JET ENGINES

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JET ENGINES
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HISTORY

• Sir Isaac Newton (18th century) the first to
  theorize that a rearward channeled explosion
  could propel a machine forward at a great rate of
  speed. This theory was based on his third law of
  motion.
• Frank Whittle (1930) British pilot designed the
  first jet engine and received a patent in 1930.
• Hans Von Ohain (1936) German airplane designer
  who patented his jet engine in 1936.

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HISTORY

• Hans Von Ohain and Frank Whittle are recognized
  as being co-inventors of the jet engine. Each
  worked without knowledge of the others work.
• Hans Von Ohains jet first flew in 1939 while
  Frank Whittles jet flew in 1941

Artists rendering of Frank Whittles prototype
jet engine.
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THE FIRST JET ENGINE
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HEINKEL HE 178 the first jet. (1939)
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GLOSTER E28-39 first British jet.(1941)
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BELL XP-59 first American jet. (1942)
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AVRO CF-100 first Canadian jet. (1950)
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SR-71 BLACKBIRD worlds fastest jet.
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AIRBUS A380
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ROLLS-ROYCE TRENT 900 TURBOFAN
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CAPABLE OF 70,000-80,000lbs THRUST
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TURBINE ENGINES

• Turbine engines are of the same heat engine
  family as the piston engine.
• All heat engines convert fuel (chemical energy)
  into heat energy.
• The basic principles of operation remain the same
  as a piston engine.
• The fuel (air/fuel mixture) is compressed and
  then ignited creating heat energy which is then
  converted to a propulsive force.
• The turbine engine uses a different method of
  compressing the gases than a reciprocating engine.

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FOUR STROKE CYCLE

• A piston engines four stroke cycle
• Intake-air/fuel mixture enters the cylinder
• Compression-the piston compresses the mixture
• Power-the mixture is ignited
• Exhaust-the gases are exhausted
• The power generated by the exploding (expanding
  gases) is converted to rotational force by the
  crankshaft, which is converted to thrust by the
  propeller.

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FOUR STROKE CYCLE

• A turbine engines process compared to the four
  stroke cycle
• Intake-the air enters the inlet
• Compression-the air is compressed by the
  compressor
• Power-the air/fuel mixture is ignited
• Exhaust-the gases are exhausted at the nozzle
• The main differences between the process taking
  place in a piston engine and a turbine engine
  are
• The method of compression. (piston/turbine)
• The point at which the fuel is added.
  (pre-compression for a piston engine, post
  compression for a turbine engine)
• A turbine engine can use a propeller to convert
  the heat energy to thrust or use the direct
  thrust generated by the expanding gases.

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FOUR STROKE CYCLE
INTAKE
COMPRESSION
POWER
EXHAUST
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NEWTONS THIRD LAW

• Both propeller driven and jet engines rely on the
  same principle in order to provide forward
  thrust.
• Newtons third law states For every action there
  is an equal and opposite reaction.
• A propeller converts heat energy created by the
  engine to thrust by accelerating a mass of air.
  This rearward force creates an equal and opposite
  force in the forward direction.
• A jet engine converts heat energy to thrust by
  expelling the expanding gases rearward which
  creates an equal and opposite reaction in the
  forward direction.

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NEWTONS THIRD LAW
Newtons third law as it applies to the
propulsive thrust created by a jet engine. The
force of the airflow from the jet nozzle is
balanced by the equal and opposite reaction of
the engine moving forward.
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THRUST VS HORSEPOWER

• The power output of jet engines is measured in
  thrust and not horsepower.
• One horsepower is defined as 550 foot-pounds of
  work accomplished in one second.
• Power is the product of force and distance over
  an interval of time. P(FD)/T
• With a turboprop engine the distance is the
  revolution of the propeller therefore a
  horsepower value can be derived.
• Although torque and rpm are produced within a jet
  engine by the turbine, the horsepower developed
  is used entirely by the engine itself.
• The definition of power would indicate a jet
  engine on an aircraft with the brakes set
  develops 0 horsepower, although it is obvious a
  propulsive force is being generated.
• This force is measured as thrust in pounds.

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GENERATION OF THRUST

• Thrust is the way we quantify the propulsive
  force developed by a jet engine.
• The way this force is developed is explained by
  Newtons second law A change in motion is
  proportional to the force applied.
• OR A force proportional to the rate of change of
  the velocity is produced whenever a body (or
  mass) is accelerated. FMa
• For a jet engine the equation can be expressed
  by FM(V2-V1)
• Where M is the mass of gases moving through the
  engine.
• V2 is the final velocity.
• V1 is the initial velocity.

