Effects of Unsteady Aerodynamics on the Flight Dynamics of an Articulated Rotor Helicopter

Abstract

We present the results of a study of the effects of unsteady airfoil aerodynamics on several flight dynamics characteristics of an articulated rotor helicopter. A new trim algorithm is presented that reduces the otherwise potentially prohibitive computational requirements. Results are presented for trim, frequency response, poles, and response to pilot inputs, including selected correlations with actual flight test data. The results indicate that when an unsteady airfoil aerodynamic model is used, minor changes occur in the trim pitch settings of the main and tail rotor, the frequency response, and the free-flight response to pilot inputs. These changes slightly improve the correlation with flight test data, and can usually be explained by specific changes in lift distribution over the rotor disk. Noticeable changes can be seen in the poles associated with rotor modes. The effect is typically that of decreasing the damping, although no mode becomes unstable or dangerously lightly damped. Neglecting the effects of the unsteady aerodynamic poles tends to be unconservative.