Fenestron

A Fenestron (or fantail, sometimes called "fan-in-fin") is a protected tail rotor of a helicopter operating like a ducted fan. Placing the fan within a duct reduces tip vortex losses, shields the tail rotor from damage, is much quieter than a conventional tail rotor, and shields ground crews from the hazard of a spinning rotor. The housing is integral with the tail skin and, like the conventional tail rotor it replaces, functions to counteract the torque of the main rotor. It was first developed by the French company Sud Aviation (now part of Airbus Helicopters) and is installed on many of their helicopters.

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.

The term Fenestron is a trademark of Eurocopter. It comes from the Occitan for a small window, and is ultimately from the Latin for window.

The concept of the Fenestron was first patented in Great Britain by the Glasgow engineering Company G. & J. Weir Ltd. It was designed by C. G. Pullin as an improvement to helicopters in British patent number 572417. Pullin's patent is dated May 1943. At that time Weir were completing the development work for the Cierva Autogiro Company as the holding company. The Fenestron was then developed in the 1960s to replace the classic tail rotor to improve security and performance for rotorcraft. Through multiple mergers and the formation of Airbus Helicopters, many light, intermediate, and medium weight helicopters use the Fenestron as a tail rotor. The Fenestron was introduced on the second experimental model of the SA 340 by Sud Aviation and on the later model Aérospatiale SA 341 Gazelle SA341/342.

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