The Art of the Helicopter John Watkinson - Karatunov.net

× T=c× D
Rotors in practice 131
Fig. 4.12 With a zero-offset head, blowback in forward flight must be opposed by a down force from the
tail plane.
and undercarriage dampers allows a further weight saving. However, it is vital that
the machine is never placed in a flight condition where the rotor thrust is downwards,
because in this negative-g situation the hull attitude is unstable, like a pencil balanced
on its point.
In the worst case the hull can roll so that the limit of travel of the teetering or flapping
hinge is reached. The side thrust from the tail rotor can initiate such a roll. The resultant
pounding of the limit stops can break the mast. In zero or negative g the rotor will
cone flat or downwards raising the possibility that if the hull pitches the tail boom can
swing into the rotor disc.
A number of Robinson helicopters have been lost due to these effects, although the
manufacturer stresses the avoidance of manoeuvres such as pushovers that can induce
the negative-g condition. In the event of getting a zero-offset helicopter into negative
g, there is only one solution and that is to return lift to the blades. This can be achieved
by the application of back cyclic or raised collective or both.
In some zero-offset helicopters, such as the Bell 222, a strong spring is fitted to the
teetering axis that minimizes the negative-g instability, but it does require a stronger
mast and gearbox to withstand the spring forces.
Zero-offset heads will not tolerate a wide range of CM movement in any direction
and are more prone to blade sailing when starting in high winds.
4.11 Rotor response
Different rotor head designs respond to cyclic control inputs in different ways
contrasted in Figure 4.13.
Consider a two-bladed rotor where the pitch control rods are at 90 ◦ to the blades. For
example, if the swashplate is tilted down at the front, the pitch angle of the blade moving
forwards will be reduced, and the pitch angle of the opposite blade will be increased.
This will cause a lift moment difference that results in a rolling couple. However, the
gyroscope-like phase lag of the rotor will convert this into a pitch forward, so the front
of the rotor disc will drop. In the hover, if the rotor is articulated, it will continue to
pitch forward until it is parallel with the swashplate. At this point there will be no more