Fenestron

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A Fenestron (or Fantail) is a totally enclosed tail rotor of a helicopter that has a tail rotor configuration and is essentially a ducted fan. The housing is integral with the tail skin, and, like the conventional tail rotor it replaces, is intended to counteract the torque of the main rotor.

The term Fenestron is a trademark of Eurocopter (the actual word, though, is a French one that refers to small windows in houses)^[1]

While conventional tail rotors typically have two or four blades, fenestrons have between eight and 18 blades. These are arranged in varying distance, so that the noise is distributed over different frequencies and thus sounds quieter. The housing allows a higher rotational speed than a conventional rotor allowing it to have smaller blades.

Advantages of the Fenestron:

• Increased safety for people on the ground. The smaller size allows it to be higher from people or objects on the ground, and the enclosure provides peripheral protection.
• Less susceptible to foreign object damage. The higher ground clearance and the enclosure also makes it less likely to strike foreign objects or to suck in loose objects such as small rocks.
• Greatly reduced noise. Since the blade tips are enclosed; this and the greater number of blades leads to reduced vibration.

Its disadvantages are those common to all ducted fans when compared to propellers. They include higher weight and air resistance of the enclosure, higher construction cost, and a higher power requirement for a given thrust.

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, a ducted fan tail rotor was also used on the US military helicopter project Boeing-Sikorsky RAH-66 Comanche, which was cancelled in 2004.

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• NOTAR