What is the primary reason helicopters typically require a tail rotor?

The primary reason helicopters typically require a tail rotor is to counteract the torque produced by the main rotor. Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. As the main rotor spins, it creates torque, a rotational force, on the helicopter fuselage in the opposite direction. Without a means of counteracting this torque, the helicopter would spin uncontrollably in the opposite direction of the main rotor. The tail rotor generates thrust in a direction that opposes the main rotor torque, allowing the pilot to maintain directional control and prevent the helicopter from spinning. By adjusting the pitch of the tail rotor blades, the pilot can vary the amount of thrust produced and control the helicopter's heading. Some helicopters use alternative anti-torque systems, such as NOTAR or fenestron, but the underlying principle remains the same: to counteract the torque of the main rotor.