Which component does NOT have a controlling function in a variable-incidence tailplane?

The elevator surface is typically responsible for controlling the pitch of the aircraft. In a variable-incidence tailplane, this component primarily alters the angle of incidence, which changes the aerodynamic characteristics of the tailplane and, consequently, the pitch of the aircraft without having a direct controlling function itself.

The autopilot systems are designed to control various flight parameters, including pitch, yaw, and roll, relying on inputs from various sensors and control surfaces, thus having a significant controlling function in overall aircraft handling.

Rudder control directly influences the yaw axis, allowing the aircraft to turn and maintain coordinated flight. Therefore, it plays a crucial role in controlling the aircraft's movement.

Stabilizing forces work in conjunction with various control surfaces to maintain stable flight. They contribute to the overall stability of the aircraft during different phases of flight and respond to various conditions.

Therefore, the only component listed that does not perform a direct controlling function in the context of a variable-incidence tailplane is the elevator surface, as it mainly serves to adjust the angle of incidence rather than actively control the aircraft's pitch during flight.