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{merge fan(turbofan)}

A turbofan is a type of airplane engine, similar to a turbojet. It essentially consists of a ducted fan with a smaller diameter turbojet engine mounted behind it that powers the fan. Part of the airstream from the ducted fan passes through the turbojet where it is burnt to power the fan, but the majority of the flow bypasses it, and produces most of the thrust.

A few designs work slightly differently and have the fan blades as a radial extension of an aft mounted low pressure turbine unit.

All of the jet-engines that power currently-manufactured commercial jet aircraft are turbofans. They are mainly used commercially because they are highly efficient and relatively quiet in operation.

Turbofans are also used in many military jet aircraft, but rarely in other vehicles (e.g. jet-powered cars) where very high speeds and lower weight are needed, while noise and efficiency are less important.

Contents 1 Introduction 2 Early turbofans 3 Low bypass turbofans 4 Afterburning turbofans 5 High-bypass turbofan engines 6 Cycle improvements 7 Thrust growth 8 Technical Discussion 9 Recent developments in blade technology 10 Turbofan engine manufacturers 10.1 General Electric 10.2 Rolls-Royce 10.3 Pratt & Whitney 11 Extreme bypass jet engines 12 Terminology 13 See also 14 Other meanings

[edit] Introduction

In a turbojet, air enters an intake before being compressed to a higher pressure by a rotating (fan-like) compressor. The compressed air passes on to a combustor where it is mixed with a fuel (e.g. kerosene) and ignited. The hot combustion gases then enter a windmill-like turbine, where power is extracted to drive the compressor. Although the expansion process in the turbine reduces the gas pressure (and temperature), there is normally sufficient energy remaining to provide a high velocity jet, as the exhaust gases expand to atmospheric pressure through the propelling nozzle. This process normally produces a net thrust opposite in direction to that of the jet. Unlike a reciprocating engine, a turbojet undertakes a continuous flow process.

The description given above is, strictly speaking, for a single spool (shaft) turbojet. After World War II, 2-spool (shaft) turbojets were developed to make it easier to throttle-back compression systems with a high design overall pressure ratio (i.e. combustor inlet pressure/intake delivery pressure). Adopting the 2-spool arrangement enables the compression system to be split into two, with a Low Pressure (LP) Compressor supercharging a High Pressure (HP) Compressor. Each compressor is mounted on a separate (co-axial) shaft, driven by its own turbine (i.e HP Turbine and LP Turbine). Otherwise a 2-spool turbojet is much like a single spool engine.

Modern turbofans evolved from the 2-spool axial-flow turbojet engine, essentially by increasing the relative size of the Low Pressure (LP) Compressor to the point where some (if not most) of the air exiting the unit actually bypasses the core (or gas generator) stream, passing through the main combustor. This bypass air either expands through a separate propelling nozzle, or is mixed with the hot gases leaving the Low Pressure (LP) Turbine, before expanding through a Mixed Stream Propelling Nozzle. Owing to a lower jet velocity, a modern civil turbofan is quieter than the equivalent turbojet. Turbofans also have a better thermal efficiency, which is explained later in the article. In a turbofan, the LP Compressor is often called a fan. Civil turbofans usually have a single fan stage, whereas most military turbofans have multi-stage fans.

Turboprop engines are gas turbine engines that deliver almost all of their power to a shaft to drive a propeller. Turboprops remain popular on very small or slow aircraft such as small commuter airliners and combat transports such as the C-130 Hercules and P-3 Orion.

If the turboprop is better at moderate flight speeds and the turbojet is better at very high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Another name sometimes used is ducted fan, though that term is also used for propellers and fans used in vertical flight applications.

The difference between a turbofan and a propeller besides direct thrust, is that the intake duct of the former slows the air before it arrives at the fan face. As both propeller and fan blades must operate at subsonic inlet velocities to be efficient, ducted fans allow efficient operation at higher vehicle speeds.

Depending on specific thrust (i.e. net thrust/intake airflow), ducted fans operate best from about 400 to 2000 km/h (250 to 1300 mph), which is why turbofans are the most common type of engine for aviation use today in airliners, as well as subsonic/supersonic military fighter and trainer aircraft. It should be noted, however, that turbofans use extensive ducting to force incoming air to subsonic velocities (thus reducing shockwaves throughout the engine).

Bypass ratio (bypassed airflow to combustor airflow) is a parameter often used for classifying turbofans, although specific thrust is a better parameter.

