The flight of insects and birds is the result of intertwined neural, sensory and actuation mechanisms, all coupled to wing kinematics and aerodynamics. The unsteady flow mechanisms responsible for lift production received a great deal of attention. Stability is as essential to flying as lift itself, but discussions of stability are limited in both number and scope, in part because it is difficult to decipher and isolate the mechanisms responsible for stability in live organisms. Passive flyers provide attractive models for examining intrinsic stability.
In this talk, I will discuss two interrelated problems of passive flyers: hovering in oscillatory flows and tumbling in quiescent fluids. Lift production in both problems depends on the shed vorticity and unsteady flow. The hovering model system consisting of an up-down asymmetric flyer in zero-mean oscillatory flows. I will analyze the conditions where the aerodynamic forces and torques are sufficient to keep the flyer aloft and balanced, and I will examine the interplay between the intrinsic stability and maneuverability of the flyer. I will then consider the problem of tumbling wings through carefully-designed experiments that non-uniformly vary the wing flexibility. I will show that non-uniform flexibility can enhance flight performance. Taken together, the two model systems provide novel insights into understanding passive flight. I will conclude by discussing the implications of these findings on live organisms and the design of man-made air vehicles.