The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 163
Abrasion is also a fact of life for rotor blades. Hovering stirs up dust and grit and
this can impact the leading edge of the blade. Composite blades may incorporate a thin
metal anti-abrasion skin wrapped around the leading edge. In many cases a sacrificial
coating made of self-adhesive tape is applied. This can be peeled off without damage
to the blade and replaced.
4.22 Blade tracking
Blade tracking is the process of adjustment that makes each blade fly identically. In the
hover with the CM of the hull at the mast the blades should all have the same coning
angle and the tips should all be in a single plane. If this is not the case there will not be
a force balance at the hub and the result will be vibration. The tracking adjustment is
performed by change to the length of the pushrod going to each blade pitch arm. The
ball joints are attached to the pushrods with a screw thread to allow length adjustment.
It should be pointed out that a tracking adjustment can only succeed if the blades
concerned are correctly statically and dynamically balanced. There may also be vibration
if a piece of blade abrasion tape has come off or if the tape application on the
blades is not identical.
The traditional way of measuring the tracking was to apply a different colour of chalk
or greasepaint to each blade tip. A sheet of stout cloth on a frame was then brought
carefully up to the edge of the spinning rotor so that coloured witness marks would be
left on the cloth. If one blade were out of track it would leave a witness mark above or
below the others. Clearly this method was not for the faint hearted and could only be
done on the ground with the helicopter supporting some of its weight with the rotor.
Subsequently a system was devised in which inward facing coloured reflectors were
temporarily fixed to the blade tips. These were illuminated by a strobe light triggered
by a sensor on the mast so that as each blade passed the same azimuth angle the strobe
light would flash. The machine could be flown with the apparatus attached and if
the tracking was incorrect the reflectors would appear at different heights. Clearly the
machine would have to be shut down each time an adjustment was required. Some
helicopters have motorized adjustments that allow the pitch rod to change its length in
flight. The optimum adjustment may be obtained by adjusting to minimize vibrations
at some specified operating condition.
In some machines trim tabs are fitted to the trailing edge of the blades, which can be
used to compensate for the effect of slight differences between the blades in forward
flight.
4.23 Blade folding
The blades of a helicopter may take up a great deal of room and in some applications,
such as operation from ships, folding blades will be needed as a practical matter.
In a small machine this may be achieved entirely manually after first removing the
locking pins. However, in large helicopters manual folding is not viable and a poweroperated
system will be needed. Figure 4.36 shows the blade folding system of the
Sea King. In this five-bladed machine, one blade is positioned over the tail boom
and does not fold. Blade folding can only take place if the rotor head is turned to
exactly the correct angle. After the rotor has stopped, an hydraulic motor will turn
the rotor to the correct angle and the shaft will then be locked.