Gust Rejection in Insect Flight

Abstract

Large commercial and military aircraft can fly in turbulent wind conditions except in extreme weather events like cyclones. Smaller man-made vehicles like Micro aerial vehicles (MAVs) and Nano aerial vehicles (NAVs) are more susceptible to normal fluctuations in wind speed encountered in the environment, and thus more difficult to control. Insects, on the other hand, stabilize themselves quickly in the presence of gustiness normally found in the atmosphere. However, very few studies have been carried out to understand the impact of gusts and turbulence on the flight performance of insects. Keeping this practical relevance in mind and to fundamentally understand the flight stability of insects under gusty environments, we investigated the flight of a freely flying insect (black soldier fly) subjected to a discrete head-on aerodynamic gust under controlled laboratory condition. Gust was generated in the form of a vortex ring which, unlike conventional methods of perturbation, is well studied and highly controllable. The flow properties of the vortex ring were characterized using flow visualization and studying the motion of a light bead. Reynolds number of the vortex ring, based on its average propagation velocity and nozzle exit diameter, was 15000, while that of fly, based on its wingtip velocity and mean wing chord, was 1100. Flight motion of the fly was recorded using two highspeed cameras, and body and wing kinematics were analyzed for cases. In response to the gust, we observed some common features in the cases analyzed: 1) asymmetry in the wing stroke amplitude, 2) large change in the body roll angle, by as much as 160°, that happened on an average, in two wing beats (~ 20 𝑚𝑠), and with the recovery in about 9 wing beats, 3) change to pitch down attitude, and 4) deceleration in flight direction. The ability to respond at such a short time scale and use of both passive and active control responses to gusts give some insight into the flight control strategies of insects. This study will help in better design of MAVs and NAVs to respond to gusts and unsteadiness in the natural environment.