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. The Bell 206 of Canadian Helicopters Robinson Helicopter Company (USA) R44, a four seat development of the R22 A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors (propellers). ...

Sources of helicopter noise

• Rotor noise
• Engine noise
• Transmission noise

The noise from a rotor can be divided into several distinct sources, which will be described as follows:

Thickness noise 

Thickness noise is dependent only on the shape and motion of the blade, and can be thought of as being caused by the displacement of the air by the rotor blades. It is primarily directed in the plane of the rotor.

Loading noise 

Loading noise is an aerodynamic source term due to the acceleration of the force distribution on the air around the rotor blade due to the blade passing through it, and is directed primarily below the rotor. In general, loading noise can include numerous types of blade loading: some special sources of loading noise are identified separately.

Blade-vortex interaction (BVI) noise 

BVI occurs when a rotor blade passes within a close proximity of the shed tip vortices from a previous blade. This causes a rapid, impulsive change in the loading on the blade resulting in the generation of highly directional impulsive loading noise. BVI noise can occur on either the advancing or retreating side of the rotor disk and its directivity is characterized by the precise orientation of the interaction. In general, advancing side BVI noise is directed down and forward while retreating-side BVIs cause noise that is directed down and rearward. It has been shown that the main parameters governing the strength of a BVI are the distance between the blade and the vortex, the vortex strength at the time of the interaction, and how parallel or oblique the interaction is (Hardin 1987, Malovrh 2005).

Broadband noise 

Another form of loading noise, broadband noise consists of various stochastic noise sources. Turbulence ingestion through the rotor, the rotor wake itself, and blade self-noise are each sources of broadband noise.

High-speed impulsive (HSI) noise 

HSI noise is caused by transonic flow shock formation on the advancing rotor blade, and is distinct from loading noise. The source of HSI noise is the flow volume around the advancing blade tip, hence it cannot be captured by examining only the acoustic sources on the surface of the blade, HSI noise is typically directed in the rotor plane forward of the helicopter, like thickness noise.

While most noise from a helicopter is generated by the main rotor, the tail rotor is a significant source of noise for observers relatively close to the helicopter, where the higher-frequency noise of the tail rotor has not yet been attenuated by the atmosphere. Tail rotor noise is particularly annoying to the human listener due to its higher frequency (as compared to the main rotor) which places it directly in the band in which the human ear is most sensitive.

Shrouded tail rotor directs noise sideways

The fenestron (tail-rotor shrouding) of Eurocopter EC120B, built 2002, photographed at the Heli-Day, Kemble Airfield, England, in August 2003. ... The fenestron (tail-rotor shrouding) of Eurocopter EC120B, built 2002, photographed at the Heli-Day, Kemble Airfield, England, in August 2003. ...

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. WWII era steam turbine used for ship propulsion. ...

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. 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. The EC 135 is a twin-engine IFR civil helicopter produced by Eurocopter, widely used amongst police and ambulance services, and for executive transport. ...

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 Coanda effect. MD 900 (German police) NOTAR, an acronym for NO TAil Rotor, is a helicopter stabilization process developed by McDonnell Douglas Helicopter Systems which eliminates the use of tail rotors on helicopters, yielding quieter and safer operation. ... Henri Marie Coandă (June 7, 1886 - November 25, 1972) was a Romanian inventor, aerodynamics pioneer and the parent of the modern jet aircraft. ...

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. The Boeing-Sikorsky RAH-66 Comanche was an advanced U.S. Army military helicopter intended for the armed reconnaissance role, incorporating stealth techniques. ...

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.

References

• J. C. Hardin and S. L. Lamkin. Concepts for reduction of blade/vortex interaction noise. Journal of Aircraft, 24(2):120–125, 1987.
• B. Malovrh and F. Gandhi. Sensitivity of helicopter blade-vortex-interaction noise and vibration to interaction parameters. Journal of Aircraft, 42(3):685–697, May-June 2005.