4.8 MANOEUVRE STABILITY
The response of a rotor to changes in AOA is unstable. Therefore a helicopter would possess negative manoeuvre stability without the addition of a surface to provide a stabilizing moment. A horizontal stabilizer is generally fitted to a helicopter to serve this purpose and thus provide a degree of manoeuvre stability. It will, however, also affect the static stability. Manoeuvre stability, or otherwise, arises from the development of pitching moments following changes in the angle of incidence of the airflow approaching the helicopter. Contributors to manoeuvre stability are:
(1) The main rotor. The main rotor provides an unstable contribution to manoeuvre stability. The size of the destabilizing pitching moment will increase with trim speed, above minimum power speed, and load factor.
(2) The horizontal stabilizer. The horizontal stabilizer provides a stabilizing contribution to manoeuvre stability. It does not matter whether the stabilizer is uploaded or downloaded when at the trim condition, since an increase in fuselage incidence will result in a nose-down pitching moment in both cases. The magnitude of the stabilizing moment increases with trim speed, but not load factor.
(3) The fuselage. The contribution to manoeuvre stability from the fuselage can be either stabilizing or destabilizing depending on the line of action of the lift and drag forces.
The destabilizing effect of aft rotor flapping (main rotor contribution to manoeuvre stability) is approximately proportional to the square of the forward speed as is the change in lift generated by the tailplane following an AOA change. Therefore, the size of stabilizer selected to provide stability at one trim speed is generally suitable for other speeds. However, since the magnitude of the destabilizing moment from the main rotor also increases with load factor, but the opposing moment from the horizontal stabilizer does not, it is highly likely that a helicopter will display manoeuvre instability if tested at both high load factor and high speed. A low set stabilizer may also suffer a variation in performance if heavily influenced by the downwash from the main rotor.
The position of the CG relative to the main rotor thrust vector and tailplane has an effect on the manoeuvre stability of a helicopter. If the CG is forward of the rotor, the increase in rotor thrust and aft tilt of the vector associated with an up gust may produce a stabilizing nose-down pitching moment, or at least a less severe nose-up moment. With an aft CG position, however, the situation is reversed and an increase in thrust and rearwards tilt of the thrust vector will generate an unstable pitching moment. The horizontal stabilizer will be required to provide a measure of manoeuvre stability, as described above. The overall stability will be weaker at higher weights and with aft CG positions since the destabilizing effect of the main rotor will be that much stronger. This is usually the reason a maximum allowable aft CG position is quoted in the operating manual. The forward limit is often established by the manufacturer either to prevent high oscillatory rotor loads or to provide adequate aft stick margin for a landing flare or rearward flight.