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Helicopter noise reduction is a topic of research into designing helicopters which can be operated more quietly, reducing the public-relations problems with night-flying or expanding an airport. In addition, it is useful for military applications in which stealth is required: long-range propagation of helicopter noise can alert an enemy to an incoming helicopter in time to re-orient defenses (see acoustic signature).
Methods of noise reduction
Almost all helicopter engines are located above the aircraft, which tends to direct much of the engine-noise upwards. In addition, with the advent of the turbine engine, noise from the engine plays a much smaller role than it once did. Most research is now directed towards reducing the noise from the main and tail rotors.
A tail-rotor which is recessed into the fairing of the tail (a fenestron) reduces the noise level directly below the aircraft, which is useful in urban areas. In addition, this type of rotor typically has anywhere from 8 to 12 blades (as compared to 2 or 4 blades on a conventional tail rotor), increasing the frequency of the noise and thus its attenuation by the atmosphere. In addition, the placement of the tail rotor within a shroud can prevent the formation of tip vortices. This type of rotor is in general much quieter than its conventional counterpart: the price paid is a substantial increase in the weight of the aircraft, and the weight that must be supported by the tail boom. For example, the Eurocopter EC-135 has such a design.
For smaller helicopters, it may be advantageous to use a NOTAR (from NO TAil Rotor) system. In this yaw-control method, air is blown out of vents along the tail boom, producing thrust via the Coandă effect.
Some designs have been done to reduce the rotor noise itself, for example the Comanche military helicopter attempted many stealth mechanisms, including attempts to quiet the rotor. One possible technique for reducing helicopter rotor noise is "modulated blade spacing". Standard rotor blades are evenly spaced, and produce greater noise at a particular frequency and its harmonics. Using varying degrees of spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies.
Helicopter pilots can select operating modes which limits the engine torque and other parameters to ensure legal limits are respected to reduce noise. Pilots can disable the restrictions in an emergency to get extra power.