1. TURBINE ENGINES
Design of a working gas turbine engine had been
under way for years prior to WWII.
The war effort had brought about many advances in
gas turbine technology.
Advantages over reciprocating engines:
1- Increased reliability.
2- longer mean times between overhaul.
3- higher airspeeds.
4- Ease of operation at high altitudes.
5- high power to engine weight ratio.
2. DESIGN AND CONSTRUCTION
Newton’s third law of motion
states that for every action there is
an equal and opposite reaction.
A squid takes sea water into its
body and uses its muscles to add
energy to the water, then expels
the water in the form of a jet. This
action produces a reaction that
propels the squid forward.
Jet propulsion take a quantity of
air and accelerating it through an
orifice or nozzle.
3. DESIGN AND CONSTRUCTION
Hero devised a toy called aeolipile used the reaction principle.
Inflated balloon is anther example of Newton’s reaction principle.
Dr. Sanford Moss submitted his master thesis on gas turbine in
1900.
He uses his concepts to develop turbo-supercharger.
Frank Whittle of England uses Dr. Moss research to develop the
first successful turbojet in 1930.
The engine completed its first flight in 1941, producing about
1000 lb at a speed over 400 miles/hr.
Hans Von Ohain, German engineer design and built a jet engine
that produced 1100 lb and made its first flight in 1939.
United states build its first jet engine which produces 1600 lb and
made its first flight in 1942.
5. JET PROPULSION TODAY
The majority of commercial aircraft utilize some
form of jet propulsion.
Development of military and commercial aircraft
that moves faster than the speed of sound.
Extremely popular for use on business jets.
6. TYPES OF JET PROPULSION
1- ROCKET.
2- RAMJET.
3- PULSEJET.
4- GAS TURBINE ENGINE.
7. ROCKET
Non air breathing engine that carries its own fuel as well
as the oxygen needed for the fuel burn.
Types of rockets:
1- solid-propellant rockets.
2- liquid-propellant rockets.
Solid- propellant rockets use a solid fuel that is mixed
with an oxidizer and shaped into a specific shape that
promotes an optimum burning rate.
Produce extremely high velocity, and used to propel some
weapons and to provide additional thrust for take off of
heavy loaded aircraft.
Liquid-fuel rocket uses fuel and oxidizing agent such as
liquid oxygen carried in tanks aboard the rocket.
When mixed the reaction is so violent that produce a
tremendous amount of heat.
9. RAMJET
Ramjet engine is an athodyd, or
aero-thermodynamic-duct.
Air breathing engine with no
moving parts.
Must be moved forward at a high
velocity before it can produce
thrust.
Limited in there use, military
weapons delivery systems.
10. PULSEJET
Similar to ramjet except that the
air intake duct is equipped with a
series of shutter valves.
Shutter valves are spring loaded
to the open position.
Air is drawn and mixed with fuel
in the combustion chamber, as
pressure build up the shutter
valves closes causing the air to
expand backward.
More useful than ramjet because
it will produce thrust prior to
being accelerated to a high speed.
11. GAS TURBINE ENGINE
The most practical form of jet engine in use today.
The standard on nearly all transport and military
aircraft.
Types of gas turbine engines:
1- Turbojet.
2- Turboprop.
3- Turboshaft.
4- Turbofan.
12. TURBOJET ENGINES
Straight forward operating principle.
Air enters through the air intake, compressed by the
compressor, fuel is added and burned in the
combustion chamber, heat causes the compressed
air to expand rearward, passes through the turbine
and spins it, which drives the compressor, and the
air then exit the engine at a much higher velocity
than the incoming air.
The difference between the entering air and the
exiting gases that produces the thrust.
EPR is the ratio of the turbine discharge pressure to
engine inlet air pressure.( EPR probes)
15. TURBOPROP ENGINES
Turboprop engine is a gas turbine engine that
delivers power to a propeller.
Power produced by a turboprop is delivered to a
reduction gear system that spins a propeller.
Used in business and commuter type aircraft
because of the combination of jet power and
propeller efficiency at speeds between 300 and 400
mph.
Provide the best specific fuel consumption of any
gas turbine engine.
17. TURBOSHAFT ENGINES
Turboshaft engine is a gas turbine
engine that delivers power to a shaft
that can drive something else.
Most of the energy produced by the
expanding gases is used to drive a
turbine.
Helicopters, auxiliary power units,
electric generators, and surface
transportation systems use turboshaft
engines.
Turboshaft engine power is measured
in shaft horsepower.
19. TURBOFAN ENGINES
Consist of a multi-bladed ducted propeller
driven by a gas turbine engine.
Provide a compromise between the best
features of the turbojet and the turboprop.
Have turbojet-type cruise speed capability, yet
retain some of the short-field takeoff capability
of a turboprop.
21. TURBOFAN ENGINES
Forward-fan engines have the fan mounted in
the front of the compressor.
Aft-fan mounted engines have the fan mounted
to the turbine section.
Inlet air is divided into two separate streams.
( engine core air, bypass air).
23. TURBOFAN ENGINES
Thrust ratio, bypass ratio, and fan pressure ratio are
the terms you should be familiar with.
Thrust ratio is the comparison of the thrust produced
by the fan to the thrust produced by the engine core
exhaust.
Bypass ratio is the ratio of incoming air to that
bypasses the core to the amount of air that passes
through the engine core.
Fan pressure ratio is the ratio of air pressure leaving
the fan to the air pressure entering the fan.
24. TURBOFAN ENGINES
Turbofan engines are divided into three classification
based on bypass ratio:
1- low bypass (1:1).
2- medium bypass (2:1 or 3:1).
3- high bypass (4:1 or greater).
Low bypass engine, bypass air could be ducted directly
overboard through a short fan duct or in a ducted fan
where the bypass air is ducted along the entire length of
the engine.
Full fan ducts reduce aerodynamic drag and noise
emissions.
Both use a converging discharge nozzle that increases
velocity and produce thrust.
26. TURBOFAN ENGINES
Medium bypass engines have thrust ratio similar
to their bypass ratios.
Fan diameter determines a fan’s bypass ratio and
thrust ratio.
High bypass ratio engines use the largest
diameter fan of any of the bypass engines.
Offer higher propulsive efficiencies and better
fuel economy of low and medium bypass
engines.
