Theory of Helicopter Damping in Pitch or Roll and a Comparison with Flight Measurements

Abstract : Calculations and flight-test measurements On a single-main-rotor helicopter indicate that the damping moment about the helicopter center of gravity produced during pitching or rolling by a rotor which has flapping hinges on the rotor shaft depends primarily upon four quantities: rotor speed, blade mass factor, height of the rotor hub above the helicopter center of gravity, and the ratio of collective pitch to the thrust coefficient - solidity ratio (cf7a). The damping moment per unit angular velocity varies inversely with rotor speed and blade mass factor, varies directly with hub height, and decreases linearly with increasing e e CT/a At values of above about 3, when 8 is in radians, the CT/a rotor moment per unit angular velocity becomes unstable (moment in the same direction as the angular velocity). These results indicate,that present-day helicopters with conventional control systems tend to have low damping at high speeds and in climbs and that unstable damping can occur q during maneuvers in which the normal acceleration falls well below 1 g.