Aerodynamics and structural mechanics of flapping flight with elastic and stiff wings

Abstract

The flapping flight mechanism is expected to provide revolutionary operation capabilities for tomorrow's Micro Air Vehicles (MAV). The unsteady aerodynamics of the flapping flight is vastly different from traditional fixed-wing flyers. Boundary layers with moving laminar-turbulent transition, three-dimensional wake vortices and fluid-structure interaction with anisotropic wing structure are only a few examples for the challenging problems. To get basic understanding of these effects, the authors develop a computational method that is validated with boundary-layer measurements on flexible and inflexible, flapping wings in a wind-tunnel. The computational method solves the unsteady Reynolds-averaged Navier-Stokes equations and is combined with both transition prediction and fluid structure interaction capability. Using generic airfoils shapes inspired by seagulls and hawks, different aerodynamic, structural and kinematic effects are systematically analyzed on their influence on thrust and propulsive efficiency of the flapping flight mechanism. In particular, we demonstrate that a slight forward-gliding motion during the flapping downstroke can increase significantly thrust and efficiency.Wing elasticity however seems to lower the propulsive efficiency in the investigated cruise flight flapping case. Beyond,we show that the wake structure of 3D flapping wings generates an efficiency loss of about 10% compared to equivalent two-dimensional flapping cases. © 2012 Springer Berlin Heidelberg.