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THRUST HORSEPOWER

• Once a jet aircraft is in motion we can start
  considering horsepower.
• Thrust horsepower is the product of thrust and
  airspeed divided by a constant.
  THP(thrustkts)/325
• Horsepower varies with speed when the thrust
  setting remains constant.

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FACTORS AFFECTING THRUST

• Air temperature as temperature decreases,
  density increases, mass flow increases, thrust
  increases. (on a cold day exceeding max. limits
  is an issue)
• Air pressure as pressure decreases, density
  decreases, mass flow decreases, thrust decreases.
• Altitude as altitude increases, temperature
  decreases, pressure decreases.
• The pressure effect is greater than the
  temperature effect so thrust shows an overall
  decrease with altitude.
• The rate of thrust decrease is greater above the
  tropopause. (isothermal layer)

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FACTORS AFFECTING THRUST

• Nozzle velocity as nozzle velocity increases,
  thrust increases up to M1.0
• The nozzle is designed to maintain a velocity
  below M1.0 so variation in nozzle velocity is
  small at higher power settings.
• Airspeed as airspeed increases, thrust decreases
  up to a speed of approx. 300kts. Nozzle
  velocity(V2) remains constant so airspeed(V1)
  directly affects thrust.
• Above this speed ram effect starts to become
  apparent.
• Pressure increases at the inlet, mass flow
  increases, thrust increases.

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TURBOPROP OR TURBOJET

• If a jet engine is capable of creating thrust on
  its own why are their turboprops in the world?
• Turboprop aircraft are more efficient at lower
  speeds and lower altitudes while turbojet engines
  are more efficient at high speeds and high
  altitudes.
• A propeller loses efficiency rapidly at airspeeds
  above 400 kts while the effect of ram air at high
  speed increases a turbojets effectiveness.
• Turboprops have a great advantage at low speeds
  associated with takeoff and climb.
• Turboprop aircraft become more economical to
  operate on shorter routes where the increased
  speed of a turbojet becomes less of a factor.
• Turbofans take advantage of the benefits of both
  propeller and jet engines.

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ENGINE COMPONENTS
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Explain how Bernoullis principle made this
possible.
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AIR INLET DUCTS

• The air inlet duct is actually considered to be
  part of the airframe, not the engine.
• Its importance to the proper operation of the
  engine is undeniable.
• The inlet duct
• guides the airflow into the engine.
• provides minimum resistance to the airstream
  flowing past the aircraft.
• delivers as much pressure as possible to the
  compressor.
• This last point is called ram recovery. Some of
  the pressure created by ram effect is lost at the
  inlet opening due to friction and turbulence.
  This is corrected by using a divergent inlet
  duct.

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DIVERGENT INLET DUCT (DIFFUSER)

• Diffuser a device which reduces the velocity and
  increases the static pressure of a moving fluid.
• The divergent inlet duct reduces the velocity of
  the incoming air mass which increases its static
  pressure.
• The goal of intake design is to obtain total
  pressure recovery.
• All of the lost pressure is regained through the
  inlet design.
• In the case of supersonic aircraft different
  designs must be used.

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CONVERGENT INLET DUCT

• We know that if a subsonic airflow decelerates
  the static pressure will increase. This is
  accomplished by a divergent inlet on subsonic jet
  engines.
• The airflow entering a jet engine at supersonic
  speeds behaves differently.
• Due to the air molecules inability to move faster
  than the speed of sound they start to compress
  together when something moves through the air at
  supersonic speeds.
• If this same supersonic airflow is moved through
  a convergent inlet the airflow will decelerate
  and the static pressure will increase.
• This deceleration occurs as the air molecules are
  incapable of further acceleration (decrease in
  volume) and the airflow is choked off at the
  point of constriction. (normal shockwave forms)
• Essentially a convergent inlet facilitates the
  formation of a normal shockwave within the inlet.
  Airflow behind a normal shockwave is always
  subsonic.

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CONVERGENT INLET DUCT

• These convergent ducts usually incorporate a
  divergent section which will reduce the subsonic
  velocity further and increase static pressure.
• Some supersonic inlets are designed to facilitate
  the formation of an oblique shockwave at the
  mouth of the inlet which smoothes the
  transitioning airflow by decreasing velocity in
  stages.
• A good example of this type of design is the
  conical center diffuser of the SR-71 Blackbird.
  (turboramjet)

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SR-71 BLACKBIRD ENGINE DESIGN
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FAN

• Turbofan jet engines utilize large diameter fan
  sections to accelerate a large air mass similar
  to the operation of a propeller.
• Some of the airflow bypasses the engine core and
  is used for cooling and generating thrust.
• The fan is usually directly connected to the
  compressor.
• The addition of a fan to the front of the
  compressor section marginally increases fuel
  burn, but greatly increases engine thrust.

