Turbofan Engine

TurbofanTurbofan Engine A turbofan is a type of aircraft engine consisting of a ducted fan which is powered by a gas turbine. Part of the airstream from the ducted fan passes through the gas turbine core, providing oxygen to burn fuel to create power. However, most of the air flow bypasses the engine core, and is accelerated by the fan blades in much the same manner as a propeller. The combination of thrust produced from the fan and the exhaust from the core is a more efficient process than other jet engine designs, resulting in a comparatively low specific fuel consumption.[1] A few designs work slightly differently and have the fan blades as a radial extension of an aft-mounted low-pressure turbine unit. Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above. All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans are also used in many military jet aircraft. 1 Turbofan Turbofan Schematic diagram of a high-bypass turbofan engine A turbofan is a type of aircraft engine consisting of a ducted fan which is powered by a gas turbine. Part of the airstream from the ducted fan passes through the gas turbine core, providing oxygen to burn fuel to create power. However, most of the air flow bypasses the engine core, and is accelerated by the fan blades in much the same manner as a propeller. The combination of thrust produced from the fan and the exhaust from the core is a more efficient process than other jet engine designs, resulting in a comparatively low specific fuel consumption.[1] A few designs work slightly differently and have the fan blades as a radial extension of an aft-mounted low-pressure turbine unit. Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above. 2 Turbofan All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans are also used in many military jet aircraft. To move an airplane through the air, thrust is generated by some kind of propulsion system. Most modern airliners useturbofan engines because of their high thrust and good fuel efficiency. On this page, we will discuss some of the fundamentals of turbofan engines. A turbofan engine is the most modern variation of the basic gas turbine engine. As with other gas turbines, there is a core engine, whose parts and operation are discussed on a separate page. In the turbofan engine, the core engine is surrounded by a fan in the front and an additional turbine at the rear. The fan and fan turbine are composed of many blades, like the corecompressor and core turbine, and are connected to an additional shaft. All of this additional turbomachinery is colored green on the schematic. As with the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary. The fan shaft passes through the core shaft for mechanical reasons. This type of arrangement is called a two spool engine (one "spool" for the fan, one "spool" for the core.) Some advanced engines have additional spools for even higher efficiency. How does a turbofan engine work? The incoming air is captured by the engine inlet. Some of the incoming air passes through the fan and continues on into the core compressor and then the burner, where it is mixed with fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle, as in a basic turbojet. The rest of the incoming air passes through the fan and bypasses, or goes around the engine, just like the air through a propeller.The air that goes through the fan has a velocity that is slightly increased from free stream. So a turbofan gets some of its thrust from the core and some of its thrust from the fan. The ratio of the air that 3 Turbofan goes around the engine to the air that goes through the core is called the bypass ratio. Because the fuel flow rate for the core is changed only a small amount by the addition of the fan, a turbofan generates more thrust for nearly the same amount of fuel used by the core. This means that a turbofan is very fuel efficient. In fact, high bypass ratio turbofans are nearly as fuel efficient as turboprops. Because the fan is enclosed by the inlet and is composed of many blades, it can operate efficiently at higher speeds than a simple propeller. That is why turbofans are found on high speed transports and propellers are used on low speed transports. Low bypass ratio turbofans are still more fuel efficient than basic turbojets. Many modern fighter planes actually use low bypass ratio turbofans equipped with afterburners. They can then cruise efficiently but still have high thrust when dogfighting. Even though the fighter plane can fly much faster than the speed of sound, the air going into the engine must travel less than the speed of sound for high efficiency. Therefore, the airplane inlet slows the air down from supersonic speeds. 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. The very first running turbofan was the GermanDaimler-Benz DB 670 (aka 109-007) which was operated on its testbed on April 1, 1943. The engine was abandoned later while the war went on and problems could not be solved. The British wartime Metrovick F.2axial flow jet was given a fan to create the first British turbofan. 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 they also led to a poor propulsive efficiency, as pure turbojets have a high specific thrust/high velocity exhaust better suited to supersonic flight. 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 production turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404 fighter engine. Civilian turbofan engines of the 1960s, such as 4 yielding a high (mixed or cold) exhaust velocity. whereas the high-pressure compressor is powered by the high-pressure turbine A high specific thrust/low bypass ratio turbofan normally has a multistage fan. The CF700 turbofan engine was also used to train Moon-bound astronauts in Project Apollo as the powerplant for the Lunar Landing Research Vehicle. . This was derived from the T-38 Talon and the Learjet General Electric J85/CJ610 turbojet (2. A smaller core flow/higher bypass ratio cycle can be achieved by raising the (HP) turbine rotor inlet temperature. developing a relatively high pressure ratio and. There are now over 400 CF700 aircraft in operation around the world. A bypass flow can only be 5 .850 lbf or 12.0 bypass ratio. The core airflow needs to be large enough to give sufficient core power to drive the fan. Say the new engine is to have the same airflow and net thrust (i. The CF700 was the first small turbofan in the world to be certified by the Federal Aviation Administration (FAA). mixed exhaust. with an experience base of over 10 million service hours. turbofan is replacing an old turbojet. in a particular military application.650 N) to power the larger Rockwell Sabreliner 75/80 model aircraft. Imagine a retrofit situation where a new low bypass ratio. low-bypass turbofan engine with a mixed exhaust. showing the low-pressure (green) and highpressure (purple) spools. Low bypass turbofans Schematic diagram illustrating a 2-spool.e. thus. but were not dissimilar to their military equivalents.200 lbf or 18. same specific thrust) as the one it is replacing. The fan (and booster stages) are driven by the low-pressure turbine.700 N). The unusual General Electric CF700 turbofan engine was developed as an aft-fan engine with a 2. as well as the Dassault Falcon 20 with about a 50% increase in thrust (4.Turbofan the Pratt & Whitney JT8D and the Rolls-Royce Spey had bypass ratios closer to 1. This improves the fan surge margin (see compressor map) in the mid-flow range. Afterburning turbofans Since the 1970s. but with a lower exhaust temperature to retain net thrust. raising the temperature of exhaust gases by a significant amount. the resulting turbofan would probably operate