Flow Structures Associated with Upper-Surface-Blown Airfoils

Abstract

Upper-surface blowing (USB) has been proposed as a means of increasing usable lift and thereby enhancing V/STOL capability at low speeds in landing configurations. At transonic Mach numbers, it has the further application of achieving low turn radii in dogfight scenarios. The attendant high accelerations are accomplished through elimination of separation by suppression of adverse pressure gradients in the viscous boundary layer, and also movement of shocks downstream of the trailing edge, thereby discouraging shock-induced separation and buffet at high maneuver incidences. Further applications of laminar-flow control through stabilization using tangential blowing to achieve favorable pressure gradients is of strong interest currently. In the application of this concept, the engine bleedoff, thrust, and structural penalties required to achieve the foregoing aerodynamic advantages is of importance to the designer. To obtain this relationship, a knowledge of the associated flow fields is required. Although attention has been given to the jet flap in theoretical investigations, relatively little analysis has been performed on upper-surface-blown configurations. For incompressible speeds, the work of Spence [4.217] represents the classical thin-airfoil treatment of the jet-flap problem. At transonic Mach numbers, a computational jet-flap solution based on small-disturbance theory was developed for airfoils, and generalized for three-dimensional wings by Malmuth and Murphy [4.218, 4.219].