A Fenestron (sometimes alternatively referred to as a fantail or a "fan-in-fin" arrangement) is an enclosed helicopter tail rotor that operates like a ducted fan. The term Fenestron is a trademark of multinational helicopter manufacturing consortium Airbus Helicopters (formerly known as Eurocopter). The word itself comes from the Occitan term for a small window, and is ultimately derived from the Latin word fenestra for window. The Fenestron differs from a conventional open tail rotor by being integrally housed within the tail boom, and like the conventional tail rotor it replaces, functions to counteract the torque generated by the main rotor. While conventional tail rotors typically have two or four blades, Fenestrons have between seven and eighteen blades; these may have variable angular spacing so that the noise is distributed over different frequencies. By placing the fan within a duct, several distinct advantages over a conventional tail rotor are obtained, such as a reduction in tip vortex losses and the potential for substantial noise reduction, while also shielding both the tail rotor itself from collision damage and ground personnel from the hazard posed by a traditional spinning rotor. It was first developed for use on an operational rotorcraft by the French company Sud Aviation (now part of Airbus Helicopters), being first adopted upon the Aérospatiale Gazelle. Since then, the company (and its successors) have installed Fenestrons upon many of their helicopters. Other manufacturers have also made limited use of the Fenestron on some of their own products, including the American aerospace corporations Bell Textron and Boeing, the Russian rotorcraft manufacturer Kamov, the Chinese Harbin Aircraft Industry Group, and the Japanese conglomerate Kawasaki Heavy Industries. History The concept of the Fenestron was first patented in Great Britain by the Glaswegian engineering company G. & J. Weir Ltd. It was designed by British aeronautical engineer C. G. Pullin as an improvement to helicopters in British patent number 572417, and is registered as having been filed during May 1943. At that time, Weir had been participated in development work for the Cierva Autogiro Company, who was the holding company for the patent. In concept, the invention was to function as a viable replacement for the conventional tail rotor arrangement, aiming to produce improvements in both safety and performance upon such equipped rotorcraft. However, this early work in Britain would not directly lead to any released product by Cierva making use of this innovation. Instead,