The physics of helicopters
For single-rotor helicopters, lift is generated through the main rotor rotating. This rotation generates torque about the main helicopter, causing the main fuselage to spin around in the opposite direction.
Early engineers designed the tail rotor to counter this torque and keep helicopters stable. Tail rotors are generally much smaller rotors mounted on a perpendicular axis to the main rotor.
By controlling the speed of the tail rotor, the pilot can stabilize the craft as well as control the direction of the helicopter.
Slowing the tail rotor would cause the helicopter body to rotate in the opposite direction of the main rotor due to excess torque in that direction.
Speeding up the tail rotor would do the opposite. Along with direction, helicopter pilots can control the yaw of the craft by adjusting the angle of the tail rotor. By pitching the tail rotor slightly up or down, the pilot creates a moment arm through the helicopter, which in turn adjusts the yaw of the craft.
