# Jet aircraft

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A jet aircraft (or simply jet) is an aircraft (nearly always a fixed-wing aircraft) propelled by jet engines.

An aircraft is a machine that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. Common examples of aircraft include airplanes, helicopters, airships, gliders, and hot air balloons.

A fixed-wing aircraft is a flying machine, such as an airplane or aeroplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinct from rotary-wing aircraft, and ornithopters. The wings of a fixed-wing aircraft are not necessarily rigid; kites, hang gliders, variable-sweep wing aircraft and aeroplanes that use wing morphing are all examples of fixed-wing aircraft.

A jet engine is a type of reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. This broad definition includes airbreathing jet engines. In general, jet engines are combustion engines.

## Contents

Whereas the engines in propeller-powered aircraft generally achieve their maximum efficiency at much lower speeds and altitudes, jet engines and aircraft achieve maximum efficiency at speeds close to or even well above the speed of sound. Jet aircraft generally cruise at faster than about Mach 0.8 (609 mph, 981 km/h or 273 m/s) at altitudes around 10,000–15,000 metres (33,000–49,000 ft) or more.

The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 metres per second, or a kilometre in 2.9 s or a mile in 4.7 s. It depends strongly on temperature, but also varies by several metres per second, depending on which gases exist in the medium through which a soundwave is propagating.

In fluid dynamics, the Mach number is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound.

Frank Whittle, an English inventor and RAF officer, developed the concept of the jet engine in 1928, [1] and Hans von Ohain in Germany developed the concept independently in the early 1930s. He wrote in February 1936 to Ernst Heinkel, who led the construction of the world's first turbojet aircraft and jet plane Heinkel He 178.

Air Commodore Sir Frank Whittle, was a British Royal Air Force air officer. He is credited with single-handedly inventing the turbojet engine. A patent was submitted by Maxime Guillaume in 1921 for a similar invention; however, this was technically unfeasible at the time. Whittle's jet engines were developed some years earlier than those of Germany's Hans von Ohain who was the designer of the first operational turbojet engine.

The English people are a nation and an ethnic group native to England who speak the English language. The English identity is of early medieval origin, when they were known in Old English as the Angelcynn. Their ethnonym is derived from the Angles, one of the Germanic peoples who migrated to Great Britain around the 5th century AD. England is one of the countries of the United Kingdom, and the majority of people living there are British citizens.

Hans Joachim Pabst von Ohain was a German physicist, and the designer of the first operational jet engine. His first design ran in March 1937, and it was one of his engines that powered the world's first flyable all-jet aircraft, the prototype of the Heinkel He 178 in late August 1939. In spite of these early successes, other German designs quickly eclipsed Ohain's, and none of his engine designs entered widespread production or operational use.

## History

After the first instance of powered flight, a large number of jet powerplants were suggested. René Lorin, Morize, Harris proposed systems for creating a jet efflux. [2] In 1910 the Romanian inventor Henri Coandă filed a patent on a jet propulsion system which used piston-engine exhaust gases to add heat to an otherwise pure air stream compressed by rotating fan blades in a duct. It was installed in his Coandă-1910 but this craft probably never flew.

René Lorin was a French aerospace engineer and inventor of the ramjet. In 1908 he patented the first subsonic ramjet design.

Henri Marie Coandă was a Romanian inventor, aerodynamics pioneer, and builder of an experimental aircraft, the Coandă-1910 described by Coandă in the mid-1950s as the world's first jet, a controversial claim disputed by some and supported by others. He invented a great number of devices, designed a "flying saucer" and discovered the Coandă effect of fluid dynamics.

The Coandă-1910, designed by Romanian inventor Henri Coandă, was an unconventional sesquiplane aircraft powered by a ducted fan. Called the "turbo-propulseur" by Coandă, its experimental engine consisted of a conventional piston engine driving a multi-bladed centrifugal blower which exhausted into a duct. The unusual aircraft attracted attention at the Second International Aeronautical Exhibition in Paris in October 1910, being the only exhibit without a propeller, but the aircraft was not displayed afterwards and it fell from public awareness. Coandă used a similar turbo-propulseur to drive a snow sledge, but he did not develop it further for aircraft.

Rocket-powered jet aircraft were pioneered in Germany. The first aircraft to fly under rocket power was the Lippisch Ente, in 1928. [3] The Ente had previously been flown as a glider. The next year, in 1929, the Opel RAK.1 became the first purpose-built rocket plane to fly.

