Compound rotorcraft can be a viable option to achieve moderately high speeds of 240 knots. However, there is no systematic wind tunnel test data in the public domain to understand the complex interactional aeromechanics involved. This work is part of a test campaign to conduct multiple high-speed wind tunnel tests of various compound configurations. The compound configuration consists of a single main rotor, a single wing on the retreating side for lift augmentation, and a single rear propeller for propulsive augmentation. In this paper, hover test results are presented. Hover tests were conducted for various configurations and at different tip Mach numbers with the maximum value of 0.5. Rotor and wing performances are measured separately using individual balances. Rotor blades are equipped with strain gauges for measuring flap, lag, and torsional moments at [Formula: see text] radial stations. The wing location is [Formula: see text] below the main rotor, and the measured wing download is only 2.8% of the rotor thrust. The wing provides a partial ground effect, making the rotor more efficient and almost compensates for the download with less than 1% reduction in net figure of merit. Blade structural loads correlate satisfactorily between the analysis and measurement except for lag bending moment. This discrepancy is a large 2/rev lag bending moment measured in the test, which needs further investigation. Propeller hover test was conducted up to 2500 revolutions per minute (tip Mach number of 0.25) at a single collective. Prediction correlates well and captures the low-Reynolds-number effect for lower revolutions per minute.
In forward flight, slowing down a rotor alleviates compressibility effects on the advancing side blade tip, extending the cruise speed limit, and inducing high-advance-ratio flight regime. 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, where the rotor is ineffective in generating thrust. A rigid hingeless rotor with lift offset may help overcoming this problem. A series of wind tunnel tests were conducted to investigate the behavior of slowed hingeless rotors at high advance ratios. Performance, control angles, hub vibratory loads, and blade structural loads were measured and compared with comprehensive analysis predictions. 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 with an articulated rotor, especially its 2∕rev flap bending moment, which can pose a critical structural constraint on the rotor.
It is a well known fact that the forward speed of a single main rotor helicopter is limited because of the compressibility effects on the advancing side and reverse flow and dynamic stall on the retreating side. Compound helicopters are a viable option which could increase the forward speed while minimizing hover penalty. Much analysis has been carried out on compound helicopters but the test data are lacking. The present research aims to systematically bridge this gap, the first step of which was to study lift compounding. Slowing down the rotor and lift offset are two key ways to increase the forward speed. Slowing down the rotor requires the rotor to fly at high advance ratios and involves more complex aerodynamic phenomena. A series of wind tunnel tests were conducted to understand rotor behavior with hingeless hub at high advance ratios with a stub wing installed on the retreating side to balance the rolling moment. This allowed the rotor to operate efficiently at high advance ratios by producing more lift on the advancing side and less on the retreating side. The rotor was tested up to an advance ratio of 0.7 at different shaft tilt angles and wing incidence angles. The experimental results including rotor performance, controls, blade structural loads, and hub vibratory loads were measured and compared to predictions with in-house comprehensive analysis, UMARC. Comparison between different wing incidences at constant total thrust provided many insights into the compounding. Significant advantages of lift compounding were identified as the helicopter could fly faster with greater efficiency, requiring lower rotor collective. The blade flap bending moment was identified as a key constraint on the maximum speed.
Slowing down the rotor in forward flight is a viable means of extending the cruise speed of a rotorcraft by alleviating compressibility effects at the advancing side blade tip. It was shown by previous wind tunnel tests that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side of the rotor needs to be trimmed against the retreating side in the reverse flow state, where 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. At the University of Maryland, a rotor test stand was modified for hingeless rotors and two wind tunnel tests were conducted to investigate the behavior of hingeless rotors at high advance ratios. The experimental results, including performance and control, hub vibratory loads and blade structural loads, are presented in this paper and compared with predictions of the in-house comprehensive analysis, UMARC. The performance results demonstrate that a hingeless rotor with lift offset is capable in generating more thrust at high advance ratios, and the blade structural load results reveal that the predominant structural constraint on the rotor is in 2/rev flap bending moment, combined with 1/rev flap bending moment from the lift offset.
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