Drag Reduction of Airfoils with Miniflaps. Can We Learn from Dragonflies?

Miniflaps at the trailing edges of airfoils (e.g., Gurney flaps or divergent trailing edges) change the Kutta condition and thus produce higher lift. Unfortunately, however, the drag is also increased due to the flow separation downstream of this particular type of trailing edge. Therefore, the trade-off between beneficial and detrimental effects is considered in this paper. Various aspects of the flow on airfoils with Gurney flaps are addressed: (1) Transonic flow. Wind tunnel experiments have been carried out with a CAST 10-2/DOA2 airfoil with Gurney flap and at high subsonic flow speed. The lift to drag ratio is improved and the test wing behaves like one having a 20% larger surface area. In addition, buffeting becomes less critical. (2) Selection of flap size. Detailed wind tunnel studies have been carried out with a low-drag glider wing at low subsonic velocities. The Gurney flap height was varied in six steps so that the most relevant parameter regime was covered. For the lift increase and for the device drag, simple empirical laws were obtained. Subsequently, for practical applications, a procedure for the selection of a suitable flap height providing a beneficial effect has been devised. (3) Drag reduction by wake stabilization. In the separation regime downstream of a Gurney flap, an absolute instability, i.e., a Karman vortex street occurs, even if the incident boundary layers are turbulent. Therefore, one approach towards drag reduction is to stabilize the wake and hence eliminate the Karman vortex street. This can be achieved with a variety of trailing edge modifications, e.g., with slits or holes in the Gurney flap. A particular structure which exhibits spadelike protrusions at the trailing edge produces also good results. Actually, the latter structure has been adopted from the trailing edge of dragonfly wings. Eliminating the Karman vortex street with these various trailing edge modifications reduces the Gurney flap device drag by about 22 – 30% without a perceivable change of the enhanced lift. Obviously, vibration and noise radiation are also reduced together with the suppression of the Karman vortex street. (4) Drag reduction with a “wakebody”. The separation regime downstream of the Gurney flap is not removed by merely stabilizing the wake flow. However, filling this regime with a tapered (twodimensional) body does produce attached flow. With this device, termed a “wake-body” it is possible to further reduce the Gurney flap device drag by more than 48%. Copyright © 2000 by D. W. Bechert, R. Meyer & W. Hage. Published by the American Institute of Aeronautics and Astronautics, Inc. With permission.