Helicopters are known to be noisy, which can create problems for pilots, passengers, and people on the ground, especially in urban areas and on military missions. Helicopter noise reduction technology has made the aircraft quieter and continues to advance.
Helicopter engines don’t cause much noise. Most engines are located above the aircraft, so noise is directed upward. Turbine engines are also less noisy than older types of helicopter engines.
Most noise from helicopters is generated by the motion of the rotors. Rotor noise can be divided into several categories.
Thickness noise depends on the shape and motion of the rotor blades and is caused when a rotor blade displaces air. Thickness noise is directed primarily in the plane of the rotor.
Loading noise is caused when a rotor blade passes through the air and accelerates the force distribution on the air around the blade. Loading noise is directed primarily below the rotor.
Blade-vortex interaction noise is caused when a rotor blade passes close to the shed tip vortices of a previous blade. This causes a rapid change in the loading of the blade and generates a highly directional impulsive loading noise. BVI noise can occur on either the advancing or retreating side of the rotor blade. The strength of BVI noise depends on the distance between the blade and the vortex, the strength of the vortex, and how parallel or oblique the interaction is.
Broadband noise is another form of loading noise. It can be caused by turbulence ingestion through the rotor, the rotor wake, and blade self-noise.
High-speed impulsive noise is different than loading noise. It is generated when transonic flow shock forms in the advancing rotor blade. HSI noise is caused when air flows around the blade tip as it advances. Like thickness noise, HSI noise is usually directed in the plane of the rotor toward the front of the helicopter.
The tail rotor is also a source of significant noise, particularly for observers close to the helicopter. Noise generated by the tail rotor has a higher frequency than noise caused by the main rotors and is in a range to which the human ear is most sensitive.
A tail rotor can be recessed into the fairing of the tail (a fenestron) to reduce noise below the helicopter. This type of rotor has eight to 12 blades, rather than the two to four in a conventional tail rotor, which increases the frequency of the noise and causes it to be reduced by the atmosphere.
The tail rotor can be placed within a shroud to prevent tip vortices. This type of rotor is much quieter but significantly increases the weight of the helicopter.
Smaller helicopters can be designed with a NOTAR (NO TAil Rotor) system. Air is blown out of vents along the tail boom to produce thrust.
Noise produced by rotors can be reduced with modulated blade spacing, in which the space between the blades varies. This spreads the noise produced by the rotors over a greater range of frequencies.
Pilots can choose operating modes to limit engine torque and other parameters to reduce noise. The restrictions can be disabled in an emergency to get additional power.