.: articles ww.helosim.com
Tail rotors and variations - by Gary 'Flyboy' Wright
Tail rotor
The tail rotor of a helicopter is mounted on the tail of a traditional single-rotor helicopter, close to perpendicular to the main rotor. It is primarily used in order to counteract the yaw motion and the torque that a rapidly turning disk naturally produces.
Design variations
There are two major variations to traditional tail rotor design concerning the placement of the tail rotor and the surrounding structure. Some companies such as Augusta Westland enclose the rotor within a fantail assembly. Such design - called fenestron - protects the tail rotor from foreign object damage better than the traditional outer mounted design but complicates the design of the tailcone to account for the enclosed mechanisms.
New developments
In some more recent helicopter designs, the tail rotor has been mounted tangential to the furthest back point of the top rotor. That is to say that it looks much like an old propeller plane, only at the back of the helicopter instead of the front of a wing. In these new designs the rotor spins in a direction opposite to the top rotor (i.e. counter-clockwise if the rotor spins clockwise and vise-versa). This in effect, cancels the spin and has the added benefit of producing forward thrust.
Most, if not all, dual-rotary helicopters do not use tail rotors, instead, the design of the two main rotors is such that they spin in the opposite directions of each other, thus each cancels out the torque and yaw produced by the other. This has been researched in the past and has been incorporated into some European designs.
Sikorsky Aircraft, a UTC subisidiary is currently researching the merger of these two concepts with a dual rotor helicopter with a a rear rotor to provide additional forward thrust and a respective increase in speed and operating range. First flight of a prototype aircraft, the X-2 Demonstrator is expected to be accomplished by the end of 2006.
Fenestron
A Fenestron (or Fantail) is a totally enclosed tail rotor of a helicopter that has a tail rotor configuration. Its purpose is to counteract the torque of the main rotor and is molded into the tail.
The term Fenestron was trademarked by the French company Sud Aviation, now known as Eurocopter.
While conventional tail rotors typically possess a maximum of 5 rotor blades, Fenestrons have between 8 and 18 blades. These are arranged in varying distance, so that the noise is distributed over different frequencies and thus appears quieter. Since the Fenestron has a smaller diameter, it has a higher circulation speed than a normal tail rotor.
Advantages of the Fenestron:
* Increased safety for people on the ground, since spinning tail rotors rank among the largest sources of danger with helicopters.
* Higher ground clearance of the tail arm and a smaller susceptibility to foreign objects.
* Greatly reduced noise, since the blade tips are enclosed; this and the greater number of blades leads to reduced vibration.
Disadvantages include higher weight of the enclosure, higher construction cost, and a higher power requirement because of the smaller size.
The Fenestron tail rotor was used for the first time at the end of the 1960s on the second experimental model of the SA 340, and on the later model Aérospatiale SA 341 Gazelle. Other than Eurocopter and its predecessors, the Fenestron was also used on the US military helicopter project Boeing-Sikorsky RAH-66 Comanche, which was cancelled in 2004.
NOTAR
NOTAR, an acronym for NO TAil Rotor, is a helicopter stabilization process developed by McDonnell Douglas Helicopter Systems which eliminates the use of tail rotors on helicopters, yielding quieter and safer operation.
Development of the NOTAR system dates back to 1975 when engineers at Hughes Helicopters began concept development work. In December 1981 Hughes flew a OH-6A fitted with NOTAR for the first time. A more heavily modified prototype demonstrator first flew in March 1986 (by which time McDonnell Douglas had acquired Hughes).
Although the concept took some time to refine, the NOTAR system is simple in theory and works to provide directional control the same way a wing develops lift. A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor transmission. This fan forces low pressure air through two slots on the right side of the tailboom, causing the downwash from the main rotor to hug the tailboom, producing lift, and thus a measure of directional control. This is augmented by a direct jet thruster and vertical stabilisers. NOTAR system benefits include far lower external noise (NOTAR-equipped helicopters are among the quietest certificated helicopters), increased safety due to the lack of a tail rotor, improved handling and performance, reduced vibration and easier maintainability.
There are three helicopters that utilize the NOTAR system, all produced by MD Helicopters:
* MD 520N - a NOTAR variant of the Hughes/MD500 series helicopter.
* MD 600N - a larger version of the MD 520N.
* MD Explorer - a twin-engine, 8-seat light helicopter.
Archived Articles:
[Ka-50 and Ka-52 Are Unrivalled in Combat]