Hummingbirds use wing inertial effects to improve maneuverability

Hummingbirds outperform other birds in terms of aerial agility at low flight speeds. To reveal the key mechanisms that enable such unparalleled agility, we reconstructed body and wing motion of hummingbird escape maneuvers from high-speed videos; then, we performed computational fluid dynamics modeling and flight mechanics analysis, in which each wingbeat was resolved. We found that the birds utilized the inertia of their wings to achieve peak body rotational acceleration within half a wingbeat before the aerodynamic forces became dominant. The aerodynamic forces instead counteracted the reversed inertial forces in the other half wingbeat, thereby to sustain body rotation, albeit at a lower acceleration. Thus, individual wingbeat cycles that generated body rotations can be split into an agility phase with rapid inertial acceleration, and a stability phase with counteracting aerodynamic and inertial acceleration. This mechanism involving inertial steering enables hummingbirds to generate instantaneous body acceleration at any phase of a wingbeat, and it is likely the key to understanding the unique dexterity distinguishing them from aircraft that solely rely on aerodynamics for maneuvering.