History of the fenestron

The Fenestron features in the majority of Eurocopter light helicopters today. We take a look back at the invention of the Eurocopter trademark rotor.

© Pablo Rada
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In the 1960s, the fragility of conventional tail rotors prompted research into reducing sensitivity to the environment. Paul Fabre, head of the Sud Aviation Aerodynamics department and specialist in seaplanes, under the supervision of head engineer René Mouille in the Design Office, designed a fanin- fin shrouded rotor for the SA340.
The Fenestron (see inset) was born, though few grasped the significance of the innovation at the time. This first metal-bladed Fenestron had one curious feature: the blade on top was the advancing blade. Aware that this would have constituted an error on a conventional rotor, Fabre believed that on the Fenestron the effect of the main rotor downwash would be negligible. His judgment proved correct for the Gazelle because the Fenestron was always positioned two or three diameters from the ground, but when the problem appeared on the SA360 Dauphin, the direction of rotation had to be reversed.

The obvious superiority of the design prompted Sud Aviation to test the Fenestron on an SA330 Puma in 1975. But on a helicopter of such tonnage (7 metric tons), the tests proved disastrous. “This failure forced us to go back to the drawing board,” explains Bernard Certain. “We tried to improve on the design by constructing a prototype, the AS350 Z, which today has pride of place at the entrance to the Marignane site. The prototype enabled the Fenestron to become the reference design for a whole new generation of helicopters, the EC135, EC130, and EC155.”

Nowadays all Fenestrons feature stators and “Chinese” (tuning) weights to reduce the power requirement and pitch control loads, with an even number of unevenly spaced blades designed to reduce noise levels.

Constantly evolving over the last 35 years, the Fenestron design remains as important as ever. Indeed, with exciting challenges such as increased tonnages awaiting the Fenestron, we can expect to see more innovative evolutions.



Born in Aix-en-Provence and fiercely loyal to his roots, Paul Fabre chose the name ‘fenestrou’, a Provencal word meaning ‘small round window’, to designate his shrouded rotor invention. However François Legrand, head of the Design Office at the time, refused to accept this deviation from the French language, and Fabre’s Fenestrou soon became the ‘Fenestron’.