Movable flaps: artificial bird feathers[63]

This concept is illustrated in Fig. 8.20. Superficially it appears similar to the Gurney flap. However, the mode of operation is quite different. And, in any case, for positive high lift the Gurney flap would be attached to the trailing edge pointing downwards. The basic idea here is that at high angles of attack when flow separation starts to occur near the trailing edge, the associated reversed flow causes the movable flap to be raised. This then acts as a barrier to the further migration of reversed flow towards the leading edge, thereby controlling flow separation.

The movable flap concept originated with Liebe* who was the inventor of the boundary-layer fence (see Section 8.4.3). He observed that during the landing approach or in gusty winds, the feathers on the upper surface of many bird wings tend to be raised near the trailing edge. (Photographs of the phenomenon on a skua wing are to be found in Bechert etal. 1997.) Liebe interpreted this behaviour as a form of biological high-lift device and his ideas led to some flight tests on a Messerchmitt Me 109 in 1938. The device led to the development of asymmetric lift distributions that made the aircraft difficult to control and the project was abandoned. Many years later a few preliminary flight tests were carried out in Aachen on a glider.* In this case small movable plastic sheets were installed on the upper surface of the wing. Apparently it improved the glider’s handling qualities at high angles of attack.

There are problems with movable flaps. Firstly, they have a tendency to flip over at high angles of attack when the reversed flow becomes too strong. Secondly, they tend not to lie flat at low angles of attack, leading to a deterioration in aerodynamic performance. This is because when the boundary layer is attached the pressure rises towards the trailing edge, so the space under the flap connects with a region of slightly higher pressure that tends to lift it from the surface. These problems were largely overcome owing to three features of the design depicted in Fig. 8.21 which was fitted to a laminar glider aerofoil (see Bechert etal. 1997). Ties limited the maximum deflection of the flaps. And making the flap porous and the trailing edge jagged both helped to equalize the static pressure on either side of the flap during attached-flow conditions. These last two features are also seen in birds’ feathers. The improvement in the aerodynamic characteristics can also be seen in Fig. 8.21.

Movable flap

Successful flight tests on similar movable flaps were carried out later on a motor glider.