fenestron

Fenestron is a branded term for an enclosed tail rotor system used on some helicopters. It consists of a ducted fan located within a circular housing at the end of the tail boom, with multiple small tail rotor blades spinning inside. The design is driven by the tail drivetrain and uses variable pitch to control yaw.

The fenestron arrangement provides several distinctive characteristics. The surrounding duct shields the rotor, reducing the risk of ground personnel being struck and protecting the rotor from debris. The enclosed rotor also helps lower noise levels compared with conventional, exposed tail rotors and can improve handling during hover and low-speed flight by smoothing yaw control and reducing wake interaction with the fuselage.

Advantages include enhanced ground safety, reduced noise, and potentially improved low-speed controllability. Disadvantages include increased weight and mechanical complexity, higher manufacturing and maintenance costs, and, in some configurations, reduced tail rotor authority at extreme yaw demands or in icy conditions. Access to and repair of components within the duct can be more challenging than for exposed rotors.

History and usage: The fenestron concept was developed by Sud Aviation (later part of Airbus Helicopters) and first used on early models such as the Gazelle. It has since been employed on several Eurocopter/Airbus Helicopters designs and related variants, with the EC135 family among the most notable modern examples. The term “fenestron” is also sometimes referenced in discussions of ducted-tail-rotor technology in other helicopter programs.