edge configuration.

Getting rid of the abrupt airfoil discontinuity ap-parently changes the wing section into one which wil lspin flatly and cannot be arrested with the use of nor-mal recovery control inputs by the pilot.

tion consisting of a retro-fitted drooped leading-edgefor the outboard portions of the swept wing. The re-sults were as NASA had ant ic ipated . The outboarddrooped leading-edge modification completely eliminatedwing-rock and departures (a roll-off tendency).

The VariEze is now unique among aircraft designsas it has a proven stall and spin resistance far superiorto that found in conventional aircraft.

NASA and the VariEze

A considerable amount of the experimentation andtesting of the outboard drooped leading edge arrange-ment by NASA revolved around the famous l i t t le home-bui l t design . . . the VariEze. NASA found, from freeflight wind tunnel tests of a large scale model of theVariEze, that the outboard drooped leading edge cuffs,when installed, would eliminate or minimize the wingtip stall which caused the test model to have a wing-rock motion in the wind tunnel free-flight tests.

This information was passed to the VariEze's de-signer. Burt Ru tan . He found NASA's conclusions tobe intr iguing and quite significant as a similar wing-rock behavior was. under some aft CG flight conditions,being encountered in the VariEzes in real life.

Burt, wi th the helpful cooperation of NASA's JoeChambers and others, ran his own tuft tests and experi-ments These ultimately led to a simple wing modifica-

And Finally . . . Some Specifics and Some Generalities

• Wing modification affects stall/spin recovery be-havior and can make an airplane have satisfactory orunsatisfactory stall/spin characteristics . . . NASA hashad its share of both experiences.

• All NASA drooped leading-edge modifications re-sulted in an improved stall behavior over that of theoriginal baseline wing.

• Test aircraft that had been modified to the fullspan drooped leading-edge configurat ion, showed im-proved stall performance, but had poor spin resistanceand a propensity to enter into moderately flat unre-coverable spins.

• The most satisfactory stall/spin recovery charac-teristics were obtained with the outboard leading-edgedroop. There is no doubt about it. the outboard leadingedge modification delays tip stall. Could it be that theabrupt discontinuity at the juncture of the two airfoils

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(sharp-edged cuffs) acts as a vortex generator providinga vortex over the wing that delays the stall to a pointwell beyond the angle of attack normally obtained withthe modified airfoil? NASA is not sure that this is whathappens but . . . it does work. (I would not be surprisedto see more homebuilts in the future with add-on, outerwing, drooped leading-edge modifications.

Here is something to think about. All configurations,even those that had unrecoverable spin tendencies ina fully developed spin, would recover from one-turnspins within one additional turn using normal spinrecovery control input. A good thought to retain, then,is that the best insurance for a positive spin recoveryis not to allow the aircraft to go past one turn beforeinitiating spin recovery. However, if the spin is kicked-off close to the ground as in making the final turn tothe 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 ac-curate to describe what happened at that time . . . underthose particular conditions . . . with that particular pilotflying that airplane (period)."

And A Final Final Note .. .

NASA is now so intrigued with the canard con-figuration and its inherent stall/spin characteristics thatthey intend to conduct additional tests with their veryown full-size VariEze. That VariEze, even now, may beundergoing extensive wind tunnel testing to establisha baseline design which may soon lead to aircraft de-signs having an inherent stall proofing. The VariEzetests were scheduled to begin late in 1980 and may evenbe underway.

Built to the exact aerodynamic and staticscale, these models might well be midgetclones of the big birds NASA uses for thepiloted tests. Wind tunnel test models andradio control test models constitute an im-portant part of the wing modification ex-periments and tests.

If you wondered how they attached thespin chute now you know. The airplane isthe Yankee.

24 MARCH 1981