Design and aerodynamic analysis of an airfoil with a bioinspired leading edge device for stall mitigation at low Reynolds number operation

Robust and predictable aerodynamic performance of unmanned aerial vehicles at the limits of their design envelope is critical for safety and mission adaptability. Deployable aerodynamic surfaces, such as flaps or slats, from the wing leading or trailing edges are often used to extend the aerodynamic envelope. One such aerodynamic device is the Alula, a feather structure attached to one of the hand digits of a bird's wing. The alula is extended by birds at high incidence angles and has been shown to improve the stall parameters of the wings. In this study, a series of wind tunnel experiments are performed to quantify the effect of various deployment parameters of an alula-like leading edge device on the aerodynamic performance of a cambered airfoil (S1223). The alula relative angle of attack, measured from the mean chord of the airfoil, is varied to modulate tip-vortex strength, while the alula deflection is varied to modulate the distance of the tip vortex to the wing surface. Boundary layer velocity profile measurements taken at x/c = 1.25 along the chord length and at three locations along the span of the airfoil show fuller BL profiles in the area of influence behind the alula. The resulting re-energizing of the BL at post stall angle of attacks delays flow reversal and separation and decreases associated drag. Results show that as alula deflection ratio increases, the lift coefficient Cl also increase. At post stall angles of attack, the wake velocity deficit zone is shown to reduce in size when the alula is deployed, confirming that the wing adverse pressure gradient is reduced. The results are in strong agreement with the measurements taken on bird wings with alulae. With the ability to change alula parameters such as location, size, deflection and angle, the complete wing configuration can be tuned for mission specific aerodynamic requirements