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FAN
Braces between the blades called mid-span
shrouds are often used to give strength to the
blades and prevent vibration.
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FAN

• ADVANTAGES
• Increased efficiency.
• Increased thrust at low speeds. (takeoff and
  climb)
• DISADVANTAGES
• Large diameter increases risk of FOD.

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SCARY ENGINE HANDLING
A pilot for a Chinese carrier requested
permission and landed at FRA (Frankfurt, Germany)
for an unscheduled refuelling stop. The reason
soon became apparent to the ground crew The
Number 3 engine had been shut down because of
excessive vibration, and because it didn't look
so good. This had apparently been no problem for
the tough guys back in China they took some
sturdy straps and wrapped them around several of
the fan blades and the structures behind, thus
stopping any unwanted wind milling (engine
spinning by itself due to airflow passing through
the blades during flight) and associated
uncomfortable vibration caused by the suboptimal
fan. Note that the straps are seatbelts....how
resourceful! After making the "repairs", off
they went into the wild blue yonder with another
revenue-making flight on only three engines! With
the increased fuel consumption, they got a bit
low on fuel, and just set it down at the closest
airport for a quick refill. That's when the
problems started The Germans, who are kind of
picky about this stuff, inspected the
malfunctioning engine and immediately grounded
the aircraft. (Besides the seatbelts, notice the
appalling condition of the fan blades.) The
airline operator had to send a chunk of money to
get the first engine replaced (took about 10
days). The repair contractor decided to do some
impromptu inspection work on the other engines,
none of which looked all that great either. The
result a total of 3 engines were eventually
changed on this plane before it was permitted to
fly again.
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ENGINE DAMAGE
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ENGINE DAMAGE
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ENGINE DAMAGE
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COMPRESSORS

• The two principal types of compressors are
• Centrifugal- air is forced away from the axis of
  rotation. (perpendicular to axis of rotation)
• Axial- air is forced along the axis of rotation.
  (parallel to axis of rotation)
• Most early jet engine designs used a centrifugal
  compressor as designers were familiar with this
  technology.
• Axial compressors are capable of much higher
  compression ratios and operate more effectively
  at low engine speeds.

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CENTRIFUGAL COMPRESSOR

• The typical centrifugal compressor consists of an
  impeller, a diffuser, and a manifold.
• Prior to reaching the impeller, incoming air
  passes through plenum chambers.
• The plenum chamber reduces the velocity of
  incoming air which increases static pressure and
  feeds the impeller.
• The impeller rotates air at high speed which
  builds up air velocity and forces it outward.
• Air leaving the impeller at high velocity flows
  through the diffuser which reduces air velocity
  and increases static pressure.
• From the diffuser air enters the manifold which
  directs the air into the combustion chamber where
  it is mixed with fuel and ignited.

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EARLY DESIGN JET ENGINE
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COMPENENTS OF A CENTRIFUGAL COMPRESOR
Is the diffuser surrounding the centrifugal
compressor divergent or convergent?
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CENTRIFUGAL COMPRESSOR

• ADVANTAGES
• Simplicity of design.
• Relative low cost.
• Low weight.
• Low starting power requirements.
• DISADVANTAGES
• Low compression ratio.
• Large frontal area.

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AXIAL COMPRESSOR

• A typical axial compressor consists of inlet
  guide vanes, rotor blades, stator vanes, and a
  diffuser.
• Airflow is delivered to the face of the
  compressor through inlet guide vanes which smooth
  the airflow and adjust airflow angle to ensure it
  is within blade angle of attack limits.
• Here the air reaches the first stage rotor blades
  which act like an airfoil to accelerate the air
  to the first stage stator blades.
• The stator blades decelerate the air which
  increases the static pressure.
• Each successive stage increases the static
  pressure with a minimal increase in air velocity.
• A diffuser at the rear of the compressor further
  decreases airflow velocity and increases static
  pressure for delivery into the combustion
  chamber.
• Bleed air for other aircraft systems is removed
  at this point.

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AXIAL COMPRESSOR

• ADVANTAGES
• High compression ratio.
• Small diameter.
• DISADVANTAGES
• Complexity of design.

