- Hush kit
A hush kit is a device for reducing noise from an
engine ; most commonly the term refers to devices which reduce noise emissions from low-bypassturbofan engine s, as fitted to older commercial aircraft (see below for examples).Jet aircraft generate a considerable amount of noise, contributing tonoise pollution nearairport s. In modernhigh-bypass turbofan engine s, the fan mounted afore the jet engine core is very large. The bigger the fan in comparison to the jet core, the more effective the bypass air is in enveloping the jet exhaust at the rear of the engine, reducing noise. The largerturbines needed to spin the large fan slow the jet exhaust, which also reduces noise. A hush kit produces a similar effect, using several modifications to the existing engine. Primarily, a device called amultilobe exhaust mixer on the rear of the engine mixes the exhaust gases of the jet core with the surrounding air and the small amount of bypass air available. Similar systems are also employed on many modernhigh-bypass turbofan engine s as standard equipment to further reduce noise. Most kits also make further modifications to the exhaust via acoustically treated tailpipes, revised inlet nacelles and guide vanes, all of which reduce forward propagating high-pitched noise caused by the small high speed fan.Modern aircraft equipped with
high-bypass turbofan engine s are able to comply with contemporaryaviation noise abatement laws and ICAO regulations. Hush kits are used on the many older freight and passenger aircraft still in service, such as theBoeing 727 and 737-200,Douglas DC-8 and DC-9, andTupolev Tu-154 . They are also deployed on smaller business and personal jets which are too small for large high-bypass turbofans.While hush kits effectively reduce noise emissions from older aircraft, noise cannot always be reduced to the level of modern planes at a reasonable cost. In
1999 this has led to a dispute between theUnited States and theEuropean Union . The EU's proposed noise ordinances effectively prevented the use of hush-kit outfitted aircraft inEurope , reducing the value of the mostly American used airplanes so equipped and hurting the profits of American hush kit manufacturers. EU Regulation 925/99 was passed over US threats to ban theConcorde , but was superseded and effectively repealed by EU Directive No. 2002/30/EC issuedMarch 26 ,2002 .External links
*
BBC News , " [http://news.bbc.co.uk/1/hi/business/the_economy/291304.stm Hell-bent on a trade war] ,"March 5 , 1999
* [http://www.nasa.gov/centers/glenn/about/fs03grc.html NASA - Quiet Aircraft Facts]ources
* [http://www.fedex.com/us/hushkit/configuration/ FedEx: Hush Kit]
* [http://www.abshushkit.com/DC-20-30.htm ABS Hush Kit.com]
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