Propulsion Systems Propulsion Systems With ControlsWith Controls

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Modern Turbofan EnginesModern Turbofan Engines

Utilize air bypassing the core to increase Utilize air bypassing the core to increase engine efficiencyengine efficiency

Trade off of kinetic energy for greater Trade off of kinetic energy for greater mass flow as bypass ratio increasesmass flow as bypass ratio increases

Two general classifications: Low Bypass Two general classifications: Low Bypass and High Bypassand High Bypass

Low Bypass TurbofansLow Bypass Turbofans

Bypass ratios less than 2 (twice as much air Bypass ratios less than 2 (twice as much air around the core as through the core)around the core as through the core)Enables much higher speeds (up to M~3), more Enables much higher speeds (up to M~3), more compact dimensions, and practical usage of compact dimensions, and practical usage of afterburners at an expense of fuel efficiency and afterburners at an expense of fuel efficiency and noise, making these engines suitable for fightersnoise, making these engines suitable for fightersTypical thrust class for this engine type is 10500-Typical thrust class for this engine type is 10500-22000lb, with the F119 putting out 35000lb with 22000lb, with the F119 putting out 35000lb with afterburnersafterburnersTypical T/W ratios of almost 8:1Typical T/W ratios of almost 8:1SFC:~.75 lb/lbf*hSFC:~.75 lb/lbf*h

High Bypass Ratio EnginesHigh Bypass Ratio Engines

Bypass Ratios 10+:1 (if any larger it is usually more Bypass Ratios 10+:1 (if any larger it is usually more efficient to mount a prop) utilizing large diameter fansefficient to mount a prop) utilizing large diameter fans

Have much larger thrust classes than low bypass Have much larger thrust classes than low bypass engines:30,000lb to as much as 130,000lbengines:30,000lb to as much as 130,000lb

The 75-100,000lb thrust class has become a popular The 75-100,000lb thrust class has become a popular choice for airliners as only two engines are requiredchoice for airliners as only two engines are required

Takes advantage of the Law of Conservation of Takes advantage of the Law of Conservation of Momentum to obtain large thrust values at the expense of Momentum to obtain large thrust values at the expense of top speed (larger air mass at a slower velocity)top speed (larger air mass at a slower velocity)

Average T/W from 5-6Average T/W from 5-6

SFC:~.37 lb/lbf*hSFC:~.37 lb/lbf*h

http://www.ae.gatech.edu/people/ptsiotra/Pictures/turbofan.gif

Engine ControlsEngine Controls

Compensators: Electronically limit an Compensators: Electronically limit an engines thrust to match an engine to engines thrust to match an engine to another in pairs, so that mounting 2 another in pairs, so that mounting 2 engines on different sides of an aircraft engines on different sides of an aircraft with not create yaw (usually utilize a with not create yaw (usually utilize a closed feed back system)closed feed back system)Engines must be individually tested to Engines must be individually tested to calibrate compensators and for information calibrate compensators and for information for FADEC usagefor FADEC usage

Engine Controls ContinuedEngine Controls Continued

FADEC: Full Authority Digital Engine Control (or FADEC: Full Authority Digital Engine Control (or DEEC Digital Electronic Engine Control)DEEC Digital Electronic Engine Control)

Controls Fuel Flow rates for precise control of Controls Fuel Flow rates for precise control of thrust, controls variable stator vanes, monitors thrust, controls variable stator vanes, monitors engines heath, and startingengines heath, and starting

Usually small enough to fit on engine (for larger Usually small enough to fit on engine (for larger engines) otherwise requires mounting in aircraft engines) otherwise requires mounting in aircraft fuselagefuselage

InletsInlets

Turbofan engines require airspeeds below Turbofan engines require airspeeds below M=1 to operate properlyM=1 to operate properlySupersonic inlet designs must incorporate Supersonic inlet designs must incorporate diffusers (normal or oblique shock) to slow diffusers (normal or oblique shock) to slow supersonic air to subsonic speedssupersonic air to subsonic speedsEngine ducting must feature smooth Engine ducting must feature smooth transitions from inlet shape to the engines transitions from inlet shape to the engines inlet shape to prevent turbulent air from inlet shape to prevent turbulent air from entering the engineentering the engine

NozzlesNozzles

The nozzle is used to The nozzle is used to accelerate the hot gas accelerate the hot gas from the combustion from the combustion chamber into the chamber into the atmosphere to produce atmosphere to produce thrustthrust

Some nozzles can adjust Some nozzles can adjust exit area to achieve the exit area to achieve the best thrust efficiency best thrust efficiency

Vectored thrust for Vectored thrust for maneuverabilitymaneuverability http://www.pr.afrl.af.mil/win/nozzle.jpg

AfterburnersAfterburners

Require high specific thrust operate efficientlyRequire high specific thrust operate efficiently

Lower specific thrust engines ie high bypass Lower specific thrust engines ie high bypass engines will not provideengines will not provide

Trade off of engine efficiency for large short term Trade off of engine efficiency for large short term gains in thrustgains in thrust

SFC:~2 lb/lbf*hSFC:~2 lb/lbf*h

www.enginehistory.org/P%26W/J58/J58_Afterburner.jpg

Necessary Propulsion HardwareNecessary Propulsion Hardware

EngineEngineStarter: takes bleed air from another source and Starter: takes bleed air from another source and transmits power through the engines gearboxtransmits power through the engines gearboxAPU: Auxiliary Power Unit provides electrical APU: Auxiliary Power Unit provides electrical power from aircraft subsystems and provides power from aircraft subsystems and provides bleed air for engine startingbleed air for engine startingCan also be started using a ground cart or Can also be started using a ground cart or explosive cartridge starting methods (B-52)explosive cartridge starting methods (B-52)Fuel tanks, lines, and possibly auxiliary fuel Fuel tanks, lines, and possibly auxiliary fuel pumps to pump from tank to tankpumps to pump from tank to tank