Computational Modeling of the Hummingbird Escape Maneuver

Abstract

Hummingbirds are perhaps the most agile flyers in nature. Studying the underlying physics of hummingbird flight may provide inspiration for developing highly maneuverable micro aerial vehicles (MAVs). In this work, we have developed a high-fidelity computational fluid dynamics (CFD) model to analyze the aerodynamics and flight mechanics of the hummingbird escape maneuver, in which hovering hummingbirds were startled and perform a rapid (less than 0.2 seconds) maneuver to back away from a perceived looming threat on the front side. Several novel mechanisms were discovered in this study. First, we found evidence across several species that hummingbirds use the inertial forces of their wings to increase their body rotational acceleration. For example, at pronation when the wings switch from upstroke to downstroke, the wing inertial forces create a torque that helps pitch up the bird’s body to move the head away from the threat. Such inertial steering effects are accompanied by aerodynamic steering of the wings to generate a fast rotational speed to improve maneuverability. Second, we studied the actuation of the wings during the escape maneuver by considering the muscle input at the shoulder joint in addition to the wings’ aerodynamic and inertial outputs. Contrary to previous thoughts that wing pitch rotation is primarily passive due to the wings’ own inertia, our results show that significant power input was required to pitch up the wings during downstroke to enhance aerodynamic force production. As a result, an active mechanism is required to pitch the wings for maximal force output. Third, we found that pitch, roll, and yaw rotations of the bird body may overlap with one another, which may create a nonlinear, inertial coupling effect that hummingbirds utilize as a passive mechanism for flight control, e.g., to stabilize body pitching during the maneuver. These novel findings have significantly improved our understanding of hummingbirds’ great maneuverability and may be useful for the future development of bioinspired MAVs.