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Self-actuating flaps on bird and aircraft wings

D.W. Bechert, W. Hage & R. MeyerDepartment of Turbulence Research, German Aerospace Center (DLR),Berlin, Germany.

Abstract

Separation control is also an important issue in biology. During the landing approach of birds andin flight through very turbulent air, one observes that the covering feathers on the upper side ofbird wings tend to pop up. The raised feathers impede the spreading of the flow separation fromthe trailing edge to the leading edge of the wing. This mechanism of separation control by birdfeathers is described in detail. Self-activated movable flaps (= artificial bird feathers) representa high-lift system enhancing the maximum lift of airfoils up to 20%. This is achieved withoutperceivable deleterious effects under cruise conditions. Several data of wind tunnel experimentsas well as flight experiments with an aircraft with laminar wing and movable flaps are shown.

1 Movable flaps on wings: artificial bird feathers

The issue of artificial feathers on wings, has an almost anecdotal origin. Wolfgang Liebe, theinventor of the boundary layer fence once observed mountain crows in the Alps in the 1930s.He noticed that the covering feathers on the upper side of the wings tend to pop up when thebirds were on landing approach or in other situations with high angle of attack, like flight throughgusts. Once the attention of the observer is drawn to it, it is comparatively easy to observe thisbehaviour in almost any bird (see, for example, the feathers on the left-hand wing of a Skua inFig. 1). Liebe interpreted this behaviour as a biological high-lift device [2]. Later, in 1938, Liebeworked as a young scientist at the former German Aeronautical Establishment (DVL—DeutscheVersuchsanstalt für Luftfahrt, the predecessor of the present DLR). Liebe attached a piece ofleather on the upper side of one wing of a fighter aircraft (see Fig. 2), a Messerschmitt BF 109.Take-off and flight of this specially outfitted aircraft were satisfactory, but landing turned out tobe tricky. At high angles of attack, the lift distribution on the wings was asymmetrical. Therefore,the pilot had to land the aircraft at a low angle of attack and at a very high speed.

Much later, Liebe presented his ideas in a journal article [2]. Liebe’s original idea was thatonce separation starts to develop on a wing, reversed flow was bound to occur in the separationregime. Under these locally reversed flow conditions, light feathers would pop up. They would

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436 Flow Phenomena in Nature

Figure 1: Skua (Catharacta maccormicki) during landing approach. Photograph byI. Rechenberg [1].

Figure 2: Schematic representation of a Messerschmitt BF 109 airplane with artificial bird featherson the right wing.

act like a brake on the spreading of flow separation towards the leading edge. Liebe was awareof the fact that flow separation is often a three-dimensional effect with variable patterns in thespanwise direction. Thus, he considered it essential to be able to interact even with local separationregimes (see Fig. 1). Therefore, Liebe suggested the name ‘reverse flow bags’(Rückstromtaschen).Following Liebe’s ideas, a few tentative flight experiments were carried out in Aachen with smallmovable plastic sheets installed on a glider wing on the upper surface near the trailing edge [3]. Itappeared as if the glider aircraft then exhibited a more benign behaviour at high angles of attack.

Beginning in early 1995, this issue was taken up again in a joint effort by four research partners:The DLR Berlin, the Institutes of Bionics and Fluid Mechanics at the Technical University ofBerlin, and the STEMME Aircraft Company in Strausberg near Berlin. Previous preliminary



Self-Actuating Flaps on Bird and Aircraft Wings 437

Figure 3: Schematic representation of a wind tunnel test section.

Figure 4: Schematic representation of the wing section with a movable flap.

experiments in the wind tunnel of the Bionics Institute with paper strips on a small wing hadhinted a favourable effect.

We embarked on two-dimensional flow experiments with a laminar glider wing section sus-pended in a low-turbulence wind tunnel. We considered it essential to prove that the aerodynamicaleffects are not confined to the low Reynolds numbers of bird flight (Re ≈ 104–105). Therefore,our experiments were carried out at the typical Reynolds numbers occurring during the land-ing approach of gliders and general aviation aircraft, i.e. at Re = 1 × 106 − 2 × 106. In addition,measurements with a wind tunnel balance (instead of surface pressure distribution and wakemeasurements) were considered crucial because (i) high angles of attack with considerable flowseparation were most interesting, (ii) the quick data processing of a balance would enable us totest a large number of configurations and (iii) hysteresis at high angles of attack can be recordedonly with a balance equipped with a quick automatic angle adjustment (see Fig. 3).