The noise of any type of jet engine is strongly related to the velocity of the exhaust gases. High bypass ratio (i.e. low specific thrust) turbofans are relatively quiet compared to turbojets and low bypass ratio (i.e. high specific thrust) turbofans. A low specific thrust engine has a low jet velocity by definition, as the following approximate equation for net thrust implies:

<math>F_n = \dot m \cdot (V_{jfe} - V_a)</math>

where:

<math>\dot m = \,</math>intake mass flow

<math>V_{jfe} =\,</math> fully expanded jet velocity (in the exhaust plume)

<math>V_a =\,</math> aircraft flight velocity

Rearranging the above equation, specific thrust is given by:

<math>\frac{F_n}{\dot m} = (V_{jfe} - V_a)</math>

So for zero flight velocity, specific thrust is directly proportional to jet velocity. Relatively speaking, low specific thrust engines are large in diameter to accommodate the high airflow required for a given thrust.

Jet aircraft are often considered loud, but a conventional piston engine or a turboprop engine delivering the same power would be much louder.

[edit] Early turbofans

Early turbojet engines were very fuel-inefficient, as their overall pressure ratio and turbine inlet temperature were severely limited by the technology available at the time. Improved materials, and the introduction of twin compressors such as in the Pratt & Whitney JT3C engine, increased the overall pressure ratio and thus the thermodynamic efficiency of engines, but led to a poor propulsive efficiency, as pure turbojets have a high specific thrust/high velocity exhaust.

The original low-bypass turbofan engines were designed to improve propulsive efficiency by reducing the exhaust velocity to a value closer to that of the aircraft. The Rolls-Royce Conway, the first turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404 fighter engine. Civil turbofan engines of the 1960s, such as the Pratt & Whitney JT8D and the Rolls-Royce Spey had bypass ratios closer to 1, but were not disimilar to their military equivalents.

The unusual General Electric CF700 turbofan engine was developed as an aft-fan engine with a 2.0 bypass ratio. This was derived from the T-38 Talon and the Learjet General Electric J85/CJ610 turbojet (2,850 lbf or 12,650 N) to power the larger Rockwell Sabreliner 75/80 model aircraft, as well as the Dassault Falcon 20 with about a 50% increase in thrust (4,200 lbf or 18,700 N). The CF700 was the first small turbofan in the world to be certificated by the Federal Aviation Administration (FAA). There are now over 400 CF700 aircraft in operation around the world, with an experience base of over 10 million service hours. The CF700 turbofan engine was also used to train Moon-bound astronauts in Project Apollo as the powerplant for the Lunar Landing Research Vehicle.

[edit] Low bypass turbofans

A high specific thrust/low bypass ratio turbofan normally has a multi-stage fan, developing a relatively high pressure ratio and, thus, yielding a high (mixed or cold) exhaust velocity. The core airflow needs to be large enough to give sufficient core power to drive the fan. A smaller core flow/higher bypass ratio cycle can be achieved by raising the (HP) turbine rotor inlet temperature.

Imagine a retrofit situation where a new low bypass ratio, mixed exhaust, turbofan is replacing an old turbojet, in a particular military application. Say the new engine is to have the same airflow and net thrust (i.e. same specific thrust) as the one it is replacing. A bypass flow can only be introduced if the turbine inlet temperature is allowed to increase, to compensate for a correspondingly smaller core flow. Improvements in turbine cooling/material technology would facilitate the use of a higher turbine inlet temperature, despite increases in cooling air temperature, resulting from a probable increase in overall pressure ratio.

Efficiently done, the resulting turbofan would probably operate at a higher nozzle pressure ratio than the turbojet, but with a lower exhaust temperature to retain net thrust. Since the temperature rise across the whole engine (intake to nozzle) would be lower, the (dry power) fuel flow would also be reduced, resulting in a better specific fuel consumption (SFC).

A few low-bypass ratio military turbofans (e.g. F404) have Variable Inlet Guide Vanes, with piano-style hinges, to direct air onto the first rotor stage. This improves the fan surge margin (see compressor map) in the mid-flow range. The swing wing F-111 achieved a very high range / payload capability by pioneering the use of this engine, and it was also the heart of the famous F-14 Tomcat air superiority fighter which used the same engines in a smaller, more agile airframe to achieve efficient cruise and Mach 2 speed.

