Droneflies combine passive and active wing pitch modulation under unilateral wing damage

The remarkable flight capabilities of insects stem from the subtle modulation of their wing kinematics. Dipteran insects can beat their wings hundreds of times per second, with each wing reversing its pitch direction twice per wingbeat. However, the neural signals hardly match up to such high frequencies, making it interesting to consider how insects regulate wing pitch in a timely manner. In this study, we investigate this problem through observations and simulations involving droneflies. Wing kinematics modulation is induced by imposing unilateral wing damage and recorded by high-speed videography. In tethered flight, the wing pitches passively without mechanical feedback, creating an imbalanced roll moment, which can be overcome by active actuation in free hover. Simulations, based on the torsional spring passive pitching model, suggest that insects modulate wing pitch through the coupling of aerodynamic, inertial, and wing hinge elastic forces. The model predicts the trend of wing pitch subjected to wing area loss as the observation and achieves a similar control effect of balancing roll moment with a slight adjustment to spring parameters. Based on these results, we show that insects combine passive and active mechanisms for wing pitch modulation.