From an Exposed Tail Rotor to the Fenestron

A helicopter’s main rotor produces torque that tends to rotate the fuselage in the opposite direction. On a conventional helicopter, a tail rotor mounted at the end of the tail boom produces the lateral thrust required to counteract this torque and provides control about the yaw axis.

The principle is effective, but an exposed tail rotor creates its own challenges, particularly when people are working around a helicopter on the ground.

During the 1960s, Sud Aviation engineers René Mouille and Paul Fabre developed an alternative: placing the tail rotor inside a protective duct. The first Fenestron® flew on the second prototype of the Gazelle on 12 April 1968. Safety was the original motivation, although subsequent development also brought acoustic and aerodynamic benefits. (Airbus)

The technology continued to evolve through the Gazelle and Dauphin families. A third generation appeared on the EC135 in 1994, using uneven blade spacing to distribute the rotor’s sound across different frequencies rather than concentrating it at a dominant frequency. (Airbus)

Today, the shrouded tail rotor remains one of the most recognisable features of the H135.

The H135 Fenestron

The Fenestron performs the same fundamental job as a conventional tail rotor: it produces lateral thrust to counter main rotor torque and allows the pilot to control yaw.

On the EC135/H135, however, the rotor is completely enclosed within a duct forming part of the tail structure. The training manual describes the system as essentially a ducted fan.

The classic EC135 system uses 10 tail rotor blades. Unlike a conventional rotor with two, three or four relatively long blades, the Fenestron uses shorter blades rotating at considerably higher speed. Variable angular spacing between the blades also helps distribute the sound they generate over different frequencies.

The principal assemblies include the rotor hub and blades, duct and stators, and fairing. At the centre, the hub carries the ten blades and contains the pitch-change mechanism.

Getting Power to the Tail Rotor

Mechanical power ultimately comes from the helicopter’s powertrain and is transmitted aft through the tail rotor drive system.

At the Fenestron, the drive enters the tail rotor gearbox. Bevel gears turn the drive through 90 degrees and establish the final tail rotor speed. The output drives the hub, blades and rotating control components.

The gearbox itself sits in the centre of the Fenestron duct, supported by ten stators. Those stators do more than provide structural support: they also act as guide vanes, influencing airflow through the duct and improving efficiency.

How the Pilot Controls the Fenestron

Yaw control is achieved by changing the pitch of all ten blades together rather than by changing rotor speed.

Movement of the cockpit pedals is transmitted through the tail rotor control system to the Fenestron. The system includes the pedal assembly, control linkage, Flexball control cable, yaw stability augmentation actuator and hydraulically boosted Fenestron actuator.

At the rotor, axial movement of the rotating control spider changes the pitch of all the blades collectively. Increasing or decreasing blade pitch changes the lateral thrust generated by the Fenestron and therefore the yawing moment acting on the helicopter.

This means the apparently simple fan visible in the H135’s tail is actually the end of an integrated system involving the powertrain, mechanical flight controls, hydraulics and stability augmentation system.

Why Put the Tail Rotor Inside a Duct?

One of the most obvious advantages is protection. The smaller rotor is positioned higher above the ground and its blades are surrounded by the duct. This reduces exposure of personnel to the rotating blades and makes the rotor less susceptible to contact with objects and debris. The greater number of shorter blades also reduces vibration.

There are trade-offs. The training material identifies additional weight, construction cost and aerodynamic drag associated with the enclosure among the disadvantages of the arrangement.

The acoustic benefits, however, have become an important part of Fenestron development. Airbus identifies the H135 as one of its benchmark helicopters for low external sound levels, something particularly relevant to aircraft operating regularly around hospitals and populated areas. (Airbus)

Inspection and Testing

The Fenestron’s condition forms part of the aircraft’s inspection regime. Preflight inspection includes checking the condition of the hub, blades, stators and duct. Blade tip clearance should be even around the duct, with no evidence of contact, while alignment marks provide an indication that the original rotor settings have not changed.

Maintenance also includes dynamic balancing of the tail rotor. The Airbus training material specifies balancing following events including blade replacement or repair, detection of an out-of-limit condition and periodic inspection. A magnetic pickup and accelerometer at the tail rotor gearbox provide the signals used to measure imbalance during a ground run.

The Fenestron Continues to Evolve

More than half a century after its first flight, the basic concept is still being developed.

Airbus has progressively refined duct geometry, blade profiles, blade spacing and stator design. The H160 introduced a larger, canted Fenestron, while the forthcoming H140 uses another optimised Fenestron design intended to further reduce sound levels. (Airbus)

The engineering principle, however, remains recognisable from that first Gazelle experiment in 1968: put the anti-torque rotor inside a carefully designed duct and make the duct part of the aerodynamic system rather than merely a guard around the blades.

On the H135, the result is one of the aircraft’s defining engineering features.