The Art of the Helicopter John Watkinson - Karatunov.net

136 The Art of the Helicopter
Enstrom use an elastomer in compression to retain the blade. Elastomers are virtually
incompressible but flexible synthetic compounds. Figure 4.15(d) shows that if a block
of elastomer is placed under compressive stress, it will bulge at the sides. If the bulging is
serious, the strength of the material will be exceeded and it will tear. Bulging would also
allow the blades to move away from the mast, causing balance problems. Figure 4.15(e)
shows that this problem is overcome by interleaving thin shims of metal with layers of
elastomer. The metal shims prevent bulging under compression, but have little effect on
the ability of the unit to twist so that pitch changes can be made. As the diagram shows
at (f), the blade is guided by sleeve bearings, but the elastomer takes the thrust. Note
that the system is failsafe because the bearing is in compression. Even if the bearing
begins to disintegrate, the blade cannot come off.
In the Enstrom only the feathering axis is elastomeric. Aerospatiale pioneered a
spherical elastomeric laminated bearing which allows full articulation in addition to
feathering.
4.13 Pitch control
There are a number of mechanisms suitable for pitch control. No two designs have
exactly the same control linkages, but the diagrams here are representative of actual
practice. One of the best ways of appreciating how these systems work is to have
someone move the controls of a helicopter one at a time whilst observing how the
linkages move.
The most common control system is the sliding swashplate shown in Figure 4.16(a).
The swashplate is made in two halves. The lower part is fitted with a spherical bearing
so that the swashplate can tilt to any angle in any direction. The sphere can also slide
up and down, often on the outside of the gearbox. A large diameter ball race is fitted
between the two halves of the swashplate. A pair of jointed scissors links is fitted. One
of these ensures that the lower half of the swashplate cannot revolve and the other
ensures that the upper part turns with the rotor shaft. Each rotor blade grip is fitted
with a pitch-operating arm. The end of each arm is connected to the periphery of the
swashplate by a pushrod fitted with spherical bearings.
Movement of the collective lever raises or lowers the swashplate, and this causes all
of the pitch operating arms to rotate all of the blades about their feathering axes by
the same amount. In addition, movement of the cyclic stick in two axes causes the
swashplate to tilt in two axes. Simultaneous lifting and tilting of the swashplate by two
controls requires a device called a mixer, an example of which is shown in Figure 4.16(b).
One end of the mixing lever is pivoted on the helicopter frame or on the gearbox and
is connected to the collective lever by a rod. The opposite end is fitted with a bell
crank operated by one axis of the cyclic control. The cyclic pitch changes take place
differentially around the average pitch determined by the collective setting.
Figure 4.16(b) shows only one dimension of the cyclic mixing for clarity. In practice
the cyclic control works in two axes and two bell cranks are fitted to the mixing lever.
These bell cranks are connected to the swashplate by vertical rods and to the cyclic stick
by horizontal rods. Moving the collective lever moves both bell cranks bodily up and
down together, and so the swashplate rises and falls. Moving the cyclic stick rocks the
bell cranks and tilts the swashplate about the collective setting in any desired direction.
Figure 4.17 shows an alternative arrangement known as a spider. The spider has
radial spokes, one for each blade. Pushrods connect the ends of the spokes to the pitch
operating arms. The hub of the spider is spherical so it can tilt and slide within the
shaft and it is fitted with a vertical post passing down through the centre of the shaft