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Propulsion Systems With Controls. http://www.hill.af.mil/museum/info/C-17.jpg. Modern Turbofan Engines. Utilize air bypassing the core to increase engine efficiency Trade off of kinetic energy for greater mass flow as bypass ratio increases
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Propulsion Systems With Controls http://www.hill.af.mil/museum/info/C-17.jpg
Modern Turbofan Engines • Utilize air bypassing the core to increase engine efficiency • Trade off of kinetic energy for greater mass flow as bypass ratio increases • Two general classifications: Low Bypass and High Bypass
Low Bypass Turbofans • Bypass ratios less than 2 (twice as much air around the core as through the core) • Enables much higher speeds (up to M~3), more compact dimensions, and practical usage of afterburners at an expense of fuel efficiency and noise, making these engines suitable for fighters • Typical thrust class for this engine type is 10500-22000lb, with the F119 putting out 35000lb with afterburners • Typical T/W ratios of almost 8:1 • SFC:~.75 lb/lbf*h
High Bypass Ratio Engines • Bypass Ratios 10+:1 (if any larger it is usually more efficient to mount a prop) utilizing large diameter fans • Have much larger thrust classes than low bypass engines:30,000lb to as much as 130,000lb • The 75-100,000lb thrust class has become a popular choice for airliners as only two engines are required • Takes advantage of the Law of Conservation of Momentum to obtain large thrust values at the expense of top speed (larger air mass at a slower velocity) • Average T/W from 5-6 • SFC:~.37 lb/lbf*h
http://www.ae.gatech.edu/people/ptsiotra/Pictures/turbofan.gifhttp://www.ae.gatech.edu/people/ptsiotra/Pictures/turbofan.gif
Engine Controls • Compensators: Electronically limit an engines thrust to match an engine to another in pairs, so that mounting 2 engines on different sides of an aircraft with not create yaw (usually utilize a closed feed back system) • Engines must be individually tested to calibrate compensators and for information for FADEC usage
Engine Controls Continued • FADEC: Full Authority Digital Engine Control (or DEEC Digital Electronic Engine Control) • Controls Fuel Flow rates for precise control of thrust, controls variable stator vanes, monitors engines heath, and starting • Usually small enough to fit on engine (for larger engines) otherwise requires mounting in aircraft fuselage
Inlets • Turbofan engines require airspeeds below M=1 to operate properly • Supersonic inlet designs must incorporate diffusers (normal or oblique shock) to slow supersonic air to subsonic speeds • Engine ducting must feature smooth transitions from inlet shape to the engines inlet shape to prevent turbulent air from entering the engine
Nozzles • The nozzle is used to accelerate the hot gas from the combustion chamber into the atmosphere to produce thrust • Some nozzles can adjust exit area to achieve the best thrust efficiency • Vectored thrust for maneuverability http://www.pr.afrl.af.mil/win/nozzle.jpg
Afterburners • Require high specific thrust operate efficiently • Lower specific thrust engines ie high bypass engines will not provide • Trade off of engine efficiency for large short term gains in thrust • SFC:~2 lb/lbf*h www.enginehistory.org/P%26W/J58/J58_Afterburner.jpg
Necessary Propulsion Hardware • Engine • Starter: takes bleed air from another source and transmits power through the engines gearbox • APU: Auxiliary Power Unit provides electrical power from aircraft subsystems and provides bleed air for engine starting • Can also be started using a ground cart or explosive cartridge starting methods (B-52) • Fuel tanks, lines, and possibly auxiliary fuel pumps to pump from tank to tank