[edit] Afterburning turbofans

Since the 1970s, most jet fighter engines have been low/medium bypass turbofans with a mixed exhaust, afterburner and variable area final nozzle – the first afterburning turbofan was the Pratt & Whitney TF30. An afterburner is a combustor located directly upstream of the nozzle. When lit, prodigious amounts of fuel are burnt in the afterburner, raising the temperature of exhaust gases by a significant amount, resulting in a higher exhaust velocity/engine specific thrust. The variable geometry nozzle must open to a larger throat area to accommodate the extra volume flow when the afterburner is lit. Afterburning gives a significant thrust boost for take off, transonic acceleration and combat maneuvers, but is very fuel intensive. Consequently afterburning can only be selected for relatively short proportion of the mission.

Unlike the main combustor, where the integrity of the downstream turbine blades must be preserved, an afterburner can operate at the ideal maximum (stoichiometric) temperature (i.e. about 2100K(3780R)). Now, at a fixed total applied fuel:air ratio, the total fuel flow for a given fan airflow will be the same, regardless of the dry specific thrust of the engine. However, a high specific thrust turbofan will, by definition, have a higher nozzle pressure ratio, resulting in a higher afterburning net thrust and, therefore, a lower afterburning specific fuel consumption. However, high specific thrust engines have a high dry SFC. The situation is reversed for a medium specific thrust afterburning turbofan: i.e. poor afterburning SFC/good dry SFC. The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period, but only has to fight fairly close to the airfield (i.e cross border skirmishes) The latter engine is better for an aircraft that has to fly some distance, or loiter for a long time, before going into combat. However, the pilot can only afford to stay in afterburning for a short period, before his/her fuel reserves become dangerously low.

Modern low-bypass military turbofans include the Pratt & Whitney F119, the Eurojet EJ200 and the General Electric F110, all of which feature a mixed exhaust, afterburner and variable area propelling nozzle. Non-afterburning engines include the Rolls-Royce/Turbomeca Adour (afterburning in the SEPECAT Jaguar) and the unmixed, vectored thrust, Rolls-Royce Pegasus.

[edit] High-bypass turbofan engines

The low specific thrust/high bypass ratio turbofans used in today's civil jetliners (and some military transport aircraft) evolved from the high specific thrust/low bypass ratio turbofans used in such aircraft back in the 60's.

Low specific thrust is achieved by replacing the multi-stage fan with a single stage unit. Unlike some military engines, modern civil turbofans do not have any stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve the desired net thrust.

The core (or gas generator) of the engine must generate sufficient Core Power to at least drive the fan at its design flow and pressure ratio. Through improvements in turbine cooling/material technology, a higher (HP) turbine rotor inlet temperature can be used, thus facilitating a smaller (and lighter) core and (potentially) improving the core thermal efficiency. Reducing the core mass flow tends to increase the load on the LP turbine, so this unit may require additional stages to reduce the average stage loading and to maintain LP turbine efficiency. Reducing core flow also increases bypass ratio (5:1, or more, is now common).

Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core. Improved blade aerodynamics reduces the number of extra compressor stages required. With multiple compressors (i.e. LPC, IPC, HPC) dramatic increases in overall pressure ratio have became possible. Variable geometry (i.e. stators) enable high pressure ratio compressors to work surge-free at all throttle settings.

The first high-bypass turbofan engine was the General Electric TF39, built to power the Lockheed C-5 Galaxy military transport aircraft. The civil General Electric CF6 engine used a related design. Other high-bypass turbofans are the Pratt & Whitney JT9D, the three-shaft Rolls-Royce RB211 and the CFM International CFM56. More recent large high-bypass turbofans include the Pratt & Whitney PW4000, the three-shaft Rolls-Royce Trent, the General Electric GE90, and the General Electric GEnx.

The significantly higher thrust provided by high-bypass turbofan engines also made civil wide-body aircraft practical and economical. In addition to the vastly increased thrust, these engines are also generally quieter. This is not so much due to the higher bypass ratio, but as to the use of low pressure ratio, single stage, fans, which significantly reduce specific thrust and, thereby, jet velocity. The combination of a higher overall pressure ratio and turbine inlet temperature improves thermal efficiency. This, together with a lower specific thrust (better propulsive efficiency), leads to a lower specific fuel consumption.

For reasons of fuel economy, and also of reduced noise, almost all of today's jet airliners are powered by high-bypass turbofans. Although modern military aircraft tend to use low bypass ratio turbofans, military transport aircraft (e.g. C-17 ) mainly use high bypass ratio turbofans (or turboprops) for fuel efficiency.

The Soviet Union's engine technology was less advanced than the West's and its first wide-body aircraft, the Ilyushin Il-86, was powered by low-bypass engines. The Yakovlev Yak-42, a medium-range, rear-engined aircraft seating up to 120 passengers was the first Soviet aircraft to use high-bypass engines.

