Helicopter Translating Tendency or Drift

During hover flight, helicopters with a single main rotor tend to move in the direction of tail rotor thrust. The direction of rotation of the main rotor and the effect of the tail rotor create this tendency. This lateral movement is called the translating tendency.

The tail rotor is designed to generate thrust to counteract the torque trying to rotate the helicopter in the opposite direction of the main rotor. Part of this thrust exerts a force on the fuselage that causes the helicopter to drift laterally in the same direction.

helicopter translating tendency
Helicopter Translating Tendency

A slope in the main rotor causes a small sideways thrust opposite the tail rotor to counteract drag. Tilt can be achieved by mounting the transmission at a slight angle or by designing the flight control system so that the rotor disc tilts when the cyclic control is centered.

The direction in which the rotor rotates makes a difference. In a system, it turns clockwise and tail rotor thrust when viewed from above causes the helicopter to slide to the left. Tilting the main rotor disc to the right in response to this causes the right slide to be low. A counterclockwise rotation system will cause a right shift and a left tilt, causing the helicopter to hover.

Not all helicopters, especially dual rotor helicopters, experience translation tendencies. In these helicopters, counter torque is created by the counter rotating rotor versus the torque generated by the other rotor. Therefore, the forces are equal in all directions and translating tendency does not occur.

helicopter movements
Helicopter Flight Direction and Rotor Relation

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