The Art of the Helicopter John Watkinson - Karatunov.net

178 The Art of the Helicopter
This represents a worst case in tail rotor performance. The result of a fast with-torque
yaw may be that when the pilot attempts to arrest it there is little response and the
required heading may be seriously overshot. This goes down badly in the military as it
spoils the weapon aiming.
It should be clear from Figure 5.8 that the tail rotor must have a wide pitch range
to allow rapid yaws in either direction and then even more to maintain that degree of
control in autorotation. A large positive pitch value is essential to permit a positive
angle of attack to be reached despite the high inflow of an against-torque yaw. This
means that when the with-torque yaw rate is high the rapid application of full opposite
pedal can result in an enormous angle of attack that may be enough to stall the blades.
This is doubly bad, first because the yaw obtained will be in the opposite direction
to that intended and second because a stalled tail rotor puts the tail transmission
under an enormous torque loading. Everything at the tail of a helicopter is designed
to be light in weight and this includes the transmission. Stalling the tail rotor may do
damage. Consequently it is considered bad practice to stamp on the tail control pedals.
A better result will be obtained if the pedals are operated gradually. In some helicopters
tail drives actually did suffer damage and the designers responded by putting viscous
dampers on the pedals.
In addition to the yaw function, effective tail power is also needed for sideways
flight. It is not obvious why anyone would want to fly sideways, but there are plenty
of examples. Large film cameras are often mounted in the main cabin and only have a
clear view to the side. Flying the machine sideways allows the camera to shoot forwards.
Flying sideways allows the pilot of an attack helicopter to dodge fire whilst keeping
his rockets aimed at the target. Pilots who wouldn’t dream of doing the above also fly
sideways as a matter of course, because this is exactly what happens when hovering
in a side wind. The helicopter only appears to be hovering; it’s actually flying along
sideways at the same speed as the wind but in the opposite direction.
In the case of a clockwise-from-the-top helicopter, the wind coming from the
starboard side is undesirable as it increases the tail rotor inflow and so requires more
power. The worst case will then be where the pilot wishes to make a maximum speed
yaw to port in a strong wind from the starboard side. The tail rotor now has to overcome
main rotor torque, boom drag due to the side wind and the yaw under conditions
of greatest inflow where its angle of attack is reduced. Needless to say this is how the
military test tail rotor performance.
Again assuming a clockwise-from-the-top helicopter, there will be a critical airspeed
in sideways flight to starboard where the tail rotor is moving at the same speed as its
induced velocity and a vortex ring is again a possibility when the pilot tries to arrest
the manoeuvre.
Sufficient shaft power is not generally a problem in a turbine helicopter as the main
rotor will be in translational lift in a side wind hover and there will be plenty of power
available for the tail rotor. The problem is whether the tail rotor can use the power
available. The blade pitch cannot be increased indefinitely or the tail rotor may stall.
The RPM cannot be increased because of compressibility in forward flight and the
diameter can’t be increased because this will need the boom to be lengthened and will
reduce ground clearance. One option is to increase the solidity of the tail rotor, typically
by adding blades. This is less efficient, but it does at least solve the tail power problem.
Other options may include an assessment of the degree of blockage and/or the amount
of rear side area.
In a piston engine helicopter the tail rotor may not be powerful enough to allow
a crosswind hover above a certain windspeed. In some light helicopters this is not a
problem because that windspeed may also exceed the maximum speed at which it is