[edit] Cycle improvements

Consider a mixed turbofan with a fixed bypass ratio and airflow. Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow there is an increase in (HP) turbine rotor inlet temperature. Although the higher temperature rise across the compression system implies a larger temperature drop over the turbine system, the mixed nozzle temperature is unaffected, because the same amount of heat is being added to the system. There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio, causing an increase in the hot mixer entry pressure. Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases. A similar trend occurs with unmixed turbofans.

So turbofans can be made more fuel efficient by raising overall pressure ratio and turbine rotor inlet temperature in unison. However, better turbine materials and/or improved vane/blade cooling are required to cope with increases in both turbine rotor inlet temperature and compressor delivery temperature. Increasing the latter may require better compressor materials.

[edit] Thrust growth

Thrust growth is obtained by increasing the thrust power of the airstream (the total mass times the total velocity of the airstream, which is the same as the momentum). There are two basic routes available:

a) hot route: increase HP turbine rotor inlet temperature

 This increases the kinetic energy of the core flow and increases the exit velocity of the exhaust stream.

b) cold route: increase bypass ratio

  This allows more air volume to pass through the engine, increasing the total momentum of the exhaust stream.

The total airflow can also be increases by using a larger core, however this is expensive and generally less effective than bypass growth.

The hot route may require changes in turbine blade/vane materials and/or better cooling. The cold route can be obtained by one of the following:

1. increasing fan diameter and intake (nacelle) size
2. adding another set of fan blades (stage) to increase compression and airflow
3. adding T-stage/s to the LP/IP compression
4. adding a zero-stage to the HP compression
5. improving the compression process, without adding stages (e.g. higher fan hub pressure ratio)
6. increasing core size (expensive, because a new turbine system is also required).

Changes must also be made to the fan to absorb the extra core power. On a civil engine, jet noise considerations mean that any significant increase in Take-off thrust must be accompanied by a corresponding increase in fan mass flow (to maintain a T/O specific thrust of about 30lbf/lb/s), usually by increasing fan diameter. On military engines, the fan pressure ratio would probably be increased to improve specific thrust, jet noise not normally being an important factor.

[edit] Technical Discussion

1. Specific Thrust (net thrust/intake airflow) is an important parameter for turbofans and jet engines in general. Imagine a fan (driven by an appropriately sized electric motor) operating within a pipe, which is connected to a propelling nozzle. Fairly obviously, the higher the Fan Pressure Ratio (fan discharge pressure/fan inlet pressure), the higher the jet velocity and the corresponding specific thrust. Now imagine we replace this set-up with an equivalent turbofan - same airflow and same fan pressure ratio. Obviously, the core of the turbofan must produce sufficient power to drive the fan via the Low Pressure (LP) Turbine. If we choose a low (HP) Turbine Inlet Temperature for the gas generator, the core airflow needs to be relatively high to compensate. The corresponding bypass ratio is therefore relatively low. If we raise the Turbine Inlet Temperature, the core airflow can be smaller, thus increasing bypass ratio. Raising turbine inlet temperature tends to increase thermal efficiency and, therefore, improve fuel efficiency.
2. Naturally, as altitude increases there is a decrease in air density and, therefore, the net thrust of an engine. There is also a flight speed effect, termed Thrust Lapse Rate. Consider the approximate equation for net thrust again:
   

