Fenestron

The fenestron of a Eurocopter EC135, with uneven blade spacing to control noise emissions.

A Fenestron (sometimes alternatively referred to as a fantail or a "fan-in-fin" arrangement) is a protected tail rotor of a helicopter operating 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 fenestra word for window.

The Fenestron differs from a conventional tail rotor by being integrally housed within the tail unit of the rotorcraft and, like the conventional tail rotor it replaces, functions to counteract the torque of the main rotor. While conventional tail rotors typically have two or four blades, Fenestrons have between eight 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, this results in several distinct advantages over a conventional tail rotor, such as a reduction in tip vortex losses, 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 corporation Boeing, the Russian rotorcraft manufacturer Kamov, and Japanese conglomerate Kawasaki Heavy Industries.

History

Fenestron on a Kawasaki OH-1 reconnaissance helicopter

Fenestron on a Kamov Ka-60 at the MAKS Air Show, 2009

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. The Fenestron was then further developed in the 1960s to function as a viable replacement for the traditional tail rotor with the aim of improving both safety and performance upon rotorcraft.

The Fenestron was introduced on the second experimental model of the SA 340 (the first being furnished with a conventional anti-torque tail unit) by the French aircraft manufacturer Sud Aviation. The SA 340's fenestron was designed by aerodynamicist Paul Fabre; unusually, this unit had its advancing blade set at the top in defiance of conventional practice. Fitted accordingly, on 12 April 1968, the SA 340 became the first rotorcraft to fly using a Fenestron tail unit. Having been detirmined to have been satifactory, this tail unit was retained and was put into production on a refined model of the rotorcraft, which was designated Aérospatiale SA 341 Gazelle.

Through multiple mergers and the formation of Airbus Helicopters, many light, intermediate, and medium weight helicopters use the Fenestron as a tail rotor. Such implimentations can be found on many of Eurocopter's helicopter range, 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), and the enlarged EC155 Super Dolphin (a wider, heavier and more advanced version of the AS365 N/N3 series of the Dolphin Helicopter).

Over time, the design and performance of the Fenestron has been improved. During the late 1970s, Eurocopter launched a second generation all-composite unit, primarily featuring a reversal of the direction of rotation as well as adopting a 20 per cent larger diameter. During the 1990s, a third generation Fenestron was equipped with unevenly-spaced blades in order to optimize its noise levels; this unit was first fitted onto the EC135, and later incorporated into the designs of the EC130 and the EC145. The Fenestron of the Airbus Helicopters H160, a medium-twin sized rotorcraft, again changed the formula, being canted by 12 degrees to achieve improved performance and greater stability when operated with higher payloads at lower speeds.

According to aviation author Ray Proudy, in a typical implementation, a Fenestron is normally paired with a larger vertical stabiliser unit that also performs the role of compensating for torque; this configuration has the effect of reducing wear on the Fenestron blades and transmission system, which in turn leads to maintenance savings. Furthermore, the adoption of larger diameter units, while posing some engineering challenges, normally increases their efficiency and decreases their power requirements. Advanced implimentations of the Fenestron are provisioned with stators and adjustable weights in order to optimise the blades for a reduction in power required and pitch control loads imposed. During the 2010s, Airbus Helicopters stated that it expected the Fenestron to continue to be refined to suit rotorcraft of increasing tonnages and for further innovations to be made in the field.

Other than Airbus Helicopters and its predecessors, other companies have also made use of Fenestron anti-torque arrangements. One such rotorcraft was the American Boeing/Sikorsky RAH-66 Comanche, a stealthy aerial reconnaissance helicopter which was canceled in 2004. Ducted fan tail rotors have also been used in the Russian Kamov Ka-60 medium-lift helicopter, and also on the Japanese military's Kawasaki OH-1 Ninja reconnaissance rotorcraft. French light helicopter manufacturer Guimbal has also used a Fenestron for their Guimbal Cabri G2, a compact reciprocating engine-powered rotorcraft.

Advantages

Detail of the pitch control mechanism of an EC135 fenestron

• Increased safety for people on the ground because the enclosure provides peripheral protection;
• Greatly reduced noise and vibration due to the enclosure of the blade tips and greater number of blades;
• A decrease in power requirements during the cruise phase of flight.
• Typically lighter and smaller than conventional counterparts.
• A lower susceptibility to foreign object damage because the enclosure makes it less likely to suck in loose objects such as small rocks;
• Enhanced anti-torque control efficiency and reduction in pilot workload.

Disadvantages

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;
• A higher construction and purchasing cost.
• An increase in power required during the hover phase of flight.

See also

• NOTAR

References

Bibliography

• Gay, Daniel. Composite Materials: Design and Applications. CRC Press, 2014. ISBN 1-4665-8487-4.
• Johnson, Wayne. "Rotorcraft Aeromechanics." Cambridge University Press, 2013. ISBN 1-1073-5528-1.
• Leishman, Gordon L. "Principles of Helicopter Aerodynamics." Cambridge University Press, 2006. ISBN 0-5218-5860-7.
• Newman, Ron. The Technical, Aerodynamic & Performance Aspects of a Helicopter. BookBaby, 2015. ISBN 1-4835-5878-9.
• Prouty, Ray. Helicopter Aerodynamics Volume I. Lulu.com, 2009. ISBN 0-5570-8991-3.

External links

• Dave's All the World Fenestrons