Poor stability

The instabilities of the helicopter fall into two categories – those at low speed due to the rotor and those at high speed due to the rotor; the designer can do very little about the first with airframe design, but he can make flight at high speed almost as stable as a fixed-wing aircraft. Unfortunately, if he chooses the latter option, he will almost cer­tainly compromise control and agility. Building large enough fixed empennage stabiliz­ers will always work but will, in turn, increase the demands on the rotor for manoeuvres. Selecting a rotor with zero or low equivalent hinge offset (e. g., most articulated rotors) will probably result in the pitch axis being marginally stable in high speed, but will again reduce the agility of the aircraft. On the other hand, a hingeless rotor, providing a roll time constant equivalent to a fixed-wing aircraft (0(0.1 s)) will also result in an unstable pitch mode with time to double of less than 1 s at high speed.

At low speed, without mechanical feedback, the single rotor helicopter is natu­rally unstable. The coupled pitch/roll, so-called pendulum instability is a product of the flapping rotor’s response to velocity perturbations. The mode is actually a fairly docile one, and is easily controlled once the required strategy is learnt, but requires consider­able attention by the pilot. However, if the outside world visual cues degrade, so that the pilot has difficulty perceiving attitude and velocity relative to the ground, then the hover task becomes increasingly difficult and Level 3 qualities are soon experienced.