   <math>F_n = m \cdot (V_{jfe} - V_a)</math>

   
   With a high specific thrust (e.g. fighter) engine, the jet velocity is relatively high, so intuitively one can see that increases in flight velocity have less of an impact upon net thrust than a medium specific thrust (e.g. trainer) engine, where the jet velocity is lower. The impact of thrust lapse rate upon a low specfic thrust (e.g. civil) engine is even more severe. At high flight speeds, high specific thrust engines can pick-up net thrust through the ram rise in the intake, but this effect tends to diminish at supersonic speeds because of shock wave losses.
3. Thrust growth on civil turbofans is usually obtained by increasing fan airflow, thus preventing the jet noise becoming too high. However, the larger fan airflow requires more power from the core. This can be achieved by raising the Overall Pressure Ratio (combustor inlet pressure/intake delivery pressure) to induce more airflow into the core and by increasing turbine inlet temperature. Together, these parameters tend to increase core thermal efficiency and improve fuel efficiency.
4. Some high bypass ratio civil turbofans use an extremely low area ratio (less than 1.01), convergent-divergent, nozzle on the bypass (or mixed exhaust) stream, to control the fan working line. The nozzle acts as if it has variable geometry. At low flight speeds the nozzle is unchoked (less than a Mach Number of unity), so the exhaust gas speeds up as it approaches the throat and then slows down slightly as it reaches the divergent section. Consequently, the nozzle exit area controls the fan match and, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake increases nozzle pressure ratio to the point where the throat becomes choked (M=1.0). Under these circumstances, the throat area dictates the fan match and, being smaller than the exit, pushes the fan working line slightly towards surge. This is not a problem, since fan surge margin is much better at high flight speeds.
5. The off-design behaviour of turbofans is illustrated under compressor map and turbine map.
6. Because modern civil turbofans operate at low specific thrust, they only require a single fan stage to develop the required fan pressure ratio. The desired overall pressure ratio for the engine cycle is usually achieved by multiple axial stages on the core compression. Rolls-Royce tend to split the core compression into two with an intermediate pressure (IP) supercharging the HP compressor, both units being driven by turbines with a single stage, mounted on separate shafts. Consequently, the HP compressor need only develop a modest pressure ratio (e.g.~4.5:1). US civil engines use much higher HP compressor pressure ratios (e.g. ~23:1 on the General Electric GE90) and tend to be driven by a two stage HP turbine. Even so, there are usually a few IP axial stages mounted on the LP shaft, behind the fan, to further supercharge the core compression system. Civil engines have multi-stage LP turbines, the number of stages being determined by the bypass ratio, the amount of IP compression on the LP shaft and the LP turbine blade speed.
7. Because military engines usually have to be able to fly very fast at Sea Level, the limit on HP compressor delivery temperature is reached at a fairly modest design overall pressure ratio, compared with that of a civil engine. Also the fan pressure ratio is relatively high, to achieve a medium to high specific thrust. Consequently, modern military turbofans usually only have 5 or 6 HP compressor stages and only require a single stage HP turbine. Low bypass ratio military turbofans usually have one LP turbine stage, but higher bypass ratio engines need two stages. In theory, by adding IP compressor stages, a modern military turbofan HP compressor could be used in a civil turbofan derivative, but the core would tend to be too small for high thrust applications.

[edit] Recent developments in blade technology

The turbine blades in a turbofan engine are subject to high heat and stress, and require special fabrication. New material construction methods and material science have allowed blades, which were originally polycrystalline (regular metal), to be made from lined up metallic crystals and more recently mono-crystalline (i.e. single crystal) blades, which can operate at higher temperatures with less distortion.

Although turbine blade (and vane) materials have improved over the years, much of the increase in (HP) turbine inlet temperatures is due to improvements in blade/vane cooling technology. Relatively cool air is bled from the compression system, bypassing the combustion process, and enters the hollow blade or vane. After picking up heat from the blade/vane, the cooling air is dumped into the main gas stream. If the local gas temperatures are low enough, downstream blades/vanes are uncooled and solid.

Strictly speaking, cycle wise the HP Turbine Rotor Inlet Temperature (after the temperature drop across the HPT stator) is more important than the (HP) turbine inlet temperature. Although some modern military and civil engines have peak RIT's of the order of 3300R (2840F) or 1833K (1560C), such temperatures are only experienced for a short time (during Take-off) on civil engines.

[edit] Turbofan engine manufacturers

The turbofan engine market is dominated by General Electric, Rolls-Royce plc and Pratt & Whitney, in order of market share.

[edit] General Electric

GE Aircraft Engines, part of the General Electric Conglomerate, currently has the largest share of the turbofan engine market. Some of their engine models include the CF6 (available on the Boeing 767, 747, Airbus A330 and more), GE90 (only the Boeing 777) and GENx (developed for the Airbus A350 & Boeing 787 currently in development) engines. Through joint ventures like CFM International and Engine Alliance, they have created the very successful CFM56 series, used on Boeing 737 and Airbus aircraft, and the GP7200, which powers the Airbus A380.

[edit] Rolls-Royce

Rolls-Royce plc is the second largest manufacturer of turbofans and is most noted for their RB211 and Trent series, as well as their joint venture engines for the Airbus A320, Airbus A380 and Boeing MD-90 families (IAE V2500), the Panavia Tornado (Turbo-Union RB199) and the Boeing 717 (BR700). As owners of the Allison Engine Company, their engines power the C-130 Hercules and several Embraer regional jets. Rolls-Royce Trent 970s were the first engines to power the new Airbus A380. It was also Rolls-Royce Olympus jets that powered the now retired Concorde although they were turbojets rather than turbofans.

