The calculated performance of a slowed-rotor compound aircraft, particularly at high flight speeds, is examined. Correlation of calculated and measured performance is presented for a NASA Langley high advance ratio test to establish the capability to model rotors in such flight conditions. The predicted performance of an isolated rotor and a wing and rotor combination are examined in detail. Three tip speeds and a range of collective pitch settings are investigated. A tip speed of 230 ft/s and zero collective pitch are found to be the best condition to minimize rotor drag over a wide speed range. Detailed rotor and wing performance is examined for both sea level and cruise altitude conditions. Rotor and wing power are found to be primarily from profile drag, except at low speed where the wing is near stall. Increased altitude offloads lift from the rotor to the wing, reducing total power required.
Stability and control of rotors at high advance ratio are considered. Stability of teetering, articulated, and gimbaled hub types is considered with a simple flapping blade analysis. Rotor control in autorotation for teetering and articulated hub types is examined in more detail for a compound helicopter (rotor and fixed wing) using the comprehensive analysis CAMRAD II. Autorotation is found to be possible at two distinct trim conditions with different sharing of lift between the rotor and wing. Stability predictions obtained using the analytical rigid flapping blade analysis and a rigid blade CAMRAD II model compare favorably. For the flapping blade analysis, the teetering rotor is found to be the most stable hub type, showing no instabilities up to an advance ratio of 3 and a Lock number of 18. Analysis of the trim controls, lift, power, and blade flapping shows that for small positive collective pitch, trim can be maintained without excessive control input or flapping angles for both teetering and articulated rotors. Nomenclature k p blade pitch-flap coupling ratio β rigid blade flap angle γ Lock number δ 3 blade pitch-flap coupling angle µ rotor advance ratio ν β fundamental flapping frequency (non-dimensional) ν θ blade fundamental torsion frequency (non-dimensional) ω dominant blade flapping frequency (non-dimensional) ( ) derivative with respect to azimuth
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