Stability and Control Analysis of the Slowed-Rotor Compound Helicopter Configuration

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.