28. TURBOFAN ENGINES
Fan pressure ratio is the ratio of air pressure leaving the
fan to the air pressure entering the fan.
Ranging from 1.5:1 on a low bypass engines up to 7:1 on
some high bypass engines.
Most high bypass ratio engines use high aspect ratio
blades.
Aspect ratio is the ratio of a blade’s length to its width,
or cord.
Low aspect ratio blades are desirable because of their
resistance to foreign object damage, especially bird
strikes.
29. UNDUCTED FAN ENGINES
Ultra high bypass (UHB) propfan and unducted
fan engine (UDF) are recently developed engines
with higher efficiencies than any engine in use.
Better fuel economy than high bypass turbofan
engines.
Achieve 30:1 bypass ratio by using single or dual
propellers made of composite blades that are 12
to 15 feet in diameter.
33. AIR INLET DUCTS
Normally considered to be a part of the airframe rather
than the engine.
Functions of air inlet duct:
1- (ram or pressure recovery) to recover as much of the
total pressure of the free airstream as possible and
deliver it to the compressor.
2- Shaped to raise the air pressure above atmospheric
pressure .
3- Provide a uniform supply of air to the compressor so
it can operate efficiently.
4- Must cause as little drag as possible.
Ram effect results from forward movement which
causes the air to pile up in the air inlet.
35. AIR INLET DUCTS
Inlet ducts are mounted :
1- On the engine.
2- In the wing.
3- On the fuselage.
Engine mounted inlets:
Used on several large commercial and military
aircraft.
The air inlet is located directly in front of the
compressor and is mounted to the engine.
37. AIR INLET DUCTS
Wing mounted inlets:
On some aircraft where the engines are mounted inside the
wings.
Typically wing mounted inlets are positioned near the wing root.
38. AIR INLET DUCTS
Fuselage-mounted inlets:
Engines mounted inside a fuselage typically use air
inlet ducts located near the front of the fuselage.
Used on many old military and modern supersonic
aircraft.
Using this type allow the manufacturer to build more
aerodynamic aircraft.
Some military aircraft use air inlet ducts mounted on
the sides of the fuselage.
40. AIR INLET DUCTS
Types of air inlet ducts:
1- Subsonic inlets.
2- Supersonic inlets.
3- Bellmouth inlets.
41.
Subsonic inlets
Fixed geometry duct whose diameter progressively
increases from front to back.
In a divergent duct as the intake air spreads out,
the velocity of the air decreases and the pressure
increases.
The increased pressure increase the engine
efficiency when it reach its designed cruise speed.
( optimum aerodynamic efficiency, best fuel
economy).
Inlet, compressor, combustor, turbine, and exhaust
duct are designed to match each other at this speed
as a unit.
43. Supersonic inlets
Fixed or variable geometry whose diameter
progressively decreases, then increases from
front to back.
Convergent-divergent duct used to slow the
incoming air to subsonic speed before it reaches
the compressor.
Movable plug or throat changes the duct
geometry to accommodate a wide range of
flight speeds.
46. Bellmouth inlets
Convergent profile that is designed for
obtaining very high aerodynamic efficiency
when stationary or in slow flight.
Used on helicopters, some slow moving
aircraft, and ground run stands.
Short in length and has rounded shoulders
offering very little air resistance.
48. FOREIGN OBJECT DAMAGE
(F.O.D)
Prevention of F.O.D. is a top priority among
turbine engine operators.
Methods of FOD prevention:
1- Inlet screen over an engine inlet duct.
2- Sand or ice separators.
3- Vortex dissipater, vortex destroyer, blowaway jet.
49. FOREIGN OBJECT DAMAGE
(F.O.D)
The use of inlet screen is common on many
rotorcraft and turboprop engines and on engine
installed in test stand.
Inlet screen is not used on high mass airflow
engines.
Sand or ice separator consists of an air intake
with at least one venturi and a series of sharp
bends.
52. VORTEX DISSIPATER
Some engines form a vortex between the
ground and the ground and the inlet during
ground operations.
This vortex can lift water and debris or small
hardware and direct it to the engine.
Vortex dissipater routes high pressure bleed air
to a discharge nozzle between the ground the
air inlet to prevent vortex from developing.
Landing gear switch arms the dissipater when
ever the aircraft is on ground.
54. COMPRESSOR SECTION
The more air that is forced into an engine, the
more thrust the engine can produce.
Modern compressors must increase the intake
air pressure 20 to 30 times above the ambient
air pressure and move the air at a velocity of
400 to 500 feet per second.
Compressor pressure ratio is the ratio of the
compressor discharge static pressure to the inlet
air static pressure.
55. COMPRESSOR SECTION
Functions of compressors:
1- Support the combustion and provide the air
necessary to produce thrust.
2- Supplies bleed air :
a- to cool the hot section.
b- heated air for anti-icing.
c- cabin pressurization and air conditioning.
d- fuel system deicing.
e- pneumatic engine starting.
56. COMPRESSOR SECTION
TYPES OF COMPRESSORS
1- CENTRIFUGAL FLOW COMPRESSOR.
2- AXIAL FLOW COMPRESSOR.
Each is named according to the direction the air
flows through the compressor.
57. CENTRIFUGAL FLOW
COMPRESSOR
Some times called radial outflow compressor.
Earliest compressor design and still in use in
some small engines and APU’s.
Consist of :
1- impeller or rotor.
2- diffuser.
3- manifold.
59. CENTRIFUGAL FLOW
COMPRESSOR
Impeller or rotor:
Consist of forged disc with integral blades, fastened by
a splined coupling to a common power shaft.
The function of the impeller is to take the air in and
accelerate it outward by centrifugal force.
Single stage compressors have only one impeller.
Two stage compressors have two impellers.
Double-sided or double entry compressors have two
impellers mounted back to back.
63. CENTRIFUGAL FLOW
COMPRESSOR
The use of more than two stages in a compressor is
impractical:
1- Energy lost when the air flow slows down as it
passes from one impeller to the next.
2- The added weight from each impeller requires more
energy from the engine to drive the compressor.
Double sided impeller allows a higher mass air flow
than single impeller compressor. But the ducting to get
the air from one side of the impeller to the other is
complicated.
64. CENTRIFUGAL FLOW
COMPRESSOR
Diffuser, a divergent
duct where the air
loses its velocity and
increases its
pressure.
In a divergent duct,
air spreads out,
slows down and
increases in static
pressure.