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COMPRESSOR STALL

• Axial compressor blades are designed to act as
  airfoils.
• The blade is effective as long as its critical
  angle of attack isnt exceeded.
• Any instability of the airflow can cause the
  compressor blades to stall.
• This design makes it necessary to maintain a
  stable airflow through the compressor at all
  times.
• During moments of compressor acceleration,
  deceleration, or interrupted inlet airflow the
  later stages of the compressor may be unable to
  handle the airflow. This can result in air piling
  up at the rear stages. (choking)
• The backflow of air results in compressor stall.
• Compressor stall is recognized by any combination
  of engine surging, intermittent popping sounds,
  or loud bangs.
• Power setting should be immediately reduced to
  avoid engine damage.
• The resultant disruption of airflow and lack of
  cooling may cause combustion chamber and turbine
  damage.

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COMPRESSOR STALL

• There are a number of design techniques employed
  to combat compressor stall
• Compressor bleed valves- at low power settings
  the bleed valves open to relieve excess pressure
  and avoid choking.
• Variable inlet and stator vanes- the initial
  stages of stator vanes automatically adjust their
  angle to match inconsistencies in airflow.
• Dual-Axial compressor (twin-spool)- two
  compressors operate mechanically independent of
  one another. They are each driven by their own
  turbine through integrated drive shafts. The rear
  (high pressure compressor) speed is mechanically
  governed by the engine fuel control. The forward
  (low pressure compressor) is allowed to rotate
  freely and find its own best operating speed. In
  this way the low pressure compressor can match
  itself to the high pressure compressor for given
  conditions.

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DUAL-AXIAL COMPRESSOR (TWIN-SPOOL)
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BURNER SECTION

• The burner section is encircled by a fuel
  manifold which injects high pressure fuel through
  fuel nozzles.
• The fuel is mixed with the compressed air in the
  combustion chamber where it is ignited.
• The resultant expanding hot gases are directed
  through the power turbines.
• Approx. 25 of the compressed air enters the
  combustion chamber while the remaining 75 is
  directed around the chamber in order to cool the
  combustion chamber and then mix with and cool the
  hot burner gases before they reach the turbine.

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TYPES OF COMBUSTION CHAMBERS

• There are three main types of combustion chamber
• Can- individual burner cans (combustion chambers)
  encircle the burner section of the engine. Each
  can is fed with its own fuel nozzle. The external
  mounting provides ease of maintenance.
• Annular- continuous, circular, inner and outer
  shrouds encircle the compressor drive shaft. This
  doughnut style combustion chamber maximizes
  space, provides even cooling, but is difficult to
  maintain.
• Can-annular- individual cans are place side by
  side in an annular chamber. This type combines
  the advantages of both designs. A removable
  shroud provides easy access to the individual
  cans.

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CAN
ANNULAR
CAN-ANNULAR
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CAN-ANNULAR
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POWER TURBINES

• Turbines are all of the axial flow type.
• The turbine consists of a stationary set of vanes
  which guide the airflow onto the turbine wheel.
• This stationary vane assembly is usually called a
  turbine nozzle while the vanes are called turbine
  nozzle guide vanes.
• The turbine rotors (wheels) directly drive the
  compressor through a shaft.
• The turbine also drives any accessories through
  the same shaft.
• In a twin spool engine each turbine drives its
  associated compressor through independent shafts
  one turning inside the other.

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POWER TURBINES

• The turbine blades themselves are subject to
  extremely high operating temperatures.
• As temperatures rise at high thrust settings a
  phenomena called creep exists.
• Creep is the expansion or stretch of the
  individual blades as they are heated.
• Designers must account for this creep, as engine
  clearance tolerances are tight.
• At cool temperatures the blades may even rattle
  and move within their attachment points, as
  operating temperatures are reached the components
  expand and create the tight fit necessary.
• Turbine blades may incorporate a series of tubes
  throughout which cool bypass air is routed in
  order to facilitate cooling.

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POWER TURBINES

• There are three main types of turbine
• Impulse- blades are shaped like buckets and
  operates like a waterwheel. With this type of
  turbine there is no change in pressure or
  velocity at the rotor. The change takes place
  through the guide vanes.
• Reaction- blades are shaped like airfoils and the
  velocity increase and pressure decrease occur at
  the rotor blade.
• Impulse-reaction- combination of both types used
  on most modern aircraft.

