An analysis of the lateral-directional stability and control of the single-rotor helicopter

Abstract

The lateral-directional stability of a single-rotor helicopter is investigated by solving the equations of motion for a typical aircraft. It is found that the motion has three constituent parts, two subsidences and an oscillation. The oscillation is slightly unstable at very low speeds but becomes more and more damped as the speed of the aircraft is increased. The response to disturbances of various kinds and to control movements is illustrated by means of examples. It is found that increasing altitude reduces the damping of the oscillatory motion and decreases the rolling effectiveness of the control. Flight measurements of the lateral-directional oscillation agree well with theory except for an overestimation of period at very low speeds. Consideration is given to the effect of varying some design parameters; in particular it is shown that a large increase in roll damping at very low speeds will stabilise the lateral-directional oscillation and that a decrease in roll damping at high speed can lead to the appearance of a second oscillatory mode which may be unstable. An Appendix shows the connection between certain stability derivatives and control positions in asymmetric flight.