65. CENTRIFUGAL FLOW
COMPRESSOR
Compressor manifold distributes the air in a smooth
flow to the combustion section.
The manifold has one outlet for each combustion
chamber so the air is evenly divided.
Outlet ducts is an elbow mounted to the outlet ports to
act as air duct.
Outlet ducts change the radial direction of the air to an
axial direction.
Turning vanes or cascade vanes help changing the
direction of the air with minimum energy loses.
67. CENTRIFUGAL FLOW
COMPRESSOR
Advantages of centrifugal compressor:
1- Simplicity of manufacture.
2- Relatively low cost, low weight, low starting power
requirements.
3- Operating efficiency over a wide range of rotational
speeds.
4- Accelerate air rapidly and immediately deliver it to
the diffuser.
5- Tip distance may reach 1.3 mach with out air flow
separation.
6- High pressure rise per stage.
68. CENTRIFUGAL FLOW
COMPRESSOR
Disadvantages of centrifugal compressor:
1- large frontal area, which mean increased
aerodynamic drag.
2- limited number of stages restrict its uses to
smaller and less powerful engines.
69. Principles of operation
The impeller is rotated at high speed by the turbine and
air is continuously induced into the centre of the
impeller.
Centrifugal action causes it to flow radially outwards
along the vanes to the impeller tip, thus accelerating
the air and also causing a rise in pressure to occur.
The engine intake duct may contain vanes that provide
an initial swirl to the air entering the compressor .
The air, on leaving the impeller, passes into the
diffuser section where the passages form divergent
nozzles that convert most of the kinetic energy into
pressure energy.
70. CENTRIFUGAL FLOW
COMPRESSOR
To maximize the airflow and pressure rise through the
compressor requires the impeller to be rotated at high
speed, therefore impellers are designed to operate at tip
speeds of up to 1,600 ft. per sec.
By operating at such high tip speeds the air velocity
from the impeller is increased so that greater energy is
available for conversion to pressure.
To maintain the efficiency of the compressor, it is
necessary to prevent excessive air leakage between the
impeller and the casing; this is achieved by keeping
their clearances as small as possible
71. AXIAL FLOW COMPRESSOR
Consist of :
1- ROTOR.
2- STATOR.
The rotor consist of rows of blades fixed on a rotating spindle.
The angle and airfoil contour forces the air backward as a
propeller.
The stator vanes are arranged infixed rows between the rows of
rotor blades and act as a diffuser at each stage.
Stators decrease the air velocity and raise the pressure.
Each pressure stage consist of one row of blades and one row of
vanes.
73. AXIAL FLOW COMPRESSOR
Single stage in an Axial compressor is capable of
rising the pressure ratio of only 1.25 : 1.
High compressor pressure ratio is obtained by adding
more stages.
Unlike centrifugal compressors, axial flow
compressors raise the air pressure rather than the
velocity.
The rotor of each stage rise the velocity of the air while
the stator vanes diffuse the air, slowing it and increase
the pressure.
The overall result is increased air pressure and
relatively constant velocity.
75. AXIAL FLOW COMPRESSOR
The space between the rotor shaft and the stator
casing gradually decreases from front to back.
This shape is necessary to maintain a constant
air velocity as air density increases.
The case of most axial compressors is
horizontally divided into two halves.
Bleed air ports are provided on the comp. case
for ancillary functions.
77. AXIAL FLOW COMPRESSOR
Disadvantage of axial compressor:
1- High weight
2- High starting power requirements.
3- Low pressure rise per stage.
4- Expensive and difficult to manufacture.
Advantages over radial flow compressors:
1- High ram efficiency.
2- The ability to obtain higher compressor pressure
ratio.
3- Reduced aerodynamic drag because of small frontal
area.
78. COMPRESSOR ROTOR BLADES
Aerofoil cross-section
with a varying angle of
incidence or twist blades.
The twist compensates for
the blade velocity
variation between the tip
and the root.
Axial flow compressors
typically have 10 to 18
compression stages.
79. COMPRESSOR ROTOR BLADES
The roots of the blades often fits loosely into the rotor, to
allow for easy assembly and vibration damping.
As the blades rotate the centrifugal force keeps the blades in
their correct position.
Bulb, Fir tree, or dovetail are the design of rotor blades roots.
The blades is secured in their position by using a pin and lock
tab or locker.
Some long fan blades have a mid-span shroud that helps
support the blades, making them more resistant to the bending
force created by airstream.
Shingling happen when the mating surfaces on a mid-span
shroud become excessively worn and the shrouds overlap.
81. COMPRESSOR ROTOR BLADES
Flat machine tip blade is cut off square at the tip.
Profile tip blade have a reduced thickness at the tips.
Profiling a compressor blade increases its natural vibration
frequency, which reduce the blade vibration tendency.
Thin trailing edge of profile tipped blades causes a vortex
which increases air velocity and prevent air from spilling back
over the blade tip.
Tight clearance around the blade tips of some newer engine is
accomplished by using a shroud strip of abradable material.
Localized increase in blade camber, both at blade tip and root
increases compressor efficiency.
The increased blade camber overcome the friction caused by
the boundary layer of air near the compressor case. (end bend).
83. COMPRESSOR STATOR VANES
Stator vanes is stationary blades located between
rows of rotating blades.
Act as diffusers for the air coming off the rotor
decreasing its velocity, increasing pressure, prevent
swirling, and direct the flow of air to the next stage.
Stator vanes are made of steel, nickel, and titanium.
Secured directly to the compressor casing or to a
stator vane retaining ring.
Stator vanes are often shrouded at their tips to
minimize vibration.
85. COMPRESSOR STATOR VANES
Inlet guide vanes are a set of stator vanes immediately
in front of the first stage rotor blades.
Inlet guide vanes direct the airflow into the first stage
rotor blades at the best angle, to improve the
aerodynamics of the compressor by reducing the drag
on the 1st rotor blades.
To maintain proper airflow through the engine,
variable IGV’s and several stator vanes are used on
some high compressor pressure ratio engines.
The outlet vane assembly is the last set of vanes that
straighten the air flow and eliminate any swirling
motion or turbulence.
87. MULTIPLE-SPOOL COMPRESSORS
Single spool compressor has only one
compressor unit connected to the turbine.
Drawback of single spool compressors:
1- rear stages operate at a fraction of their capacity,
while the forward stages are overloaded.
