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Tuesday, November 23, 2010

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 10,000 to 15,000 meters (about 33,000 to 49,000 ft). At these altitudes, jet engines achieve maximum efficiency over long distances. The engines in propeller powered aircraft achieve their maximum efficiency at much lower altitudes. Jet aircraft can move faster than sound.
Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently during the late 1930s. The concept had already been discussed as early as August 1928 by Frank Whittle at Flying School, Wittering, but Hans von Ohain also wrote in February 1936 to Ernst Heinkel, telling him of the design and its possibilities. However, it can be argued that A. A. Griffith, who published a paper in July 1926 on compressors and turbines, which he had been studying at the RAE, also deserves priority credit.

History

Heinkel He 178, the world's first aircraft to fly purely on turbojet power


Caproni Campini N1 in flight
A number of jet powerplants were suggested from the first instances of powered flight. René Lorin, Morize, Harris proposed systems for creating a jet efflux.
The "turbojet", the most common type of jet in use today, was invented in the 1930s, independently by Frank Whittle and Hans von Ohain. The first turbojet aircraft to fly was the Heinkel He 178 prototype of the German Air Force, the Luftwaffe, piloted by Erich Warsitz on August 27, 1939.
The first flight of a jet engined aircraft to come to popular attention was the Italian Caproni Campini N.1 motorjet prototype that flew on August 27, 1940. Test pilot Major Mario De Bernardi of the Regia Aeronautica was at the controls. It was the first jet aircraft recognised by the Fédération Aéronautique Internationale (at the time the German He 178 program was still kept secret). Campini had proposed the motorjet in 1932.
The British experimental Gloster E.28/39 first took to the air on May 15, 1941, powered by Sir Frank Whittle's turbojet, and piloted by Glosters test pilot Flt Lt PG Sayer. After the United States was shown the British work, it produced the Bell XP-59A with a version of the Whittle engine built by General Electric, which flew on September 12, 1942, piloted by Col L. Craigie.
The first operational jet fighter was the Messerschmitt Me 262., made by Germany during late World War II. It was the fastest conventional aircraft of World War II – although the rocket-powered Messerschmitt Me 163 Komet was faster. It had first flown in 1941 but mass production started in 1944 with the first squadrons opeational that year, too late for a decisive effect on the outcome of the war. About the same time, mid 1944, the United Kingdom's Gloster Meteor was being committed to defense of the UK against the V1 flying bomb – itself a jet powered aircraft – and then ground-attack operations over Europe in the last months of the war. USSR tested its own Bereznyak-Isayev BI-1 in 1942, but the project was scrapped by Stalin in 1945. The Imperial Japanese Navy also developed jet aircraft in 1945, including the Nakajima J9Y Kikka, a crude copy of the Me-262.
The US introduced the North American B-45 Tornado, their first jet bomber, into service in 1948. Although capable of carrying nuclear weapons it was used for reconnaissance over Korea.
On November 8, 1950, during the Korean War, United States Air Force Lt. Russell J. Brown, flying in an F-80, intercepted two North Korean MiG-15s near the Yalu River and shot them down in the first jet-to-jet dogfight in history.
The UK put the English Electric Canberra into service in 1951. Designed to fly higher and faster than any interceptor it carried no defensive armament.

BOAC operated the first commercial jet service, from London to Johannesburg, in 1952 with the de Havilland Comet jetliner. The Comet was initially ahead of rivals, but a series of crashes gave time for the Boeing 707 to enter service in 1958 and dominate the market for civilian airliners.
The fastest military jet plane was the SR-71 Blackbird at Mach 3.35 (2,275 mph, 3,661 km/h). The fastest commercial jet plane was the Tupolev Tu-144 at Mach 2.35 (1,555 mph, 2,503 km/h).

Modern jet aircraft



Bahrain Royal Flight Boeing 747SP
Modern airliners cruise at speeds of 0.75 to 0.85 Mach, or 75% to 85% of the speed of sound.

Other jets

Most people use the term 'jet aircraft' to denote gas turbine based airbreathing jet engines, but rockets and scramjets are both also propelled by them.
The fastest airbreathing jet aircraft is the unmanned X-43 scramjet at around Mach 9–10. The fastest manned (rocket) aircraft is the X-15 at Mach 6.85.
The Space Shuttle, while far faster than the X-43 or X-15, is not regarded as an aircraft during ascent. During re-entry it is classed (like a glider) as an unpowered aircraft.


The shape of most airliners is usually designed to have nearly the same cross-sectional area at each point along its length as the Sears-Haack body

Aerodynamics

Many jet aircraft fly at high speeds, either supersonic or speeds just below the speed of sound ("transonic"). Aerodynamics is therefore an important consideration.
Jet aircraft are usually designed using the Whitcomb area rule, which says that the cross-section of the aircraft at any point must be approximately the same as the Sears-Haack body. This minimises the production of shockwaves which would waste energy.

Jet engines

Main article: jet engine
Jet engines come in several main types:
turbojet
turbofan (which come in two main forms low bypass turbofan and high bypass turbofan)
rocket
The types are used for different aircraft. Turbojets are seldom used, but was used on Concorde; it has a high exhaust speed and low frontal cross-section, and so is best suited to high-speed flight. Low bypass turbofans have a lower exhaust speed than turbojets and are used for transonic and low supersonic speeds. High bypass turbofans are used for subsonic aircraft and are quite efficient and are widely used for airliners.
Rockets have extremely fast exhaust speeds and are mainly used when high speeds or extremely high altitudes are needed.


(ource:wikipedia)

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