June 4, 2012 by Tim McAdams

A helicopter’s tail rotor is necessary to counteract the torque of the main rotor. Without it, the fuselage would spin the opposite direction of the main rotor (Newton’s third law). However, it also creates issues from consuming power that could be used for lift to safety for ground personnel. Over the years engineers have developed different designs to address some of these concerns. One idea that was first used in the late 1960s was a ducted fan.

A conventional tail rotor typically has two or four blades, while a ducted fan design can have eight to thirteen blades. The blades are also much smaller, spin at higher speeds and are mounted within a shroud that forms part of the vertical tail fin of the helicopter. Called a fantail (or sometimes a fan-in-fin) the housing and vertical fin is integrated into the tail boom. Another term is Fenestron and is trademarked by French helicopter manufacture Eurocopter.

Some of the main advantages of a ducted fan design include good protection against ground obstacles and foreign object damage, increased safety for ground personnel working around the tail boom, and increased aerodynamic efficiency. Also, a ducted fan reduces noise and vibration levels. However, the system is more complex than a traditional tail rotor adding weight and cost. Moreover, a ducted fan needs to have sufficient width to be efficient which adds drag.  A large cambered vertical fin helps efficiency in forward flight, but can make crosswind hovering more challenging.  

Eurocopter’s Fenestron has been constantly evolving over the last 35 years and is currently used on the EC120, EC130, EC135, AS365 and EC155 helicopters. The Fenestron has features like stators and tuning weights to reduce the power requirement and pitch control loads. Also used in the design is an even number of unevenly spaced blades designed to reduce noise levels. Although currently Eurocopter is the predominate user of ducted fans in their tail rotor designs other manufactures have also built helicopters with this design. For example, the Boeing/Sikorsky RAH-66 Comanche, the Russian Kamov Ka-60, and the Japanese military helicopter, the Kawasaki OH-1 Ninja.

EC135 Fenestron


Eurocopter's Fenestron

  • Avi Weiss

    Earlier Fenestron implementation also had somewhat “damped” or “spongy” response compared to classic tail-rotor design, though most seem to provide superior tail-rotor authority overall. Adding up the additional safety margin (reduced possibility of tail strike), and reduced noise signature due to edge shrouding, I think the trade-offs are worth it.

    Hopefully improvements in design and complexity reduction will continue over time, and other manufacturers will embrace fan design for counter-torque.

  • Alan Barnes

    “Eurocopter’s Fenestron has been constantly evolving over the last 35 years and is currently used on the EC120, EC130, EC135, AS365 and EC155 helicopters”

    Also the newer EC145 T2.

    Another helicopter that uses a Fenestron tail is the Guimbal Cabri G2. I *really* wish they would bring those to the US.

  • http://AOPA Jim Borger

    My experience has been less tail rotor authority, especially when doing hovering turns with a cross wind.. It is quieter and safer for ground operations but I still prefer the old style tail rotors.

  • Aaron q

    How are the fan tails driven? I am under the impression that they are electrically driven. If so, your emergency battery power is significantly reduced requiring an emergency landing is case of generator fail, or worse engine fail. Someone please clarify.

  • http://AOPA Jim Borger

    They are driven by a drive shaft just like a regular tail rotor.

  • Alan Barnes

    Drive shaft like normal. At least for the EC120 and EC135.

  • Mike

    I would not consider the fenestron to be a ducted fan system. The true ducted fan helo’s would be the md500 & 600n notars.

  • Michael Flitterman

    I have always thought the “shrouded” tail rotor great for safety etc. But it is a propeller and those stators do impede the rotor authority when you need a swift kick from that tail. Like Jim Borger says, it requires more leg work when doing low speed and/or hovering turns especially if the wind isn’t co-operating. Although I do think some of that cross-wind annoyance comes from both the somewhat slower anti-torque response and that big tail being blown around. Nonetheless, it is quieter, far safer and, apparently, has less maintenance needs per flight hour. So it gets my vote as better than an outright (right out there) tail rotor.

  • Evan

    I’m not sure what aerodynamic sacrifices were made developing the fenestron, but I can say that I’ve never run out of left pedal in the EC135. I also never ran out of pedal in a 76 or 64 (only happened once in a 206B3). Also, the EC135 operators manual doesnt list an EP for LTE, like it did in another aircraft I flew. While the EC definitely feels different that the others I’ve flown, I really can’t compare it to the others because of the differences in gross weight, hydraulic systems, and stabilization/augmentation equipment. The EC135 also seems to have a good bit more vertical fin surface area than most others, relative to it’s size. The EC135 has made me work the pedals a bit more to keep that tail in check, but it’s always been up to the task. It’s different, but it works, and looks good doing it!

  • Alan Barnes

    @Mike Sure it’s a ducted fan. The duct isn’t that large but it’s still there.

    @others This copied and pasted section is from the US Navy’s “Introduction to Helicopter Aerodynamics” training book.

    Use of a ducted fan instead of a tail rotor – Some designers have chosen to use a ducted fan (fenestron) that is imbedded in the tail to generate thrust, rather than the traditional tail rotor. Such an arrangement offers many advantages (at the possible expense of added weight). A ducted fan is shielded by the surrounding structure from forward flight generated winds. This negates the dissymmetry of lift problem. It also tends to be quieter (depending on speed and blade layout) because the fan generates little, if any, tip vortices and is shielded from interaction with the main rotor’s vortices. Additionally, the fan operates efficiently because it has reduced tip losses and pressure effects on the lip of the shroud help draw in air. The performance benefit realized as a result allows designers to use a fenestron that is 30% smaller than a conventional tail rotor that produces the same effect. Finally, a shrouded tail rotor provides improved safety for personnel operating around the aircraft on the flight line while the aircraft is turning.