Helicopter noise reduction
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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).
Sources of helicopter noise
- Rotor noise
- Thickness noise
- Loading noise
- Broadband noise
- Blade-vortex interaction noise
- High-speed impulsive (HSI) noise
- Tail 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 adverse effect 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.
Changes in blade-section motion relative to the observer as the steadily loaded propeller rotates, generally referred to as "loading" noise. This source tends to dominate at low blade speed.[citation needed]
Blade-vortex interaction noise
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.
Tail rotor 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.
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
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
See also
- Aircraft noise
- Aviation and the environment
- Acoustic quieting
- Aeronautical engineering
- BERP rotor
- Helicopter rotor
- Noise pollution
- Noise regulation
- QTOL
- Stealth helicopter
References
- ^ Hardin 1987, Malovrh 2005
- ^ "Stealth Helicopter: MH-X Advanced Special Operations Helicopter". GlobalSecurity.org. Retrieved 28 April 2012.
- CiteSeerX 10.1.1.15.3782. Retrieved 30 January 2013.
- 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.