The Art of the Helicopter John Watkinson - Karatunov.net

168 The Art of the Helicopter
fitted with couplings to accommodate small angular errors and plunging. This avoids
putting unnecessary stress on bearings and on the shaft itself. Often the shaft will be
run on the outside of the tail boom to allow easy inspection. The shaft may be exposed,
or covered by a D-shaped detachable cowling as in, for example, the Bell 206.
The tail rotor gearbox is relatively simple since it has only to turn the drive through
90 ◦ . The output shaft bearings will be designed to support the tail rotor and withstand
flight loads. An oil level sight glass will be provided, and generally a chip detector. In
large machines an oil temperature gauge may be fitted.
The tail rotor only requires collective pitch control and so it will have a swashplate
which moves along the shaft axis but without tilting. There are various ways of moving
the swashplate via the foot pedals. The pedals usually communicate with the tail using a
pair of stranded steel wires, since these are light in weight. Figure 5.1 shows that there is
some variety in the mechanisms used to convert the wire motion into movement of the
swashplate. Figure 5.1(a) shows an example where the swashplate runs on the gearbox
casing between the rotor and the gearbox. A simple bell crank and fork arrangement
will move the swashplate. Figure 5.1(b) shows a system in which a pitch control rod
runs through the hollow tail rotor shaft to a swashplate which is outboard of the rotor.
This may be operated by a bell crank as before. The pitch control rod may be terminated
in a coarse screw thread. The wires from the foot pedals are wound round a drum that
(a)
(b)
Fig. 5.1 (a) The tail rotor pitch may be controlled by a swashplate between the gearbox and the rotor. (b) The
swashplate may also be found outboard of the rotor.