AOPA Hover Power

The X3 is a hybrid aircraft that offers the speed of a turboprop-powered airplane and the full hover flight capabilities of a helicopter. It’s equipped with two Rolls-Royce Turbomeca RTM322 turboshaft engines, producing 2,309 hp each, powering a standard EC155 five-bladed main rotor system and two propellers on short-span fixed wings. The X3 first flew September 6^th, 2010 and then in May of 2011 achieved a speed of 232 knots while using only 80 percent of prop shaft torque limit. The X3 is a proof of concept aircraft intended to demonstrate advanced technologies that could be used on high-speed helicopters in the next ten years.

The X3’s rotor speed is slowed down to reduce the Mach number of the advancing blade tip. To prevent retreating blade stall at high airspeeds the load on the rotor is reduced while the short span wings provide up to 40% lift instead. Also, the X3 does not have a tail rotor. So to counter the torque of the main rotor at low speed prop differential thrust is used. At high speed, remaining anti-torque function is realized by vertical fin flaps.

In May of 2012 the American Helicopter Society awarded Eurocopter’s X3 development team the Howard Hughes award for an outstanding improvement in fundamental helicopter technology brought to fruition during the preceding year.

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