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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.

Propelling nozzles accelerate the available gas to subsonic, transonic, or supersonic velocities depending on the power setting of the engine, their internal shape and the pressures at entry to, and exit from, the nozzle. The internal shape may be convergent or convergent-divergent (C-D). C-D nozzles can accelerate the jet to supersonic velocities within the divergent section, whereas a convergent nozzle cannot accelerate the jet beyond sonic speed.[1]

Propelling nozzles may have a fixed geometry, or they may have variable geometry to give different exit areas to control the operation of the engine when equipped with an afterburner or a reheat system. When afterburning engines are equipped with a C-D nozzle the throat area is variable. Nozzles for supersonic flight speeds, at which high nozzle pressure ratios are generated,[2] also have variable area divergent sections.[3] Turbofan engines may have an additional and separate propelling nozzle which further accelerates the bypass air.

Propelling nozzles also act as downstream restrictors, the consequences of which constitute an important aspect of engine design.[4]

The thrust reversers on some engines are incorporated into the nozzle itself and are known as target thrust reversers. The nozzle opens up in two halves which come together to redirect the exhaust partially forward. Since the nozzle area has an influence on the operation of the engine (see below), the deployed thrust reverser has to be spaced the correct distance from the jetpipe to prevent changes in engine operating limits.[16] Examples of target thrust reversers are found on the Fokker 100, Gulfstream IV and Dassault F7X.

Noise-reducing

Jet noise may be reduced by adding features to the exit of the nozzle which increase the surface area of the cylindrical jet. Commercial turbojets and early by-pass engines typically split the jet into multiple lobes. Modern high b

The thrust reversers on some engines are incorporated into the nozzle itself and are known as target thrust reversers. The nozzle opens up in two halves which come together to redirect the exhaust partially forward. Since the nozzle area has an influence on the operation of the engine (see below), the deployed thrust reverser has to be spaced the correct distance from the jetpipe to prevent changes in engine operating limits.[16] Examples of target thrust reversers are found on the Fokker 100, Gulfstream IV and Dassault F7X.

Noise-reducing

Jet noise may be reduced by adding features to the exit of the nozzle which increase the surface area of the cylindrical jet. Commercial turbojets and early by-pass

Jet noise may be reduced by adding features to the exit of the nozzle which increase the surface area of the cylindrical jet. Commercial turbojets and early by-pass engines typically split the jet into multiple lobes. Modern high by-pass turbofans have triangular serrations, called chevrons, which protrude slightly into the propelling jet.

Further topics

The nozzle, by virtue of setting the back-pressure, acts as a downstream restrictor to the compressor, and thus determines what goes into the front of the engine. It shares this function with the other downstream restrictor, the turbine nozzle.[17] The areas of both the propelling nozzle and turbine nozzle set the mass flow through the engine and the maximum pressure. While both these areas are fixed in many engines (i.e. those with a simple fixed propelling nozzle), others, most notably those with afterburning, have a variable area propelling nozzle. This area variation is necessary to contain the disturbing effect on the engine of the high combustion temperatures in the jet pipe, though the area may also be varied during non-afterburning operation to alter the pumping performance of the compressor at lower thrust settings.[4]

For example, if the propelling nozzle were to be removed to convert a turbojet into a turboshaft, the role played by the nozzle area is now taken by the area of the power turbine nozzle guide vanes or stators.[18]

Reasons for C-D nozzle over-expansion and examplesFor complete expansion to ambient pressure, and hence maximum nozzle thrust or efficiency, the required area ratio increases with flight Mach number. If the divergence is too short giving too small an exit area the exhaust will not expand to ambient pressure in the nozzle and there will be lost thrust potential[20] With increasing Mach number there may come a point where the nozzle exit area is as big as the engine nacelle diameter or aircraft afterbody diameter. Beyond this point the nozzle diameter becomes the biggest diameter and starts to incur increasing drag. Nozzles are thus limited to the installation size and the loss in thrust incurred is a trade off with other considerations such as lower drag, less weight.

Examples are the F-16 at Mach 2.0[21] and the XB-70 at Mach 3.0.[22]

Another con

Examples are the F-16 at Mach 2.0[21] and the XB-70 at Mach 3.0.[22]

Another consideration may relate to the required nozzle cooling flow. The divergent flaps or petals have to be isolated from the afterburner flame temperature, which may be of the order of 3,600 °F (1,980 °C), by a layer of cooling air. A longer divergence means more area to be cooled. The thrust loss from incomplete expansion is traded against the benefits of less cooling flow. This applied to the TF-30 nozzle in the F-14A where the ideal area ratio at Mach2.4 was limited to a lower value.[23]

A divergent section gives added exhaust velocity and hence thrust at supersonic flight speeds.[24]

The effect of adding a divergent section was demonstrated with Pratt &Whitney's first C-D nozzle. The convergent nozzle was replaced with a C-D nozzle on the same engine J57 in the same aircraft F-101. The increased thrust from the C-D nozzle (2,000 lb, 910 kg at sea-level take-off) on this engine raised the speed from Mach 1.6 to almost 2.0 enabling the Air

