The Art of the Helicopter John Watkinson - Karatunov.net

140 The Art of the Helicopter
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Fig. 4.20 Where the advance angle is less than 90 ◦ , the stationary part of the swashplate is advanced by
rotation of the points at which the pushrods from the cockpit are attached. In this example, a four-blade head
has an advance angle of 45 ◦ . The pushrods from the cyclic stick approach the swashplate with a further 45 ◦
advance. In a head with a phase lag of less than 90 ◦ , the swashplate advance could be reduced to compensate.
the pushrods from the cockpit must now connect at different angles. By consensus,
all rotational angles are measured in degrees from the tail boom in the direction of
rotation as shown.
The control axis can stay parallel to the cyclic stick, but the axis of the swashplate
will tilt with a phase shift with respect to both. For example, a four-bladed head may
have an advance angle of only 45 ◦ . The swashplate tilt will need to be advanced a
further 45 ◦ with respect to the cyclic stick. Pushing the stick forward would cause the
swashplate to tilt down midway between the front and the advancing side, i.e. at a phase
angle of 135 ◦ , so the lowest point on the control orbit is reached 45 ◦ before the straight
ahead position. At this point the pushrod going straight up to the pitch-operating
arm will set the advancing blade at 90 ◦ to minimum pitch. The same blade will reach
maximum pitch 180 ◦ later, and so there will be a roll couple towards the advancing
side. The phase lag of the rotor will convert this to a forward tilt.
In other words the swashplate linkage has advanced the stick movement with respect
to the control axis by 45 ◦ , the pitch-operating arms have advanced it a further 45 ◦ , and
the 90 ◦ rotor lag cancels both, making the rotor tilt to follow the cyclic stick. Inspection
of the cyclic pushrods in the Enstrom or the Lynx will show that they approach the
spider at 45 ◦ either side of the centreline. The pitch-operating arms have an advance
of a further 45 ◦ . The Chinook, having three-bladed heads, has an advance angle of
60 ◦ . In contrast the JetRanger, having only two blades, can have an advance angle
of 90 ◦ in the pitch operating arms, and the swashplate moves parallel to the control
axis. Since the advance angle of the pitch-operating arms and the phase advance of the