Climbing and descending flight

In a steady climb the cyclic stick is often held forward to counter the nose-up pitching moment from the tailplane as well as that from the main rotor. A rapid entry into flight idle glide (FIG) or autorotation will have the opposite effect. The amount of aft stick required during a rapid entry is of particular interest especially if there is a danger of infringing control margins. Sometimes even during a steady autorotation there is still concern over the amount of aft cyclic required to counter the net nose – down moment from the tailplane. In such cases the designer may arrange for a reduction in the effectiveness of the stabilizer, thereby reducing the upsetting moment. Measurement of the position of the cyclic stick in steady climbs or descents is part of a test technique called ‘Trimmed Flight Control Positions’ (TFCP) which is discussed later (Section 5.2).

4.7.3 Forward flight

The response of a rotor to changes in speed is stable. Therefore a tail-less helicopter should possess a degree of positive static stability. Since, however, the fuselage contribution to static stability is variable and the addition of a horizontal surface will provide manoeuvre stability most helicopters are fitted with a tailplane at some location on the tail boom. Static (speed) stability, or otherwise, arises from the development of pitching moments following changes in the speed of the airflow approaching the helicopter. In summary, the contributors to static stability are:

(1) The main rotor. The main rotor provides a stable contribution to static stability. The size of the stabilizing moment increases with speed and rotor thrust.

(2) The horizontal stabilizer. The horizontal stabilizer produces a stabilizing contri­bution to static stability provided that it is downloaded. Therefore the inherent stability of the isolated main rotor (with speed) can be increased, or reduced, by the addition of a suitably sized downloaded, or uploaded, tailplane. The magnitude of the moment from the tailplane increases with speed.

(3) The fuselage. The contribution to static stability from the fuselage can be either stabilizing or destabilizing depending on the line of action of the lift and drag forces.