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EXHAUST DUCT AND NOZZLE

• The exhaust duct guides and smoothes the airflow
  as it is directed through the nozzle.
• The nozzle is the restricted exit point of the
  exhaust gases.
• The nozzle is of convergent design to further
  accelerate the airflow to increase thrust.
• Nozzle design allows the airflow to accelerate to
  just below Mach 1 to maximize thrust. (subsonic
  aircraft)
• The nozzle area must be exact and is fixed at
  time of manufacture. (some older designs allow
  for trimming of the nozzle maintenance can
  slightly adjust the opening size.)

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AFTERBURNER

• An afterburner is essentially a ramjet engine
  attached to the rear of a turbojet or turbofan.
• The afterburner consists of the afterburner duct,
  spray bars, flame holders, and a variable-area
  exhaust nozzle.
• Afterburners take advantage of the unburned 75
  of compressed air.
• Fuel is added to the exhaust gases, and ignited
  to provide added thrust.
• Afterburner operation greatly increases fuel
  consumption and is only used for short durations.
• Increases fuel consumption by 2-3 times and
  improves thrust by 50-100.
• Used to provide high climb rates to more
  efficient operating altitudes, and for high speed
  sprint capabilities.
• The nozzle must be variable to accommodate the
  increased volume of air during afterburner
  operation.

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VARIABLE DIAMETER NOZZLE
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THRUST VECTORING NOZZLE
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THRUST REVERSERS

• The high speeds and high weights of jet aircraft
  require powerful stopping capability.
• The typical way to take pressure off of aircraft
  brakes and tires is to employ thrust reversers.
• There are two main types of reverser
• Mechanical blockage (clamshell, bucket)- deploys
  mechanical obstructions to the airflow and
  redirects it to apply reverse thrust.
• Aerodynamic blockage (cascade)- used on by-pass
  (turbofan) engines a mechanism located internally
  (inside the by-pass ducts) directs airflow from
  the nacelle to provide reverse thrust.

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MECHANICAL REVERSER(CLAMSHELL)
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AERODYNAMIC REVERSER(CASCADE)
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THRUST REVERSERS

• Thrust reverser requirements
• Able to withstand high temperatures.
• Mechanically strong.
• Streamlined when not in use.
• Fail safe. (in air deployment of thrust reversers
  has had devastating results)

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TYPES OF TURBINE ENGINES

• Turboprop- Efficient at low airspeeds and
  altitude. Added weight and mechanical complexity
  is a disadvantage.
• Turbojet- Efficient at high airspeeds and
  altitude. Relatively poor performance and
  inefficiency in lower altitudes is the main
  disadvantage.
• Turbofan- Combines the positive attributes of
  turboprop and turbojet engines to improve overall
  efficiency. Expensive and have a large inlet
  diameter. (risk of FOD)
• Afterburning turbojet- utilizes combustion of
  unburned airflow to improve thrust capability.
  Used for specific applications (military), as
  efficiency is greatly reduced.

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TURBOPROP
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TURBOJET
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TURBOFAN
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AFTERBURNING TURBOJET
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TURBINE ENGINE INSTRUMENTATION

• Engine pressure ratio indicator (EPR) pronounced
  eeper- shows the ratio of total pressure of
  turbine discharge to total pressure at the
  compressor inlet. The primary instrument used
  when setting thrust.
• Torquemeter (turboprop)- measures the rotational
  force developed by the engine. The torque
  developed is proportional to horsepower and the
  torquemeter is an indicator of SHP. If we add the
  thrust portion of the exhaust gases we get ESHP.
• Tachometer- measures the rotational speed of the
  compressor. For dual-axial compressor engines it
  usually indicates high pressure compressor speed.
  This rpm value is indicated in a percentage. N1
  normally refers to low pressure compressor speed
  while N2 normally refers to high pressure
  compressor speed.

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TURBINE ENGINE INSTRUMENTATION

• Exhaust gas temperature indicator (EGT) or
  Interstage turbine temperature (ITT)- temperature
  must be monitored in order to ensure engine
  component integrity is maintained.
• Fuel flow indicator- indicates the fuel flow in
  pounds per hour.
• Oil pressure
• Oil temperature

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A-320 ENGINE INSTRUMENTS
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AUXILLARY POWER UNIT

• Small turbine engine usually installed in the
  tail of an aircraft used to supply
• Electrical
• Pneumatic
• Power to aircraft systems while on the ground
  with engines shutdown, or in the case of some
  aircraft for the supply of emergency power.
• Pneumatic may be used for environmental systems
  on the ground or for pressurization in an
  emergency.
• Electrical provides power for lights and starting
  on the ground and emergency electricity in the
  air.

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AUXILLARY POWER UNIT