In our first wind tunnel trials, it turned out that the naïve approach to just emulate bird feathersby attaching plastic strips to the wing surface produced rather confusing results. Therefore, wecontinued our experiments with a simpler device, i.e. thin movable flaps on the upper rear sur-face of our glider airfoil. After a variety of different materials were tested, flaps made of eitherelastic plastic material or thin sheet metal were used. The flaps were attached to the rear part ofthe airfoil and could pivot on their leading edges (see Fig. 4). Under attached flow conditions,the movable flap is very slightly raised. This is due to the fact that the static pressure increasesin the downstream direction in the rear part of the upper surface of the airfoil. Thus, the spaceunder the flap is connected to a regime of slightly elevated static pressure. Consequently, in mostplaces, the pressure beneath the movable flap is higher than above it. This is the reason why theflap is slightly lifted in this case. As a matter of fact, this behaviour proved to be a disadvantage.




The drag is obviously slightly increased due to the small separation regime at the end of the flap.In addition, there is a slight decrease in lift because the curvature of the airfoil at the trailing edgeis decreased. Thus, the effective angle of attack of the airfoil is also decreased. Therefore, theimpact of the movable flap is slightly deleterious.

However, there are several ways to deal with this problem. The first and obvious one would beto lock the movable flap onto the airfoil surface under attached flow conditions. The second oneis also rather simple: make the flap porous in order to obtain equal static pressure on both sidesof the flap for attached flow conditions. A third method is to make the trailing edge of the flapjagged, as shown in Fig. 5. This leads to an exchange of pressures as well. Incidentally, the lattertwo ‘inventions’ are indeed found on bird wings.

Now, how do the movable flaps respond to reversed flow? First, it should be mentioned that theflow velocities of the reversed flow are considerably smaller than the mean flow velocity. Thus,the movable flaps have to be very light and should respond with high sensitivity to even weakreversed flows. A very soft trailing edge of the movable flaps facilitates a sensitive response there.Again, this feature is found on bird feathers.

Once the flow starts to separate, the movable flap follows gradually. It does not, however,protrude into the high-speed flow above the separation wake. This high-speed flow would push

Figure 5: Data for movable flaps installed on a laminar glider airfoil (HQ41). All numbers givepercentage of airfoil chord.




the flap back to a lower elevation. At this point, we would like to stress the marked differencebetween our movable flaps and a conventional rigid spoiler on a wing [4]: a spoiler protrudes intothe high-speed flow regime and increases the width of the wake. In this way, it increases the dragand reduces the lift. In contrast, at high angles of attack, our device will do the opposite: reducedrag and increase lift.At the same time, the effective shape of the airfoil changes due to the slightlyelevated flap and a lower effective angle of attack ensues. Thus, the pressure distribution on theairfoil is adjusted in such a way that the tendency for flow separation is reduced. Consequently,the flow remains attached to higher (real) angles of attack and the lift of the wing is increased.

Nevertheless, there are limits for this favourable behaviour of movable flaps. At very highangles of attack, the reversed flow would cause the flap to tip over in the forward direction, andthe effect of the flap would vanish. This can however be prevented by limiting the opening angleof the flap. We achieved this very simply by attaching limiting strings to the movable flap. In ourexperiments, we determined the optimal maximum opening angle of the flaps. It was found tolie between 60◦ (for solid and porous flaps) and about 90◦ (for flaps with jagged trailing edges).Once the full opening angle is reached, the separation jumps forward over the flap. Hence, forvery high angles of attack, the effect of the movable flap finally decreases and vanishes. Theseexperimental findings were also confirmed by numerical studies, where simulations with two-and three-dimensional static flaps as well as two-dimensional simulations of free-movable flapswere carried out [5].

