Application of leading-edge vortex manipulations to reduce wing rockamplitudes

Abstract

HE extension of an airplane's performance envelope into the high angle-of-attack region to improve maneuverability often carries the penalty of undesirable instabilities. One frequently encountered lateral instability is the limit-cycle roll oscillation, rock, which is driven by strong, concentrated vortices originating from the leading edges of highly swept lifting surfaces.!~8 Mathematical models917 at various levels of complexity have clarified the role of these vortices in driving the rolling motion. Simultaneously accumulated experimental evidence18-19 indicates that a wing's rolling moment can be affected by mechanical or pneumatic manipulation of these vortices. Since these vortices generate the much needed high-lift coefficients at larger angles of attack, it is logical to assume that control of this undesirable roll oscillation is possible by manipulating the strength and/or location of the vortices1820 that cause the wing rock. In the present study, a simple flap system was incorporated into the front section of a slender double-delta wing where the leading-edge vortex motion could be influenced by moving these flaps (Fig. 1). Since analytical studies21 on the control of wing rock indicate that a feedback of the roll oscillation rate, or a frequency shift between the wing motion and the control device, can generate damping of the motion, the proposed flap system was activated (mechanically) at the same frequency of the rolling motion. Also, instead of feeding back the roll rate, a relative phase shift (between the oscillatory motions of the flaps and wing) was imposed due to the mechanical simplicity required for such a wind-tunnel model. Experimental Apparatus A schematic description of the free-to-roll wind-tunnel model is shown in Fig. 1 (top) and additional geometrical details of the model are provided in the bottom of Fig. 1. Each leading-edge flap was mounted on two roller bearings to reduce friction, and the size of these bearings dictated the wing thickness. Intuitively, the flaps should have been placed at the most forward position, starting at the apex, but because of the large size of the bearings the flaps were moved approximately 45% of the chord behind the appex. Consequently, to increase the likelihood of being able to manipulate leading-edge vortex location by such simple leading-edge flap deflections, a double-delta wing shape was used. This choice is based on experimental evidence2224 indicating that a second vortex forms near the leading-edge break point and, therefore, the flaps were placed behind this point. (As most highspeed airplanes have thin, highly swept strakes that cannot house movable control surfaces, the above placing permits