Refined measurement and validation of performance and loads of a mach-scaled rotor at high advance ratios

Abstract

Slowed-rotor compound helicopters support high speed flight by reducing the main rotor RPM as cruise speed increases to maintain sub-critical tip Mach numbers on the advancing side, resulting in high advance ratio (?) flight regimes. This work investigates the performance and vibratory loads of a Mach-scale rotor with highly similar, non-instrumented blades (untwisted, untapered) at advance ratios up to 0.9. A 4-bladed, articulated rotor with a diameter of 5.6 ft. was tested in the Glenn L. Martin Wind Tunnel at 30%, 40%, and 50% of nominal speed (corresponding to advancing tip Mach numbers up to 0.53). For each test point, the rotor is trimmed to minimize cyclic flap angles. Collective sweeps from -2° to 12° were performed for each flight condition and longitudinal shaft tilt angles of -4°, 0°, and 4° (positive aft) were tested. Blade root pitch and flap motion, trim cyclics, shaft power, and steady and vibratory hub loads were measured. Blade similarity was shown to improve rotor track and trim at high ?. Thrust reversal was observed at an advance ratio of 0.9, but positive shaft tilt increased lift, and overall performance at high ?. Vibratory hub loads are shown to increase with advance ratio. Correlations of the measured data with predictions from the comprehensive analysis code UMARC are presented, and show satisfactory agreement for rotor performance over the entire range of advance ratios.