at a higher nozzle pressure ratio than the turbojet. which burns fuel from afterburner-specific fuel injectors. and it was also the heart of the famous F-14 Tomcat air superiority fighter which used the same engines in a smaller. an afterburner can operate at the 6 . Unlike the main combustor. resulting from a probable increase in overall pressure ratio. Afterburning is often designed to give a significant thrust boost for take off. A few low-bypass ratio military turbofans (e. Consequently afterburning can only be used for short portions of a mission. When lit. F404) have Variable Inlet Guide Vanes. An afterburner is a combustor located downstream of the turbine blades and directly upstream of the nozzle. most jet fighter engines have been low/medium bypass turbofans with a mixed exhaust. However the Mach 3 SR-71 was designed for continuous operation and to be efficient with the afterburner lit. Improvements in turbine cooling/material technology would facilitate the use of a higher turbine inlet temperature. despite increases in cooling air temperature. where the downstream turbine blades must not be damaged by high temperatures. prodigious amounts of fuel are burnt in the afterburner. the (dry power) fuel flow would also be reduced. afterburner and variable area final nozzle. more agile airframe to achieve efficient cruise and Mach 2 speed. with piano-style hinges.g. The variable geometry nozzle must open to a larger throat area to accommodate the extra volume flow when the afterburner is lit. resulting in a higher exhaust velocity/engine specific thrust. transonic acceleration and combat maneuvers.Turbofan introduced if the turbine inlet temperature is allowed to increase. to direct air onto the first rotor stage. Since the temperature rise across the whole engine (intake to nozzle) would be lower. Efficiently done. The swing wing F-111 achieved a very high range / payload capability by pioneering the use of this engine. to compensate for a correspondingly smaller core flow. resulting in a better specific fuel consumption (SFC). but is very fuel intensive. vectored thrust. At a fixed total applied fuel:air ratio. cross border skirmishes) The latter engine is better for an aircraft that has to fly some distance. resulting in a higher afterburning net thrust and. A. Rolls-Royce Pegasus.e. high specific thrust engines have a high dry SFC. Stationary components 1. However. the total fuel flow for a given fan airflow will be the same.Turbofan ideal maximum (stoichiometric) temperature (i.e. Modern low-bypass military turbofans include the Pratt & Whitney F119. all of which feature a mixed exhaust. Non-afterburning engines include the Rolls-Royce/Turbomeca Adour (afterburning in the SEPECAT Jaguar) and the unmixed. by definition. before his/her fuel reserves become dangerously low. poor afterburning SFC/good dry SFC. Low pressure spool B. before going into combat. about 2100K/3780Ra/3320F). regardless of the dry specific thrust of the engine. the pilot can only afford to stay in afterburning for a short period. However.g. but only has to fight fairly close to the airfield (e. have a higher nozzle pressure ratio. a high specific thrust turbofan will. therefore. afterburner and variable area propelling nozzle. or loiter for a long time. However. The situation is reversed for a medium specific thrust afterburning turbofan: i. a lower afterburning specific fuel consumption. The former engine is suitable for a combat aircraft which must remain in afterburning combat for a fairly long period. High pressure spool C. High-bypass turbofan engines . the Eurojet EJ200 and the General Electric F110 and F414. Nacelle 7 . Through improvements in turbine cooling/material technology. Again. Core nozzle 9. 8 . Low pressure turbine 8. or more. High pressure turbine 7. A mixed exhaust is often employed nowadays 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 1960s. the fan (and booster stages) are driven by the low-pressure turbine. Low pressure compressor 4.Turbofan 2. modern civil turbofans do not have any stationary inlet guide vanes in front of the fan rotor. is now common). IPC. high-bypass turbofan engine with an unmixed exhaust. Improved blade aerodynamics reduces the number of extra compressor stages required. Fan nozzle Schematic diagram illustrating a 2-spool. LPC. High pressure compressor 5. HPC) dramatic increases in overall pressure ratio have become possible. thus facilitating a smaller (and lighter) core and (potentially) improving the core thermal efficiency. but more stages are required. Reducing the core mass flow tends to increase the load on the LP turbine. Fan 3. Combustion chamber 6. The fan is scaled to achieve the desired net thrust.e. Unlike some military engines. Further improvements in core thermal efficiency can be achieved by raising the overall pressure ratio of the core. Low specific thrust is achieved by replacing the multi-stage fan with a single stage unit. a higher (HP) turbine rotor inlet temperature can be used. so this unit may require additional stages to reduce the average stage loading and to maintain LP turbine efficiency.e. 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. With multiple compressors (i. stators) enable high pressure ratio compressors to work surge-free at all throttle settings. Variable geometry (i. Reducing core flow also increases bypass ratio (5:1. This enables heavily laden. military transport aircraft (e. leads to a lower specific fuel consumption. Although modern combat aircraft tend to use low bypass ratio turbofans. the General Electric GE90/GEnx and the GP7000. wide body aircraft to accelerate quickly during take-off and consequently lift-off within a reasonable runway length. together with a lower specific thrust (better propulsive efficiency). This is not so much due to the higher bypass ratio. thereby. This. Because of the implied low mean jet velocity. Other high-bypass turbofans are the Pratt & Whitney JT9D. the static (i. as to the use of a low pressure ratio. single stage. Mach 0) thrust is consequently relatively high. jet velocity. and also of reduced noise.83). Consequently the engine must be over-sized to give sufficient thrust during climb/cruise at high flight speeds (e. 9 . almost all of today's jet airliners are powered by high-bypass turbofans. fan. For reasons of fuel economy.g.g. designed in mid 1960s to power the Lockheed C-5 Galaxy military transport aircraft. Mach 0.Turbofan Cutaway diagram of the General Electric CF6-6 engine The first high-bypass turbofan engine was the General Electric TF39. C-17 ) mainly use high bypass ratio turbofans (or turboprops) for fuel efficiency.e. More recent large high-bypass turbofans include the Pratt & Whitney PW4000. the three-shaft Rolls-Royce Trent. The civil General Electric CF6 engine used a derived design. the three-shaft Rolls-Royce RB211 and theCFM International CFM56. The combination of a higher overall pressure ratio and turbine inlet temperature improves thermal efficiency. High-bypass turbofan engines are generally quieter than the earlier low bypass ratio civil engines. which significantly reduces specific thrust and. Because of the high thrust lapse rate. a high bypass ratio/low specific thrust turbofan has a high thrust lapse rate (with rising flight speed). produced jointly by GE and P&W. which reduces the aircraft's net thrust by 50%. being on a different coaxial shaft and driven by a separate (IP) turbine. with the IP compressor. fan pressure ratio.g. As the design overall pressure ratio of an engine cycle increases. by progressively: 1) opening interstage/intercompressor blow-off valves (inefficient) and/or 2) closing variable stators within