Effects of Eigenmodes, Forward Velocity and Flapping Frequency in Force Generation Mechanisms of a Flapping-Wing MAV

Abstract

Wind tunnel testing is recurrent for the characterization of the time resolved evolution of the forces acting during a flap cycle of a flapping wing micro aerial vehicle (FWMAV). However, this characterization is often influenced by the lack of knowledge of the typical flapping kinematics and flight regime of the platform, as well as by the effects of attaching a cyclic force system to a force balance. In this regard, the current study analyzes the impact of the previous factors on the sub-flap forces acting longitudinally on a clap-and-fling flapping wing robot. This was done by replicating, in a wind tunnel setup, eight conditions that represent the typical real flight regime of an 17.4g ornithopter. Three force transducer clamping locations are studied, complemented with mechanical impulse and finite element analyses of the natural modes of the structure. The effects of forward flight and flapping frequency are determined by comparing the typical flight conditions with zero free-stream velocity force measurements. It was observed that eigenfrequencies of the ornithopter  are close to the first and second harmonics of the flapping frequency for two clamping locations, thus contaminating the force measurements. Furthermore, the forward velocity was found to impact the forces, by affecting the wing shape during flap. An increase in flight velocity reduces the amplitude of the forces acting along the body of the ornithopter (X force). Conversely, the forces along the stroke plane of the wings (Z force) change in phase with an increase in the flight velocity, increasing peak magnitude and the component of Lift force.