Active and Passive Damping Effects During Hummingbird Escape Maneuvers

When hummingbirds detect a threat during hovering, they perform rapid body rotations in combination with high linear accelerations to escape. Fast and precise control of their body rotations is critical in such escape maneuvers. In this work, we computationally modelled the full-body aerodynamics of the hummingbird escape maneuver and identified the damping mechanisms that the birds utilize for rotation control after the top rotational velocities have been reached. We performed numerical simulations of the hummingbird for both the free-body flight and the "fixed-body" flight, where the bird's body rotation was removed and only the wing kinematics relative to the body were incorporated into the simulation. The comparison of two simulations allows us to identify the passive damping effects inherent in flapping-wing flight, e.g., the well-known flapping counter torque (FCT) that is induced when the body rotation is coupled with wing stokes. Our results show that in producing passive damping torques, the body rotation not only modifies the wings' velocities but also their effective angles of attack, an effect that is not included in the FCT. In addition to the passive damping mechanisms, we found that the bird also uses significant active damping to control the body rotations.