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

Understanding the uncontrolled passive dynamics of flying insects is important for evaluating the constraints under which the insect flight control system operates and for developing biomimetic robots. Passive dynamics is typically analyzed using computational fluid dynamics (CFD) methods, relying on the separation of the linearized hovering dynamics into longitudinal and lateral parts. While the longitudinal dynamics are relatively understood across several insect models, our current understanding of the lateral dynamics is lacking, with a nontrivial dependence on wing-wing interaction and on the details of wing kinematics. Particularly, the passive stability of the fruit fly, D. melanogaster, which is a central model in insect flight research, has so far been analyzed using simplified quasi-steady aerodynamics and synthetic wing kinematics. Here, we perform a CFD-based lateral stability analysis of a hovering fruit fly, using accurately measured wing kinematics, and considering wing-wing interaction. Lateral dynamics are unstable due to an oscillating-diverging mode with a doubling time of 17 wingbeats. These dynamics are determined by wing-wing interaction and the wing elevation kinematics. Finally, we show that the fly's roll controller, with its one wingbeat latency, is consistent with the lateral instability. This work highlights the importance of accurate wing kinematics and wing-wing interactions in stability analyses and forms a link between such passive instability and the insects' controller.