AIRBORNE AND GROUND RESONANCE OF HINGELESS ROTORS

A hingeless rotor helicopter, with in-plane blade frequency less than rotor speed, is analyzed for its airborne and ground resonance characteristics. The analysis, which accounts for the fundamental blade and fuselage flexible modes as well as for the rigid-air frame modes of motion, uses the quasi-steady representation of aerodynamic forces in hover to show that hingeless rotors such as that of the BO-105 helicopter are stable, even at the resonant rotor speed nearest the operating range in the airborne state, because of inherent aerodynamic damping. Although not used by the BO-105 helicopter, the effects of cyclic feedbacks proportional to the airframe attitudes are explored. The analysis reveals that, if this type of feedback control is used for aircraft flight stability, the inherent aerodynamic damping of hingeless rotors is deteriorated.