What is "fenestron"

n. (label en aviation) a protected tail rotor of a helicopter operating like a ducted fan.

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 fenestra for window.