The Art of the Helicopter John Watkinson - Karatunov.net

drag. However, the slots are emitting downward jets and this causes some upthrust that
nearly cancels the induced drag leaving a side thrust to counteract the torque.
About 60% of the anti-torque thrust comes from the boom. The remainder comes
from a nozzle at the extreme end of the boom. This has a stationary inner part and an
outer rotating sleeve. The inner part has a port on both sides, whereas the outer part
has a single port. The pilot’s foot pedals rotate the sleeve so that it will uncover one or
other port to allow yaw control. The degree to which the sleeve turns determines the
amount of port opening and so controls the thrust.
The boom is pressurized by a variable pitch fan. The pitch required is not a simple
function of the pedal position and never becomes negative. Instead it is a function of
how much thrust is needed. In other words the application of right or left pedal will
cause the fan pitch to increase to provide extra airflow to the yaw control ports. In the
case of autorotation the boom will be in upwash and the boundary layer control is no
longer effective, but nor is it needed. The fan is driven from the main rotor and yaw
control is by rotating the tail sleeve as usual.
The NOTAR system is remarkably quiet and safe and demonstration pilots have
made their point by backing the tail boom into bushes and using back cyclic so that
the machine rests on the rear of the skids and the end of the boom.
In the absence of a tail rotor to weathercock, the NOTAR machines, not surprisingly,
need a substantial amount of fin area. In the production machine, minor yaw instability
is handled with a gyroscopically controlled rudder tab on one of the fins. It is possible
that this is not a fundamental problem and further development may eliminate it.
5.12 Tail rotor failure
It is often overheard that loss of tail rotor drive will cause the machine to go out of
control, but this is not necessarily the case. If the tail rotor drive fails without causing
other damage and the machine is put promptly into autorotation, the torque reaction
of the main rotor will cease, and the tail rotor will be unnecessary. In some machines
the tail boom and fin area are large enough to counteract engine torque by yawing
into the slipstream at reasonable translational speed and then autorotation will only
be necessary for landing. Nevertheless most pilots would rather lose the engine than
the tail rotor drive.
Unfortunately tail rotor failures can result in more than loss of drive. A failed component
in the rotor can cause serious imbalance and this can tear the whole tail gearbox
out of the boom. A broken drive shaft may flail and destroy hydraulics and wiring or
even sever the boom. In these cases the machine will suffer a serious forward CM shift
and a loss of rear side area. A machine with a zero-offset rotor head probably wouldn’t
retain control. Figure 5.19 shows a zero-offset helicopter that has developed a serious
nose down trim having lost its tail rotor in a collision with a ship. A machine with a
substantial tail boom and a rotor head having a wide CM tolerance would have a much
better chance.
Despite all the theory, retaining control requires the pilot to take the correct actions
and in order to take good decisions the pilot has to know the situation. In the case of
tail rotor problems the pilot cannot see what is wrong and has to deduce the problem
from whatever symptoms of vibration, noise, yaw and pitch he experiences. When this
happens suddenly it is asking a lot of the pilot to do exactly the right thing. Nevertheless
many pilots have walked away from tail rotor failure, in some cases with no further
damage to the machine.
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