[edit] Pratt & Whitney

Pratt & Whitney is behind GE and Rolls-Royce, the JT9D has the proud distinction of being chosen by Boeing to power the original 747 "Jumbo jet". They current supply the PW4000 to some Airbus A330 and Boeing 777. Their engines can also be found on 747 and other old jets.

[edit] Extreme bypass jet engines

In the 1970s Rolls-Royce/SNECMA tested a M45SD-02 turbofan fitted with variable pitch fan blades to improve handling at ultra low fan pressure ratios and to provide thrust reverse down to zero aircraft speed. The engine was aimed at ultra quiet STOL aircraft operating from city centre airports.

In a bid for increased efficiency with speed, a development of the turbofan and turboprop known as a propfan engine, was created that had an unducted fan. The fan blades are situated outside of the duct, so that it appears like a turboprop with wide scimitar-like blades. Both General Electric and Pratt & Whitney/Allison demonstrated propfan engines in the 1980s. Excessive cabin noise and relatively cheap jet fuel prevented the engines being put into service.

[edit] Terminology

Afterburner

extra combustor immediately upstream of final nozzle (also called reheat)

Average stage loading

constant * (delta temperature)/[(blade speed) * (blade speed) * (number of stages)]

Bypass

airstream that bypasses the combustor

Bypass ratio

bypass airflow /combustor inlet airflow

Core

turbomachinery handling the airstream that passes through the combustor.

Core power

residual shaft power from turbine expansion to ambient pressure after deducting core compression power

Core thermal efficiency

core power/power equivalent of fuel flow

Dry

afterburner (if fitted) not lit

Fan

turbofan LP compressor

Fan pressure ratio

fan outlet total pressure/intake delivery total pressure

Gas generator

engine core

HPC

high pressure compressor

HP compressor

high pressure compressor

HPT

high pressure turbine

HP turbine

high pressure turbine

Intake ram drag

penalty associated with jet engines picking up air from the atmosphere (conventional rocket motors do not have this drag term, because the oxidiser travels with the vehicle)

IPC

intermediate pressure compressor

IP compressor

intermediate pressure compressor

IPT

intermediate pressure turbine

IP turbine

intermediate pressure turbine

LPC

low pressure compressor

LP compressor

low pressure compressor

LPT

low pressure turbine

LP turbine

low pressure turbine

Net thrust

nozzle total gross thrust - intake ram drag (excluding nacelle drag, etc, this is the basic thrust acting on the airframe)

Overall pressure ratio

combustor inlet total pressure/intake delivery total pressure

Overall thermal efficiency

thermal efficiency * propulsive efficiency

Propulsive Efficiency

propulsive power/rate of production of propulsive kinetic energy (maximum propulsive efficiency occurs when jet velocity equals flight velocity, which implies zero net thrust!)

SFC

Specific fuel consumption

Specific fuel consumption

total fuel flow/net thrust (proportional to flight velocity/overall thermal efficiency)

Static pressure

normal meaning of pressure. Excludes any kinetic energy effects

Specific thrust

net thrust/intake airflow

Thermal efficiency

rate of production of propulsive kinetic energy/fuel power

Total fuel flow

combustor (plus any afterburner) fuel flow rate (e.g. lb/s or g/s)

Total pressure

static pressure plus kinetic energy term

Turbine rotor inlet temperature

gas absolute mean temperature at principal (e.g. HP) turbine rotor entry

[edit] See also

• Jet Engine Performance
• Jet aircraft
• Jetboat
• Jet engine
• Variable Cycle Engine
• Turbojet
• Turboprop
• Propfan
• Turboshaft
• Ramjet
• Spacecraft propulsion
• Supercharger
• Turbocharger
• Gas turbine
• Kurt Schreckling who built practical jet engines for model aircraft

• Wikibooks: Jet propulsion

[edit] Other meanings

The Unicode standard includes a turbofan character, #274B, in the dingbats range. Its official name is "HEAVY EIGHT TEARDROP-SPOKED PROPELLER ASTERISK = turbofan". In appropriately-configured browsers, it should appear in quotes here: "❋";

cs:Dvouproudový motor

de:Turbofan es:Turbofan fa:توربوفن fr:turboréacteur ko:터보팬 it:Turboventola nl:Turbofan no:Turbofan pl:Silnik turbowentylatorowy pt:Turbofan zh:渦輪扇發動機