Effects of wingtip local sweep on the aerodynamic performance of flapping wings

This study focuses on the wingtip local sweep commonly found in bird wings and aims to systematically clarify its aerodynamic effect on flapping-wing vehicles. To this end, a representative flapping-wing model with a 20° local sweep at two-thirds of the span was designed, and both rigid CFD simulations and flexibility-resolved simulations driven by Digital Image Correlation (DIC) measured deformation were conducted to comparatively analyze the aerodynamic differences and evolution between the swept wing and the unswept baseline wing. The results show that under rigid conditions, the wingtip local sweep significantly enhances the aerodynamic efficiency of the flapping-wing: compared to the baseline wing, the average lift coefficient increases by 10.15%, the drag coefficient decreases by 10.71%, the power coefficient exhibits only a slight increase of 0.63%, and the lift efficiency rises by 10.78%, primarily due to the stable attachment of the Leading-Edge Vortex (LEV). Further comparison under flexible conditions shows that the structural flexibility does not weaken but rather enhances the aerodynamic benefit of the swept wing by adaptively responding to flow field variations, resulting in an increased lift efficiency up to 28.86%. Mechanistic analysis suggests that the local wingtip sweep reduces the local effective inflow velocity and increases the reduced frequency ( k ), thereby delaying LEV shedding while strengthening spanwise flow and vorticity transport.This mechanism aids in maintaining LEV stability and optimizing aerodynamic load distribution. This study confirms the significant aerodynamic advantages of wingtip local sweep during flexible flapping flight and provides important theoretical support and engineering references for the efficient aerodynamic design of Flapping-Wing Micro Air Vehicles(FW-MAVs).