The Ente was the world’s first rocket-powered full-size aircraft. It was designed by Alexander Lippisch as a sailplane and first flown under power on June 11, 1928, piloted by Fritz Stamer.

The Opel RAK.1 was the world's first purpose-built rocket-powered aircraft. It was designed and built by Julius Hatry under commission from Fritz von Opel who flew it on September 30, 1929 in front of a large crowd at Rebstock airport near Frankfurt-am-Main.

The turbojet was invented in the 1930s, independently by Frank Whittle and later Hans von Ohain. The first turbojet aircraft to fly was the Heinkel He 178 V1 first prototype of the German Air Force, the Luftwaffe , on August 27, 1939 in Rostock (Germany). [4]

The turbojet is an airbreathing jet engine, typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine. The compressed air from the compressor is heated by the fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.

The Heinkel He 178 was the world's first aircraft to fly under turbojet power, and the first practical jet aircraft. It was a private venture by the German Heinkel company in accordance with director Ernst Heinkel's emphasis on developing technology for high-speed flight. It first flew on 27 August 1939, piloted by Erich Warsitz. This flight had been preceded by a short hop three days earlier.

A prototype is an early sample, model, or release of a product built to test a concept or process or to act as a thing to be replicated or learned from. It is a term used in a variety of contexts, including semantics, design, electronics, and software programming. A prototype is generally used to evaluate a new design to enhance precision by system analysts and users. Prototyping serves to provide specifications for a real, working system rather than a theoretical one. In some design workflow models, creating a prototype is the step between the formalization and the evaluation of an idea.

The first flight of a jet-engined aircraft to come to public attention was the Italian Caproni Campini N.1 motorjet prototype that flew on August 27, 1940. [5] 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. [6] The United States produced the Bell XP-59A using two examples of a version of the Whittle engine built by General Electric, which flew on October 1, 1942. The Meteor was the first production jet as it entered production a few months before the Me 262,[ citation needed ] which itself had been in development since before the start of the war as Projekt 1065.

The first operational jet fighter was the Messerschmitt Me 262, [7] manufactured by Germany during World War II, which entered service on 19 April 1944 with Erprobungskommando 262 at Lechfeld just south of Augsburg. It was the fastest conventional aircraft of World War II  – although there were faster aircraft propelled by unconventional means, such as the rocket-powered Messerschmitt Me 163 Komet. The Messerschmitt Me 262 had first flown on April 18, 1941, with initial plans drawn up by Dr Waldemar Voigt's design team in April 1939, but mass production did not start until early 1944 with the first squadrons operational that year, too late for a decisive effect on the outcome of the war.

Around this time, mid 1944, the United Kingdom's Gloster Meteor was being committed to defence of the UK against the V-1 flying bomb  – itself a pulsejet-powered aircraft and direct ancestor of the cruise missile– and then ground-attack operations over Europe in the last months of the war. In 1944 Germany introduced into service the Arado Ar 234 jet reconnaissance and bomber, though chiefly used in the former role, with the Heinkel He 162 Spatz single-jet light fighter premiering as 1944 ended. USSR tested its own Bereznyak-Isayev BI-1 in 1942, but the project was scrapped by Joseph Stalin in 1945. The Imperial Japanese Navy also developed jet aircraft in 1945, including the Nakajima J9Y Kikka, a modified, and slightly smaller version of the Me 262 that had folding wings. By the end of 1945, the US had introduced their first jet fighter, the Lockheed P-80 Shooting Star into service and the UK its second fighter design, the de Havilland Vampire.

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 as a light bomber. It was designed to fly higher and faster than any interceptor.

BOAC operated the first commercial jet service, from London to Johannesburg, in 1952 with the de Havilland Comet jetliner. This highly innovative aircraft travelled far faster and higher than the propeller aircraft, was much quieter, smoother, and had stylish blended wings containing hidden jet engines. However, due to a design defect, and use of aluminium alloys, the aircraft suffered catastrophic metal fatigue which led to several crashes. [8]

The series of crashes gave time for the Boeing 707 to enter service in 1958 and this came to dominate the market for civilian airliners. The underslung engines were found to be advantageous in the event of a propellant leak, and so the 707 looked rather different from the Comet: the 707 has a shape that is effectively the same as that of contemporary aircraft, with marked commonality still evident today for example with the 737 (fuselage) and A340 (single deck, swept wing, four below-wing engines).

Turbofan aircraft began entering service in the 1950s and 1960s, bringing far greater fuel efficiency, and this is the type of jet in widespread use today.