In birds tipping-over of the feathers is not observed. Probably, the feather shafts are sufficientlystiff and well-anchored to prevent this deleterious situation.

An important question is where on the airfoil should a movable flap be installed. We started ourexperiments with movable flaps being located at the downstream end of the airfoil. This appearedreasonable because on laminar airfoils such as ours, the first 60–70% of the upper surface isdesigned to be laminar. For bird wings, which operate at lower Reynolds numbers, however,surface smoothness is not so important. Any attachment or other deviation from a perfectlysmooth surface in this laminar regime would cause transition, entailing significant additionaldrag. In contrast, on the rear part of the airfoil and downstream of the laminar regime, minorchanges in the surface quality do not produce a detectable increase of the drag.

In our experiments, we found that the trailing edge of the movable flap should be located slightlyupstream (≥1% chord) of the trailing edge of the airfoil. Otherwise, it would not respond properlyto flow separation. On the other hand, the farther upstream the flap is located, the farther upstreamthe flow separation would have already spread once the flap starts to respond. Thus, if one wantsto interfere with incipient separation, the trailing edge of the flap should be located close to thetrailing edge of the airfoil.

Another intriguing question is what should be the size of the movable flap. We started our windtunnel experiments with comparatively small flaps having a length of about 12% of the airfoilchord length. The effect was significant (see Fig. 5) and resulted in an increase of maximum lift of10%. Increasing the flap length produced a further increase of maximum lift. For instance, a flaplength of 22% resulted in an increase of 18% of the maximum lift. However, for large movableflaps (which are not flexible), the self-adjustment to the flow situation becomes less satisfactory.Typically, a movable flap starts to rise when the flow separation has already reached its upstreamedge. On the other hand, full reattachment of the flap is obtained at the lowered angle of attackwhen the reattachment line of a reference wing (without movable flap) has moved downstream tothe location of the flap trailing edge. This creates a significantly different behaviour for increasingangles as compared with decreasing angles. This hysteresis in the airfoil data is not desired becauseit would make the aircraft difficult to handle. One way to avoid this problem is to divide the flapinto movable parts that are attached to each other (see Fig. 5). Indeed, this double flap adjusts itself




much better and the hysteresis is practically eliminated. Nevertheless, the impressive increase inmaximum lift is still maintained.

Going back to bird feathers: obviously, they are flexible and are likely to have the requiredproperties. Birds, however, possess several consecutive rows of covering feathers on their wingsand, as can be seen in Fig. 1, several of them pop up at once during the landing approach.

Our experiments with more than one movable flap, however, turned out to be tricky. In somecases, when the rear flap rose, the additional forward flap also popped up immediately. Thus,the forward flap tended to behave like a conventional spoiler, causing a sudden drop in the liftforce of the airfoil. Things seemed to work better when thin plastic flaps were used. This drewour attention to the significance of fluttering of the flimsy flaps. As a preliminary conclusion, wenow think that a combination of two movable flaps work best if the first flap flutters when beingactivated. We actually managed to demonstrate the effect of fluttering with a single fluttering flapin a relatively forward position on the airfoil. It is a comparative experiment between a flutteringflap and a very similar one that did not flutter. For comparison, data on a ‘naked’ reference airfoilare also provided (see Fig. 6). The slight increase of drag under attached flow conditions is causedby the fact that the first (upstream) movable flap causes earlier transition (in the laminar regime)even when it is still attached.

Figure 6: The lift-enhancing effect of fluttering and non-fluttering dual flaps. All numbers givepercentage of airfoil chord. The perforation ratio of the flap was about 1%.




Apart from this minor effect, the differences in performance are rather dramatic. The slightlyshorter non-fluttering flap (lower diagram in Fig. 6) pops up when the separation reaches it athigh angles of attack; it makes things worse by indeed acting as a spoiler. On the other hand, theslightly longer fluttering flap (upper diagram in Fig. 6) reacts in a completely different manner.With increasing angle of attack, it raises rather gradually. The flutter vibration on the longer (10 cm)flap had a peak-to-peak magnitude of about 3 cm, and the observed frequency was about 40 HzIn addition, it vibrates with a very large amplitude. By this interaction, the lift again increases byabout 6% (Fig. 6) above the value obtained by the rear flap alone (10%).

