Hush kit
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A hush kit is an
aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
device used to help reduce the noise produced by antique aircraft
jet engine A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
s. These devices are typically installed on older
turbojet The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
and
low-bypass turbofan The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engines, as they are much louder than later
high-bypass turbofan engine The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which achieves mechanical ...
s. Hush kits are used because the engines on
jet aircraft A jet aircraft (or simply jet) is an aircraft (nearly always a fixed-wing aircraft) propelled by jet engines. Whereas the engines in propeller-powered aircraft generally achieve their maximum efficiency at much lower speeds and altitudes, je ...
generate a large amount of
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
, which significantly increases
noise pollution Noise pollution, also known as environmental noise or sound pollution, is the propagation of noise with ranging impacts on the activity of human or animal life, most of them are harmful to a degree. The source of outdoor noise worldwide is main ...
near
airport An airport is an aerodrome with extended facilities, mostly for commercial air transport. Airports usually consists of a landing area, which comprises an aerially accessible open space including at least one operationally active surface ...
s.


Design

The most common form of hush kit is a multi-lobe
exhaust mixer In aviation, exhaust mixer is a feature of many turbofan engines, where the bypass (cold/slow) air is mixed with the core (hot/fast) exhaust gases, before exhausting to atmospheric pressure through a common (mixed flow) propelling nozzle. Benef ...
. This device is fitted to the rear of the engine and mixes the jet core's exhaust gases with the surrounding air and a small amount of available bypass air. Modern high-bypass
turbofan The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engines build on this principle by utilizing available bypass air to envelop the jet-core exhaust at the rear of the engine, reducing noise. Most hush kits make further modifications to the exhaust, including acoustically treated tailpipes, revised inlet
nacelle A nacelle ( ) is a "streamlined body, sized according to what it contains", such as an engine, fuel, or equipment on an aircraft. When attached by a pylon entirely outside the airframe, it is sometimes called a pod, in which case it is attached ...
s and guide vanes. They reduce the forward propagating, high-pitched noise caused by the small, high-speed fan. This kind of high-pitched noise is much less of an issue on modern high-bypass turbofan engines as the significantly larger front fans they employ are designed to spin at much lower speeds than those found in older
turbojet The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
and low-bypass turbofan engines.


Use

Modern aircraft equipped with mid and high-bypass turbofan engines are designed to comply with contemporary aviation
noise abatement Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. Overview The main areas of noise mitigation or abatement are: transportation noise control, ...
laws and
ICAO The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international a ...
regulations. Several older aircraft that are still in service (typically in a cargo capacity) have hush kits retrofitted so that they are able to conform with noise regulations needed to operate in many commercial airports. Some of the examples include: *
Boeing 707 The Boeing 707 is an American, long-range, narrow-body airliner, the first jetliner developed and produced by Boeing Commercial Airplanes. Developed from the Boeing 367-80 prototype first flown in 1954, the initial first flew on December 20, ...
*
Boeing 727 The Boeing 727 is an American narrow-body airliner that was developed and produced by Boeing Commercial Airplanes. After the heavy 707 quad-jet was introduced in 1958, Boeing addressed the demand for shorter flight lengths from smaller airpo ...
*
Boeing 737-100 The Boeing 737 is a narrow-body aircraft produced by Boeing at its Renton Factory in Washington. Developed to supplement the Boeing 727 on short and thin routes, the twinjet retains the 707 fuselage width and six abreast seating with two un ...
*
Boeing 737-200 The Boeing 737 is a narrow-body aircraft produced by Boeing at its Boeing Renton Factory, Renton Factory in Washington (state), Washington. Developed to supplement the Boeing 727 on short and thin routes, the twinjet retains the Boeing 707, 7 ...
*
Douglas DC-8 The Douglas DC-8 (sometimes McDonnell Douglas DC-8) is a long-range narrow-body airliner built by the American Douglas Aircraft Company. After losing the May 1954 US Air Force tanker competition to the Boeing KC-135, Douglas announced in Ju ...
*
Douglas DC-9 The McDonnell Douglas DC-9 is an American five-abreast single-aisle aircraft designed by the Douglas Aircraft Company. It was initially produced by the developer company as the Douglas DC-9 until August 1967 and then by McDonnell Douglas. After ...
*
Gulfstream II The Gulfstream II (G-II) is an American twin engine business jet designed and built by Grumman and then in succession, Grumman American and finally Gulfstream American. Its Grumman model number is G-1159 and its US military designation is C-11A. ...
*
Gulfstream III The Gulfstream III, a business jet produced by Gulfstream Aerospace, is an improved variant of the Grumman Gulfstream II. Design and development The Gulfstream III was built at Savannah, Georgia, in the United States and was designed as an im ...
*
Learjet 23 The Learjet 23 (originally Lear Jet 23) is an American six-to-eight-seat (two crew and four to six passengers) twinjet, high-speed business jet manufactured by Learjet. Introduced in 1964, it was Learjet's first model and created a new market f ...
*
Learjet 24 The Learjet 24 is an American six-to-eight-seat (two crew and four to six passengers) twin-engine, high-speed business jet, which was manufactured by Learjet as the successor to the Learjet 23. History The Learjet 24 was designed as an improved ...
*
Learjet 25 The Learjet 25 is an American ten-seat (two crew and eight passengers), twin-engine, high-speed business jet aircraft manufactured by Learjet. It is a stretched version of the Learjet 24. Development The first Model 25 flew on August 12, 196 ...
*
Learjet 28 The Learjet 28 is an American eight-to-ten-seat (two crew and six to eight passengers), twin-engine, high-speed business jet, intended to be the successor to the Learjet 25. The Learjet 29 is identical except for the addition of a long-range fuel t ...


