Wind Tunnel Test on Slowed Rotor Aeroechanics at High Advance Ratios

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Wang, Xing
Chopra, Inderjit
In forward flight, slowing down a rotor alleviates compressibility effects on the advancing side, extending the cruise speed limit and inducing high advance ratio flight regime. To investigate the aerodynamic phenomena at high advance ratios and provide data for the validation of analysis tools, a series of wind tunnel tests were conducted progressively with a 33.5-in radius, 4-bladed Mach-scaled rotor in the Glenn L. Martin Wind Tunnel. In the first stage of the research, a wind tunnel test was carried out at high advance ratios with highly similar, non-instrumented blades and on-hub control angle measurements, in order to gain a baseline performance and control dataset with minimum error due to blade structural dissimilarity and pitch angle discrepancy. The tests were conducted at advance ratios of 0.3 to 0.9, and a parametric study on shaft tilt was conducted at $0^\circ$ and $\pm 4^\circ$ shaft tilt angles. The test data were then compared with those of previous tests and with the predictions of the in-house comprehensive analysis UMARC. The airload results were investigated using comprehensive analysis to gain insights on the influences of advance ratio and shaft tilt angle on rotor performance and hub vibratory loads. Results indicate that the thrust benefit from backward shaft tilt is dependent on the change in the inflow condition and the induced angle of attack increment, and the reverse flow region at high advance ratios is the major contributor to changes in shaft torque and horizontal force. In the second stage of the research, the rotor blades were instrumented with pressure sensors and strain gauges at 30\% radius, and pressure data were acquired to calculate the sectional airloads by surface integration up to an advance ratio of 0.8. The test results of blade airloads and structural loads were compared with the predictions of comprehensive analysis (UMARC and PrasadUM) and CFD/CSD coupled analysis (PrasadUM/HAMSTR). The focus was on the data correlation between experimental pressure, airload and structural load data and the CFD/CSD predicted results at various collective and shaft tilt settings. Overall, the data correlation was found satisfactory, and the study provided some insights into the aerodynamic mechanisms that affect the rotor airload and performance, in particular the mechanisms of backward shaft tilt, hub/shaft wake and the formation of dynamic stall in the reverse flow region. The next stage focused on hingeless rotor with lift offset. Previous wind tunnel tests have shown that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side needs to be trimmed against the retreating side with significant reverse flow, in which the rotor is ineffective in generating thrust. Therefore, a hingeless rotor that allows the advancing side to generate more thrust can be rewarding in overall thrust potential. Wind tunnel tests were conducted up to an advance ratio of 0.7 to investigate the behavior of hingeless rotors at high advance ratios with lift offsets. Performance, control angles, hub vibratory loads and blade structural loads were compared with comprehensive analysis predictions from UMARC, plus the wing performance predictions from AVL. The results demonstrate that a hingeless rotor with lift offset is more efficient in generating thrust and exhibits higher lift-to-drag ratio at high advance ratios. The blade structural load level is significantly higher compared to an articulated rotor, especially for 2/rev flap bending moment, which can pose a critical structural constraint on the rotor.