What is a single-rotor helicopter?

A single-rotor helicopter is a rotorcraft with a main rotor and, in most cases, a tail rotor for torque compensation and control about the vertical axis. In this case, the main rotor is driven by a motor and the power transmission to the rotor is generated by a gearbox and a drive shaft; thus, a torque is transmitted to the main rotor, which is supported by the airframe of the helicopter.

In the case of direct drives by small motors or by nozzles on the main rotor blades, no torque is transmitted to the airframe, but the drive is supported by the surrounding air; action = reaction. No tail rotor is needed here. The control around the vertical axis is done by means of a 45 degree hinged rudder which is generated by the downwash (air flowing down through the main rotor) during hovering flight and by the driving wind during forward flight.