Hush kit
Encyclopedia
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-bypass turbofan engines, as fitted to older commercial aircraft (see below for examples).
Jet aircraft
generate a considerable amount of noise
, contributing to noise pollution
near airport
s. In modern high-bypass turbofan engines, 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 larger turbines 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 a multilobe 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 modern turbofan engines 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 engines are able to comply with contemporary aviation noise abatement laws and ICAO
regulations. Hush kits are used on the many older freight and passenger aircraft still in service, such as the Boeing 727
and 737-200, Douglas DC-8
and DC-9, and Tupolev Tu-154
. While a hushkit can reduce jet noise, no hushkit can completely silence a jet engine. While hushkits are normally found on older aircraft, small business jets and other aircraft too small to be fitted with large, high bypass turbofan engines will be manufactured with hushkits installed, a more economical way to meet noise restrictions than expense engine or design changes.
Hush kits add weight to aircraft with them. They sometimes reduce engine efficiency, which can lower aircraft range and/or cruise speed. For example, a hush kit for the Gulfstream II might weigh 234 pounds (total airplane weight 65,500 pounds) and cause about a 1.6% reduction in aircraft range due to the hush kit's reduction in engine efficiency. Another example is Federal Express's hush kit for its Boeing 727
s, which add 900 pounds of extra weight (total airplane weight up to 190,000 pounds), cause a 0.5% increase in fuel burn for short trips and no measurable increase for long flights.
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 the United States
and the European Union
. The EU's proposed noise ordinances effectively prevented the use of hush-kit outfitted aircraft in Europe
, 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 the Concorde
, but was superseded and effectively repealed by EU Directive No. 2002/30/EC issued March 26, 2002.
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
; most commonly the term refers to devices which reduce noise emissions from low-bypass turbofan engines, as fitted to older commercial aircraft (see below for examples).
Jet aircraft
Jet aircraft
A jet aircraft is an aircraft propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes – as high as . At these altitudes, jet engines achieve maximum efficiency over long distances. The engines in propeller-powered aircraft...
generate a considerable amount of noise
Aircraft noise
Aircraft noise is noise pollution produced by any aircraft or its components, during various phases of a flight: on the ground while parked such as auxiliary power units, while taxiing, on run-up from propeller and jet exhaust, during take off, underneath and lateral to departure and arrival paths,...
, contributing to noise pollution
Noise pollution
Noise pollution is excessive, displeasing human, animal or machine-created environmental noise that disrupts the activity or balance of human or animal life...
near airport
Airport
An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...
s. In modern high-bypass turbofan engines, 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 larger turbines 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 a multilobe 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 modern turbofan engines 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 engines are able to comply with contemporary aviation noise abatement laws and ICAO
International Civil Aviation Organization
The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...
regulations. Hush kits are used on the many older freight and passenger aircraft still in service, such as the Boeing 727
Boeing 727
The Boeing 727 is a mid-size, narrow-body, three-engine, T-tailed commercial jet airliner, manufactured by Boeing. The Boeing 727 first flew in 1963, and for over a decade more were built per year than any other jet airliner. When production ended in 1984 a total of 1,832 aircraft had been produced...
and 737-200, Douglas DC-8
Douglas DC-8
The Douglas DC-8 is a four-engined narrow-body passenger commercial jet airliner, manufactured from 1958 to 1972 by the Douglas Aircraft Company...
and DC-9, and Tupolev Tu-154
Tupolev Tu-154
The Tupolev Tu-154 is a three-engine medium-range narrow-body airliner designed in the mid 1960s and manufactured by Tupolev. As the mainstay 'workhorse' of Soviet and Russian airlines for several decades, it serviced over a sixth of the world's landmass and carried half of all passengers flown...
. While a hushkit can reduce jet noise, no hushkit can completely silence a jet engine. While hushkits are normally found on older aircraft, small business jets and other aircraft too small to be fitted with large, high bypass turbofan engines will be manufactured with hushkits installed, a more economical way to meet noise restrictions than expense engine or design changes.
Hush kits add weight to aircraft with them. They sometimes reduce engine efficiency, which can lower aircraft range and/or cruise speed. For example, a hush kit for the Gulfstream II might weigh 234 pounds (total airplane weight 65,500 pounds) and cause about a 1.6% reduction in aircraft range due to the hush kit's reduction in engine efficiency. Another example is Federal Express's hush kit for its Boeing 727
Boeing 727
The Boeing 727 is a mid-size, narrow-body, three-engine, T-tailed commercial jet airliner, manufactured by Boeing. The Boeing 727 first flew in 1963, and for over a decade more were built per year than any other jet airliner. When production ended in 1984 a total of 1,832 aircraft had been produced...
s, which add 900 pounds of extra weight (total airplane weight up to 190,000 pounds), cause a 0.5% increase in fuel burn for short trips and no measurable increase for long flights.
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 the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and the European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
. The EU's proposed noise ordinances effectively prevented the use of hush-kit outfitted aircraft in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, 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 the Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
, but was superseded and effectively repealed by EU Directive No. 2002/30/EC issued March 26, 2002.
External links
- BBC NewsBBC NewsBBC News is the department of the British Broadcasting Corporation responsible for the gathering and broadcasting of news and current affairs. The department is the world's largest broadcast news organisation and generates about 120 hours of radio and television output each day, as well as online...
, "Hell-bent on a trade war," March 5, 1999 - NASA - Quiet Aircraft Facts