Methods for Improving the Modeling of Rotor Aerodynamics for Flight Mechanics Purposes

The application of the aerodynamic-phase-lag off-axis response correction technique to blade-element models of the UH-60 helicopter is presented. First, frequency domain techniques are used to find the optimal aerodynamic-phase-lag angle for the best correlation between the responses of a higher order linearized model of the aircraft and flight test data. Then, significant improvement in the off-axis response correlation of a complete nonlinear model of the UH-60 is shown using this optimal value. For cases where flight test data are not available a priori, a comprehensive analytical wake model which correctly predicts the off-axis responses will need to be used to provide the aircraft responses for optimizing the aero-phase-lag values used in blade-element models. A complete unsteady aerodynamic vortex model is therefore configured for the UH-60 and applied to study the influence of various unsteady aerodynamic effects. Good match between the responses of this model and UH-60 flight test data, in both on and off-axis, is shown. Least squares techniques are then used to find the optimal aerodynamic-phase-lag angle that provides the best correlation between a dynamic-inflow-type model and the complete unsteady model. Good agreement of the equivalent aerodynamic-phase-lag is shown between the comprehensive-wake-model-based results and the flighttest- tuned results. Finally, the vortex model is used to study the sensitivity of the optimal phase-lag angle to various rotor parameters, including flapping frequency, thrust coefficient, and number of blades.