2- does not respond quickly to sudden control input
changes.
Single-spool compressors are relatively
simple and inexpensive.
89. MULTIPLE-SPOOL COMPRESSORS
Single spool compressor Drawbacks were overcome by
splitting the compressor into two or three sections.
Each compressor is connected to its turbine by shafts that run
coaxially, one within the other.
Dual-spool, twin-spool compressors has two compressors
connected to two turbines.
Front section is called low pressure, low speed, or N1
compressor. Driven by 2 stage low pressure turbine (rear
turbine).
Second compressor is called high pressure, high speed, or N2
compressor. Driven by single stage high pressure turbine
(front turbine).
The low pressure compressor is driven by the high pressure
turbine by a shaft that rotate inside the high pressure
compressor shaft.
91. MULTIPLE-SPOOL COMPRESSORS
Since the spools are not connected together, each is free to
seek its own best operating speed.
High pressure compressor speed is relatively constant.
Low pressure compressor speeds up or slows down with
changes in the inlet sir flow caused by flight condition.
N1 increases at high altitude and decreases at low altitude to
supply the high pressure compressor with constant air
pressure and mass flow for each power setting.
Triple-spool compressor turbo-fan engine has three
compressors connected to three turbines.
The fan, low pressure, or N1 compressor, the next in line is
called intermediate or N2 compressor, and the inner most is
called high pressure or N3 compressor.
93. COMPRESSOR STALL
Compressor blades are airfoils, so its subjected to the
same aerodynamic principles as aircraft wings.
Compressor blade has an angle of attack which is : an
acute angle between the chord line and the relative
wind.
The angle of attack of a compressor blade is the
result of inlet air velocity and the compressor’s
rotational velocity.( vector ) Quantity to the
approaching inlet air.
Compressor stall is an imbalance between the two
vector quantities, inlet velocity and compressor
rotational speed.
94. COMPRESSOR STALL
Compressor stall occur when the compressor
blade’s angle of attack exceeds the critical
angle of attack.
Smooth airflow is interrupted and turbulence
is created with pressure fluctuations.
During stall airflow in the compressor slow
down and stagnate sometimes reverse
direction.
Heard as pulsating or fluttering sound in its
mildest form to a loud explosion in its most
developed state.
95. COMPRESSOR STALL
Cockpit indications for compressor stall:
1- fluctuations in rpm.
2- increase in exhaust gas temperature.
Transient stall are mild and not harmful to the
engine, and often correct themselves easily.
Sever or hung stall can significantly impair engine
performance, cause loss of power and can damage
the engine.
Reducing the angle of attack on the rotor blades is
the only way to overcome a stalled condition.
96. COMPRESSOR STALL
Methods of preventing compressor stall:
1- Variable inlet guide vanes and stator vanes.
2- Air-bleed valves.
Reasons of compressor stall:
1-When A/C flies in sever turbulence or performs
abrupt flight maneuvers.
2- Excessive fuel flow caused by sudden engine
acceleration.
3- Contamination or damaged compressor blades,
stator vanes or turbine components. (FOD)
98. COMBINATION COMPRESSORS
Axial flow-centrifugal flow compressors were developed to
combine the best features of centrifugal and axial compressors.
Currently used in some smaller engines installed on business
jets and helicopters.
99. COMPRESSOR AIR BLEED
Compressor supplies high pressure, high
temperature air for various secondary functions
such as:
Cabin pressurization.
2. Heating.
3. Cooling.
4. Deicing.
5. Anti-icing.
6. Pneumatic engine starting.
1.
100. COMPRESSOR AIR BLEED
Bleed air or customer bleed air is tapped from the
compressor through bleed ports at various stages.
Bleed port is a small opening adjacent to the
compressor stage selected for bleed air supply.
The required air pressure and temperature determine
the compressor stage to bleed air from.
The air bled from the last stage often need cooling
because the air temperature would be very high
because of compression. (650ْ F).
Bleeding air dose cause a small drop in engine
power, power loss can be detected by observing EPR,
and EGT.
102. DIFFUSER
The divergent shape of a
diffuser slows compressor
discharge while at the same
time increase its pressure to
the highest value in the
engine.
Air speed must be slowed
to support combustion.
Diffuser is a separate
section bolted to the rear of
the compressor ahead of the
combustion section.
103. COMBUSTION SECTION
Located directly between the compressor diffuser
and turbine section.
Basic components of combustion section:
1- one or more combustion chambers (combustors).
2- fuel injection system.
3- ignition source.
4- fuel drainage system.
104. COMBUSTION SECTION
Combustion chamber or combustors is where
the fuel and air are mixed and burned.
Consist of an outer casing with a perforated
inner liner.
Perforation are various shapes and sizes that
effect the flame propagation within the liner.
105. COMBUSTION SECTION
Fuel injection system meters the right amount
of fuel through the fuel nozzles.
Fuel nozzles are located in the combustion
chamber case or compressor outlet elbow.
Fuel is sprayed in a finely atomized spray into
the liner.
The finer the spray the more rapid and
efficient the combustion process.
106. COMBUSTION SECTION
High energy capacitor discharge system is
typically used as ignition source for turbine
engine.
Ignition system produces 60 to 100 sparks per
minute.
A ball of fire results at the igniter electrodes.
Some systems can shoot sparks several inches.
Care must be taken to avoid lethal shock
during maintenance.
107. COMBUSTION SECTION
Unburned fuel is drained out after engine shut
down.
Draining the unburned fuel eliminates engine
fire after shutdown, and reduces the possibility
of exceeding tail pipe or turbine inlet
temperature.
Helps to prevent gum deposits in the fuel
manifold and the combustion chamber.
108. COMBUSTION SECTION
To accomplish the task of burning the fuel air
mixture efficiently the C.C must:
1- Mix fuel and air effectively in the best ratio
for good combustion.
2- Burn the mixture as efficiently as possible.
3- Cool the hot combustion gases to a
temperature the turbine blades can tolerate.
4- Distribute hot gases evenly to the turbine
section.
109. COMBUSTION SECTION
Air flow through the combustor is divided into
primary and secondary paths.
25 to 35 % of the incoming air is primary.
65 to 75 % of the incoming air is secondary.
Primary or combustion air is directed inside the liner,
passing through a set of swirl vanes which give the
air a radial motion.
