In what flight configuration is the tail rotor required most?

The tail rotor is most critically needed in a hovering crosswind condition, particularly when the weathercock effect is added to the torque effect. When a helicopter is hovering in a crosswind, its fuselage will naturally tend to weathercock into the wind due to aerodynamic forces. This weathercocking creates additional yawing moments that affect directional control.

In these conditions, the torque produced by the main rotor can create a tendency for the helicopter to rotate in the opposite direction, requiring the tail rotor to exert sufficient thrust to maintain a stable hover. The tail rotor adjusts to counteract both the yaw from the torque effect and the additional weathercocking force created by the wind. This makes the tail rotor essential for controlling the helicopter's heading and preventing unwanted rotation during such a flight configuration.

In contrast, a climbing left turn or a simple left turn relies less on the tail rotor's additional input since the main rotor's thrust vector is already providing a degree of directional control. Therefore, the need for the tail rotor is most pronounced in the hovering crosswind scenario with the added complexity of the weathercocking effect.