Thus, we hypothesize how this new lift-enhancing effect works: It is clear that this mechanismextracts energy from the mean potential flow. Therefore, it works best if the fluttering flap is nottoo small, because a small device cannot interact through the boundary layer with the outsidepotential flow. Obviously, the fluttering requires elastic compliance of the flap and would not workwith rigid or too short flaps. The instability draws energy from the mean flow and feeds energy intothe near-wall region by a non-linear pumping process. In the upstroke, air is displaced upwardsabove the flap, but, at the same time, air is sucked into the opening gap under the flap. In thedownstroke, air is expelled near the wall, in the downstream direction. The latter expelling motionis the one that feeds energy into the near-wall region, virtually operating like an intermittent walljet. This helps to keep the flow attached and, in turn, produces higher lift of the airfoil. Thus, thevelocities near the flap trailing edge are possibly lower than the mean flow velocity, but probablynot small compared with it.

Fluttering of feathers also occurs in bird wings under high-lift conditions, which can be seenin video documentations of landing birds. Nevertheless, we are the first to prove that vibrationactually increases the lift. This novel effect combines previous scientific knowledge on the effectof unsteady blowing [6, 7] or oscillating flaps [8] with an effect channelling energy from the meanflow into these separation control mechanisms. The observed flutter is an instability, remotelyrelated to that of a flag fluttering in the wind. Obviously, this device is lightweight, extremelysimple and it requires no additional external energy. However, implementation on aircraft willobviously require additional research. We wish to add here that we have also carried out experi-ments even with three movable flaps. The highest increase of maximum lift was 23%. Incidentally,the operation of movable flaps is independent of the flow status; the boundary layer can be eitherlaminar or turbulent.

In contrast, the self-activated single flaps in the aft regime of airfoils have a good chance forapplication in aircraft at high-lift conditions. They do not require any vibration in order to workproperly and they are remarkably stable and reliable in their operation. Therefore, after selecting aparticularly reliable and hysteresis-free configuration, we proceeded to prepare flight experiments.

1.1 Flight experiments with movable flaps

Our flight experiments were performed with a STEMME S10 motor glider. With its piston engine,it can take off by itself. The foldable propeller can be retracted into the nose of the cockpit. Duringflight, the motor can be re-started if necessary. With the propeller retracted, the aircraft is a fast,high-performance glider. The laminar wing is equipped with conventional flaps that also operateas ailerons.

As a specific preparation for flight experiments with this aircraft, we made sure that our movableflaps would also work effectively in combination with the conventional flaps on the wings. Figure 7shows data with both types of flaps combined. The movable flap is actually mounted on theconventional flap. As can be seen in Fig. 7, the increase of lift caused by the movable flap persists.




Figure 7: Combination of conventional and self-activated flaps for three different flap angles γ .Dotted curves: with movable flap.

During the flight experiments, the intention was to fly at very high angles of attack just intothe regime of total stall. Usually, for tests of high-lift systems, one does not go that far in orderto avoid dangerous situations like spinning of the aircraft. Our flight tests, however, includedsuch situations with the purpose of demonstrating the inherent safety of our movable flaps. Thisrequired:

(i) a very skilled pilot, familiar with the behaviour of the aircraft;(ii) sufficient altitude during critical flight phases so that the pilot has sufficient time to handle

the arising situations or, in the worst case (which did not occur), to exit with a parachute;(iii) the introduction of the changes in the aircraft in a gradual, step-by-step fashion in order to

avoid unfamiliar situations for the pilot;(iv) a ‘special preparation’ of the aircraft to keep it controllable at high angles of attack.

The ‘special preparation’mentioned in point (iv) can be seen in Fig. 8. The elevator was equippedwith vortex generators on the upper surface in order to extend its angular regime of attached flow.The same vortex generators were installed on the outer parts of the wings. That caused an increaseof maximum lift of 31%. There were, however, some peculiar flight-dynamical effects causedby this: the return to normal flight attitude out of the stall-spinning sequence sometimes resulted




Figure 8: STEMME S10 test aircraft, equipped with self-activated flaps.

