Abstract: Yaw control device for helicopters with contra-rotating rotors which controls the anti-torque action of the rotors on the helicopter fuselage by means of a variable speed reduction ratio drive transmission comprising a differential traction drive. At one setting of the yaw controls the rotor torques are balanced by means of the differential in which the input shaft drives a planet carrier and the output shafts are driven by traction discs.The yaw control is accomplished by means of tilting the axes of rotation of the planets thus changing the effective radia of driving the discs which upsets the rotor torques equilibrium. In a stabilized flight of the helicopter the rotations of the planets on their carrier is practically nil, hence the power loss due to the use of the traction drive is small.