Active and Passive Damping Effects During Hummingbird Escape Maneuvers

Fast and precise control of body rotations is crucial to the amazing agility of hummingbirds. However, many questions remain unclear, especially when it comes to complex maneuvers such as escaping from perceived threats. In this work, we computationally modelled the full-body aerodynamics of the hummingbird escape maneuver that was evoked from the frontal side of the bird while hovering. Our goal is to identify the aerodynamic damping mechanisms that the birds utilize for rotational control. We performed numerical simulations of a hummingbird for both the free-body flight and the "fixed-body" flight, in which the bird's body rotation was removed and only the wing kinematics relative to the body were incorporated into the simulation. The comparison of the 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 the body rotation not only modifies the wings' velocities but also their effective angles of attack when there is rotation about all three body axes (a feature in the escape maneuver under consideration), an effect that was not included in previous analysis of the FCT. In addition, we found that the bird also uses significant active damping to control the body rotations. We have used a quasi-steady model that incorporates the effect of body rotation on the angle of attack to confirm our findings.