Experimental Characterization of the Downwash Generated by an Insect-Like Flapping-Wing System

Flight dynamics and control of tailed flapping-wing aerial vehicles are expected to strongly depend on the flow conditions in which the tail is positioned. This work experimentally characterizes the far wake from an insect-like flapping-wing system in hovering conditions. Particle image velocimetry is used to create a high-resolution visualization of the instantaneous and time-averaged flowfield at various locations below the flapping wings. The flow velocity is concentrated directly below the wings, with a large region of near-zero velocity between the wing roots. Directly below the area covered by the wing stroke, the wake is highly unsteady. Wake velocity is strongest below the outer half of the wingspan and in the first few chord lengths below the wing. Beyond that range, velocity amplitude diminishes quickly. Part of the wing momentum is imparted to the wake, causing the vortices created during up- and downstroke to diverge longitudinally as they travel downstream. The wakes produced by the two wings appear to attract each other as they converge laterally. The observed wake characteristics are relevant to wake modeling efforts used in tailed flapping-wing drone design. Implications for wake modeling and tailed vehicle flight dynamics and control are discussed.