Aerodynamic role of rapid wing pitch-up at mid-stroke in hovering mosquitoes

Mosquito wings execute rapid pitch-up during mid-stroke, markedly differing from the nearly constant-angle-of-attack flapping observed in many other insects. However, the aerodynamic role of this motion in mosquito hovering remains incompletely understood. Here, we numerically compare the aerodynamic performance using real measured wing kinematics with that under an artificially constructed trapezoidal pitch profile. The results show that both cases generate sufficient cycle-averaged vertical force to balance body weight, with nearly identical cycle-averaged energy expenditure. Nevertheless, the temporal distributions of force and aerodynamic power differ significantly: the real case produces large force peaks during the middle of the stroke, which are absent in the trapezoidal case, where large force peaks instead occur at stroke reversals. Further flow field analysis reveals that rapid pitch-up not only facilitates the rapid generation of vortices but also enhances force generation during mid-stroke by keeping the leading-edge vortex continuously and closely attached to the wing leeward surface, thereby enabling the delayed stall mechanism to function effectively. This rapid pitch-up can also prevent abrupt force fluctuations at stroke reversals, alleviating the potential risk of damage to the slender mosquito wing. These findings deepen our understanding of mosquito hovering aerodynamics and provide guidance for designing small-stroke-amplitude flapping-wing micro air vehicles.