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)).