Coherent relationship between passive rotation and active stroke of a flapping wing

By referring to the flapping flight of insects, human has realized the flight of micro aerial vehicles (MAVs). To reduce the system mass and decrease the mechanical complexity of MAVs, the wings are generally designed to be passively rotated by means of aerodynamic and wing inertial forces. However, it is an open question how to modulate the passive rotation of the flapping wing during flight and steering maneuvers. Therefore, we designed a passive-rotation flapping apparatus and measured the time varying wing kinematics by using the high-speed camera. Experimental results showed that the passive rotation strongly depends on the kinematics of active stroke of wing. By modulating the stroke amplitude and frequency, one obtained different rotating amplitudes and velocities, as well as the phases between stroke reversal and wing rotation, which is of particular importance for unconventional force generation. Based on the amplitude and frequency of wing stroke, a driving Reynolds number is applied to characterize the active stroke. To characterize passively rotating motion, a rotational Reynolds number is proposed. Furthermore, a coherent relationship between the driving Reynolds number and the rotational Reynolds number was demonstrated.