Experimental Investigation of Double-Hinged Vortex Flap Configurations

Abstract

D ELTA wings are poor lift generators. An often-employed design approach to ameliorate the low lift production is to sharpen thewing’s leading edge. A consequence of the enforced flow separation is the formation of leading-edge vortices. The vortices induce high surface velocities over the wing and may significantly augment lift, especially as the delta becomes more slender [1]. Leading-edge flaps (formed by downward rotation of a segment of the wing’s leading edge) may be used to lessen the performance loss by recovering thrust from the vortices by positioning them over wing elements with a forward-facing area: so-called leading-edge vortex flaps (LEVFs) [2]. Studies of vortex flaps (VFs) have shown the concept to be effective [2,3]; however, their performance is constrained by the characteristics of the leading-edge vortices with incidence. The vortices expand in size and migrate inboard off the flap. As a result, the flaps lose effectiveness. A study by Rinoie and Stollery [3] showed that the maximum lift-to-drag ratio L=D for the 60 and 70 deg delta wings they tested with vortex flaps coincided with smooth flow onto the flap with no separation. As a result, increasing wing incidence requires larger flap deflections to maintain maximized L=D values. Deflecting the flap reduces lift for a given geometric angle of attack as the vortex strength is reduced, as well as to a lesser extent, the attached flow lift [4]. An additional byproduct of flap deflection is a moderate increase in the minimum-drag coefficient. As a result, the initialL=D is attenuated, butmay increase significantly beyond that of the sharp planar delta for moderate to high lift coefficients. An attempt to reduce the drag penalty as well as limit the expansion of thevortex off theflapswas documented byRao [5]. Various flap configurations were investigated, variants including constant length and segmented configurations. The segmented flaps generated two smaller distinct leading-edge vortices so as to minimize vortex spillage. Rao’s results suggested that generating multiple leading-edge vortices was effective in delaying vortex spillage and thus suction loss from the flap. Generally, experimental observation has indicated that the leading-edge vortices on a planar delta lie along a trajectory of approximately 70% of the local semispan [6]. It may be feasible to design a vortex flap that is located further inboard, such that the flap hingeline runs along a ray of approximately 65% semispan. To limit the flap size, a tab may be employed, where the outer wing surface is inclined so as to be parallel to the inner wing section (see Fig. 1). A small tab was evaluated by Hoffler and Rao [7]; some of their designs resulted in the tab having a rearward inclination such that vortex formation over the tab would contribute additional drag. Other tabbed configurations that were tested were not deemed as successful [7]. In this paper, so-called double-hinged vortex flaps (HVF) are evaluated, where the configuration consists of a planar inboard portion, a vortex flap and a planar outboard panel. HVFs may reduce vortex loss with incidence (as its hingeline orientation is closer to the natural vortex trajectory, i.e., the flap is located further inboard), the minimum-drag penalty and structural complication. The undeflected outboard panel of the HVF may also augment the strength of the primary vortex compared with a conventional LEVF, where the effective leading-edge incidence is diminished. The proposed design would mitigate against a movable flap; thewing configuration would be fixed. The basis of comparison of the flaps in this study is different from that of Hoffler and Rao [7]. In the present study the flap area is conserved for a given flap area ratio, and does not include the area of the outboard panel (or tab). The configuration and size of the outboard panels are also conceptually different from those in [7], where the tabs were approximately constant chord and were not an integral part of the wing design, but an add-on. A low-speed windtunnel investigation was conducted to evaluate the concept, characterized through force-balance, pressure measurement, surface flow visualization, and vortex burst trajectories.