A Semi‐Empirical Model for Dynamic Stall

A semi‐empirical model is formulated to represent the unsteady lift, drag, and pitching moment characteristics of an airfoil undergoing dynamic stall. The model is presented in a form which is consistent with an indicial formulation for the unsteady aerodynamics under attached flow conditions. The onset of vortex shedding during dynamic stall is represented using a criterion for leading edge or shock induced separation based on the attainment of a critical leading edge pressure. The induced vortex lift is represented empirically along with the associated pitching moment which is obtained by allowing the center of pressure to move in a time dependent manner during dynamic stall. Significant nonlinearities in the airfoil behavior associated with trailing edge separation are represented using a Kirchhoff flow model in which the separation point is related to the airfoil behavior. These effects are represented in such a way as to allow progressive transition between the dynamic stall and the static stall characteristics. It is shown how the above features may be implemented as an algorithm suitable for inclusion within rotorcraft airloads or aeroelasticity analyses. Validation of the model is presented with force and moment data from two‐dimensional unsteady tests on the NACA 0012, HH‐02, and SC‐1095 airfoils.