Extraction of a Dynamic Inflow Model from Wake Measurements on a Hovering Rotor

The parameters of a Pitt-Peters dynamic inflow model for a rotor undergoing collective inputs were extracted from experimental measurements on a hovering rotor. The four-bladed rotor of 2 m diameter featured straight, untwisted blades and a solidity of σ = 0.010. The nominal trim condition was CT /σ = 0.07 at a speed of 840 RPM. The rotor wake was measured using phase-resolved, 2D, 3-component particle image velocimetry (PIV) over a large region of interest (0.84 m x 0.77 m), and the integrated rotor aerodynamic forces were obtained from simultaneous hub loads measurements. The frequency response of rotor inflow to rotor thrust was found by measuring the system response to a stepped-sine collective input, which included frequencies of 0.2, 0.3, 0.4, 0.6, and 0.7/rev. The thrust amplitude increased with input frequency, reaching 27.4% of the steady thrust at the highest input frequency. The inflow amplitude was 4.3% of the steady inflow at 0.2/rev and decreased to 2.0% at 0.7/rev. A first-order transfer function was fit to the discrete frequency response to compute the parameters of the Pitt-Peters dynamic inflow model. The apparent mass term for a rotor undergoing collective inputs was found to be M11 = 0.0288±4.4% and the gain term was found to be ¯L11 = 0.118±6.3%. The results agreed well with examples in literature extracted from high-fidelity numerical models. However, there were differences in the steady-state gain and the frequency at which the inflow magnitude is attenuated. The methodology for extending the present approach to cyclic inflow components is presented.