the compressor Most modern American civil turbofans employ a relatively high pressure ratio High Pressure (HP) Compressor. Turbofan configurations Turbofan engines come in a variety of engine configurations. the climb-out is much shallower. Modern twin engined airliners normally climb very steeply immediately after take-off. This occurs when some of the compressor aerofoils stall (like the wings of an aircraft) causing a violent change in the direction of the airflow.e. without employing variable geometry. a medium-range.Turbofan The turbofans on twin engined airliners are further over-sized to cope with losing one engine during take-off. was powered by lowbypass engines. SFC). with many rows of variable stators to control surge margin at part-throttle. As the HP Compressor has a modest pressure ratio it can be throttled-back surgefree. the choice of turbofan configuration has little impact upon the design point performance (e. the Ilyushin Il-86. However. Off-design performance and stability is. which supercharges the HP compressor. In the threespool RB211/Trent the core compression system is split into two. because a shallow 10 . as long as overall component performance is maintained. rear-engined aircraft seating up to 120 passengers introduced in 1980 was the first Soviet aircraft to use high-bypass engines. net thrust. For a given engine cycle (i. same airflow. affected by engine configuration. however. at throttled conditions. but sufficient to clear obstacles in the flightpath. The Yakovlev Yak-42. bypass ratio. without encountering an instability known as compressor surge. If one engine is lost. However. The Soviet Union's engine technology was less advanced than the West's and its first wide-body aircraft. overall pressure ratio and HP turbine rotor inlet temperature). it becomes more difficult to throttle the compression system. compressor stall can be avoided. the air going into the engine must travel less than the speed of sound for high efficiency. high bypass ratio turbofans are nearly as fuel efficient as turboprops. known as the CJ805. [edit]Single shaft turbofan Although far from common. comprising a fan and high pressure compressor driven by a single turbine unit. the Single Shaft Turbofan is probably the simplest configuration. the fan blades being a radial extension of the turbine blades. You can explore the design and operation of an afterburning turbojet engine by using the interactive EngineSim Java applet. One of the problems with the Aft Fan configuration is hot gas leakage from the LP turbine to the fan. Therefore. The SNECMA M53. Aft fan turbofan One of the earliest turbofans was a derivative of the General Electric J79 turbojet. Hot gas from the turbojet turbine exhaust expanded through the LP turbine. That is why turbofans are found on high speed transports and propellers are used on low speed transports. a turbofan generates more thrust for nearly the same amount of fuel used by the core. Many modern fighter planes actually use low bypass ratio turbofans equipped with afterburners. Even though the fighter plane can fly much faster than the speed of sound.Turbofan IP compressor working line is inevitable. Set the Engine Type to "Turbofan" and you can vary any of the parameters which affect thrust and fuel flow. Because the fuel flow rate for the core is changed only a small amount by the addition of the fan. the IPC requires at least one stage of variable geometry. which featured an integrated aft fan/low pressure (LP) turbine unit located in the turbojet exhaust jetpipe. which powers Mirage fighter aircraft. the airplane inlet slows the air down from supersonic speeds. is an example of a Single Shaft Turbofan. Because the fan is enclosed by the inlet and is composed of many blades. the M53 requires a variable area mixer to facilitate part-throttle operation. Low bypass ratio turbofans are still more fuel efficient than basic turbojets. This means that a turbofan is very fuel efficient. 11 . In fact. it can operate efficiently at higher speeds than a simple propeller. This Aft Fan configuration was later exploited in the General Electric GE36 UDF (propfan) Demonstrator of the early 80's. Despite the simplicity of the turbomachinery configuration. all on the same shaft. They can then cruise efficiently but still have high thrust when dogfighting. while the inlet does no work on the flow. inlets come in a variety of shapes and sizes with the specifics usually dictated by the speed of the aircraft. 12 . which are also called jet engines.Turbofan Most modern passenger and military aircraft are powered by gas turbine engines. straight. A subsonic aircraft has an inlet with a relatively thick lip. the surface of the inlet from outside to inside is a continuous smooth curve with some thickness from inside to outside. short inlet works quite well. SUBSONIC INLETS For aircraft that cannot go faster than the speed of sound. a simple. There are several different types of gas turbine engines. The most upstream portion of the inlet is called the highlight. All turbine engines have an inlet to bring free stream air into the engine. As shown in the figures above. The inlet sits upstream of the compressor and. inlet performance has a strong influence on engine net thrust. On a typical subsonic inlet. but all turbine engines have some parts in common. or the inlet lip. like large airliners. the inlet includes the entire lower surface of the aircraft forward of the cowl lip. the inlet must bring the high speed external flow down to subsonic conditions in the burner. For a supersonic aircraft. But because inlet operation is so important to engine performance. More exotic inlet shapes are used on some aircraft for a variety of reasons. In England. not the engine manufacturer. hot boundary layers are usually present on the compression surfaces of hypersonic inlets. At very low aircraft speeds. all engine manufacturers also employ 13 . Because the inlet is so important to overall aircraft operation. This variable geometry inlet is used on the F-14 and F-15 fighter aircraft. the heat environment is even worse because the flight Mach number is higher than that for a ramjet-powered aircraft. For scramjet-powered aircraft. The inlets of the SR-71 actually produce thrust during flight. The inlets of the Mach 3+ SR-71 aircraft are specially designed to allow cruising flight at high speed. use a central cone to shock the flow down to subsonic speeds. Scramjet inlets are highly integrated with the fuselage of the aircraft. like the one at the upper right. Thick. The inlet lip is sharpened to minimize the performance losses from shock waves that occur during supersonic flight. The flow exiting a scramjet inlet must remain supersonic. use flat hinged plates to generate the compression shocks.Turbofan SUPERSONIC INLETS An inlet for a supersonic aircraft. HYPERSONIC INLETS Inlets for hypersonic aircraft present the ultimate design challenge. For ramjet-powered aircraft. the inlet must slow the flow down to subsonic speeds before the air reaches the compressor. which is a more accurate description of their function at low aircraft speeds. On the X-43A. At high speeds. Some supersonic inlets. has a relatively sharp lip. or when just sitting on the runway. Other inlets. INLET EFFICIENCY An inlet must operate efficiently over the entire flight envelope of the aircraft. a good inlet will allow the aircraft to maneuver to high angles of attack and sideslip without disrupting flow to the compressor. inlets are called intakes. it is usually designed and tested by the airframe company. on the other hand. free stream air is pulled into the engine by the compressor. with the resulting