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 center 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 maneuver 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
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 tendency.
- Flight control rigging may be designed so the rotor disk is
tilted slightly left when the cyclic control is centered.
- 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
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