USE OF ACTUATOR-DISC DYNAMIC INFLOW FOR HELICOPTER FLAP-LAG STABILITY.

A hierarchy of thirteen unsteady and qasi-steady inflow models is used for prediction of flap-lag damping in hover and in foward flight. It includes variations in inflow model that range from a simple, lift-deficiency function to a dynamic, five-degree-of-freedom inflow model. The models are based on two general inflow formulations. The first formulation is a time-delayed, unsteady momentum theory with a first-harmonic description of inflow. the second formulation is an unsteady, actuator-disc theory with both first-and second-harmonic variations in inflow. Based on previous analyses, the first-harmonic, unsteady actuator-disc theory is known to be reasonably accurate, thus providing an objective measure for comparison. The results show an interesting relationship between the number of blades on a rotor and the choice of the inflow model to be used.