Inertial coupling in the flight mechanics of hummingbirds during an escape maneuver

When responding to a perceived threat, hummingbirds perform an impressive escape maneuver from hovering to backing off and then flying away. In the process, their body goes through simultaneous pitch, yaw, and roll rotations. The rotational dynamics of the body therefore involves inertial coupling from these multi-axis rotations. In this study, we analyzed high-speed videos and performed high-fidelity computational fluid dynamics modeling of the maneuver. We found that while a bird's pitch-up was occurring, inertial coupling between yaw and roll helped slow down and terminate the pitch, thus serving as a passive control mechanism for the maneuver. Furthermore, an inertial coupling between pitch-up and roll can help accelerate yaw before the yaw-roll coupling. Compared with aircraft and other animals that primarily rely on aerodynamic forces for flight control, hummingbirds seem to utilize inertial coupling as a novel mechanism to achieve superb aerial agility.