Effects of Timing and Magnitude of Wing Stroke-Plane Tilt on the Maneuverability of Flapping Flight*

Hummingbirds perform a variety of agile maneuvers and one of them being the escape maneuver as they steer away from the threats using only 3-4 wingbeats in less than 150 ms. One kinematic feature that enables this maneuverability is the tilt of wing stroke plane. Here we investigate how timing and magnitude of the stroke-plane tilt affect this maneuverability using a flapping wing model with AR = 3 and Re=1000. The wing stroke plane is initially horizontal and then begins to tilt backward. Under different flapping amplitude, we quantify the effects of timing (relative to the phase of flapping cycle) and magnitude of this stroke-plane tilt on wing aerodynamic forces and moments using experimental, CFD and quasi-steady methods. Results show that backward thrust and pitching moment are maximized when the tilting occur near the end of downstroke and in the middle of upstroke, respectively. Good agreement is obtained among experimental, CFD, and quasi-steady model results, indicating that the effect of unsteady wake is negligible. Comparing the optimal stroke-plane tilting kinematics with those measured in hummingbirds suggests that hummingbirds attempt to maximize the backward acceleration in the beginning of the escape maneuver.