Impact

Hush kits can adversely affect the range and performance of the aircraft they are fitted to because of the extra weight. It also reduces engine performance and aerodynamic efficiency. For example, the hush kit fitted to the
Gulfstream II The Gulfstream II (G-II) is an American twin engine business jet designed and built by Grumman and then in succession, Grumman American and finally Gulfstream American. Its Grumman model number is G-1159 and its US military designation is C-11A. ...
adds to the total airplane weight of , causing around a 1.6% reduction in aircraft range. Meanwhile, the hush kit for the Gulfstream G-III jets weighs about and cause a calculated 2% drop in range, although the reduction in range has not been noticed during actual flights, which are rarely made to the very limit of the aircraft's range. On a larger aircraft, like FedEx's
Boeing 727 The Boeing 727 is an American narrow-body airliner that was developed and produced by Boeing Commercial Airplanes. After the heavy 707 quad-jet was introduced in 1958, Boeing addressed the demand for shorter flight lengths from smaller airpo ...
s, the hush kits add of extra weight (total airplane weight up to ) and this results in an overall 0.5% increase in fuel burn for short trips (but no measurable increase for long flights).


Regulation

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 concern led to a regulatory dispute between the United States and the
European Union The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been des ...
, where the EU proposed a new noise ordinance which effectively prevented the use of hush kits in
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
. By December 31, 1999, all Stage 2 noise jetliners 75,000 pounds or more operating in the US must have Stage 3 hush kits in order to continue to fly in the US after January 1, 2000. This regulation threatened to reduce the value of the mostly-American used airplanes that employed hush kits and hurt the profits of American hush kit manufacturers. EU Regulation 925/99 was passed over US threats to ban
Concorde The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC). Studies started in 1954, and France an ...
but was superseded (and effectively repealed) by EU Directive No. 2002/30/EC issued March 26, 2002. In 2013, the FAA modified 14 CFR part 91 rules to prohibit the operation of jets weighing 75,000 pounds or less that are not Stage 3 noise compliant after December 31, 2015. Any Stage 2 Business jets 75,000 pounds or below that have not been modified by installing Stage 3 noise compliant engines or have not had Stage 3 hush kits installed for non-compliant engines will not be permitted to fly in the contiguous 48 states after December 31, 2015. ''14 CFR §91.883 Special flight authorizations for jet airplanes weighing 75,000 pounds or less'' – lists special flight authorizations that may be granted for operation after December 31, 2015.


See also

*
Aircraft noise pollution Aircraft noise pollution refers to noise produced by aircraft in flight that has been associated with several negative stress-mediated health effects, from sleep disorders to cardiovascular ones. Governments have enacted extensive controls that a ...
*
Chevron (aerospace) The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which achieves mechanic ...
*
Winglets Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft' ...


References


External links

*
BBC News BBC News is an operational business division of the British Broadcasting Corporation (BBC) responsible for the gathering and broadcasting of news and current affairs in the UK and around the world. The department is the world's largest broadca ...
,
Hell-bent on a trade war
" March 5, 1999

{{DEFAULTSORT:Hush Kit Aircraft engines Aircraft noise reduction