Experimental Characterization of a Rotor to Dynamic Collective Pitch Inputs in Hover

Abstract Experiments were performed on a two-meter diameter, four-bladed rotor to extract the coefficients of a dynamic inflow model. The frequency response of rotor induced velocity to rotor thrust was found by introducing stepped-sine collective pitch inputs at frequencies of 0.2 per revolution to 0.7 per revolution, where the rotor rotational frequency was 14 Hz. The induced velocity field was measured using phase-resolved, two-dimensional, three-component particle image velocimetry (PIV) over a large region of interest (0.84 m x 0.77 m) in a radial slice of the rotor flow field. The rotor thrust was measured using a hub-mounted load cell. Limited by the frame rate of the cameras, PIV was performed using an under-sampling technique in which one image pair was captured during each rotor revolution, and the time history was recreated from the phase-resolved measurements. The thrust amplitude was found to increase with input frequency, reaching 27.4% of the steady thrust at the highest input frequency. The induced velocity amplitude followed the opposite trend, decreasing to 2.0% of the steady value at the highest input frequency. Dynamic inflow states were extracted and fit to a first-order transfer function. The gain and corner frequency of the transfer function were found to be K = 34.7 +/- 1.52 dB and b = 4.08+/-0.19 rad/s, respectively, which followed a similar trend to computational studies reported in the literature.