inlet geometry having a rectangular cross section. like the one shown at the lower left. High stagnation temperatures are present in this speed regime and variable geometry may not be an option for the inlet designer because of possible flow leaks through the hinges. but all jet engines have some parts in 14 . Most modern passenger and military aircraft are powered by gas turbine engines. Different airframers use different indices. If the airflow demanded by the engine is much less than the airflow that can be captured by the inlet. The airflow mis-match can producespillage drag on the aircraft. then the difference in airflow is spilled around the inlet. For hypersonic inlets the value of pressure recovery is very low and nearly constant because of shock losses. There are several different types of jet engines. so hypersonic inlets are normally characterized by their kinetic energy efficiency. Pressure recovery is another inlet performance index. The ratio of the average total pressure at the compressor face to the free stream total pressure is called the total pressure recovery. the better the inlet.Turbofan inlet aerodynamicists. the higher the value. The amount of disruption of the flow is characterized by a numerical inlet distortion index. which are also called jet engines. but all of the indices are based on ratios of the local variation of pressure to the average pressure at the compressor face. called rotors. and centrifugal compressors . are connected to the central shaft and rotate at high speed. are fixed and do not rotate. called stators. All jet engines have a compressorto increase the pressure of the incoming air before it enters the burner. which has been removed and is not shown. In the axial compressor. In the figure on the right. Other rows. At the upper left is a picture of a single rotor stage for a different compressor so that you can see how the individual blades are shaped and aligned. The job of the stators is to increase pressure and keep the flow from spiraling around the axis by bringing the flow back parallel to the axis. the air flows parallel to the axis of rotation. There are two main types of compressors used in modern jet engines. The stators of this compressor are connected to the outer casing. Some of the rows. Compressor performance has a large influence on total engine performance.Turbofan common. Here is an animated version of the axial compressor: 15 . we see a picture of the rotors of an axial compressor. The compressor is composed of several rows of airfoil cascades. axial compressors . The compressor is attached to a shaft which is connected to the power turbine on the right end of the blue shaft. At the bottom of the figure is a computer generated figure of an entire axial compressor with both rotors and stators. that will be added to these pages in the future. unsteady. But unlike a propeller blade. Each blade on a rotor or stator produces a pressure variation much like the airfoil of a spinning propeller. the blades of an axial compressor are close to one another. 16 .Turbofan How does an axial compressor work? The details are quite complex because the blade geometries and the resulting flows are three dimensional. Compressor blades continuously pass through the wakes of upstream blades that introduce unsteady flow variations. and can have important viscous and compressibility effects. like stall and surge. which seriously alters the flow around each blade. the rotational speed of the shaft necessary to produce the pressure increase. The performance is characterized by the pressure ratio across the compressor CPR. Compressor designers must rely on wind tunnel testing and sophisticated computational models to determine the performance of an axial compressor. There are additional important compressor topics. and an efficiency factor that indicates how much additional work is required relative to an ideal compressor. and the cans are arranged around the central shaft. The design of ramjet and scramjet burners are slightly different than the burners used on gas turbine engines. Many modern burners have an annular design. Burners are made from materials that can withstand the hightemperatures of combustion. shown in orange. as shown by the three burner configurations at the lower left. On this page. This is a canannular design. The liner is often perforated to enhance mixing of the fuel and air. There are several different types of gas turbine engines. The resulting high temperature exhaust gas is used to turn the power turbine and produce thrust when passed through a nozzle. The central shaft that connects the turbine and compressor passes through the center hole. and serviced. as shown in the photo at the upper right. and all turbine engines have some parts in common. 2. The burner is arranged like anannulus. shown in red. The advantage to the can-annular design is that the individual cans are more easily designed. Each can has both a liner and a casing. The burner at the left is an annular combustor with the liner sitting inside the outer casing which has been peeled open in the drawing. 3. The photo at the top left shows some actual burner cans. we discuss the operation of a gas turbine burner. All turbine engines have a combustor. in which the fuel is combined with high pressure air and burned. Burners are also used on ramjet and scramjet propulsion systems. The burner is shown in red on the computer graphic at the lower right of the figure. or a doughnut. and all three designs are found in modern gas turbines: 1. in which the casing is annular and the liner is canshaped. There are three main types of combustors. or burner. A compromise design is shown at the right. 17 . tested. The burner sits between the compressor and the power turbine. although the basic thermodynamic principles are the same. which are also called jet engines. and an inner liner. The burner in the middle is an older can or tubular design. A burner usually has an outer casing.Turbofan Most modern passenger and military aircraft are powered by gas turbine engines. There are several different types of gas turbine engines. All gas turbine engines have a power turbine located downstream of the burner to extract energy from the hot flow and turn thecompressor. Description of Images The bottom of the figure shows: • computer drawings of a turbojet with the location of the turbine relative to the other engine components.Turbofan The details of mixing and burning the fuel are quite complex and require extensive testing for a new burner. on the right 18 . but all turbine engines have some parts in common. Most modern passenger and military aircraft are powered by gas turbine engines. which are also called jet engines. For our purposes. we can consider the burner as simply the place where combustion occurs and where the working fluid (air) temperature is raised with a slight decrease in pressure. Work is done on the power turbine by the hot exhaust flow from the burner. turbofan engine. Turbofan and turboprop engines usually employ a separate turbine and shaft to power the fan and gear box respectively. Here is an animated version of the turbine section: The upper left of the figure shows an actual power turbine. Other rows. the turbine is magenta in color and the shaft is colored blue. is composed of several rows of airfoil cascades. This arrangement produces a three spool engine. Some of the rows. which is colored cyan in the drawing on the right. Such an arrangement is termed atwo spool engine. Design Details 19 . are connected to the central shaft and rotate at high speed. like the compressor. called rotors. an additional turbine and shaft is present to power