Figure 9: In-flight video recording. The picture on the left shows an attached flap and attachedflow. In the picture on the right, the woollen threads indicate partial separation and themovable flap has risen by itself.

in spinning in the opposite direction. This was probably caused by the tremendously increaseddifferences in lift between attached and fully separated flow conditions on the outer wing, due tothe vortex generators.

A reduction to half the previous number of vortex generators (i.e. a reduced vortex generatordensity) reduced the increase of maximum lift to merely 15% on the outer wings. This turned out tobe more compatible with the original flight-dynamical layout of the aircraft and thus eliminatedthe problem. In addition, the performance of the outer wing was then closer to the one withmovable flaps on the inner part of the wing. Incidentally, to find the solution for these safety-relevant problems, it was very useful to have available the full-scale wind tunnel experiment inparallel to the flight tests.

In order to highlight the flow situation on the wing, woollen threads were attached to its surface.These and the motion of the movable threads were recorded by a video camera on the empennage.The flight speed was also recorded on videotape. Typical flow situations can be seen in Fig. 9.The video pictures in Fig. 9 are fully consistent with parallel experiments in the wind tunnel atan identical air speed and Reynolds number.

In flight experiments, the increase in the maximum lift coefficient cL can be documented byrecording the minimum attainable speed before stall. Therefore, during the tests, the flight speed




was reduced very gradually until total stall occurred. The reduction in minimum speed due to themovable flaps was recorded in this way. For comparison, test flights were also carried out withlocked movable flaps. The reduction in minimum speed due to the movable flaps was 3.5%. Thiscorresponds to a 7% increase of lift. Taking into account that only 61% of the wing area wasequipped with movable flaps, one obtains an 11.4% increase of maximum lift for the airfoil. Thisis exactly the same value that had been obtained previously in the wind tunnel with the same typeof movable flap.

The comments of the pilot were also positive. Permanent spinning did not occur following astraight-flight stall situation. In contrast, with locked movable flaps, permanent spinning developedfrom the same situation. However, due to our cautiousness, the flaps were installed only in the innerpart of the wing. Therefore, the changes in flight behaviour were only moderate, albeit positive.Another observation was that keeping the flight speed at low and near-stall values appeared tobe easier with movable flaps. More detailed information can be found in a recent report on thissubject [9].

1.2 Further experiments with self-activated flaps

All the above-mentioned investigations were done on wings with a large aspect ratio with a two-dimensional flow field. On the examined airfoils, the flow separation starts at the trailing edgeand develops with increasing angle of attack gradually towards the leading edge. This favourablebehaviour of the flow separation is not necessarily found in the wings of large transport aircraft. Itwas also of interest to find out whether it was possible to use self-activated flaps on swept wingsas well as swept and tapered wings, which are common to these aircraft types. Experiments (seeFig. 10, left) with an aircraft model with a swept wing of constant chord were carried out. The tested

Figure 10: Wind tunnel tests with self-actuating flaps on different aircraft models; left: half modelwith swept wing of constant chord; right: half model of an AIRBUS A320.




configurations of both models were equipped with the extracted high-lift devices consisting ofslats and fowler flaps.

It turned out that the flow field on the swept wing was dominated, especially at higher anglesof attack, by secondary flow. The design of our flaps, which was optimized for a two-dimensionalflow separation that starts at the trailing edge of the airfoil, did not perform well under thesethree-dimensional flow conditions. A slight improvement to this situation was achieved by theuse of boundary layer fences. With these boundary layer fences, the cross-flow component ofthe secondary flow on the wing was inhibited. The performance of the self-activated flaps wasimproved, but the parasitic drag of the boundary layer fences makes this configuration unsuitable.

A wind tunnel test with a half model of an Airbus A320 (see Fig. 10, right) showed clearly thatthe three-dimensional flow field of the swept and tapered wing was strongly adverse to the use ofself-activated flaps. Although the flaps reacted to the incipient flow separation, no improved liftbehaviour was observed.

