Self-Actuating Flaps on Bird and Aircraft Wings 437
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Self-actuating flaps on bird and aircraft wings D.W. Bechert, W. Hage & R. Meyer Department 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 and in flight through very turbulent air, one observes that the covering feathers on the upper side of bird wings tend to pop up. The raised feathers impede the spreading of the flow separation from the trailing edge to the leading edge of the wing. This mechanism of separation control by bird feathers is described in detail. Self-activated movable flaps (= artificial bird feathers) represent a high-lift system enhancing the maximum lift of airfoils up to 20%. This is achieved without perceivable deleterious effects under cruise conditions. Several data of wind tunnel experiments as 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, the inventor 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 the birds were on landing approach or in other situations with high angle of attack, like flight through gusts. Once the attention of the observer is drawn to it, it is comparatively easy to observe this behaviour in almost any bird (see, for example, the feathers on the left-hand wing of a Skua in Fig. 1). Liebe interpreted this behaviour as a biological high-lift device [2]. Later, in 1938, Liebe worked as a young scientist at the former German Aeronautical Establishment (DVL—Deutsche Versuchsanstalt für Luftfahrt, the predecessor of the present DLR). Liebe attached a piece of leather 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 to be 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 that once separation starts to develop on a wing, reversed flow was bound to occur in the separation regime. Under these locally reversed flow conditions, light feathers would pop up. They would WIT Transactions on State of the Art in Science and Engineering, Vol 4, © 2006 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) doi:10.2495/1-84564-095-0/5e 436 Flow Phenomena in Nature Figure 1: Skua (Catharacta maccormicki) during landing approach. Photograph by I. Rechenberg [1]. Figure 2: Schematic representation of a Messerschmitt BF 109 airplane with artificial bird feathers on the right wing. act like a brake on the spreading of flow separation towards the leading edge. Liebe was aware of the fact that flow separation is often a three-dimensional effect with variable patterns in the spanwise direction. Thus, he considered it essential to be able to interact even with local separation regimes (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 small movable plastic sheets installed on a glider wing on the upper surface near the trailing edge [3]. It appeared 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 of Berlin, and the STEMME Aircraft Company in Strausberg near Berlin. Previous preliminary WIT Transactions on State of the Art in Science and Engineering, Vol 4, © 2006 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) 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 had hinted 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 aerodynamical effects 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 wake measurements) were considered crucial because (i) high angles of attack with considerable flow separation were most interesting, (ii) the quick data processing of a balance would enable us to test a large number of configurations and (iii) hysteresis at high angles of attack can be recorded only 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 feathers by attaching plastic strips to the wing surface produced rather confusing results. Therefore, we continued 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 either elastic plastic material or thin sheet metal were used. The flaps were attached to the rear part of the 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 increases in the downstream direction in the rear part of the upper surface of the airfoil. Thus, the space under the flap is connected to a regime of slightly elevated static pressure. Consequently, in most places, the pressure beneath the movable flap is higher than above it. This is the reason why the flap is slightly lifted in this case. As a matter of fact, this behaviour proved to be a disadvantage. WIT Transactions on State of the Art in Science and Engineering, Vol 4, © 2006 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) 438 Flow Phenomena in Nature 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 edge is decreased. Thus, the effective angle of attack of the airfoil is also decreased. Therefore, the impact of the movable flap is slightly deleterious. However, there are several ways to deal with this problem. The first and obvious one would be to lock the movable flap onto the airfoil surface under attached flow conditions. The second one is also rather simple: make the flap porous in order to obtain equal static pressure on both sides of the flap for attached flow conditions. A third method is to make the trailing edge of the flap jagged, as shown in Fig. 5. This leads to an exchange of pressures as well. Incidentally, the latter two ‘inventions’ are indeed found on bird wings. Now, how do the movable flaps respond to reversed flow? First, it should be mentioned that the flow 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 weak reversed 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 give percentage of airfoil chord. WIT Transactions on State of the Art in Science and Engineering, Vol 4, © 2006 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) Self-Actuating Flaps on Bird and Aircraft Wings 439 the flap back to a lower elevation. At this point, we would like to stress the marked difference between our movable flaps and a conventional rigid spoiler on a wing [4]: a spoiler protrudes into the high-speed flow regime and increases the width of the wake. In this way, it increases the drag and reduces the lift. In contrast, at high angles of attack, our device will do the opposite: reduce drag and increase lift.At the same time, the effective shape of the airfoil changes due to the slightly elevated flap and a lower effective angle of attack ensues. Thus, the pressure distribution on the airfoil 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 high angles of attack, the reversed flow would cause the flap to tip over in the forward direction, and the effect of the flap would vanish. This can however be prevented by limiting the opening angle of the flap.