Lateral flight instability in fruit flies is determined by wing-wing interaction and wing elevation kinematics

Understanding the passive dynamics of flying insects is important for evaluating the constraints under which the insect flight control system evolved and for developing biomimetic robots. Previous studies using computational fluid dynamics (CFD) simulations on several insects found that they are passively unstable due to a non-oscillatory diverging mode, with positive coupling between roll and sideways motion. These studies assumed simplified and sinusoidal wing kinematics, with zero elevation angle and negligible wing-wing interaction. Here, we revisit these assumptions, by performing CFD-based linear stability analysis of a fruit fly with accurate experimental wing kinematics and wing-wing interactions. We find that passive lateral dynamics are unstable, but with an oscillating-diverging mode that stems from a negative coupling between roll and sideways motion: e.g., when the fly slides to the right, it rolls to the left. Due to low damping, this seemingly restoring effect results in an oscillating instability. This coupling stems from wing elevation that induces a drag-based restoring torque, and from wing-wing interaction, that reduces lift-based diverging torque. These results highlight the importance of real wing kinematics and wing-wing interactions in such analyses.