The Art of the Helicopter John Watkinson - Karatunov.net

safe to start the rotors. However, some piston engine helicopters, such as the Enstrom,
can safely start the rotors in a gale and on moving to the hover it will be found that
the machine simply weathercocks into wind with the pedals only allowing a certain
yaw angle either way. The tail rotor is defeated by inflow and cannot obtain an angle
of attack.
In any tail rotor operation that increases the inflow, the blade reaction will tilt back
and the tail rotor will require more torque to drive it. This torque is provided by the
engine, and delivered by the long tail drive shaft. If the tail rotor absorbs more power
and nothing else happens, there will be less power for the main rotor. In a simple piston
engine machine, where there is seldom any sophisticated engine governing, yawing in
the same direction as the rotor turns will result in the machine also tending to descend.
The pilot has to compensate by raising the collective lever slightly, which will then
require more engine power. The tail rotor takes roughly 15% of the total power in
the still hover and more in the circumstances described above. Many piston-engine
helicopters don’t have a lot of surplus power and the pilot soon learns that large pedal
inputs in the hover in one direction are to be avoided. Given a choice, the pilot will
always prefer to yaw in the direction requiring the least tail power, using smooth and
gradual pedal applications. With limited power, yawing must be done slowly as there
is no point initiating a yaw in the ‘easy’ direction if it cannot be stopped.
In a turbine-powered helicopter the RRPM is accurately governed and so if a yaw in
the direction of main rotor rotation is initiated, the extra torque needed by the tail rotor
is automatically provided by action of the governor. As a result the RRPM relative to
the helicopter does not change. However, the whole helicopter is now turning with the
yaw and so the RRPM relative to the air has slightly increased. One might not expect
this to have much effect, but the yaw RPM can raise the effective RRPM by a few
percent and as lift is proportional to the square of the RRPM, a significant increase in
lift can occur. Thus the turbine helicopter will climb under the same conditions that
caused the piston engine helicopter to descend.
5.6 The tail plane
The tail plane is a necessity in forward flight for two reasons. First, because the main
rotor alone doesn’t have stability in pitch; and second, because the drag of the hull acts
some way below the rotor head, a moment results which tends to pitch the hull down.
The nose-down attitude of the hull will result in higher drag than if it is aligned with
the RAF. Figure 4.12 showed that an aft-mounted tail plane developing a down force
produces a moment in the opposite direction to the hull drag moment.
In a machine with a teetering or zero-offset head there can be no moments from the
rotor and so this mechanism determines the hull attitude. The hull will adopt a pitch
angle where the drag moment, the tail plane moment and any moment due to an offset
CM all sum to zero. The system is stable because if the hull pitches down the (negative)
angle of attack increases and produces a restoring moment.
In Figure 5.9 the machine has rotor head offset and the hull drag moment can be
balanced by any combination of the tail plane moment and a couple from the main
rotor due to the tip path axis being tilted with respect to the shaft axis. In practice a
couple from the main rotor will be obtained with the penalty of increased mast stress
and some vibration and so it is beneficial to trim the hull attitude in cruise with the tail
plane so that a minimal rotor couple exists.
In the hover the tail plane is an unnecessary weight to be lifted and may actually
produce significantly more down force than its weight if it is in the rotor downwash.
The tail 179