Flexibility increases lift for passive fluttering wings

We examine experimentally the influence of flexibility on the side-to-side fluttering motion of passive wings settling under the influence of gravity. Our results demonstrate the existence of an optimal flexibility that allows flexible wings to remain airborne twice as long as their rigid counterparts of identical mass and size. Flow visualization and measurements allow us to elucidate the role of flexibility in generating increased lift and wing circulation by shedding additional vorticity at the turning point. Theoretical scalings are derived from a reduced model of the flight dynamics and yield quantitative agreement with experiments. These scalings rationalize the strong positive correlation between flexibility and flight time. Our experimental results and theoretical scalings represent an ideal system for the validation of computational approaches to model biologically inspired fluid-structure interaction problems.