Dynamic Stall Modeling and Correlation with Experimental Data on Airfoils and Rotors

Two methods for modeling dynamic stall have been developed at United Aircraft. The a, A, B Method generates lift and pitching moments as functions of angle of attack and its first two time derivatives. The coefficients are derived from experimental data for oscillating airfoils . The Time Delay Method generates the coefficients from steady state airfoil characteristics and an associated time delay in stall beyond the steady state stall angle. Correlation with three types of test data shows that the a, A, B Method is somewhat better f o r use in predicting helicopter rotor response in forward flight. Correlation with lift and moment hysteresis loops generated for oscillating airfoils was good for both models. Correlation with t e s t data in which flexibly mounted two-dimensional airfoils were oscillated to simulate the 1P pitch variation of a helicopter rotor blade showed that both methods overpredicted the response, and neither gave a clear advantage. The a, A, B Method gave better correlation of torsional response of full scale rotors and remains the method in general use. The Time Delay Method has the potential t o be applied more easily and probably can be improved by consideration of spanwise propagation of stall effects.