The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 141
swashplate always add up to the phase lag of the rotor, they can generally be ignored
for the purpose of studying rotor response, and all that is of interest is the control axis.
The direction of main rotor rotation is quite unimportant. On most American
machines the blades rotate anticlockwise when viewed from above, whereas most
French and Russian machines use the opposite rotation.
4.14 Cyclic trim
The controls of a fixed-wing aircraft tend to be blown to the neutral position by the
slipstream, and this results in the pilot feeling resistance roughly proportional to control
deflection. This feel is important so the pilot knows how much control power he is using.
In the helicopter, there is no equivalent of feel in the cyclic control. Forces fed back to
the cyclic stick from the rotor head bear little relationship to the deflection, so feel must
be provided artificially. This is done by springs that tend to return the cyclic control to
a central position.
The cyclic displacement needed in translational flight to counteract lift dissymmetry
and inflow roll would be tiring to maintain against spring pressure, so a trim system is
used to alter the neutral position of the springs. Figure 4.21shows how a trim servo
works. The trim servo has telescopic cylinders containing strong springs that normally
(a) (b)
Fig. 4.21 A cyclic trim servo. (a) Springs 6 and 8 bias the stick to a neutral point which can be moved by an
electric motor 1 which drives a worm 2, a wheel 3 and a leadscrew 4. If the pilot moves the stick away from
the trim servo, sleeve 5 and rod 7 move together, compressing spring 6. In the opposite direction, spring 8
compresses and sleeve 5 remains stationary. (b) Circuit of trim motor drive. The motor direction is selected by
choice of field coils which are wound in opposite senses.