Wave drag reduction by means of aerospikes on transonic wings

Abstract

Wave drag and shock induced boundary layer separation are important issues of flows around transonic wings. At transonic speeds the supersonic flow regime that is formed locally above wings, is terminated by a shock wave. This happens especially under offdesign flight conditions and results in a wave drag. In addition, the shock-boundary layer interaction can cause a separation of the boundary layer and can thus lead to further losses and, eventually, to buffeting. These phenomena limit the maximum economic cruise speed of airplanes. The negative effect of the transonic flow regime can be mitigated by controlling the shock terminating the supersonic region above the wing. In the past many different concepts based, for example, on passive ventilation (perforated plates, slots, grooves), active suction, contour bumps or adaptive walls have been persued [1-3].Mostly, these control methods are based on a two-dimensional approach, i.e., control devices are applied unifomly along the whole span. More recently, also three-dimensional control devices have been shown to positively affect lift and drag [4-7]. All these approaches have in common that measures for controlling the shock are applied directly at the surface of the wing. However, a control of the shock wave is also possible by placing external devices above the surface of the wing into the supersonic flow regime. The latter concept that is related to the one of aerospikes on blunt bodies, is studied in the present paper. As in the case of flow control measures that are applied directly at the surface of a wing the basic idea of aerospikes is achieving the pressure rise across a system of oblique and normal shocks instead of across a single normal shock thus reducing wave losses. In the present investigation oblique shocks were produced by disturbing the supersonic flow above the wing. In a test series the effectiveness of a variety of different spikeshaped bodies placed above a transonic wing was tested in the DNW-TWG, Göttingen. In addition to pressure measurements a colour schlieren system was set up for providing information about the influence of spikes on the flow field. In the following, first basics of aerospikes on transonic wings are explained. Then, wind tunnel experiments are described and results of measurements are presented and discussed. This is followed by a conclusion.