As air is swirled the speed is reduced to about five to
six feet per second.
Its important to slow the air to prevent flameout.
110. COMBUSTION SECTION
Radial motion generate a vortex in the flame area which
properly mix the fuel and air.
The combustion process is completed in the first third of
a combustor.
The secondary air flow at high speed (several hundred
feet per sec.) around the combustor’s periphery.
Secondary air forms a cooling blanket on both sides of
the liner and centers the combustion flames.
Some air enters the combustors through the perforations
to ensure the burning of any remaining fuel.
Secondary air mix with the combustion gases to provide
an even distribution of energy to the turbine nozzle.
112. COMBUSTION SECTION
Types of combustion chambers:
1- Multiple-can type.
2- Annular type and reverse flow annular type.
3- Can-annular type
113. MULTIPLE-CAN TYPE
Consist of a series of individual combustor cans which
acts as individual burner units.
Well suited to centrifugal compressor engines.
Each Can has a case and a perforated stainless steel liner.
Inner liner is heat resistant and easily removed for
inspection.
Each Can has a large curvature to provide high resistance
to warpage.
Tow igniter plugs in two cans start the combustion, then
the flame is traveled to the other cans by flame
propagation tubes (interconnectors).
Each flame propagation tubes is a small tube surrounded
with larger tube or jacket.
116. ANNULAR TYPE
Commonly used on small and large engines.
The most efficient for thermal efficiency, weight, and
physical size.
Consist of a housing and a perforated inner liner or
basket.
The liner is single unit that encircle the turbine shaft.
An annular combustor with two baskets is known as a
double annular combustion chamber.
Two igniters are used to ignite the fuel/air mixture.
118. ANNULAR TYPE
Air flow enters at the front and is discharged at the rear
with primary and secondary airflow.
Must be removed as one unit for repair or replacement.
Reverse flow combustors are designed so the airflow can
reverse direction.
The combustion gases enters from the rear and flowing in
the opposite direction of the normal airflow through the
engine.
The turbine wheels are inside the combustor area, which
allow for a shorter and lighter engine.
Compressor discharge air is preheated as it passes around
the combustion chamber.
Lighter weight and air preheat make up for the losses
caused by the reversing of the direction of the air.
120. CAN-ANNULAR TYPE
Combination of the multiple-can and annular type
combustors.
Consist of a casing that encircles multiple cans (liners)
assembled radially around the engine axis.
A fuel nozzle cluster is attached at the forward end of
each burner can.
Pre-swirl vanes are placed around each fuel nozzle.
(through fuel mixing and slow the air).
Tow igniter plugs initiate the combustion and
propagation tubes connect the liners.
Each can and its liner removed individually for
maintenance.
Combine the ease of overhaul and testing of multiple-can
combustors with the compactness of annular combustors.
122. FLAME OUT
High air flow rate or excessively slow airflow can
extinguish the combustion flame.
Flameout is uncommon in modern engine but if the
correct set of circumstances can cause engine die out.
Turbulent weather, high altitude, slow acceleration, and
high speed maneuvers can induce a flameout.
Lean die-out occurs at high altitude where low engine
speeds and low fuel pressure form a weak flame that can
die out in normal airflow.
Rich blow-out occurs during rapid engine acceleration
when an overly rich mixture causes the fuel temperature
to drop below the combustion temperature or when there
is insufficient airflow to support combustion.
123. TURBINE SECTION
Transforms a portion of the kinetic energy in
the hot exhaust gases into mechanical energy
to drive the compressor and accessories.
In a turbojet engine the turbine absorbs
approximately 60 to 80 % of the total pressure
energy from exhaust gases.
Consist of:
1- case. 2- stator. 3- shroud. 4- rotor.
125. TURBINE SECTION
TURBINE CASING
Encloses the turbine rotor and stator assembly,
support the stator elements.
Has flanges on both ends that provide a means
of attaching the turbine section to the
combustion section and the exhaust assembly.
126. TURBINE SECTION
TURBINE STATOR
Stator element, turbine nozzle, turbine guide vanes, and nozzle
diaphragm.
Located directly aft of the combustion section and
immediately forward of the turbine wheel.
Exposed to the highest temperatures in a gas turbine engine.
Function: To collect the high energy airflow from the
combustors and direct the flow to strike the turbine rotor at the
appropriate angle.
The stator vanes form a converging nozzles which convert
some of the pressure energy to velocity energy.
The velocity energy of the exhaust gases is converted to
mechanical energy by the rotor blades.
127. TURBINE SECTION
TURBINE SHROUD
Turbine nozzle assembly consist of an inner and outer shroud
that retains and surround the nozzle vanes.
The vanes are assembled between the inner and outer shroud
in deferent methods.
The nozzle vanes must be constructed to allow for thermal
expansion, to prevent distortion or warping of the nozzle
assembly.
Installing the vanes loosely in the inner and outer shrouds and
encase them in an inner and outer support rings allow thermal
expansion of the vanes.
Rigidly weld or rivet the vanes into the inner and outer
shrouds which are cut into segments that have gaps between
them allow for expansion.
129. TURBINE SECTION
TURBINE ROTOR
Consist of a shaft and a turbine rotor, or wheel.
Turbine wheel is a dynamically balanced unit
consisting of blades attached to a rotating disk.
The disk is the anchoring component for the turbine
blades and bolted or welded to the shaft.
The shaft rotates in bearing that are lubricated by oil
between the outer race and the housing to reduce
vibration and allows for a slight misalignment in the
shaft.
130. TURBINE SECTION
TURBINE ROTOR
The high velocity gases pass through the
turbine nozzle to rotate the turbine wheel.
Many engines use multiple turbine stages to
absorb sufficient energy to drive the
compressor.
The turbine is exposed to high rotational speed
and elevated operating temperature stress.
This stress could lead to turbine bleed growth
or creep.
132. TURBINE SECTION
TURBINE BLADES
Airfoil shaped designed to extract the maximum
amount of energy from the hot gases.
Blades are either forged or cast.
Steel forged or cast nickel-based alloys.
Development of reinforced ceramic holds promise.
Blades fit loosely into turbine disk when cold, and
expand to fit tightly when hot.
Fir tree slots is the most commonly used method for
attaching turbine blades.
The blade may be retained in its groove by peening,
welding, rivets, or lock tabs.
