The Art of the Helicopter John Watkinson - Karatunov.net

5.1 Introduction
5
The tail
Helicopters with a single main rotor must have some means of balancing the torque
reaction due to driving the rotor. Whilst the anti-torque rotor could be mounted anywhere
for hovering, in forward flight the most stable location is aft, supported on some
kind of structure called a boom. In addition to the anti-torque function helicopters
need some means of yaw control and the tail rotor also serves that purpose. In order to
balance the weight of machinery at the tail, the helicopter cabin usually extends some
way forward of the mast. This large forward side area is unstable in yaw and generally
some fin area is needed to give directional stability in forward flight. A further consideration
is that the main rotor on its own is unstable in pitch in forward flight and a
tail plane is usually required.
As a result the tail of the conventional helicopter will be a structure supporting a
variable pitch tail rotor, its transmission and controls, some fin area and a tail plane. In
reality tail booms are also encrusted with antennae, flare launchers, navigation lights,
strobe beacons, static vents, registration letters and warning notices, not to mention
tail skids and occasionally part of the undercarriage.
In a helicopter with more than one rotor the torque may be cancelled in a different
way and a great variety of yaw control mechanisms will be found. These multi-rotor
yaw mechanisms are considered in Chapter 9. In this chapter the conventional type
of tail rotor will be considered, along with a number of alternatives having various
advantages and drawbacks. The conventional tail rotor is well understood and relatively
inexpensive owing to its wide use.
The far aft location of the tail rotor assembly means that it must be lightly built in
order to avoid the machine becoming tail heavy. The thrust needed from the tail rotor is
much smaller than that needed in the main rotor. These two features conspire to ensure
that the tail rotor is considerably more fragile than the main rotor. Unfortunately the
fragile tail rotor is in a more exposed location where the pilot cannot see it. During
certain manoeuvres, such as quick stops and rearward flight it can get very close to
the ground, and it may come off worse in any encounter with vegetation the pilot has
not seen. Tail rotor blades are usually fitted with soft aluminium ‘telltales’ at the tips
which will be distorted by any impact and indicate that a close inspection for damage
is required.
Generally the tail boom will be fitted with some kind of skid intended to impact
the ground before the tail blades. In some cases this is a tubular structure partially
encircling the rotor and known as a D-ring. This gives better protection as well as