Longitudinal stability and control of the single-rotor helicopter

Abstract

The equations of motion of the helicopter are presented and reduced to non-dimensional form. The force and moment derivatives for the single-rotor helicopter (including tailplane if required) are given as simple formulae or in the form of charts. Comparisons are made with wind-tunnel and flight tests where possible and agreement is generally quite good. In the development of the theory, static and manoeuvre stabilities are introduced in a manner analogous to fixed wing aircraft practice. It is shown that the static stability of the helicopter is proportional to the coefficient E in the stability quartic whilst the manoeuvring qualities are represented by coefficient C. The N.A.C.A. 'divergence requirement' is expressed in terms of the 'short-period' motion. Calculations show that the poor damping in pitch of the single-rotor helicopter without a tailplane results in poor manoeuvring qualities, i.e., considerable time taken to reach steady acceleration following a control movement, but that the fitting of even a small tailplane provides a great improvement in stability and control.