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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. Apositive 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.
Figure 4-5. Antitorque pedals compensate for changes in
torque and control heading in a hover.
Tail Moves Tail Moves
Negative or Low

Positive Pitch
Medium

Positive Pitch
High Positive

Pitch
Figure 4-6. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.
4-4
If the left pedal is in a forward position, the tail rotor
has a high positive pitch position. In this position, tail
rotor thrust exceeds the thrust needed to overcome
torque effect during cruising flight so the helicopter
yaws to the left.
The above explanation is based on cruise power and airspeed.
Since the amount of torque is dependent on the
amount of engine power being supplied to the main rotor,
the relative positions of the pedals required to counteract
torque depend upon the amount of power being used at
any time. In general, the less power being used, the
greater the requirement for forward displacement of the
right pedal; the greater the power, the greater the forward
displacement of the left pedal.
The maximum positive pitch angle of the tail rotor is
generally somewhat greater than the maximum negative
pitch angle available. This is because the primary
purpose of the tail rotor is to counteract the torque of
the main rotor. The capability for tail rotors to produce
thrust to the left (negative pitch angle) is necessary,
because during autorotation the drag of the transmission
tends to yaw the nose to the left, or in the same
direction the main rotor is turning.
5-1
By knowing the various systems on a helicopter, you
will be able to more easily recognize potential problems,
and if a problem arises, you will have a better understanding
of what to do to correct the situation.
ENGINES
The two most common types of engines used in helicopters
are the reciprocating engine and the turbine
engine. Reciprocating engines, also called piston
engines, are generally used in smaller helicopters. Most
training helicopters use reciprocating engines because
they are relatively simple and inexpensive to operate.
Turbine engines are more powerful and are used in a
wide variety of helicopters. They produce a tremendous
amount of power for their size but are generally
more expensive to operate.
RECIPROCATING ENGINE
The reciprocating engine consists of a series of pistons
connected to a rotating crankshaft. As the pistons move
up and down, the crankshaft rotates. The reciprocating
engine gets its name from the back-and-forth movement
of its internal parts. The four-stroke engine is the most
common type, and refers to the four different cycles the
engine undergoes to produce power. [Figure 5-1]
　
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