The Tu-144 supersonic transport was the fastest commercial jet plane at Mach 2.35 (1,555 mph, 2,503 km/h). It went into service in 1975, but soon stopped flying. The Mach 2 Concorde aircraft entered service in 1976 and flew for 27 years.

The fastest military jet plane was the SR-71 Blackbird at Mach 3.35 (2,275 mph, 3,661 km/h).

## 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 jet propulsion.

Cruise missiles are single-use unmanned jet aircraft, powered predominately by ramjets or turbojets or sometimes turbofans, but they will often have a rocket propulsion system for initial propulsion.

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, was not regarded as an aircraft during ascent as it was carried ballistically by rocket thrust, rather than the air. During re-entry it was classed (like a glider) as an unpowered aircraft. The first flight was in 1981.

The Bell 533 (1964), Lockheed XH-51 (1965), and Sikorsky S-69 (1977-1981) are examples of compound helicopter designs where jet exhaust added to forward thrust. [9] The Hiller YH-32 Hornet and Fairey Ultra-light Helicopter were among the many helicopters where the rotors were driven by tip jets.

Jet-powered wingsuits exist - powered by model aircraft jet engines - but of short duration and needing to be launched at height. [10]

## Aerodynamics

Because of the way they work, the typical exhaust speed of jet engines is transonic or faster, therefore most jet aircraft need to fly at high speeds, either supersonic or speeds just below the speed of sound ("transonic") so as to achieve efficient flight. Aerodynamics is therefore an important consideration.

Jet aircraft are usually designed using the Whitcomb area rule, which says that the total area of cross-section of the aircraft at any point along the aircraft from the nose must be approximately the same as that of a Sears-Haack body. A shape with that property minimises the production of shockwaves which would waste energy.

## Jet engines

Jet engines come in several main types:

The different types are used for different purposes.

Rockets are the oldest type and are mainly used when extremely high speeds or extremely high altitudes are needed. Due to the extreme, typically hypersonic, exhaust velocity and the necessity of oxidiser being carried on board, they consume propellant extremely quickly. For this reason, they are not practical for routine transportation.

Turbojets are the second oldest type; it has a high, usually supersonic, exhaust speed and low frontal cross-section, and so is best suited to high-speed, usually supersonic, flight. Although once widely used, they are relatively inefficient compared to turboprop and turbofans for subsonic flight. The last major aircraft to use turbojets were Concorde and Tu-144 supersonic transports.

Low bypass turbofans have a lower exhaust speed than turbojets and are mostly used for high sonic, transonic, and low supersonic speeds. High bypass turbofans are used for subsonic aircraft and are quite efficient and are widely used for airliners.

## Flying characteristics

Jet aircraft fly considerably differently to propeller aircraft.

One difference is that jet engines respond relatively slowly. This complicates takeoff and landing maneuvers. In particular, during takeoff, propeller aircraft engines blow air over their wings and that gives more lift and a shorter takeoff. These differences caught out some early BOAC Comet pilots. [8]

## Propulsive efficiency

In aircraft overall propulsive efficiency${\displaystyle \eta }$ is the efficiency, in percent, with which the energy contained in a vehicle's propellant is converted into useful energy, to replace losses due to air drag, gravity, and acceleration. It can also be stated as the proportion of the mechanical energy actually used to propel the aircraft. It is always less than 100% because of kinetic energy loss to the exhaust, and less-than-ideal efficiency of the propulsive mechanism, whether a propeller, a jet exhaust, or a fan. In addition, propulsive efficiency is greatly dependent on air density and airspeed.

Mathematically, it is represented as ${\displaystyle \eta =\eta _{c}\eta _{p}}$ [11] where ${\displaystyle \eta _{c}}$ is the cycle efficiency and ${\displaystyle \eta _{p}}$ is the propulsive efficiency. The cycle efficiency, in percent, is the proportion of energy that can be derived from the energy source that is converted to mechanical energy by the engine.

For jet aircraft the propulsive efficiency (essentially energy efficiency) is highest when the engine emits an exhaust jet at a speed that is the same as, or nearly the same as, the vehicle velocity. The exact formula for air-breathing engines as given in the literature, [12] [13] is

${\displaystyle \eta _{p}={\frac {2}{1+{\frac {c}{v}}}}}$

where c is the exhaust speed, and v is the speed of the aircraft.

