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Questions about reducing the impact of aircraft noise, either by reducing the amount of noise generated, changing aircraft flightpaths so that more of the noise is generated farther away from anyone who might complain, or both.

Noise reduction, in the context of aviation, refers to reducing the amount of generated by aircraft, and/or reducing the impact of what noise is generated.

Most types of aircraft generate a great deal of noise (especially during and , when the aircraft is much much closer to the ground and its resident discerning ears), which causes local NIMBYs to complain loudly about s, aircraft, and aviation in general. This makes it important to reduce the noise footprint of aircraft as much as possible, in order to keep airports from being closed and flights banned from a deluge of lawsuits.

There are several ways of reducing the noise footprint of aviation on the local population. These fall into two main categories.

Reducing the amount of noise generated

These approaches target the problem at the source, by causing the aircraft to simply produce less noise in the first place; they mostly rely on advances in or changes in the methods of .

For large aircraft, which form the bulk of the world's commercial-aviation fleet, the vast majority of the problem is noise (s are loud). How this is dealt with depends on the specific type of engine used, and on whether we're talking about minimising the amount of noise that will be generated by a new aircraft or engine, or reducing the amount of noise generated by an already-existing aircraft/engine:

  • Exhaust jet noise (a high-pitched shrieking or screaming sound produced by the airflow turbulence resulting from the interaction of the engine's very hot, very fast exhaust jet with the ambient air), which is worst for engines and not much better for low-bypass s (although it is still a problem - albeit a milder one - even for high-bypass turbofans), can be reduced in one of two ways:
    • Increasing the engine's will, all else being equal, make it quieter (and also improve its [reducing its consumption], as a bonus); however, this is not always practical, especially if the aircraft has already been designed and built, and especially especially if its engines are mounted on (or in) its or buried within the , rather than being slung under the wing. As a result, the use of higher-bypass engines is something usually mentioned in the context of designing new aircraft (although success stories of reengining existing aircraft - either by physically replacing the aircraft's engines with a different type, or by building a new variant of the aircraft using the new engines rather than the old - do abound).
    • Another option is to use an exhaust mixer, which (as the name implies) mixes the exhaust jet (and bypass flow, if any) into the ambient air, reducing the amount of noise it generates. This can be done for almost any jet engine, and is much easier to retrofit onto an existing aircraft (such a retrofit is known as a hushkit or hush kit) than a whole new engine would be, but has the drawback of making the engine slightly less efficient. Mixers come in several types; older turbojets and low-bypass turbofans used instantly-recognisable multi-lobed mixers (with the more complex designs resembling an exotic flower sprouting from the engine's tailpipe), which were incredibly good at mixing the different airflows but had a relatively high impact on engine performance, while newer high-bypass turbofans generally either use which help to kick-start exhaust mixing, or else simply route the engine's core and bypass airflow down a single long tailpipe and hope that they mix somewhat before going out the end.
  • For high-bypass turbofans, the main problem is instead fan noise, a deep buzzsaw roar generated at high thrust settings when the blades of a large-diameter fan spin fast enough for their tips to travel at speeds, generating hundreds upon hundreds of s per second. As sonic booms are generally loud, this produces a great deal of noise (a problem exacerbated by the fact that the highest thrust settings are generally only used when close to the ground, during a or ). As the speed of sound in the ambient air is not generally amenable to being arbitrarily increased, the only solution is to reduce the speed at which the engines' fan blade tips travel:
    • One option would be to reduce the size of the fan (as the speed of the fan's blade tips at a given RPM is directly proportional to the fan's diameter). This is not a widely-used solution, as reducing the fan size without also reducing the size of the engine core will reduce the engine's bypass ratio, resulting in an increase in exhaust jet noise (see above), while reducing both the fan size and the core size will make the engine considerably less powerful, requiring the use of more engines per aircraft, and, thereby, considerably increasing the costs of (engines being one of the greatest contributors to aircraft maintenance costs), as well as making the aircraft less fuel-efficient. Nevertheless, a few aircraft have gone this route (although usually along with also going down other noise-reduction avenues simultaneously), a notable example being the .
    • The more attractive option is to reduce the fan's RPM, usually by means of a . For a long time, this was only an option for small aircraft (such as the aforementioned BAe 146), due to the considerable engineering challenges of developing a gearbox for a large geared turbofan, but geared turbofans can now be found on aircraft as large as the , with even larger examples in the pipelines.
  • For all types of jet engines (although it works best with turbofans - the higher-bypass, the better), noise can be decreased by using an acoustic liner in the engine nacelle; this is a layer of a special material that absorbs sound, dissipating it as a (relatively) small amount of heat energy.

Another possibility, which works for both jet and aircraft, is to change how the aircraft is operated, so that it spends less (or, ideally, no) time in very-noisy configurations.

  • One way of doing this is a reduced-thrust takeoff, where the aircraft takes off with its engines at a thrust setting lower than (and, thus, less noisy than) the maximum available; this has the added benefit of reducing engine wear and prolonging the engines' lifespan, as operating at very high thrust levels is quite hard on essentially any kind of aircraft engine. (Obviously, as a lower thrust setting causes the aircraft to accelerate more slowly and require more runway to reach takeoff speed, a reduced-thrust takeoff is only possible if the aircraft is significantly lighter than its maximum allowable weight and taking off from a fairly long runway.)
  • For landing, one way of reducing noise generation is to have the set to a lesser setting than would otherwise be used (for instance, landing with the flaps at 20° rather than 45°). Large flap deflections considerably increase the produced by the aircraft, thus requiring a higher engine thrust setting in order to maintain controlled flight; reducing the amount of flap used allows the engines to be run closer to idle, reducing noise. The drawback of landing with reduced flaps is a greater landing distance, as the reduction in drag causes the aircraft to decelerate more slowly following touchdown, and the reduced flap setting also causes the aircraft to generate less at a given , requiring it to fly the at a higher airspeed and touch down at a higher than would otherwise be the case.

Reducing the impact of the noise that is generated

These approaches focus on making it so that whatever noise does get generated is less objectionable to the surrounding population. In practice, this mainly boils down to "go make noise somewhere else".

  • One obvious way to do this is to route approach and departure paths, whenever possible, over unpopulated or sparsely-populated areas (where there are no, or only a few, people to be bothered) or industrial zones (where large amounts of noise are part and parcel of everyday work and the additional noise from aircraft flying overhead would scarcely be noticed). Unfortunately, this is difficult or impossible for many airports, especially those situated in dense urban areas.
    • This is one of several reasons that newer airports tend to be situated, wherever possible, some distance away from the cities they serve (up to and including, for coastal cities, creating new land to put the airport on), rather than in the city centre.
  • If aircraft must pass over residential areas, as many of them as possible should do so in the daytime, when fewer people are trying to sleep. Many airports, especially in crowded countries like the , have government-imposed curfews that either ban nighttime flights altogether (except in an ), or restrict them to using newer, quieter aircraft.
  • In some cases, the impact of aircraft noise can be decreased by using steeper approach and departure profiles, so that the aircraft spends less time close to the ground (where its noise is the most objectionable). However, some aircraft (especially the older and/or larger ones, which tend to be the noisiest of the lot) may not be able to fly these steeper climbs and descents safely, especially if the approach and/or departure was already fairly steep to begin with.

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