Effect of Rotation Kinematics and Angle of Attack on Flapping Flight

Abstract

Unsteady aerodynamics of a rigid flapping wing at a Reynolds number of 10,000 for forward flight with an advance ratio of 0.5 is analyzed. A spiral leading-edge vortex with a strong spanwise flow along its core is formed during the downstroke, resulting in a peak lift and thrust. A negative spanwise flow formed due to the tip vortex prevents the removal of vorticity from the leading-edge vortex, leading to instability and separation of the leading-edge vortex. Analysis of different rotation timings shows that supination results in the leading-edge vortex formation near the base and its strength depends on the flapping velocity. A stronger vortex is formed for advanced rotation and it generates high lift. Delayed rotation affects thrust production during translation and results in low propulsive efficiency. Analysis of rotation duration shows that shorter rotation results in high instantaneous lift values, whereas continuous long-duration rotation results in high thrust and propulsive efficiency. Analysis of different angles of attack show that a moderate angle of attack, which results in a high thrust-to-lift ratio and complete shedding of the leading-edge vortex at the end of translation, is required for high propulsive efficiency.