Flapping counter torque and active control in the escape maneuvers of hummingbirds

Flapping counter torque (FCT) is an intrinsic mechanism in flapping-wing flight of animals, where the rotation of an animal's body creates asymmetric left-right wing motion, leading to a counter torque that opposes the body rotation. FCT corresponds to a passive damping effect that could be harnessed for disturbance rejection and flight stabilization, but its role in fast maneuvers remains unclear. In this work, we used the reconstructed escape flight of hummingbirds to test the effects of FCT in fast maneuvers, which features rapid and simultaneous body pitch, roll, and yaw, and linear accelerations. In addition to computational fluid dynamics (CFD) simulation of free-body flight, we also performed a fixed-body CFD simulation by removing the body-rotation induced wing velocities while retaining the wing kinematics relative to the body. The aerodynamic torques from the fixed-body flight are considered active torques, and the differences between the free and fixed-body flights are considered the FCTs. Our results show that the FCT in the roll axis is particularly strong during downstroke, due to the large bilateral wing velocity asymmetry associated with the body roll, as well as changes to the wings' angle of attack by body rotations around the other axes. To overcome the strong damping and sustain the rotation, the bird utilizes an active torque to overcome the FCT during downstrokes and also employs the wing kinematics that would incur less FCT during upstrokes. Overall, the hummingbird is able to alleviate and control the FCT and still achieve great agility in the maneuver.