Highly deformable flapping membrane wings suppress the leading edge vortex in hover to perform better

Airborne insects flap their wings to create a leading edge vortex on top of their wings which aids to keep them aloft. The leading edge vortex is a coherent accumulation of vorticity and a low pressure region which enhances the lift that is generated by flapping wings compared to fixed wings with the same geometries. In this study, we present a flexible membrane wing design for flapping wing flight that passively deforms under fluid dynamic loading and maintains a positive camber during the front and backstroke in hover. We observe experimentally that the unsteady deformation of the wing suppresses the formation of a coherent leading edge vortex as flexibility increases. At the lift and energy optimal aeroelastic conditions, there is no leading edge vortex anymore. Instead, the vorticity accumulates in a bound shear layer covering the wing's upper surface from the leading to the trailing edge. Despite the absence of a leading edge vortex, the optimal flexible membrane wings show enhanced lift and power economy compared to their rigid counterparts. We relate the force production on the wings with their deformation through scaling analyses. Additionally, we identify the geometric angles at the leading and trailing edges as observable indicators of the flow state and use them to map out the transitions of the flow topology and their aerodynamic performance for a wide range of aeroelastic conditions.