135. TURBINE SECTION
IMPULSE TURBINE BLADES
The total pressure drop across each stage
occurs in the fixed nozzle guide vanes which,
because of their convergent shape, increase the
gas velocity whilst reducing the pressure.
Turbine blades absorb the force required to
change the direction of airflow and change it
to rotary motion.
137. TURBINE SECTION
REACTION TURBINE BLADES
Turning force is produced based on an
aerodynamic action.
The turbine blades form a series of converging
duct that increase gas velocity and reduce
pressure.
Reduced pressure produces a lifting force that
rotate the turbine wheel.
139. TURBINE SECTION
IMPULSE REACTION TURBINE BLADES
Most modern engines uses impulse-reaction
turbine blades.
Evenly distribute the workload along the
length of the blade.
The blade base is impulse shaped while the
blade tip is reaction shaped.
Creates a uniform velocity and pressure drop
across the entire blade length.
141. TURBINE SECTION
TURBINE BLADES
Can be open or shrouded at their tips.
Open ended are used on high speed turbines, shrouded ended
are used on slower rotational speed turbines.
The end of each blade has a shroud attached to its end, once
installed the shrouds contact each other and provide support.
The shroud reduces the vibration and prevent the air from
escaping over the blades tips.
The added weight cause the turbine blades to be more
susceptible to blade growth.
A knife edge seal is machined around the outside of the shroud
which reduces air losses at the blade tip.
143. TURBINE SECTION
COOLING
The most limiting factor in running a gas turbine
engine is the temperature.
The higher the temperature raises, the more power or
thrust an engine can produce.
The effectiveness of a turbine engine’s cooling
system plays a big role in engine performance.
Cooling systems allow the turbine to operate 600 to
800 ْ F above the temperature limits of their metal
alloys.
Engine bleed air is used to cool the components in the
turbine section.
144. TURBINE SECTION
COOLING
Turbine disk absorb heat from the hot gases passing around
their rim and the heat conducted from the turbine blades.
Cooling air is directed over each side of the disk.
Convection cooling or film cooling is the type of cooling used
to cool turbine blades and vane by directing compressor bleed
air through the hollow blades and out through holes in the tip,
leading edge, and trailing edge.
Some vanes are constructed of a porous high temp material,
bleed air is ducted into the vane and exits through the porous
material (transpiration cooling).
The turbine vane shrouds may also be perforated with cooling
holes.
148. EXHAUST SECTION
Exhaust section determine to some extent the
amount of thrust developed.
The size and shape of exhaust section affect:
1- Turbine inlet temperature.
2- the mass air flow through the engine.
3- The velocity and pressure of the exhaust jet.
Exhaust section extend from the rear of the
turbine section to the point where the exhaust
gases leave the engine.
150. EXHAUST SECTION
EXHAUST CONE
Consist of:
1- Outer duct or shell. 2- Inner cone or tail cone.
3- Hollow struts.
4- Tie rods.
The outer duct is made of stainless steel and attached to the
rear flange of turbine section.
Purpose of the tail cone is to channel and collect turbine
discharge gases into a single jet.
The outer duct and the inner cone form a divergent duct, so the
air pressure increases and velocity decreases.
Hollow struts support the inner cone and help straighten the
swirling exhaust gases.
The tie rods assist the struts in centering the inner cone within
the outer duct.
152. EXHAUST SECTION
TAIL PIPE
An extension of the exhaust section that directs
exhaust gases safely from the exhaust cone to the
nozzle.
Tail pipe cause heat and friction losses that causes
drop in exhaust gas velocity and thrust.
Used with engines that are installed within the
fuselage to protect the surrounding airframe.
On engine that require no tailpipe, the nozzle is
mounted directly to the exhaust cone assembly.
153. EXHAUST SECTION
EXHAUST NOZZLE
Provides the exhaust gases with the final boost in
velocity.
Converging design and the converging-diverging
design used on aircraft.
Converging design produces a venturi that accelerates
the exhaust gases and increases engine thrust.
Converging-diverging diameter decrease then
increase from front to back which increase the
velocity of exhaust gases above the speed of sound.
154. EXHAUST SECTION
The flow of cool and hot gases in a ducted low
by pass turbofan engine combined in a mixer
unit.
High bypass turbofan engines exhaust the two
streams separately through two sets of nozzles
arranged coaxially around the exhaust nozzle.
On some high pass engines a common or
integrated nozzle is used to mix the hot and
cold gases prior to their ejection.
Exhaust nozzle opening can be fixed or
variable geometry.
156. AFTERBURNERS
Used to accelerate the exhaust gases to increase
thrust.
Installed after the turbine and in front of exhaust
nozzle.
Consist of fuel manifold, ignition source and flame
holder.
The gases in the tailpipe sill contain a large quantity
of oxygen.
Fuel manifold consist of fuel nozzle or spray bars
inject fuel into the tailpipe.
158. THRUST REVERSERS
The brakes are unable to slow the A/C
adequately during landing.
Brake wear would be prohibitive and heat
buildup could lead to brake fire.
Most turbojet and turbofan powered A/C are
fitted with thrust reversers to assist in braking.
Thrust reversers redirect the flow of gases to
provide thrust in the opposite direction.
161.
ACCESSORY SECTION
Functions of accessory drive section
1- Used to power both engine and aircraft accessories.
2- Act as an oil reservoir or sump and housing the accessory
drive gears and reduction gears.
Accessory drive could located at engine’s midsection or front
or rear of the engine.
Rear mounted gear boxes allow the narrowest engine diameter
and lowest drag configuration.
Bevel gear drive the gear box using engine main power shaft.
The gear box distributes power to each accessory drive pad.
Reduction gear is necessary to provide the appropriate drive
speed for the accessories.
Intermediate or transfer gearbox is used on some engines to
obtain the needed reduction gearing.
The more accessories an engine has the more power is needed
to drive the gearbox.
165. ENGINE STATION NUMBERING
Engine manufacturers assign station number to
several points along a turbine engine’s gas path.
Station number provide a mean of rapidly locating
certain engine areas during maintenance.
Establish locations for taking pressure and
temperature readings.
Engine inlet pressure station is pt2 while turbine
discharge pressure station is pt7.
Engine pressure ratio is pt7 : pt2.
Pt2 is total pressure at station 2 and Tt2 is total
temperature at station 2.
