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Thickness noise, load noise... Do you know why helicopters are so noisy? Let's take a look at the unknown sources of noise!
The noise problem of helicopters is not only a nuisance to people in urban life, but also affects the decision-making of adjusting flight paths and landing times. From a military perspective, noise issues are a potential safety hazard that can be detected by enemy forces, forcing military units to seek solutions to reduce noise. This article will explore the sources of helicopter noise and the methods used by today's technology to reduce noise.
The sources of helicopter noise mainly come from the movement of the blades, the operation of the engine and various aerodynamic effects.
Noise Sources
Helicopter noise can be divided into several distinct sources, each with different characteristics and explanations. Here are some of the main sources of noise:
Thickness Noise
Thickness noise is mainly caused by the shape and movement of the rotor blade. This noise is generated by the displacement of air caused by the rotor blade, primarily in the direction of the plane of the rotor. It is a constant noise when the helicopter is in flight and is directly related to the design and speed of the blade.
Load noise
Load noise is caused by the change in force distribution of the rotor blade as it passes through the air. This noise is generally emitted downward from the rotor and is more noticeable at low speeds. Load noise can also include various types of blade loading, which must be considered when monitoring helicopter noise.
Blade vortex interaction noise
Blade vortex interaction noise (BVI) is caused when a rotor blade approaches a vortex created by the previous blade. This results in rapid changes in load on the blade, creating a highly directional noise. BVI noise may be detected on either side of the rotor wear, and its directivity is affected by the precise direction of the interaction.
Broadband Noise
Another type of load noise is broadband noise, which originates from various random noise sources. This noise can be composed of turbulence through the rotor, self-noise and rotor wake. These noises are factors that are difficult to separate specifically but need to be considered together.
High-speed impact noise
High Speed Impact Noise (HSI) is the exclamation sound of fluid moving at supersonic speeds as the advancing blades move. This is also a noise source that is distinct from load noise, as HSI noise is usually directed toward the front of the helicopter, consistent with thickness noise. This highlights the aerodynamic nature of the design and operation.
Tail rotor noise
While most of the noise comes from the main rotor, the tail rotor is also a significant noise source when the helicopter is viewed up close. The high frequency noise characteristics of the tail rotor make it more annoying to spectators than the main rotor noise.
Noise reduction methods
With the advancement of technology, many noise reduction methods have been proposed. These approaches mainly focus on reducing the noise of the main and tail rotors. Most helicopters have their engines located above the machinery, which helps direct the engine noise upward. Engine noise is relatively flat with today’s technology, and the focus is gradually shifting to reducing rotor noise.
Many designs have aimed to embed the tail rotor into the fuselage to reduce noise over urban areas. Such a design has more rotor blades, which in turn generates noise at a higher frequency, which is better absorbed by the environment.
For example, the Eurocopter EC-135 uses this design, while also taking into account ease of control throughout the entire flight process. For smaller helicopters, designs using the NO TAil Rotor (NO-TAR) system will become increasingly popular, which uses the Coandă effect to create a rotating airflow, further reducing noise.
In addition, some helicopter designs are also being tested with the goal of reducing rotor noise, and many military helicopters, such as the Comanche, are looking for multiple measures to achieve noise suppression. For example, by adjusting the blade spacing and changing the blade layout, vibration noise can be dispersed, thereby reducing specific noise frequencies and their harmonics.
Faced with the endless noise problems, engineers and designers still need to continue to explore more innovative and effective solutions. This is not only for the quality of human life, but also related to various military strategies and environmental considerations. Whether helicopters will be able to operate in the future without hampering the quality of human life is a question we need to ponder.
