Tail rotor
Methods of noise reduction
Helicopter engines are normally located above the aircraft anyway, which directs a lot of the engine-noise upwards
A tail-rotor which is recessed into the fairing of the tail reduces the apparent noise directly under the aircraft, which may be useful in certain urban areas. For example the Eurocopter EC-135 has such a design.
Some designs take this a step further, and use a ducted fan or pressure from below the main rotor, and use it to blow air out the side of the tail to produce torque. As with other tail rotor designs, the yaw torque increases with main rotor torque, balancing the handling.
Some designs have been done to reduce the rotor noise itself, for example the Comanche military helicopter attempted many stealth mechanisms, including attempts to quieten the rotor.
Helicopters can select a particular mode ("Category") which ensures the engine torque and other parameters never exceeds that for a particular legal limit designed to reduce noise. The pilot can switch off this restriction in an emergency to get extra power.