167. NOISE SUPPRESSION
Noise produced by a turbine engine results when hot,
high velocity gases mix with cold, low velocity air
surrounding the engine.
Turbofan engines reduce the noise levels both inside
the cabin and on ground .
Turbofan engines seldom require noise suppressors
because the hot gases mix with cold gas prior to their
release to atmosphere.
Turbojet engine require additional noise suppression
equipment.
168. NOISE SUPPRESSION
A device that breaks up the airflow behind the
tail cone and sound insulating material are
used as noise suppressors.
The sound intensity is measured in decibels.
Decibel is the ratio of one sound to another.
One decibel is the smallest change in sound
intensity that the human ear can detect.
FAA establish rules for aircraft operators that
specify maximum noise levels.
171. ENGINE MOUNTS
Gas turbine engine relatively produce little torque so
they do not need heavily constructed mounts.
The mounts support the engine weight and allow for
transfer of stresses created by the engine to the
aircraft structure.
Wing mounted turbofan engine, the engine is attached
to the A/C by two to four mounting brackets.
Turboprop and turboshaft engines use heaver mounts
because of the torque developed.
173. BEARINGS
Engine main bearing support the compressor and
turbine rotor, and located along the length of the rotor
shaft.
The number of bearing is determined by the length
and weight of the rotor shaft.
Spilt spool axial compressor require more main
bearing than a centrifugal compressor.
Ball and roller bearing are used to support an engine’s
main rotor shaft.
Consist of inner and outer races that provide support
and hold lubricating oil.
174. BEARINGS
Advantages of ball and roller bearings:
Offer little rotational resistance.
2.Enable precision alignment of rotating elements.
3.Tolerate high momentary overloads.
4.Are easily replaced.
5.Are relatively inexpensive.
6.Are simple to cool, lubricate, and maintain.
7.Accommodate both radial and axial loads.
8.Are relatively resistant to elevated temperatures.
1.
175. BEARINGS
Disadvantages of ball and roller bearings:
Vulnerability to damage caused by foreign
matter.
2.Tendency to fail without appreciable warning.
Proper lubrication and sealing against entry of
foreign matter is essential.
Labyrinth, helical thread, and carbon seal are
used to seal the bearings from foreign matter.
1.
176. BEARINGS
Labyrinth seal does not rub against an outer
surface, instead each seal consist of a series of
rotating fins that come very close but do not touch a
fixed abradable race.
Air pressure on one side prevent the oil from
coming out of the bearing.
Helical seals depend on reverse threading to stop
oil leakage.
Carbon seals are spring loaded to hold the carbon
ring against the rotating shaft.
178. TURBOPROP ENGINES
Gas turbine engine that drives a propeller to produce
thrust.
The turbine of a turboprop engine extract up to 85%
of the engine’s total power output to drive the
propeller.
Multiple stages turbine and special design blades to
extract more energy from the exhaust gases.
Most turboprop engines use a free turbine to drive the
propeller.
179. TURBOPROP ENGINES
Free turbine is an independent turbine that is not
mechanically connected to the main turbine.
Power turbine is placed in the exhaust stream after the
main turbine and dedicated to drive the propeller.
Fixed shaft engines is used to extract the gas energy
to drive the propeller by adding more turbine stages
to the main shaft.
High speed low torque turbine output is converted to
low speed high torque by a reduction gear to drive the
propeller.
Constant speed propellers are used to maintain a
constant engine rpm.
181. TURBOSHAFT ENGINES
Gas turbine engine that operate something
other than a propeller.
Use almost all the energy in the exhaust gases
to drive an output shaft.
Power may be taken from the engine turbine or
from a free turbine.
Free turbine is not mechanically coupled to the
main turbine and may operate at its own speed.
Used to power helicopters and APUs.
182. AUXILIARY POWER UNITS
Turbine powered aircraft require large amounts of
power for starting and operation.
Electrical power is needed for passenger amenities
such as lighting, entertainment, and food preparation.
High pressure, high volume pneumatic air source is
needed to start the engine and ground air
conditioning.
Auxiliary power units meet these demands for ground
power when the engines are not running.
183. AUXILIARY POWER UNITS
Consist of a small turbine powerplant driving an electric
generator identical to aircraft generators.
APU compressor supplies bleed air for heating cooling, antiicing and engine starting.
APU is started using its own electric starter motor and aircraft
battery power using the fuel of the aircraft.
APU fuel control unit automatically adjust the fuel flow to
operate the APU at its rated speed.
Load control valve protect the APU from overheating by
modulate the pneumatic load automatically.
Cool down period is specified by the manufacturer to keep the
APU from being damaged because of thermal shock.
185. OPERATING PRINCIPLES
Gas turbine engine is a heat engine that
converts the chemical energy of fuel into heat
energy.
Heat energy is converted into kinetic energy in
the form of a high velocity stream of air.
The kinetic energy is converted into
mechanical energy by the turbine that drive the
compressor and the accessories and/or the
propeller or gearbox.
186. ENERGY TRANSFORMATION CYCLE
Energy transformation cycle in a gas turbine engine is
known as the Brayton or constant pressure cycle.
Intake, compression, combustion, and exhaust event
occur in both piston and turbine cycle.
In turbine engine all four events happen
simultaneously and continuously.
Gas turbine engine produce power continuously.
Gas turbine engine must burn a great deal of fuel to
support the continuous production of power.
188. ENERGY TRANSFORMATION CYCLE
The air is continuously drawn into the engine thorough the
inlet to the first compressor stage.
The compressor increase the static air pressure of the air.
Fuel is sprayed in the combustion chamber and ignited
resulting in continuous combustion.
The heat increase the air’s volume while maintaining a
relatively constant pressure.
Exhaust gases leave the combustion gases through the turbine
where pressure decreases and the velocity increases
dramatically.
Gas turbine engine produces thrust based on Newton’s third
law of motion.
The acceleration of a mass of air by the engine is the action
while forward movement is the reaction.
189. ENERGY TRANSFORMATION CYCLE
The working cycle upon which the gas
turbine engine functions is represented
by the cycle shown on the pressure
volume diagram
Point A represents air at atmospheric
pressure that is compressed along the
line AB. From B to C heat is added to
the air by introducing and burning fuel
at constant pressure, thereby
considerably increasing the volume of
air. Pressure losses in the combustion
chambers are indicated by the drop
between B and C. From C to D the
gases resulting from combustion
expand through the turbine and jet pipe
back to atmosphere. During this part of
the cycle, some of the energy in the
expanding gases is turned into
mechanical power by the turbine.
