Hovering hawkmoths exploit unsteady circulation to produce aerodynamic force

This study employs an integrated approach, combining three-dimensional flow visualization and two-dimensional flow measurement to investigate the underlying unsteady aerodynamic mechanisms of hovering hawkmoths. Using a single vortex ring model, three aerodynamic force components, such as aerodynamic force induced by unsteady circulation, vortex loop size variation and added mass, are estimated within a dimensionless time (normalized by one wing beat cycle) range of 0.418 < T^* < 0.455, where both the vortex loop circulation and loop size data are available. The force analysis reveals that the unsteady circulation-induced aerodynamic force dominates the overall force production and contributes 67% of the total force while the vortex loop size variation and added mass effect-induced aerodynamic forces only count for 25% and 8%, respectively. These findings suggest the hawkmoth primarily relies on unsteady circulation to generate aerodynamic forces.