In accordance with Newton's law
of action and reaction, the helicopter fuselage tends to rotate in the direction
opposite to the rotor blades. This effect is called torque. Torque must
be counteracted and or controlled before flight is possible. In tandem rotor and
coaxial helicopter designs, the rotors turn in opposite directions to neutralize
or eliminate torque effects. In tip-jet helicopters, power originates at the
blade tip and equal and opposite reaction is against the air; there is no torque
between the rotor and the fuselage. However, the torque problem is especially
important in single main rotor helicopters with a fuselage mounted power source.
The torque effect on the fuselage is a direct result of the work/resistance of
the main rotor. Therefore torque is at the geometric centre of the main rotor.
Torque results from the rotor being driven by the engine power output. Any
change in engine power output brings about a corresponding change in torque
effect. Furthermore, power varies with the flight manoeuvre and results in a
variable torque effect that must be continually corrected.
Compensation for torque in the
single main rotor helicopter is accomplished by means of a variable pitch
antitorque rotor (tail rotor) located on the end of a tail boom extension at the
rear of the fuselage. Driven by the main rotor at a constant ratio, the tail
rotor produces thrust in a horizontal plane opposite to torque reaction
developed by the main rotor. Since torque effect varies during flight when power
changes are made, it is necessary to vary the thrust of the tail rotor.
Antitorque pedals enable the pilot to compensate for torque variance. A
significant part of the engine power is required to drive the tail rotor,
especially during operations when maximum power is used. From 5 to 30 percent of
the available engine power may be needed to drive the tail rotor depending on
helicopter size and design. Normally, larger helicopters use a higher percent of
engine power to counteract torque than do smaller aircraft. A helicopter with
9,500 horsepower might require 1,200 horsepower to drive the tail rotor, while a
200 horsepower aircraft might require only 10 horsepower for torque correction.
In addition to counteracting
torque, the tail rotor and its control linkage also permit control of the
helicopter heading during flight. Application of more control than is necessary
to counteract torque will cause the nose of the helicopter to swing in the
direction of pedal movement. To maintain a constant heading at a hover or during
takeoff or approach, the pilot must use antitorque pedals to apply just enough
pitch on the tail rotor to neutralize torque and hold a slip if necessary.
Heading control in forward trimmed flight is normally accomplished with cyclic
control, using a coordinated bank and turn to the desired heading. Application
of antitorque pedals will be required when power changes are made.
In an autorotation, some degree
of right pedal is required to maintain correct trim. When torque is not present,
mast thrust bearing friction tends to turn the fuselage in the same direction as
main rotor rotation. To counteract this friction, the tail rotor thrust is
applied in an opposite direction to counter the frictional forces.
During hovering flight, the
single rotor helicopter has a tendency to drift laterally to the right due to
the lateral thrust being supplied by the tail rotor. The pilot may prevent right
lateral drift of the helicopter by tilting the main rotor disk to the left. This
lateral tilt results in a main rotor force to the left that compensates for the
tail rotor thrust to the right.
Helicopter design usually
includes one or more features which help the pilot compensate for translating
Flight control rigging may be
designed so the rotor disk is tilted slightly left when the cyclic control is
The collective pitch control
system may be designed so that the rotor disk tilts slightly left as
collective pitch is increased to hover the aircraft.
The main transmission may be
mounted so that the mast is tilted slightly to the left when the helicopter
fuselage is laterally level.