Fenestron

A Fenestron (or fantail, sometimes called "fan-in-fin") is a shrouded tail rotor of a helicopter that is essentially a ducted fan. The housing is integral with the tail skin and like the conventional tail rotor it replaces, it is intended to counteract the torque of the main rotor. It was originally conceived by Sud Aviation (now Eurocopter, part of the European Aeronautic Defence and Space Company (EADS)).

While conventional tail rotors typically have two or four blades, Fenestrons have between eight and 18 blades. These may have variable angular spacing, 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.

The term Fenestron is a trademark of Eurocopter. It comes from the modern French for a small window, and is ultimately from the Latin fenestra ("window")