What does the tail rotor do on a helicopter?

The tail rotor on a helicopter primarily serves the essential function of counteracting torque generated by the main rotor. When the main rotor spins, it creates a torque reaction that tends to rotate the helicopter's fuselage in the opposite direction. The tail rotor is positioned at the rear of the helicopter and is designed to produce a horizontal thrust that counteracts this torque, ensuring the stability of the aircraft in flight.

By adjusting the pitch of the blades on the tail rotor, the pilot can control the amount of thrust it generates, which allows for maneuverability and directional control during flight. Consequently, the tail rotor is critical for maintaining control and preventing unwanted yaw, enabling the helicopter to fly efficiently and safely.

In contrast, the other options do not accurately describe the primary role of the tail rotor. It does not decrease lift, increase speed, or enhance roll control; rather, its primary purpose is to balance the forces acting on the helicopter due to the main rotor's torque.