Inertial and aerodynamic effects in hummingbird escape maneuvers.

In this study, we use a high-fidelity CFD approach to investigate the underlying flight mechanics of hummingbirds during rapid escape maneuvers that are characterized by pitch-up and roll rotations in addition to a backward linear acceleration. The full-body kinematics of two Rivoli's hummingbirds transitioning from hovering to escaping were recorded on video in the lab and then reconstructed for 3D CFD simulation. In addition to detailed aerodynamics forces from the simulation, inertial forces were also calculated based on wing mass distribution. The results show that hummingbirds generate peak pitch or roll rotational accelerations within a wingbeat cycle, primarily using inertial torques of the wings. The aerodynamic torques also play an important role within the same wingbeat by counteracting the opposite inertial torques following the rotational acceleration phase, thereby reducing the deceleration and maintaining a high angular rate of rotation. Another important finding is that the birds may make use of inertial coupling of the body (i.e., the cross-product terms of the moment of inertia) to transfer body momentum from one body axis to another.