Rotorcraft Flying Handbook, FAA-H-8083-21

The rotor disc tilts in the direction that pressure is applied
to the cyclic pitch control. If the cyclic is moved forward,
the rotor disc tilts forward; if the cyclic is moved aft, the
disc tilts aft, and so on. Because the rotor disc acts like a
gyro, the mechanical linkages for the cyclic control rods
are rigged in such a way that they decrease the pitch angle
of the rotor blade approximately 90° before it reaches the
direction of cyclic displacement, and increase the pitch
angle of the rotor blade approximately 90° after it passes
the direction of displacement. An increase in pitch angle
increases angle of attack; a decrease in pitch angle
decreases angle of attack. For example, if the cyclic is
moved forward, the angle of attack decreases as the rotor
blade passes the right side of the helicopter and increases
on the left side. This results in maximum downward
deflection of the rotor blade in front of the helicopter and
maximum upward deflection behind it, causing the rotor
disc to tilt forward.
ANTITORQUE PEDALS
The antitorque pedals, located on the cabin floor by the
pilot’s feet, control the pitch, and therefore the thrust,
of the tail rotor blades. [Figure 4-5] . The main purpose
of the tail rotor is to counteract the torque effect of the
main rotor. Since torque varies with changes in power,
the tail rotor thrust must also be varied. The pedals are
connected to the pitch change mechanism on the tail
rotor gearbox and allow the pitch angle on the tail rotor
blades to be increased or decreased.
Figure 4-5. Antitorque pedals compensate for changes in
torque and control heading in a hover.
HEADING CONTROL
Besides counteracting torque of the main rotor, the tail
rotor is also used to control the heading of the helicopter
while hovering or when making hovering turns. Hovering
turns are commonly referred to as “pedal turns.”
In forward flight, the antitorque pedals are not used to
control the heading of the helicopter, except during portions
of crosswind takeoffs and approaches. Instead they
are used to compensate for torque to put the helicopter in
longitudinal trim so that coordinated flight can be maintained.
The cyclic control is used to change heading by
making a turn to the desired direction.
The thrust of the tail rotor depends on the pitch angle of
the tail rotor blades. This pitch angle can be positive, negative,
or zero. A positive pitch angle tends to move the tail
to the right. A negative pitch angle moves the tail to the
left, while no thrust is produced with a zero pitch angle.
With the right pedal moved forward of the neutral position,
the tail rotor either has a negative pitch angle or a
small positive pitch angle. The farther it is forward, the
larger the negative pitch angle. The nearer it is to neutral,
the more positive the pitch angle, and somewhere
in between, it has a zero pitch angle. As the left pedal is
moved forward of the neutral position, the positive pitch
angle of the tail rotor increases until it becomes maximum
with full forward displacement of the left pedal.
If the tail rotor has a negative pitch angle, tail rotor
thrust is working in the same direction as the torque of
the main rotor. With a small positive pitch angle, the
tail rotor does not produce sufficient thrust to overcome
the torque effect of the main rotor during cruise flight.
Therefore, if the right pedal is displaced forward of
neutral during cruising flight, the tail rotor thrust does
not overcome the torque effect, and the nose yaws to
the right. [Figure 4-6]
With the antitorque pedals in the neutral position, the tail
rotor has a medium positive pitch angle. In medium positive
pitch, the tail rotor thrust approximately equals the
torque of the main rotor during cruise flight, so the helicopter
maintains a constant heading in level flight.
Tail Moves
Negative or Low
Positive Pitch
Medium
Positive Pitch
High Positive
Pitch
Tail Moves
Figure 4-6. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.
4-3