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Propulsion SystemsWith Controls
http://www.hill.af.mil/museum/info/C-17.jpg
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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
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Low Bypass Turbofans
Bypass ratios less than 2 (twice as much airaround the core as
through the core)
Enables much higher speeds (up to M~3), morecompact dimensions,
and practical usage of
afterburners at an expense of fuel efficiency andnoise, making
these engines suitable for fighters
Typical thrust class for this engine type is 10500-22000lb, with
the F119 putting out 35000lb with
afterburnersTypical T/W ratios of almost 8:1
SFC:~.75 lb/lbf*h
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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
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http://www.ae.gatech.edu/people/ptsiotra/Pictures/turbofan.gif
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Engine Controls
Compensators: Electronically limit anengines thrust to match an
engine toanother in pairs, so that mounting 2
engines on different sides of an aircraftwith not create yaw
(usually utilize aclosed feed back system)
Engines must be individually tested tocalibrate compensators and
for informationfor FADEC usage
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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, monitorsengines heath,
and starting
Usually small enough to fit on engine (for larger
engines) otherwise requires mounting in aircraft
fuselage
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Inlets
Turbofan engines require airspeeds belowM=1 to operate
properly
Supersonic inlet designs must incorporate
diffusers (normal or oblique shock) to slowsupersonic air to
subsonic speeds
Engine ducting must feature smooth
transitions from inlet shape to the enginesinlet shape to
prevent turbulent air fromentering the engine
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Nozzles
The nozzle is used to
accelerate the hot gas
from the combustion
chamber into the
atmosphere to producethrust
Some nozzles can adjust
exit area to achieve the
best thrust efficiencyVectored thrust for
maneuverabilityhttp://www.pr.afrl.af.mil/win/nozzle.jpg
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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 termgains in
thrust
SFC:~2 lb/lbf*h
www.enginehistory.org/P%26W/J58/J58_Afterburner.jpg
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Necessary Propulsion Hardware
Engine
Starter: takes bleed air from another source andtransmits power
through the engines gearbox
APU: Auxiliary Power Unit provides electricalpower from aircraft
subsystems and providesbleed 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 fuelpumps to pump from tank to tank
