Stall flutter
As the name implies, stall flutter is a phenomenon which occurs with partial or complete separation of the flow from the airfoil occurring periodically during the oscillation. In contrast to classical flutter (i.e., flow attached at all times) the mechanism for energy transfer from the airstream to the oscillating airfoil does not rely on elastic and/or aerodynamic coupling between two modes, nor upon a phase lag between a displacement and its aerodynamic reaction. These latter effects are necessary in a linear system to account for an airstream doing positive aerodynamic work on a vibratingwing. The essential feature of stall flutter is the nonlinear aerodynamic reaction to the motion of the airfoil /structure. Thus, although coupling and phase lag may alter the results somewhat, the basic instability and its principal features must be explained in terms of nonlinear normal force and moment characteristics.