separate parts of the compressor. In both drawings. are fixed and do not rotate. on the left. The turbine. Depending on the engine type. For some high performance engines. The left end of the shaft would be attached to the compressor. The job of the stators is to keep the flow from spiraling around the axis by bringing the flow back parallel to the axis. The power turbine shown on the upper left of the figure is for a two spool.Turbofan • the turbine section alone with the central shaft attached to the turbine. there may be multiple turbine stages present in the engine. called stators. as shown in the picture at the upper left. The pressure gradient helps keep the boundary layer flow attached to the surface of the turbine blades. which is bled off the compressor. Turbine blades must be made of special materials that can withstand the heat. Because of the high pressure change across the turbine. the blades experience flow temperatures of more than a thousand degrees Fahrenheit. Sitting just downstream of the burner. the pressure drop across a single turbine stage can be much greater than the pressure increase across a corresponding compressor stage. or they must be actively cooled. The tips of turbine blades are often connected by a thin metal band to keep the flow from leaking. 20 . A single turbine stage can be used to drive multiple compressor stages. Turbine blades exist in a much more hostile environment than compressor blades. The blade is hollow and cool air. the pressure decreases across the turbine. Since the turbine extracts energy from the flow. At the upper right of the figure. actively cooled turbine blade.Turbofan There are several interesting turbine design details present on this slide. we show a picture of a single. the flow tends to leak around the tips of the blades. is pumped through the blade and out through the small holes on the surface to keep the surface cool. Since the boundary layer is less likely to separate on a turbine blade than on a compressor blade. often have a fixed geometryconvergent nozzle as shown on the left of the figure. to conduct the exhaust gases back to the free stream. As shown above. Mixing of the two flows provides some thrust enhancement and these nozzles also tend to be quieter than convergent 21 . but all turbine engines have some parts in common. A nozzle is a relatively simple device. The core flow exits the center nozzle while the fan flow exits the annular nozzle. just a specially shaped tube through which hot gases flow. Simple turbojets.Turbofan Most modern passenger and military aircraft are powered by gas turbine engines. nozzles come in a variety of shapes and sizes depending on the mission of the aircraft. However. There are several different types of gas turbine engines. and turboprops. The nozzle sits downstream of the power turbine. which are also called jet engines. Turbofan engines often employ a co-annular nozzle as shown at the top left. the mathematicswhich describe the operation of the nozzle takes some careful thought. All gas turbine engines have a nozzle to produce thrust. and to set the mass flow rate through the engine. large drag penalties can occur near the nozzle exits. This allows the exhaust flow to be easily deflected. Changing the direction of the thrust with the nozzle makes the aircraft much more maneuverable. Because the nozzle conducts the hot exhaust back to the free stream.1 Single shaft turbofan 6. but variable geometry provides efficient engine operation over a wider airflow range than a simple fixed nozzle. You can explore the design and operation of nozzles with our interactive nozzle simulator program which runs on your browser.Turbofan nozzles. Recently. in particular. then is expanded through the divergent section to the exit at the right. Rocket engines usually have a fixed geometry CD nozzle with a much larger divergent section than is required for a gas turbine. On fighter aircraft. The variable geometry causes these nozzles to be heavier than a fixed geometry nozzle. or vectored. the external nozzle configuration is often designed by the airframer. All of the nozzles we have discussed thus far are round tubes. however. As with the inlet design. The internal nozzle is usually the responsibility of the engine manufacturer. engineers have been experimenting with nozzles with rectangular exits. there can be serious interactions between the engine exhaust flow and the airflow around the aircraft. Contents [hide] • • • • • • 1 Introduction 2 Early turbofans 3 Low bypass turbofans 4 Afterburning turbofans 5 High-bypass turbofan engines 6 Turbofan configurations o o 6.CD nozzle as shown on the left. the flow first converges down to the minimum area or throat. as shown in the middle of the figure. Afterburning turbojets and turbofans require a variable geometry convergent-divergent . In this nozzle.2 Aft fan turbofan 22 . Rocket engines also use nozzles to accelerate hot exhaust to produce thrust. A typical nozzleafterbody configuration is shown in the upper right for an F-15 with experimental maneuvering nozzles. 4 Pratt & Whitney 11.Turbofan o o o o o o o • • • • • 6.8 High Pressure Turbine 6.3 Rolls-Royce 11.9 Low Pressure Turbine 7 Cycle improvements 8 Thrust growth 9 Technical Discussion 10 Recent developments in blade technology 11 Turbofan engine manufacturers o o o o o • • • • 11.5 Three spool 6.2 CFM International 11.5 Aviadvigatel 12 Extreme bypass jet engines 13 Terminology 14 Other meanings 15 Notes and references Welcome to the Beginner's Guide to Propulsion 23 .4 Boosted two spool 6.6 Geared fan 6.7 Military turbofans 6.1 General Electric 11.3 Basic two spool 6. and the reaction to this acceleration produces a force on the engine. is accelerated by the engine. A propulsion system is a machine that producesthrust to push an object forward. or working fluid.Turbofan What is propulsion? The word is derived from two Latin words: pro meaning before or forwards andpellere meaning to drive. Propulsion means to push forward or drive an object forward. 24 . A gas. On airplanes. thrust is usually generated through some application of Newton's third law of action and reaction. spend most of their life in a cruise condition. require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. we realize that an airplane propulsion system must serve two purposes. The pages were originally prepared as teaching aidsto support EngineSim. excess thrust is not as important as high engine efficiency and low fuel usage. like fighter planes or experimental high speed aircraft.html) for teachers and students. Some aircraft. the ramjet. There is a special section of the Beginner's Guide which deals with compressible. Some aircraft. like airliners and cargo planes. Other slides were prepared to support LTP videoconferencing workshops(http://www. we can generate high thrust by accelerating a large mass of gas by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners. 25 . and the rocket. the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. Future hypersonic aircraft will employ some type of ramjet or rocket propulsion. or by accelerating a small mass of gas by a large amount. the turbofan which is used on almost all commercial airliners. Modern military aircraft typically employ afterburners on a low bypass turbofan core. Common types There are two types of jet engine that are seen commonly today. First.gov/WWW/K-12/CoE/Coemain. an interactive educational computer program that allows students to design and test jet engines on a personal computer. And other slides were prepared as part of Power Point Presentations for the Digital Learning Network.nasa.grc. it is more fuel efficient to accelerate a large