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A fenestron is a type of rear rotor for a helicopter with a shrouded design intended to make the rotor more efficient. This design was developed in Europe and can be seen on a range of helicopter models as an alternative to the traditional exposed tail rotor. The purpose of the tail rotor is to counteract torque created by the main rotor of the aircraft, keeping the helicopter stable and on course while in the air; without a tail rotor, the helicopter would tend to spin, and this is not desirable.
There are a number of advantages to the fenestron design. Having a shroud over the rotor reduces risks for ground crews and also limits foreign object design. The numerous small blades are less noisy and the use of the shrouding reduces turbulence. This makes the helicopter less noisy and also reduces vibration, creating a more comfortable ride. The design, essentially that of a ducted fan, also works more efficiently than a traditional tail rotor.
Disadvantages to the fenestron primarily surround the increased weight of the design. The shrouding in the housing adds weight to the tail and can imbalance a helicopter unless it is designed to accommodate it. Having more weight also increases operating expenses, as more fuel is required to lift the helicopter, maintain altitude, and power it. These tradeoffs may be acceptable in some cases when the benefits of the fenestron design are weighed.
While the fenestron is safer than a regular rotor, there are still some risks for ground crews. Clothing and hair need to be secured while working around the tail to avoid hazards, and people also need to be careful with their extremities, as they can be sucked into the fenestron and severely injured. When helicopters are checked over by technicians, the blades must be inspected to make sure they are in good working order, checking for issues like dents and cracks caused by foreign objects, as well as loose bolts and other potential problems.
Some helicopters do away with the tail rotor altogether, using a different method to work against the torque from the overhead blades. No matter what system is used, learning to fly a helicopter requires some special training, as these aircraft handle very differently from planes. While some pilots train on both types of aircraft for flexibility, it is more common for people to focus on learning to fly one or the other.