During autorotation, what does the rear rotor still need to control?

During autorotation, a helicopter relies on the lift generated by the main rotor while descending without engine power. The rear rotor, which is part of the tail rotor system, is crucial for maintaining yaw control. Yaw refers to the rotation of the helicopter around its vertical axis; this is important for maintaining directional stability and control.

In an autorotation scenario, with the main rotor losing power, the tail rotor's primary responsibility shifts to counteracting the torque produced by the main rotor. This torque tends to rotate the helicopter in the opposite direction of the main rotor's spin. Thus, the tail rotor must actively control yaw to keep the helicopter flying straight and prevent unwanted rotation.

The other aspects like pitch, altitude, and roll do not fall under the purview of the tail rotor’s control in this context. Pitch is primarily managed by the pilot through cyclic input, altitude is naturally lost during autorotation because the helicopter descends, and roll is managed by the lateral control of the main rotor. Therefore, the tail rotor's essential role in maintaining yaw control is critical to ensuring a safe autorotation landing.