mass by a small amount. The site was prepared at NASA Glenn by the Learning Technologies Project (LTP) to provide background informationon basic propulsion for secondary math and science teachers.Turbofan A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. engine efficiency is not as important as very high thrust. And second. the turbine (or jet) engine. or high speed. Because of the aerodynamic efficiency of propellers and fans. flares. and rocket engines which are used for spaceflight and other terrestrial uses such as ejector seats. This section is intended for undergraduates who are studying shock waves or isentropic flows and contains severalcalculators and simulators for that flow regime. We will discuss four principal propulsion systems: the propeller. the faster the airplane will accelerate. the greater the difference between the thrust and the drag. called theexcess thrust. For these airplanes. Different propulsion systems generate thrust in slightly different ways. the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. aerodynamics. For these airplanes. Since thrust depends on both the amount of gas moved and the velocity. In fact. Why are there different types of engines? If we think about Newton's first law of motion. fireworks etc. In the 1960s there was little difference between civil and military jet engines. 26 .Turbofan [edit]Turbofan engines Main article: Turbofan Most modern jet engines are actually turbofans. Multistage fans are normally needed to reach the relatively high fan pressure ratio needed for high specific thrust. but to a bypass duct. supplying supercharged air not only to the engine core. however. jet noise being of less concern in military uses relative to civil uses. better fuel consumption is achieved as well as higher thrust at low speeds. Today's military turbofans. and this wastes energy. the bypass ratio tends to be low. at airliners flight speed conventional turbojet engines generate an exhaust that ends up travelling very fast backwards. Turbofans are used for airliners because they give an exhaust speed that is better matched for subsonic airliners. By emitting the exhaust so that it ends up travelling more slowly. Although high turbine inlet temperatures are often employed. Civil turbofans today have a low exhaust speed (low specific thrust -net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. In addition. where the low pressure compressor acts as a fan.0. The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases. before expanding through a 'mixed flow nozzle'. Only a single fan stage is required. have a relatively high specific thrust. apart from the use of afterburning in some (supersonic) applications. Consequently the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common). the lower exhaust speed gives much lower noise. because a low specific thrust implies a low fan pressure ratio. to maximize the thrust for a given frontal area. usually significantly less than 2. An approximate equation for the net thrust of a rocket engine is: Where F is the thrust. and P is the atmospheric pressure. space exploration and manned access. Rocket engines are used for high altitude flights because they give very high thrust and their lack of reliance on atmospheric oxygen allows them to operate at arbitrary altitudes. the high exhaust speed and the heavier. oxidiser-rich propellant results in more propellant use than turbojets. and their use is largely restricted to very high altitudes. Isp(vac) is the specific impulse. is the propellant flow in kg/s. Jet engines make their jet from propellant from tankage that is attached to the engine (as in a 'rocket') as well as in duct engines (those commonly used on aircraft) by ingesting an external 27 . very high speeds. g0 is a standard gravity. or where very high accelerations are needed as rocket engines themselves have a very high thrust-to-weight ratio. and permitted landing on the moon in 1969. Ae is the area of the exhaust bell at the exit. This is used for launching satellites. However. [edit]General physical principles All jet engines are reaction engines that generate thrust by emitting a jet of fluid rearwards at relatively high speed.Turbofan [edit]Rocket engines Main article: Rocket engine A common form of jet engine is the rocket engine. The forces on the inside of the engine needed to create this jet give a strong thrust on the engine which pushes the craft forwards. (In practice parts of the exhaust may be faster than others. the thrust characteristics of a rocket motor are different from that of an air breathing jet engine. when a vehicle moves with certain velocity v. Therefore. a vehicle gets the same thrust if it outputs a lot of exhaust very slowly. is greater than the vehicle velocity. So the thrust is actually equal to 28 . which provides the bulk of the fluid exiting the exhaust. and thrust is independent of speed. c.c here. In other words. as the net engine thrust is the same as if the gas were emitted with the velocity c-v. the gross thrust of the nozzle is the net thrust of the engine. creating an opposing ram drag at the intake: mv Most types of jet engine have an intake. and thus the important quantity is called the effective exhaust speed .) However. rocket motors do not have ram drag. The jet engine with an intake duct is only useful if the velocity of the gas from the engine. the oxidizer and fuel both being carried within the vehicle.Turbofan fluid (very typically air) and expelling it at higher speed. do not have an intake. Consequently. however. Conventional rocket motors. but it's the average momentum that matters. v. [edit]Thrust The motion impulse of the engine is equal to the fluid mass multiplied by the speed at which the engine emits this mass: I=mc where m is the fluid mass per second and c is the exhaust speed. the fluid moves towards it. or a little exhaust very quickly. a greater mass of fluid must go through the engine to continue to accelerate at the same rate. the vehicle velocity as this gives the smallest residual kinetic 29 . the equation implies that the vehicle can't accelerate past its exhaust velocity as it would have negative thrust. and propulsive efficiency(ηp)-how much of the energy of the jet ends up in the vehicle body rather than being carried away as kinetic energy of the jet. Even though overall energy efficiency η is simply: η = η pη c For all jet engines the propulsive efficiency is highest when the engine emits an exhaust jet at a speed that is the same as. Additionally. but all engines have a designed limit on this.Turbofan S = m (c-v) This equation implies that as v approaches c. cycle efficiency (ηc). [edit]Energy efficiency Dependence of the energy efficiency (η) upon the vehicle speed/exhaust speed ratio (v/c) for airbreathing jet and rocket engines Energy efficiency (η) of jet engines installed in vehicles has two main components.how efficiently the engine can accelerate the jet. or nearly the same as. the practical combustion temperatures and nozzle efficiencies are much lower.Turbofan energy. another factor is cycle efficiency. 30 .