2 Concluding remarks

We think that movable flaps look promising for small aircraft with simple flaps and for glideraircraft. They work very well on wings with a large aspect ratio. The flow separation is suppressedefficiently when it starts at the trailing edge and develops gradually with increasing angles of attacktowards the leading edge. Other applications, such as use on rudders of boats and ships as well ason sails of surfboards and sailing boats, may be possible.

On the other hand, we would also like to present the cases where we were not successful:

(i) For sophisticated high-lift systems with slats and Fowler flaps, we achieved only marginaleffects with additional movable flaps, which suggest that this is not a useful application.

(ii) Movable flaps cannot be combined with vortex generators being installed upstream. Thisis because vortex generators generate warped, free shear layers with which movable flapscannot interact in a meaningful way.

(iii) For swept and tapered wings, it is very difficult to obtain a positive effect with self-activatedmovable flaps. The complicated secondary flow in such wing configurations is significant,and it would seem, only birds can handle this.

Acknowledgements

This research is funded by the German National Science Foundation (DFG) under the umbrellaof the Special Research Activity (Sonderforschungsbereich, SFB 557, ‘Beeinflussung komplexerturbulenter Scherströmungen’) at the Technical University Berlin. The person who took the highestpersonal risk in this project was the test pilot P. Montag of the STEMME Aircraft Company,Strausberg. In addition, we appreciate the support of Dr. R. Stemme and M. Lang of the STEMMEAircraft Company. A. Quast, DLR Braunschweig, supplied a test wing. Prof. W. Liebe, Berlin,and Dr. J. Mertens, DaimlerChrysler Aerospace Airbus GmbH, Bremen, provided very valuablecomments and advice. Financial support was provided by the German Federal Ministry of Science,Technology and Education (BMBF) and is gratefully acknowledged.

Dr. D.W. Bechert passed away in December 2004. He was a pioneer in the field of drag-reducingriblets derived from sharkskin and self-actuating flaps derived from bird feathers, and was alwaysopen to inspiration from nature. To him, nature was a source of stimulation for technical inventions.He also thought that nature may serve as evidence for the functionality of a technical inventionwhen the same mechanism ‘turns up’ when taking a close look at biology.




References

[1] Rechenberg, I., Bannasch, R., Patone, G. & Müller, W., Aeroflexible Oberflächenklappenals ‘Rückstrombremsen’ nach dem Vorbild der Deckfedern des Vogelflügels, Statusbericht1995 für das BMBF-Vorhaben 13N6536, Inst. f. Bionik u. Evolutionstechnik, TU Berlin,1995.

[2] Liebe, W., Der Auftrieb am Tragflügel: Entstehung und Zusammenbruch. Aerokurier, Heft12, pp. 1520–1523, 1979.

[3] Malzbender, B., Projekte der FV Aachen, Erfolge im Motor- und Segelflug. Aerokurier,Heft 1, p. 4, 1984.

[4] Erk, P., Separation control on a post-stall airfoil using acoustically generated perturbations,PhD Thesis, Hermann-Föttinger-Institut für Strömungsmechanik, TU Berlin, 1997.

[5] Schatz, M., Knacke, T., Thiele, F., Meyer, R., Hage, W. & Bechert, D.W., Separation controlby self-activated movable flaps, AIAA-Paper 2004-1243, 2004.

[6] Greenblatt, D., Seifert, A. & Wygnanski, I., Dynamic stall management by oscillatory forc-ing, Paper presented at the EUROMECH 361 Colloquium ‘Active Control of Turbulent ShearFlows’, Berlin, 17–19 March 1997.

[7] Parkinson, G.V., Brown, G.P. & Jandali, T., The aerodynamics of two-dimensional airfoilswith spoilers. V/STOL Aerodynamics, AGARD-CP-143, pp. 14/1–14/10, 1974.

[8] Katz, Y., Nishri, B. & Wygnanski, I., The delay of turbulent boundary layer separation byoscillatory active control, AIAA-Paper 89-0975, 1989.

[9] Meyer, R., Experimentelle Untersuchung von Rückstromklappen auf Tragflügeln zurBeeinflussung von Strömungsablösungen, PhD Thesis, Mensch & BuchVerlag, Berlin, 2001.



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