An analysis of the longitudinal stability and control of a single-rotor helicopter

Abstract

The general theory of longitudinal stability and control for a single-rotor helicopter is presented in a form similar to that for fixed-wing aircraft. It is shown to be possible to establish for the helicopter in forward flight, in the same way as for fixed-wing aircraft, stick-fixed static and manoeuvre margins, on which the stability and handling qualities depend to a marked extent. If the static margin Kn > 0 the helicopter is mathematically statically stable, and the pilot requires a forward stick displacement to hold increased speed and conversely. If the manoeuvre margin Hm > 0, the helicopter is unlikely to be subject to rapid divergence in a disturbance, and the pilot requires a backward stick displacement for positive normal acceleration in a pull-out. Theoretical relations are derived for Kn and Hm in a general form covering the case of a tailplane linked to the rotor control. Relations are given also for determining Kn and Hm from measured control changes to trim. An analysis is given of the growth of acceleration in a pull-out and assessment of estimated acceleration curves in terms of the National Advisory Committee for Aeronautics 'divergence requirement' suggests that the latter may be satisfied if Hm has a small positive value. Further evidence on this point will be obtained in tests now being made on a number of helicopters to study the correlation of stability and control characteristics and pilots' impressions of the handling qualities. Extension of the theory to stick-free longitudinal stability depends on knowledge of the rotor forces on the control plane and the analysis of these forces is being considered.