(Note:[20]) The exact formula for air-breathing engines moving at speed v with an exhaust velocity c is given in the literature as:[21] is And for a rocket: [22] In addition to propulsive efficiency. energy efficient nozzles. as they can achieve extremely high combustion temperatures and can have very large. Cycle efficiency is highest in rocket engines (~60+%). Heat engine efficiency is determined by the ratio of temperatures that are reached in the engine to that they are exhausted at from the nozzle. essentially a jet engine is typically a form of heat engine. which in turn is limited by the overall pressure ratio that can be achieved. Cycle efficiency in turbojet and similar is nearer to 30%. Turbofan Specific impulse as a function of speed for different jet types with kerosene fuel (hydrogen Isp would be about twice as high). Propellant consumption in jet engines is measured by Specific Fuel Consumption. They all measure the same thing. In rocketry. For airbreathing engines such as turbojets energy efficiency and propellant (fuel) efficiency are much the same thing. since the propellant is a fuel and the source of energy. Although efficiency plummets with speed. and this means that a high energy propellant gives better 31 . it turns out that efficiency per unit distance (per km or mile) is roughly independent of speed for jet engines as a group. the propellant is also the exhaust. whereas specific fuel consumption is inversely proportional to the others. however airframes become inefficient at supersonic speeds [edit]Fuel/propellant consumption A closely related (but different) concept to energy efficiency is the rate of consumption of propellant mass. specific impulse and effective exhaust velocity are strictly proportional. greater distances are covered. Specific impulse or Effective exhaust velocity. 400 0. SFC Effective SFC in Engine scenari in Isp i exhaust lb/(lbf· type o g/(kN· n s velocity ( h) s) m/s) NK-33 roc vacuu 10.8 1.2 1.605[2 80C2B1F 17.8 cruise 3] (dry) 3.240 225 453 4.012 29.587 Concor de M2 1.5 309 330 3.553 Boeing CF6747.950 58.1 3] 400 turbofan cruise General Electric sea CF6 turbo level fan [23] 5.195[2 33.877 SR-71 J-58 turboj at M3.70 115.Turbofan propellant efficiency but can in some cases actually can give lower energy efficiency.423 127 800 7.9 et (wet) RollsRoyce/Sne cma Olympus 593 53.307 8.95 ket engine vacuu m Ramjet M1 4.900 18.9 ket engine m Space SSME roc Shuttle 7.0.696 11.000 0 32 . This is primarily because rockets almost universally use dense liquid or solid reaction mass which gives a much smaller volume and hence the pressurisation system that supplies the nozzle is much smaller and lighter for the same performance. Clearly for a given engine. or to accelerate the mass of the engine. rocket engines generally achieve very much higher thrust to weight ratios than duct engines such as turbojet and turbofan engines. and which in turn results in more engineering materials being needed to hold 33 . Duct engines have to deal with air which is 2-3 orders of magnitude less dense and this gives pressures over much larger areas. but mostly is a function of engine construction technology. As can be seen in the following table. the better the thrust to weight is. the less fuel is used to compensate for drag due to the lift needed to carry the engine weight.Turbofan [edit]Thrust-to-weight ratio Main article: Thrust-to-weight ratio The thrust to weight ratio of jet engines of similar principles varies somewhat with scale. the lighter the engine. m/s.2[25] Space shuttle's SSME rocket engine RD-180 rocket engine NK-33 rocket engine 73. Engine Thrust-toweight ratio 4.66[27] [edit]Comparison of types Comparative suitability for (left to right) turboshaft.12[26] 73. Turboprops obtain little thrust from jet effect. but are useful for comparison. low bypass andturbojet to fly at 10 km altitude in various speeds.Turbofan the engine together and for the air compressor. Horizontal axis . Vertical axis displays engine efficiency. They are gas turbine 34 .0 with reheat[24] Concorde's Rolls-Royce/Snecma Olympus 593 turbojet J-58 (SR-71 Blackbird jet engine) 5.speed.4 136. propellers no longer provide any thrust (c-v < 0). but they emit it at the much higher speeds possible with a de Laval nozzle.4 (0.Turbofan engines that have a rotating fan that takes and accelerates the large mass of air but by a relatively small change in speed. Such engines are effective at lower speeds. being blown from the both exhausts.v) where m1 and m2 are the air masses.4 times the 35 . turbojets and other similar engines accelerate a much smaller mass of the air and burned fuel. turboshafts are most effective at about Mach 0. For instance. The thrust of such engine is S = m1 (c1 . but at higher speeds than the turboshafts and propellers in general. When the plane speed exceeds this limit.v) + m2 (c2 . at the 10 km altitude. turboprops are very efficient. Low bypass turbofans have the mixed exhaust of the two air flows. than the pure jets. because they accelerate a large mass of air. This low speed limits the speed of any propeller driven airplane. This is why they are suitable for supersonic and higher speeds. running at different speeds (c1 and c2). However. deprived of their inlet systems can only accept air at around half the speed of sound. whereas with a turbojet (or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to decrease with increasing altitude. low bypass turbofans become more effective at about Mach 0. At any given throttle. jet engines.Turbofan speed of sound).75 and turbojets become more effective than mixed exhaust engines when the speed approaches Mach 2-3.[28] [edit]Altitude and speed With the exception of scramjets. Rocket engines have extremely high exhaust velocity and thus are best suited for high speeds (hypersonic) and great altitudes. The 36 . the thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure falls thus increasing net thrust at the nozzle exit plane). Rocket engines are more efficient than even scramjets above roughly Mach 15. even after compression. For turbojet engines altitudes of about 40km appear to be possible. This can be eased by flying faster which helps compress the air in at the front of the engine. Scramjets may theoretically manage 75km.[29] Rocket engines of course have no upper limit.Generally then.at very high altitudes the air becomes too thin to burn. but ultimately the engine cannot go any faster without melting. The intensity of the noise is proportional to the thrust as well as proportional to the fourth power of the jet velocity. whereas for ramjet engines 55km may be achievable. the lower speed exhaust jets emitted from engines such as high bypass turbofans are the 37 .Turbofan inlet system's job for transonic and supersonic aircraft is to slow the air and perform some of the compression. The limit on maximum altitude for engines is set by flammability. [edit]Noise Noise is due to shockwaves that form when the exhaust jet interacts with the external air. whereas the fastest jets are the loudest.4). military jets and rockets inherently need to have supersonic exhaust at top speed. They could convert in flight from being largely a turbojet to being largely a compressor-assisted ramjet. Therefore since engines for supersonic vehicles such as Concorde. so these vehicles are especially noisy even at low speeds.Turbofan quietest. the engine used variable geometry vanes to direct excess air through 6 bypass pipes from downstream of the fourth compressor stage into the 38 . Although some variation in jet speed can often be arranged from a jet engine (such as by throttling back and adjusting the nozzle) it is difficult to vary the jet speed from an engine over a very wide range. [edit]Advanced designs [edit]J-58 combined ramjet/turbojet The SR-71 Blackbird's Pratt & Whitney J58 engines were rather unusual. At high speeds (above Mach 2. The downside of hydrogen is its density. Hydrogen is a highly desirable fuel. jet engines running on hydrogen are quite easy to build—the first ever turbojet was run on hydrogen.