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A rotor shaft can be found in one of two places on a helicopter: the main rotor blade and the tail. The main rotor shaft is attached to the main rotor blade and transfers the power from the engine to the blades. The tail rotor shaft is located along the length of the tail section and powers the tail rotor. The tail rotor makes flight possible for the helicopter by providing power to maintain straight flight and altitude. In order for the helicopter to move forward and backward, the blades of the main rotor must be able to pivot on the rotor shaft.
The main blades on a helicopter are not affixed directly to the aircraft's engine. In most cases, the main rotor shaft is affixed to the engine via a gearbox. This gearbox allows the engine to operate at a reduced speed while maintaining rotor speed. The tail rotor shaft also operates off of a gearbox that matches the speed of the tail rotor to that of the larger main rotor. The pilot is able to manipulate the speed of the tail rotor by adjusting a hand control mounted beside the pilot seat. Without this small rotor controlling the position of the helicopter's tail, the helicopter would simply spin violently out of control.
The ability to control the flight characteristics of the helicopter makes the tail rotor shaft a primary target for enemy fire. The relatively small diameter of the shaft makes it prone to small arms fire as well as larger anti-aircraft weaponry. In many combat designated helicopters, the tail rotor as well as the tail rotor shaft are armor protected. In cases in which the tail rotor is shot away, the helicopter is going to come down violently no matter how good the pilot is. The instances of successfully landing a helicopter with the tail rotor damaged is so rare that when a military pilot is successful in doing so, it nearly always results in receiving a medal or citation.
Maintenance of the rotor shafts is scheduled on a frequent basis. Bearings and bearing mounts are replaced after a predetermined number of hours of use. This schedule varies by manufacturer and model of helicopter. A worn bearing can lead to an unbalanced shaft and subsequent vibration. This vibration can lead to bearing failure and thus tail rotor failure and the possible crash of the helicopter. Routinely changing the bearings reduces the risk of this type of failure causing problems for the helicopter and its crew.