Aerodynamics and dynamic stability of micro-air-vehicle with four flapping wings in hovering flight

Recently,a novel concept of flapping Micro-Air-Vehicles(FMAVs)with four wings has been proposed,which potentially utilizes the clap-and-fling effect for lift enhancement and agile maneuvers through an adjustment of wing kinematics.However,the application of the clap-and-fling effect in the four-winged FMAVs is underexplored and the dynamic stability is still unclear.In this paper,aerodynamics and flight dynamic stability of the four-winged FMAVs are studied experimentally and numerically.Results show that the clap-and-fling effect is observed when the flapping frequency is above 18 Hz.Due to the clap-and-fling effect,the lift generation and aerodynamic efficiency are both improved,which is mainly attributed to the fling phase.Further studies show that the clap-and-fling effect becomes weaker as the wing root spacing increases and is almost absent at a wing root spacing of 1.73 chord length.In addition,a wing with an aspect ratio of 3 can increase both lift generation and efficiency due to the clap-and-fling effect.Finally,according to the dynamic stability analysis of the four-winged FMAV,the divergence speed of the lateral oscillation mode is about 4 times faster than that of the longitudinal oscillation mode.Our results can provide guidance on the design and control of four-winged FMAVs.