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.
The Antitorque Rotor
Compensation for torque in the single main rotor helicopter is accomplished by means of a variable pitch anti-torque 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 (keeping the aircraft in trim, the tail is not used to turn the helicopter IN forward flight. 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 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 laterally level.