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.