An experimental study of a leading-edge alula-inspired device (LEAD) for moderate aspect ratio wings at low Reynolds numbers

Even though Unmanned Aerial Vehicles (UAVs) operating at low Reynolds numbers are becoming common, their performance and maneuverability are still greatly limited due to aerodynamic phenomena such as stall and flow separation. Birds mitigate those limitations by adapting their wings and feather shapes during flight. Equipped with a set of small feathers, known as Alula, located near the leading edge and covering 5% to 20% of the span, bird wings can sustain the lift necessary to fly at low velocities and high angles of attack. The proposed alula-inspired leading-edge device (LEAD) increases the capability of a wing to maintain higher pressure gradients by modifying the near-wall flow close to the leading-edge. It also generates tip vortices that modify the turbulence on the upper-surface of the wing, delaying flow separation. The effect of the LEAD can be compared to traditional slats or vortex generators on two-dimensional wings. For finite wings, on the other hand, the effect depends on the interaction between the LEAD’s tip vortices and those from the main structure. This research presents the effect on lift generation of different placements of the LEAD along the span of a moderate aspect-ratio wing. Wind tunnel experiments were conducted on a wing with an S1223 airfoil at post-stall and deep-stall angles of attack and at low Reynolds numbers of 100,000 and 135,000. To quantify the aerodynamic effect of the device, the lift generated by the wing with and without the LEAD were measured using a 6-axis force and torque transducer, and the resulting lift coefficients were compared. Results show that, in general, the location of the LEAD root yielding the highest lift enhancement was 50% semi-span away from the wing root. Lift improvements of up to 29% for post stall and 32% for deep stall were obtained at the best location, demonstrating that the three-dimensional effects of the LEAD are important. The lift enhancement was also more prominent on a finite moderate aspect-ratio wing (3D) than on an airfoil (2D), confirming that the LEAD is a three-dimensional device. Wake boundary layer sampling through hot-wire anemometry showed that stall forms at the root of the rectangular test wing and propagates toward the tip. The addition of the LEAD to the wing resulted in a reduction of velocity deficit, indicating the attenuated flow separation, in the region along the wingspan that is covered by the device. Identifying the configurations and deployment parameters that improve lift generation and mitigate stall the most is needed to design an adaptive LEAD that can be implemented on a UAV wing for increased mission-adaptability