Propulsion Systems With Controls

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 lb, 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 ,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

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

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

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