Passive rotation of a flapping wing with an inhomogeneous mass distribution

 By flapping wings, insects obtain lift to sustain flight. According to experimental and theoretical investigation, it is found that the wing of insects can be rotated passively owing to the aerodynamic and wing inertial forces. In addition, the wings of insect have various shapes and an inhomogeneous mass distribution, which introduces an additional torque owing to mismatch of rotation axis and the action point of the resultant of gravity force and buoyancy force. In this paper, we experimentally investigated the passive rotation of the flapping wing with an inhomogeneous mass distribution with a Reynolds number around 250 and measured the time varying kinematics by using high-speed camera. Experimental results showed that larger additional torque results in larger stroke angle, which generates larger conventional lift force due to the wing translation. Moreover, increasing additional torque can lead to advanced wing rotation, which is also beneficial to lift generation. Then, we compared the experimental results to the prediction of a quasi-steady numerical model. Our work demonstrates that the passive rotation of flapping wing depends sensitively on the additional torque, which potentially is used as an effective control method for design micro aerial vehicles (MAVs).