Tail rotor
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The tail rotor, or anti-torque rotor, is a necessary component of helicopters that have a single main rotor. The tail rotor is a fan mounted on the tail boom of the helicopter, with a lateral axis of rotation. The thrust it creates is offset from the center of gravity, producing torque which counters the torque created by the main rotor. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, which allows the pilot to rotate the helicopter around its vertical axis, providing directional control.
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[edit] Technical details
The tail rotor is essentially an un-ducted fan with blades that vary in pitch to vary the amount of thrust they produce. The blades most often utilize a composite material construction, such as a core made of aluminum honeycomb or plasticized paper honeycomb, covered in a skin made of aluminum or carbon fiber composite. The blades use a symmetrical airfoil, and their pitch angle can be adjusted both positive and negative to produce thrust in either direction. The pitch change mechanism is almost always a cable system that runs from the anti-torque pedals in the cockpit to a mechanism mounted on the tail rotor gearbox. In larger helicopters the pitch change mechanism is powered by a hydraulic power control servo. In the event of a hydraulic system failure, the mechanical cable system will still be able to control the tail rotor pitch, though the control resistance felt by the pilot will be considerably greater.
The tail rotor is powered by the helicopters main powerplant, and rotates at a speed proportional to that of the main rotor. As an example, in the case of the Rolls-Royce Model 250 such as used on the Bell 206, the engine has two output shafts, one out of each end of the engine's gearbox. The front shaft is a short one that connects to the main rotor transmission. The rear shaft is long and lightweight and passes down the tail boom to the tail rotor transmission. In both piston and turbine powered helicopters, the main rotor and the tail rotor are mechanically connected so that in the event of an engine failure, the momentum of the main rotor will provide power to turn the tail rotor to allow directional control during autorotation.
Unlike a propeller used for forward propulsion, the blades of a tail rotor are not twisted, because the tail rotor is mounted with its axis of rotation lateral and so it need not be optimized for forward flight.
[edit] Reliability and safety
The tail rotor and the systems that provide power and control for it are considered critically important for safe flight. As with many parts on a helicopter, the tail rotor, its transmission, and many parts in the drive system are often life-limited, meaning they are arbitrarily replaced after a certain number of flight hours, regardless of condition. Between replacements, parts are subject to frequent inspections utilizing visual as well as chemical methods such as fluorescent penetrant inspection to detect weak parts before they fail completely.
Despite the emphasis on reducing failures, they do occasionally occur, most often due to hard landings and tailstrikes, or foreign object damage. Though the tail rotor is considered essential for safe flight, the loss of tail rotor function does not necessarily result in a fatal crash. In cases where the failure occurs due to contact with the ground, the aircraft is already at low altitude and the pilot may be able to reduce collective and land the helicopter before it spins completely out of control. Should the tail rotor fail randomly during cruise flight, forward momentum will often provide some directional stability, as many helicopters are equipped with a vertical stabilizer. The pilot would then be forced to autorotate and make an emergency landing with significant forward airspeed, which is known as a running landing or roll-on landing.
The tail rotor itself is a hazard to ground crews working near a running helicopter. For this reason, tail rotors are painted with stripes of alternating colors to increase their visibility to ground crews while the tail rotor is spinning.
[edit] Alternative technologies
There have been three major alternative designs which attempt to solve the shortcomings of the tail rotor system.
The first is to use a ducted fan rather than an un-ducted fan. This design is referred to as a fantail, or by the trade name Fenestron, a trademark of Eurocopter. Placing the fan within a duct reduces tip vortex losses, shields the tail rotor from damage, shields ground crews from the hazard of a spinning rotor, and is much quieter than a conventional tail rotor. The ducted fan uses shorter and more numerous blades, but otherwise it is very similar to a conventional tail rotor.
McDonnell Douglas developed the NOTAR system, which eliminates having any rotating parts out in the open. The NOTAR system uses a variable pitch ducted fan driven by the helicopter's powerplant, but the ducted fan is mounted inside the fuselage, and the exhaust passes through the tail boom to the end, where it is expelled out one side to provide thrust. The advantages of the system are similar to the Fenestron system discussed above.
A way to eliminate the necessity of a tail rotor altogether is to use two main rotors which turn in opposite directions, so that the torque created by one rotor cancels out the torque created by the other. Such a design is commonly seen on heavy lift helicopters such as the CH-47 Chinook and V-22 Osprey. Other designs such as the Kamov Ka-50 use coaxial contra-rotating main rotors, which means that both rotors spin around the same axis but in opposite directions. The complexity of any dual main rotor system almost invariably requires the addition of a fly-by-wire flight control system, which increases costs drastically.
