The fenestron of an Eurocopter EC135, with uneven blade spacing to control noise emissions.
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. It is especially useful on rescue helicopters that have to land in crowded areas. The housing is integral with the tail skin and, like the conventional tail rotor it replaces, is intended 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 housing allows a higher rotational speed than a conventional rotor, allowing it to have smaller blades.
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.
The Fenestron can be found on many Eurocopter helicopters, such as the Eurocopter EC120 Colibri, EC130 ECO Star, EC135 (and EC635 the military version of the EC135), the AS365 N/N3 Dolphin (also known as the HH-65C widely used by the United States Coast Guard), the EC155 Super Dolphin (a wider, heavier and more advanced version of the AS365 N/N3 series of the Dolphin Helicopter), and more recently on the small reciprocating engine powered Guimbal Cabri G2.
Other than Airbus Helicopters and its predecessors, a Fenestron was also used on the U.S. Boeing/Sikorsky RAH-66 Comanche, which was canceled in 2004. Ducted fan tail rotors have also been used in the Russian Kamov Ka-60, and also on the Japanese military's Kawasaki OH-1 Ninja helicopter.
- Increased safety for people on the ground, the enclosure provides peripheral protection;
- Greatly reduced noise and vibration, due to the enclosure of the blade tips and the greater number of blades;
- A lower susceptibility to foreign object damage, as the enclosure makes it less likely to suck in loose objects such as small rocks;
- Enhanced anti-torque control efficiency.
A computational simulation has suggested that maximum achievable thrust is twice as high, and that at identical power, thrust was slightly greater, than for a conventional rotor of the same diameter. 
The Fenestron's disadvantages are those common to all ducted fans when compared to propellers. They include:
- An increase in weight, power requirement and air resistance brought by the enclosure;
- Higher construction and purchasing cost.
- "History of the fenestron" Rotor Online, Eurocopter
- Prouty, Ray, Helicopter Aerodynamics, Helobooks, 1985, 2004, pg 266
- 30 Years of Innovation
- Hover and wind-tunnel testing of shrouded rotors for improved micro air vehicle design p65-66. University of Maryland, 2008. Accessed: 15 March 2013.
- Johnson, Wayne. "Rotorcraft Aeromechanics" page 282. Cambridge University Press 2013. Accessed: 15 April 2014. ISBN 1107355281, 9781107355286
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