Leading-Edge Vortex Flap
As noted previously, the delta wing is a planform, that was developed for supersonic flight, but it also must operate at subsonic speeds. At high angle of attack, the wing lift is increased by virtue of vortex lift (see Fig. 6.28), but the wing drag also increases with increasing vortex lift. This is because the presence of a separation vortex at the wing leading edge effectively rotates the leading-edge suction through 90° (see Fig. 6.30c). This means that there is no longer a leading-edge suction to supply thrust and alleviate some of the drag.
The L/D ratio is a measure of the aerodynamic efficiency of a wing. Although vortex lift increases CL, the simultaneous increase in CD can result in the effective wing L/D ratio being lower. This reduced L/D may have a major impact on performance parameters such as takeoff and climb, where the L/D ratio has an important role.
The leading-edge vortex flap (LEVF) is a device that improves L/D for a delta wing at a high angle of attack at some penalty in lift. The LEVF is a small deflecting surface mounted at the leading edge of a delta wing, as shown in Fig. 6.33.
When the LEVF is deflected downward, the leading-edge separation acts on the flap surface and a thrust component is generated (compare Figs. 6.30c and 6.33). Thus, wing drag is decreased (i. e., thrust is increased) at the cost of a decrease in vortex lift. At very high values of CL, the vortex on the flap moves downstream and the full suction force F in Fig. 6.33 is not recovered. The experimental results in the paper by Rinoie and Stollery, 1994, indicate that at low speed, a 60° delta wing with sharp leading and trailing edges has a dramatic (i. e., 40%) improvement in L/D at CL = 0.45 with a vortex flap deflection of 30°.