Leading-edge curvature effect on aerodynamic performance of flapping wings in hover and forward flight.

This study investigates the role of leading-edge (LE) curvature in flapping wing aerodynamics considering hovering and forward flight conditions. A scaled-up robotic model is towed along its longitudinal axis by a rack gear carriage system. The forward velocity of the robotic model is changed by varying the advance ratioJfrom 0 (hovering) to 1.0. The study reveals that the LE curvature has insignificant influence on the cycle-average aerodynamic lift and drag. However, the time-history lift coefficient shows that the curvature can enhance the lift around the middle of downstroke. This enhanced lift is reduced from 5% to 1.2% asJchanged from 0 to 1.0. Further flow examinations reveal that the LE curvature is beneficial by enhancing circulation only at the outboard wing sections. The enhanced outboard circulation is found to emanate from the less stretched leading-edge vortices (LEVs), weakened trailing-edge vortices (TEVs), and the coherent merging of the tip vortices (TVs) with the minor LEVs as observed from the phase-lock planar digital particle image velocimetry measurements. The far-wake observation shows that the LE curvature enhances the vorticity within the TV, helping to reduce the overall flow fluctuations in the far field. These findings can be extended to explain the predominantly straight LE wing shape with a small amount of curvature only observed near the wing tip for flapping fliers with Re from 103to 104.

Aerodynamic performance of flexible flapping wings deformed by slack angle.

Wing flexibility is unavoidable for flapping wing flyers to ensure a lightweight body and for higher payload allowances on board. It also effectively minimizes the inertia force from high-frequency wingbeat motion. However, related studies that attempt to clarify the essence of wing flexibility remain insufficient. Here, a parametric study of a flexible wing was conducted as part of the effort to build an aerodynamic model and analyze its aerodynamic performance. The quasi-steady modeling was adopted with experimentally determined translational forces. These forces were determined from 84 flexible wing cases while varying the angle of attack at the wing rootαrand the flexibility parameter, slack angleθS, with 19 additional rigid wing cases. This study foundαrfor optimum lift generation to exceed 45° irrespective ofθS. The coefficient curves were well-fitted with a cubed-sine function. The model was rigorously validated with various wing kinematics, giving a good estimation of the experimental results. The estimated error was less than 5%, 6%, and 8% for the lift, drag, and moment, respectively, considering fast to moderate wing kinematics. The study was extended to analyze the pure aerodynamic performance of the flexible wing. The most suitable wing for a flapping-wing micro-aerial vehicle wing design with a simple vein structure was found to be the 5° slack-angled wing. The inference from this study further shows that a small amount of deformation is needed to increase the lift, as observed in natural flyers. Thus, wing deformation could allow living flyers to undertake less pitching motion in order to reduce the mechanical power and increase the efficiency of their wings.