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The mysterious source of helicopter noise: Do you know what's causing all that loud noise?
With the rapid development of the aviation industry, the use of helicopters has become more and more common, but their noise has become a major problem for many residents, especially in urban areas. Where does all this noise come from? In an in-depth study of helicopter noise, experts found that the sources of this noise are diverse and intertwined.
According to the research, helicopter noise can be mainly divided into eight different sources.
Noise source analysis
During helicopter operation, the interaction of various noise sources makes the helicopter sound more destructive, and these noises are classified as follows:
1. Thickness noiseThickness noise is mainly generated by the shape and movement of the rotor blades, and this noise is closely related to the direction of blade movement. When the blades rotate, they change the state of the surrounding air, so thickness noise is mainly used to describe the noise caused by air displacement.
2. Load noiseLoad noise comes from the acceleration of the blades as they pass through the air due to the distribution of forces. This noise is usually directed primarily toward the underside of the rotor and is the most noticeable source of noise at low blade speeds.
3. Blade vortex interaction noise
When one rotor blade passes through the vortex created by the previous blade, it causes rapid changes in blade loading, which in turn produces a highly directional transient noise. This noise causes different noise patterns on different rotor sides, affecting how the noise propagates.
4. Broadband noise
Broadband noise includes various random noise sources, such as vortex inhalation, rotor noise, etc.
5. High-speed impact noiseHigh-speed impact noise comes from the supersonic flow generated by the rapid movement of the rotor blades, and this noise is usually directed toward the front of the rotor.
6. Tail rotor noiseTail rotor noise is particularly noticeable to observers close to the helicopter. Due to its high frequency, this noise has a relatively large impact on the human ear.
The combination of these noise sources makes the helicopter produce strong and widespread sound during flight, which can greatly affect the surrounding environment.
Methods of noise reduction
Over the past few decades, the aviation industry has made significant progress in research on reducing helicopter noise. Helicopter designers began working on improving the design of the rotor and tail rotor to reduce their noise. For example, burying the tail rotor deep inside the tail shell can effectively reduce the noise directly below it, which is particularly meaningful in urban environments.
Another approach worth noting is the use of the NO TAil Rotor (NOTAR) system. In this design, propulsion is generated by sending air along the side tail, eliminating the need for a traditional tail rotor and further reducing the source of noise.
A series of design improvements, including the use of variable blade pitch, effectively reduce specific frequencies of rotor noise, making it sound softer.
In addition, the engine noise of helicopters is usually directed upwards, and with the popularization of turbine engines, this part of the noise has been reduced to a relatively low level in the overall noise. New noise reduction technologies and designs are continually being explored to meet future demands for quieter air traffic.
This series of research and technological innovations not only helps meet the public's expectations for helicopter noise, but also becomes a critical requirement in military operations, where reducing the range of sound propagation has further enhanced the stealth capabilities of helicopters.
Perhaps in the future, with the advancement of technology, helicopters will operate more quietly. What kind of technology do you think will be the key to changing the rules of the noise management game?
