Wing rapid responses and aerodynamics of fruit flies during headwind gust perturbations.

Insects are the main source of inspiration for flapping-wing micro air vehicles (FWMAVs). They frequently encounter wind gust perturbations in natural environments, and effectively cope with these perturbations. Here, we investigated the rapid gust response of flies to instruct the gust stability design of FWMAVs. A novel method to produce impulsive wind gusts that lasted less than 30 ms was designed to observe flies' rapid responses. Headwind gust perturbations were imposed on 14 tethered fruit flies, and the corresponding wing motions during perturbations were recorded by three high-speed cameras. The numerical simulation method was then applied to analyze aerodynamic forces and moments induced by the changes in wing kinematics. Results shows that flies mainly utilize three strategies against headwind gust perturbations, including decreasing the magnitude of stroke positional angle at ventral stroke reversal, delayed rotation and making the deviation angles in upstroke and downstroke closer (i.e. the wing tip trajectories of upstroke and downstroke tend be closer). Consequently, flies resist increments in lift and drag induced by the headwind gusts. However, flies seem to care little about changes in pitch moment in tethered conditions. These results provide useful suggestions for the stability control of FWMAVs during headwind gust perturbations.