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A hush kit is an aerodynamic device used to help reduce the noise produced by older aircraft jet engines. These devices are typically installed on older turbojet and low-bypass turbofan engines, as they are much louder than later high-bypass turbofan engines. [1]
Hush kits are used because the engines on jet aircraft generate a large amount of noise, which significantly increases noise pollution near airports. [2]
The most common form of hush kit is a multi-lobe exhaust mixer. 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 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 nacelles and guide vanes. They reduce the forward propagating, high-pitched noise caused by the small, high-speed fan. [1]
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 and low-bypass turbofan engines.
Modern aircraft equipped with mid and high-bypass turbofan engines are designed to comply with contemporary aviation noise abatement laws and ICAO 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. [1] Some of the examples include:
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 adds 106 kilograms (234 lb) to the total airplane weight of 65,500 lb (29,700 kg), causing around a 1.6% reduction in aircraft range. [3]
Meanwhile, the hush kit for the Gulfstream G-III jets weighs about 170 kg (370 lb) 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. [4]
On a larger aircraft, like FedEx's Boeing 727s, the hush kits add 410 kg (900 lb) of extra weight (total airplane weight up to 86,000 kg (190,000 lb)) and this results in an overall 0.5% increase in fuel burn for short trips (but no measurable increase for long flights). [5] [6]
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. [7]
In 1999, this concern led to a regulatory dispute between the United States and the European Union, where the EU proposed a new noise ordinance which effectively prevented the use of hush kits in Europe. 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. [1] EU Regulation 925/99 was passed over US threats to ban Concorde 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.
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 airports. On December 5, 1960, the 727 was launched with 40 orders each from United Airlines and Eastern Air Lines. The first 727-100 rolled out November 27, 1962, first flew on February 9, 1963, and entered service with Eastern on February 1, 1964.
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 energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
The 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 July 1955 its derived jetliner project. In October 1955, Pan Am made the first order along with the competing Boeing 707, and many other airlines followed. The first DC-8 was rolled out in Long Beach Airport on April 9, 1958, and flew for the first time on May 30. FAA certification was achieved in August 1959 and the DC-8 entered service with Delta Air Lines on September 18.
A business jet, private jet, or bizjet is a jet aircraft designed for transporting small groups of people. Business jets may be adapted for other roles, such as the evacuation of casualties or express parcel deliveries, and some are used by public bodies, government officials, VIPs, or the armed forces.
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The Rolls-Royce BR700 family of turbofan engines for regional jets and corporate jets. It was developed by BMW and Rolls-Royce plc through the joint venture BMW Rolls-Royce AeroEngines GmbH, established in 1990. The BR710 first ran in 1995. It is manufactured in Dahlewitz, Germany. Rolls-Royce took full control of the company in 2000, which is now known as Rolls-Royce Deutschland. The military designation of the series is F130.
The General Electric F414 is an American afterburning turbofan engine in the 22,000-pound thrust class produced by GE Aviation. The F414 originated from GE's widely used F404 turbofan, enlarged and improved for use in the Boeing F/A-18E/F Super Hornet. The engine was developed from the F412 non-afterburning turbofan planned for the A-12 Avenger II, before it was canceled.
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The Learjet 23 is an American six-to-eight-seat twinjet, high-speed business jet manufactured by Learjet. Introduced in 1964, it was Learjet's first model and created a new market for fast and efficient small business aircraft. Production ended in 1966 after 101 aircraft had been delivered.
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The General Electric Passport is a turbofan developed by GE Aviation for large business jets. It was selected in 2010 to power the Bombardier Global 7500/8000, first run on June 24, 2013, and first flown in 2015. It was certified in April 2016 and powered the Global 7500 first flight on November 4, 2016, before its 2018 introduction. It produces 14,000 to 20,000 lbf of thrust, a range previously covered by the General Electric CF34. A smaller scaled CFM LEAP, it is a twin-spool axial engine with a 5.6:1 bypass ratio and a 45:1 overall pressure ratio and is noted for its large one-piece 52 in (130 cm) fan 18-blade titanium blisk.
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