190. VELOCITY AND PRESSURE
Velocity and pressure of the air passing through a gas turbine
engine must change to produce thrust.
Pressure is increased in the compressor while velocity remains
relatively constant.
Gas velocity must be increased after combustion to rotate the
turbine.
Bernoulli’s principle stats that; when a fluid or gas is supplied
at a constant flow rate through a duct, the sum of the potential,
or pressure energy, and kinetic, or velocity energy is constant.
The pressure and velocity of a mass of air flowing in a
divergent or convergent duct must increase or decrease
accordingly. (energy can not be created or destroyed)
The temperature of the air will change too.
192. THRUST CALCULATIONS
Jet engine produces thrust by accelerating an
air mass to a velocity higher than that of the
incoming air.
Newton’s 2nd law of motion stats that force is
proportional to the product of mass and
acceleration or acceleration is directly
proportional to force and inversely
proportional to mass.
F=MXA
F = force.
M = mass.
A = acceleration.
193. THRUST CALCULATIONS
The acceleration of air mass through a gas turbine
engine is the difference between the exiting jet
exhaust and the intake air.
The acceleration must be compared to a constant.
(gravitational constant = 32.2 f/sec²)
Applying this to the formula
F = Ms (V2 – V1)/g
F =force. Ms = mass airflow through the engine
V2 =air velocity at the exhaust.
V1 = forward velocity of the engine.
g = acceleration of gravity which is 32.2 ft./sec².
194. THRUST CALCULATIONS
Example:
Given
Ms = 50 pounds per sec. V1 = 0 feet per sec.
V2 = 1,300 feet per sec. g = 32.2 ft./sec².
F gross = Ms x (V2 – V1 )/g
= 50 lb./sec x (1300 ft/sec. – 0 )/ 32.2 ft./sec².
=65,000 lb ft./sec²/ 32.2 ft./sec².
= 2,018.6 pounds
195. THRUST CALCULATIONS
Example 2:
Given
Ms = 50 pounds per sec. V1 = 734 feet per sec.
V2 = 1,300 feet per sec. g = 32.2 ft./sec².
F net = Ms x (V2 – V1 )/g
= 50 lb./sec x (1300 – 734)/ 32.2 ft./sec².
=50 x 566 / 32.2.
= 878.9 pounds net thrust.
Thrust can be increased by increasing mass flow of
air or by increasing the exhaust velocity.
As the aircraft speed increase more air enters the
engine resulting in an increase in exhaust velocity.
196. THERMAL EFFICIENCY
1.
2.
3.
Thermal efficiency is the ratio of the actual power
an engine produces divided by the thermal energy
in the fuel consumed.
Gas turbine engine can operate with thermal
efficiency as high as 50 % while the thermal
efficiency of reciprocating engine is between 30
to 40 %.
Factors which determine thermal efficiency:
Turbine inlet temperature.
Compression ratio.
Component efficiency of the compressor and the
turbine.
197. THERMAL EFFICIENCY
The higher a gas turbine engine raises the temperature
of the incoming air, the more thrust the engine can
produce,.
The limiting factor to increasing the temperature of
the air is the amount of heat the turbine section can
withstand.
The more a gas turbine engine compresses the
incoming air, the more thrust the engine can produce.
Engine with high compression ratio force more air
into the engine, so more heat energy transferred to
internal airflow thus increasing the thermal efficiency.
Compressor and turbine efficiency directly impact the
compression ratio of the engine which has a direct
impact on the thermal efficiency.
200. FACTORS AFFECTING THRUST
TEMPERATURE
The more dense the air passing through an engine is, the
more thrust the engine can produce.
Air density is inversely proportional to temperature, as
outside temperature increases (OAT), air density decreases.
As the density of the air entering a gas turbine engine
decreases, engine thrust also decreases.
Thrust augmentation system is used to compensate for the
effect of hot weather on the amount of thrust.
Water injection system inject water, or a mixture of water
and alcohol into the compressor inlet or in the combustion
chamber.
Water will cool the air mass, allow more fuel to be burned,
and increase the air mass to maintain air pressure in the
engine.
202. FACTORS AFFECTING THRUST
ALTITUDE
As altitude increases the air pressure drops.
The pressure at 18,000 feet is about 7.34 psi.
The pressure at 20,000 feet is about 6.75 psi.
The pressure at 30,000 feet is about 4.36 psi.
As altitude increases the temp. also decreases.
The decrease in the temp. increases the air density which
increase the thrust.
But the drop in pressure has a greater effect on decreasing
the thrust.
At 36,000 the temp. stabilizes at -69.7 deg. F, so the density
of air stop increasing.
Long range jet aircraft find 36,000 feet an optimum altitude
to fly.
204. FACTORS AFFECTING THRUST
AIRSPEED
As forward airspeed increases, the air mass
acceleration in the engine decreases, so less thrust
is produced.
As the aircraft speeds up, more air is forced into the
engine (ram effect), results in an increase in air
pressure within the engine, which produces more
thrust.
The result of the thrust reduced by increasing the
airspeed and the thrust increased by ram effect is
known as ram recovery.
206. FACTORS AFFECTING THRUST
ENGINE RPM
Early engines had a linear relationship between compressor
rpm and engine thrust.
Engine power output could be set using an rpm gauge.
Modern turbofan engines have a non-linear relationship
between compressor rpm and thrust produced.
Power is set using an engine pressure ratio EPR since thrust
and EPR have more proportional relationship than thrust
and rpm.
At low engine speeds, large increases in rpm produce
relatively small increase in thrust and vise versa.
207. FACTORS AFFECTING THRUST
ENGINE RPM
Compressor aerodynamics limits engine rpm
because the efficiency begins to drop when
the blade tip speed reach the speed of sound.
The longer the blade is, the higher the tip
rotational speed.
Large diameter compressors turn at a
relatively slow rotational speed, while small
diameter compressors could reach 50,000
rpm.
208. FACTORS AFFECTING THRUST
FAN EFFICIENCY
The more efficient the fan is, the more thrust
the engine can produce.
Turbofan replaced turbojet engines on most
transport and business jet aircraft.
Turbofan is quieter and much more fuel
economic.
