Forward flapping flight from flexible fins

The mechanics and energetics of aquatic flight by the clearnose skate (Raja eglanteria) are examined with cinefilm and a new theoretical approach toward flight mechanics. Film analyses show that these animals move with a flapping, flexing wing that has a propulsive wave travelling rearward at twice the forward speed of the animal. A combination of blade-element theory and unsteady airfoil theory is used to examine the mechanics and energetics of this mode of locomotion. The theoretical analysis shows that (i) unsteady effects determine the overall performance of the wings, and (ii) there exist wing shapes that minimize the cost of transport or maximize the thrust. The theory indicates that the wings of swimming skates closely approach the minimum cost of transport. The results are extended to explore other modes of flapping wing propulsion, including those in animals whose wings deform passively in response to hydro- or aero-dynamic loads.