Propulsion Systems With Controls - AOE

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22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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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.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