Unsteady Aerodynamics Applied to Flutter and Propulsion.

Abstract

The meaning of "unsteady aerodynamics" is briefly discussed. With regard to the physical simplifications implied, incompressible flow is completely governed by the vorticity-transport-equation. L. Prandtl's concept of an infinitely thin boundary layer (1922) furnishes the key to computation of pressure. Bernoulli's equation for unsteady boundary conditions imposed on a streamlined obstacle in uniform flow provides the pressure, hence the forces and moments acting on the obstacle. The balance of the mean powers occuring at the individual degrees of freedom during one cycle of the unsteady motion determines the basic mechanical behaviour of an aerodynamic configuration. "Flutter" is characterized by the (undesired) intake of energy from the flow passing the obstacle, whereas "propulsion" by means of unsteady motion requires internal sources of energy. The concept of a unified approach to both phenomena is discussed and applied to current aeroelastic problems aswell as to animal flight.