[30] 80% of the SR71's thrust at high speed was generated in this way.Turbofan afterburner. the molecule is very much lighter than other molecules. although not duct engines. improving specific impulse by 10-15%. in gaseous form the tanks are impractical for flight. Also. However.2. in almost every other way. hydrogen-fueled rocket engines have seen extensive use. [edit]Hydrogen fuelled air-breathing jet engines Jet engines can be run on almost any fuel. giving much higher thrust. The name coined for this setup is turbo-ramjet. but even 39 . although the energy per mole is not unusually high. hydrogen is problematic. and permitting continuous operation at Mach 3. The energy per kg of hydrogen is twice that of more common fuels and this gives twice the specific impulse. In addition. as. It is also deeply cryogenic and requires very significant insulation that precludes it being stored in wings. a higher mass-flow through the engines. The low temperature allows lighter materials to be used. Finally. and difficult for most airports to accommodate. research is ongoing and hydrogen-fueled aircraft designs do exist that may be feasible. pure hydrogen is not found in nature. and permits combustors to inject more fuel 40 . Nevertheless. and must be manufactured either via steam reforming or expensive electrolysis. [edit]Precooled jet engines Main article: Precooled jet engine An idea originated by Robert P.Turbofan in the form of liquid hydrogen it has a density one fourteenth that of water. Carmichael in 1955[31] is that hydrogen-fueled engines could theoretically have much higher performance than hydrocarbon-fueled engines if a heat exchanger were used to cool the incoming air. The overall vehicle would end up being very large. there was no obvious way to stop it once it had taken off. [edit]Nuclear- powered ramjet Project Pluto was a nuclearpowered ramjet. unshielded nuclear reactor. The idea is also being researched by the EU for a concept to achieve non-stop antipodal supersonic passenger travel at Mach 5 (Reaction Engines A2). Rather than combusting fuel as in regular jet engines. which would be a great disadvantage in any 41 .Turbofan without overheating the engine. and the ramjet was predicted to be able to cover any required distance at supersonic speeds (Mach 3 at tree-top height). intended for use in a cruise missile. which could permit jet engines to be used up to hypersonic speeds and high altitudes for boosters for launch vehicles. This dramatically increased the engine burn time.[32] and ATREX. air was heated using a hightemperature. that might permit single-stage-to-orbit launch vehicles. However. This idea leads to plausible designs like Reaction Engines SABRE. [33] Due to aerodynamic heating at these high speeds. Scramjets. which it was being designed for. however.Turbofan non-disposable application. it was dangerous to be in or around the flight path of the vehicle (although the exhaust itself wasn't radioactive). Scramjets start working at speeds of at least Mach 4. and have a maximum useful speed of approximately Mach 17. Also. operate with supersonic airflow through the entire engine. They share a similar structure with ramjets. Thus. being a speciallyshaped tube that compresses air with no moving parts through ram-air compression. 42 . scramjets do not have the diffuser required by ramjets to slow the incoming airflow to subsonic speeds. because the reactor was unshielded. [edit]Scramjets Main article: Scramjet Scramjets are an evolution of ramjets that are able to operate at much higher speeds than any other kind of airbreathing engine. These disadvantages limit the application to warhead delivery system for all-out nuclear war. failures do sometimes occur. are a net source of carbon to the atmosphere. but for supersonic aircraft that fly in the stratosphere some destruction of ozone may occur. Nitrogen compounds are also formed from the combustion process from atmospheric nitrogen. [edit]Safety and reliability Main article: Air safety Jet engines are usually very reliable and have a very good safety record.Turbofan cooling poses a challenge to engineers. At low altitudes this is not thought to be especially harmful. Some scientists believe that jet engines are also a source of global dimming due to the water vapour in the exhaust causing cloud formations. 43 . Sulphates are also emitted if the fuel contains sulphur. However. and in that case. [edit]Environmenta l considerations Jet engines are usually run on fossil fuel propellant. all 10 people on board were killed.Turbofan [edit]Compressor blade containment Main article: Blade off testing The most likely failure is compressor blade failure. a Dassault Falcon 20 crashed at aParis airport during an emergency landing attempt after sucking lapwings into an engine. It is a common threat to aircraft safety and has caused a number of fatal accidents. 35 died and 21 were injured. of the 104 people aboard. and modern jet engines are designed with structures that can catch these blades and keep them contained within the engine casing. which caused an engine failure and a fire in the airplane fuselage. In 1988 an Ethiopian Airlines Boeing 737 sucked pigeonsinto both engines during take-off and then crashed in an attempt to return to the Bahir Dar airport. [edit]Bird strike Bird strike is an aviation term for a collision between a bird and an aircraft. Verification of a jet engine design involves testing that this system works correctly. A US Airways Airbus A320 44 . In another incident in 1995. Military aircraft designed for highspeed flight typically have pure turbojet.Turbofan aircraft sucked in one bird in each engine. rather than through the core of the engine. increasing the risk that ingested materials will get into the core of the engine to cause damage. The plane landed in the Hudson River after taking off from LaGuardia International Airport in New York City. ice. typical on transport aircraft. As a result. which contains the smaller and more delicate compressor blades. etc. such materials go through the relatively unobstructed bypass duct. This is due to the fact that the fan of a highbypass turbofan engine. are at higher risk than big heavy multi-engine ones. such as military jet fighters. [34] Modern jet engines have the capability of surviving an ingestion of a bird. acts as a centrifugal separator to force ingested materials (birds. Small fast planes. or low-bypass turbofan engines.) to the outside of the fan's disc. The highest risk of the bird strike is during the takeoff 45 . There were no fatalities. which is in the vicinity of the airports. where rotary parts of the engine break off and exit through the case. However. nor the possibility that an engine failure would release many fragments in 46 .Turbofan and landing. Although fuel and control lines are usually duplicated for reliability. in low altitudes. the statistical models used to come up with this figure did not account for the fact that the number-two engine was mounted at the tail close to all the hydraulic lines. [edit]Uncontained failures One class of failures that has caused accidents in particular is uncontained failures. and can penetrate the cabin. These can cut fuel or control lines. the crash of United Airlines Flight 232 was caused when hydraulic fluid lines for all three independent hydraulic systems were simultaneously severed by shrapnel from an uncontained engine failure. Prior to the United 232 crash. the probability of a simultaneous failure of all three hydraulic systems was considered as high as a billion-to-one. [edit] 47 . and have increasingly utilized highstrengthcomposite materials to achieve the required penetration resistance while keeping the weight low.Turbofan many directions. Since then. more modern aircraft engine designs have focused on keeping shrapnel from penetrating the cowling or ductwork.