The effect of adding a divergent section was demonstrated with Pratt &Whitney's first C-D nozzle. The convergent nozzle was replaced with a C-D nozzle on the same engine J57 in the same aircraft F-101. The increased thrust from the C-D nozzle (2,000 lb, 910 kg at sea-level take-off) on this engine raised the speed from Mach 1.6 to almost 2.0 enabling the Air Force to set a world's speed record of 1,207.6 mph (1,943.4 km/h) which was just below Mach 2 for the temperature on that day. The true worth of the C-D nozzle was not realised on the F-101 as the intake was not modified for the higher speeds attainable.[25]

Another example was the replacement of a convergent with a C-D nozzle on the YF-106/P&W J75 when it would not quite reach Mach 2. Together with the introduction of the C-D nozzle, the inlet was redesigned. The USAF subsequently set a world's speed record with the F-106 of 1526 mph (Mach 2.43).[25] Basically, a divergent section should be added whenever flow is choked within the convergent section.

Some very early jet engines that were not equipped with an afterburner, such as the BMW 003 and the Jumo 004 (which had a design known as a Zwiebel [wild onion] from its shape),[26] had a translating plug to vary the nozzle area.[27] The Jumo 004 had a large area for starting to prevent overheating the turbine and a smaller area for take-off and flight to give higher exhaust velocity and thrust. The 004's Zwiebel possessed a 40 cm (16 in) range of forward/reverse travel to vary the exhaust nozzle area, driven by an electric motor-driven mechanism within the body's divergent area just behind the exit turbine.

Afterburner-equipped engines may also open the nozzle for starting and at idle. The idle thrust is reduced which lowers taxi speeds and brake wear. This feature on the J75 engine in the F-106 was called 'Idle Thrust Control' and reduced idle thrust by 40%.[28] On aircraft carriers, lower idle thrust reduces the hazards from jet blast.

In some applications, such as the J79 installation in various aircraft, during fast throttle advances, the nozzle area may be prevented from closing beyond a certain point to allow a more rapid increase in RPM[29] and hence faster time to maximum thrust.

In the case of a 2-spool turbojet, such as the Olympus 593 in Concorde, the nozzle area may be varied to enable simultaneous achievement of maximum low-pressure compressor speed and maximum turbine entry temperature over the wide range of engine entry temperatures which occurs with flight speeds up to Mach 2.[30]

On some augmented turbofans the fan operating line is controlled with nozzle area during both dry and wet operatio

Afterburner-equipped engines may also open the nozzle for starting and at idle. The idle thrust is reduced which lowers taxi speeds and brake wear. This feature on the J75 engine in the F-106 was called 'Idle Thrust Control' and reduced idle thrust by 40%.[28] On aircraft carriers, lower idle thrust reduces the hazards from jet blast.

In some applications, such as the J79 installation in various aircraft, during fast throttle advances, the nozzle area may be prevented from closing beyond a certain point to allow a more rapid increase in RPM[29] and hence faster time to maximum thrust.

In the case of a 2-spool turbojet, such as the Olympus 593 in Concorde, the nozzle area may be varied to enable simultaneous achievement of maximum low-pressure compressor speed and maximum turbine entry temperature over the wide range of engine entry temperatures which occurs with flight speeds up to Mach 2.[30]

On some augmented turbofans the fan operating line is controlled with nozzle area during both dry and wet operation to trade excess surge margin for more thrust.

The nozzle area is increased during afterburner operation to limit the upstream effects on the engine. To run a turbofan to give maximum airflow (thrust), the nozzle area may be controlled to keep the fan operating line in its optimum position. For a turbojet to give maximum thrust, the area may be controlled to keep the turbine exhaust temperature at its limit.[31]

What happens if the nozzle doesn't open when the afterburner is selected?

Certain aircraft, like the German Bf-109 and the Macchi C.202/205 were fitted with "ejector-type exhausts". These exhausts converted some of the waste energy of the (internal combustion) engines exhaust-flow into a small amount of forward thrust by accelerating t

Certain aircraft, like the German Bf-109 and the Macchi C.202/205 were fitted with "ejector-type exhausts". These exhausts converted some of the waste energy of the (internal combustion) engines exhaust-flow into a small amount of forward thrust by accelerating the hot gasses in a rearward direction to a speed greater than that of the aircraft. All exhaust setups do this to some extent, provided that the exhaust-ejection vector is opposite/dissimilar to the direction of the aircraft movement.

Ejector exhausts were devised by Rolls-Royce Limited in 1937.[34] On the 1944 de Havilland Hornet's Rolls-Royce Limited in 1937.[34] On the 1944 de Havilland Hornet's Rolls-Royce Merlin 130/131 engines the thrust from the multi-ejector exhausts were equivalent to an extra 450bhp per-engine at full-throttle height.[35]