Experimental characterization of the response of single and coaxial counter-rotating rotors to dynamic collective pitch inputs



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Although several analytical and numerical models of dynamic inflow are available in the literature, limited experimental validation has been performed on single rotors and none for coaxial, counter-rotating (CCR) rotors. A comprehensive understanding of this response is necessary to accurately predict rotor performance, flight dynamics, and guide control system design. This work performs detailed flow field measurements on a scaled rigid single and CCR rotor system in hover subject to dynamic collective pitch excitation. Specifically, the study seeks to fill in gaps in experimental data available for correlation with low-order inflow models. To this end, a model-scale, closely spaced rotor system with instrumentation to measure individual rotor loads is used in addition to a fully characterized PIV setup to measure the rotor flow field. Hover tests consist of stepped-sine collective pitch inputs with an amplitude of Δθ₀ = 1° over a range of actuation frequencies of f = 0.0/rev, 0.2/rev, 0.3/rev, 0.4/rev, 0.6/rev, and 0.7/rev. The stepped-sine inputs are implemented on the isolated single rotor and CCR rotor configurations. There are four main test conditions for the CCR rotor configuration: (i) upper rotor-only sweeps, (ii) lower rotor-only sweeps, (iii) upper and lower rotor in-phase sweeps, and (iv) upper and lower rotor out-of-phase sweeps. Frequency domain system identification is performed on results from both configurations to characterize the system and extract low-order inflow models from the measured data to be compared to the numerical models available in the literature. The single rotor testing resulted in an increase in the thrust coefficient amplitude of 19.1% to 27.1% of the nominal thrust C [subscript T] = 0.00742 over an actuation frequency range of f = 0.2/rev - 0.7/rev. In contrast, the change in the inflow ampli tude decreased with actuation frequency from ∆λ₀ = 0.0028 at f = 0.2/rev to ∆λ₀ = 0.0013 at f = 0.7/rev. The resulting frequency response relating the change in inflow to the change in thrust had a first-order response with a steady-state gain of K = 18.6 dB. Numerical models available in the literature captured the general trends in the frequency response but underpredicted the magnitude and overestimated the corner frequency. The CCR rotor testing revealed a second-order response between the change in inflow to a change in thrust coefficient for the upper and lower rotor-only test cases and the upper and lower rotor in-phase test case. Overall, the magnitude of the frequency responses was captured well by the current numerical models for the low-frequency range. The models showed a first-order response at the high-frequency range due to the assumption that the coaxial rotor can be treated as a system of first-order governing equations and did not capture the responses shown in the measurements.


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