Design, modelling, and experimental validation of a self-rotating flapping wing rotorcraft with motor-spring resonance actuation system.

Compared with traditional flapping motion, Flapping Wing Rotor (FWR) frees the rotating freedom by installing the two wings asymmetrically, which introduces rotor's motion characteristics and enables FWR to have higher lift and aerodynamic efficiency at low Reynolds number. However, most of the proposed FWRs contain linkage mechanical transmission structures, the fixed degrees of freedom (DoFs) of which prohibit the wings from achieving variable flapping trajectories, limiting the further optimization and controller design of FWRs. In order to fundamentally address the above challenges of FWRs, this paper presents a new type of FWR with two mechanically decoupled wings, which are directly driven by two independent motor- spring resonance actuation systems. The proposed FWR has 12.4 grams of system weight and 165-205mm wingspan. In addition, a theoretical electromechanical model based on the DC motor model and quasi-steady aerodynamic forces is established, and a series of experiments are conducted in order to figure out the ideal working point of the proposed FWR. It is notable that both our theoretical model and experiments exhibit uneven rotation of the FWR during flight, i.e., rotation speed dropping in the downstroke while boosting in the upstroke, which further tests the proposed theoretical model and uncover the relationship between flapping and passive rotation in the FWR. To further validate the performance of the design, free flight tests are conducted, and the proposed FWR demonstrates stable liftoff at the designed working point.