An improved method of operating rotorcraft capable of achieving high speeds
such that stability is maintained as the craft speed exceeds 0.75 times
the rotor tip speed. These high speeds are achieved by varying collective
pitch, including to negative values, to maintain acceptable levels of
flapping at high aircraft forward speeds and low rotor rotation rates, or
adjusting or maintaining the rotor rotation rate by automatically
controlling the tilt of the rotor disk relative to the airstream or
aircraft, or a combination of these techniques. More specifically, by
utilizing these techniques the forward aircraft speeds can be high enough,
and the rotor rotation rates low enough, that an advance ratio, Mu,
greater than 0.75 can be achieved while maintaining rotor stability.