## Range

For a long range jet operating in the stratosphere, the speed of sound is constant, hence flying at fixed angle of attack and constant Mach number causes the aircraft to climb, without changing the value of the local speed of sound. In this case:

${\displaystyle V=aM}$

where ${\displaystyle M}$ is the cruise Mach number and ${\displaystyle a}$ the local speed of sound. The range equation can be shown to be:

${\displaystyle R={\frac {aM}{c_{T}}}{\frac {C_{L}}{C_{D}}}ln{\frac {W_{1}}{W_{2}}}}$

which is known as the Breguet range equation after the French aviation pioneer Louis Charles Breguet.

## Related Research Articles

A rocket is a missile, spacecraft, aircraft or other vehicle that obtains thrust from a rocket engine. Rocket engine exhaust is formed entirely from propellant carried within the rocket before use. Rocket engines work by action and reaction and push rockets forward simply by expelling their exhaust in the opposite direction at high speed, and can therefore work in the vacuum of space.

A ramjet, sometimes referred to as a flying stovepipe or an athodyd, is a form of airbreathing jet engine that uses the engine's forward motion to compress incoming air without an axial compressor or a centrifugal compressor. Because ramjets cannot produce thrust at zero airspeed, they cannot move an aircraft from a standstill. A ramjet-powered vehicle, therefore, requires an assisted take-off like a rocket assist to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around Mach 3. This type of engine can operate up to speeds of Mach 6.

A turboprop engine is a turbine engine that drives an aircraft propeller.

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 accelerate air rearwards. Thus, whereas all the air taken in by a turbojet passes through the turbine, in a turbofan some of that air bypasses the turbine. 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.

An aircraft engine is a component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines, except for small multicopter UAVs which are almost always electric aircraft.

Thrust-specific fuel consumption (TSFC) is the fuel efficiency of an engine design with respect to thrust output. TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust. It is thus thrust-specific, meaning that the fuel consumption is divided by the thrust.

A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion, but whereas a ramjet decelerates the air to subsonic velocities before combustion, the airflow in a scramjet is supersonic throughout the entire engine. That allows the scramjet to operate efficiently at extremely high speeds.

A rocket engine uses stored rocket propellant mass for forming its high-speed propulsive jet. Rocket engines are reaction engines, obtaining thrust in accordance with Newton's third law. Most rocket engines use combustion, although non-combusting forms also exist. Vehicles propelled by rocket engines are commonly called rockets. Since they need no external material to form their jet, rocket engines can perform in a vacuum and thus can be used to propel spacecraft and ballistic missiles.

An afterburner is a component present on some jet engines, mostly those used on military supersonic aircraft. Its purpose is to provide an increase in thrust, usually for supersonic flight, takeoff, and combat situations. Afterburning is achieved by injecting additional fuel into the jet pipe downstream of the turbine. Afterburning significantly increases thrust without the weight of an additional engine, but at the cost of very high fuel consumption and decreased fuel efficiency, limiting its practical use to short bursts.

The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.

A rocket-powered aircraft or rocket plane is an aircraft that uses a rocket engine for propulsion, sometimes in addition to airbreathing jet engines. Rocket planes can achieve much higher speeds than similarly sized jet aircraft, but typically for at most a few minutes of powered operation, followed by a glide. Unhindered by the need for oxygen from the atmosphere, they are suitable for very high-altitude flight. They are also capable of delivering much higher acceleration and shorter takeoffs.

A propelling nozzle is a nozzle that converts the internal energy of a working gas into propulsive force; it is the nozzle, which forms a jet, that separates a gas turbine, being gas generator, from a jet engine.

This article outlines the important developments in the history of the development of the air-breathing (duct) jet engine. Although the most common type, the gas turbine powered jet engine, was certainly a 20th-century invention, many of the needed advances in theory and technology leading to this invention were made well before this time.

In aircraft and rocket design, overall propulsive efficiency is the efficiency with which the energy contained in a vehicle's propellant is converted into kinetic energy of the vehicle, to accelerate it, or to replace losses due to aerodynamic drag or gravity. It can also be described as the proportion of the mechanical energy actually used to propel the aircraft. It is always less than one, because conservation of momentum requires that the exhaust have some of the kinetic energy, and the propulsive mechanism is never perfectly efficient. Overall propulsive efficiency is greatly dependent on air density and airspeed.

A reaction engine is an engine or motor that produces thrust by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."

An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is forced into the engine by several stages of centrifugal, axial or ram compression, which is then heated and expanded through a nozzle. They are typically gas turbine engines. The majority of the mass flow through an airbreathing jet engine is provided by air taken from outside of the engine and heated internally, using energy stored in the form of fuel.

## References

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