The Art of the Helicopter John Watkinson - Karatunov.net

long. The cross-section of the duct matches the stream tube contraction as the fan is
approached and so the reduced pressure caused by the fan is applied to the duct. As a
result the effective area of the fenestron is the area of the duct mouth. The close fitting
duct effectively prevents blade tip loss. The direction of rotation of the fenestron should
be chosen on the same basis as that of a conventional tail rotor, namely the bottom
blade going towards the nose.
An advantage of the fenestron is that as it is set in the fin, there can be no fin blockage
as happens with a conventional tail rotor. In fact the fin is augmenting the effect of
the fan. The fenestron presents a much reduced hazard to ground personnel and is
quieter. However, as the fenestron is inseparable from its duct assembly, a powerful
fenestron can become quite large and ground clearance may be an issue. Some machines
equipped with fenestrons have a sacrificial collapsing bumper intended to absorb the
energy of an unintended ground contact after which it must be replaced.
5.11 NOTAR
A helicopter without a tail rotor has a number of advantages. It can land in scrub
without damage, ground personnel are less likely to be injured and there may be a
reduction in noise. The first attempt at a tail rotorless machine was the experimental
Weir W-9, shown in Figure 5.17, that flew in the UK in 1944. This machine had a piston
engine that, in addition to driving the main rotor, drove a variable pitch fan to blow air
Fig. 5.17 The Weir W-9 had no tail rotor but instead ducted cooling air and exhaust gases down the tail boom
to provide an anti-torque jet reaction. Twenty-five per cent of engine power was lost in the anti-torque system,
which was not pursued.
The tail 187