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DETAILED TRAINING
VAR Part 7 - Aircraft Maintenance Basic Cat B1+B2
TRAINING MANUAL
M15.04
Issue: 01
Rev: 00
Date: 25/04/2014
© VAECO Training Center
Training Manual
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EJAMF
Gas Turbine Engine
MODULE 15
M15.04
COMPRESSORS
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M15 GAS TURBINE ENGINE
HAM US/F
SwD
01.04.2008
ATA DOC
Page 1
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Gas Turbine Engine
MODULE 15
COMPRESSOR TYPES
COMPRESSOR COMPONENTS
There are 2 ways to compress air in engines.
You can decrease the volume in a cylinder by a piston or you can make use of
Bernoulli’s Principle to get compressed air. This is the method which is used in
the compressors of gas turbine engines.
The compressor of a gas turbine engine supplies a continuous flow of air to the
engine combustor.
The compressor has 2 main components. The compressor rotor and the
compressor stator.
Each rotor has rotor blades mounted on it. As it rotates around the axis of the
compressor, the rotor blades suck in air and then push it to the outlet side of
the rotor. This action increases the energy of the airflow.
The compressor stator is a fixed component. The stator also has blades
attached to it, which are named stator vanes. The stator vanes guide and slow
the airflow to cause an increase in pressure.
The combination of a rotor and stator is named a compressor stage.
Note: The rotor is always the first part of a compressor stage.
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M15.04 COMPRESSORS
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01|Compr Comp|A,B1
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M15.04 COMPRESSORS
COMPRESSOR TYPES
Figure 1
HAM
HAM US/F-4
US/F
DoJ
SwD
Nov
01.04.2008
30, 2010
Axial Compressor Stage
01|Compr Comp|A,B1
Page 3
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MODULE 15
Compressor Components cont.
If there is a set of stator vanes in front of a compressor stage, they are always
named inlet guide vanes or just IGVs.
These inlet guide vanes improve the airflow into the first compressor stage.
They are either fixed or movable.
The stator vanes in the fan duct are usually named outlet guide vanes or
OGVs. They improve the airflow into the fan nozzle.
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Figure 2
HAM US/F
SwD
01.04.2008
IGVs and OGVs
02|Compr Comp|A,B1
Page 5
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MODULE 15
Compressor Components cont.
Other compressor parts are the compressor inlet case and the compressor
outlet case. These cases give support for the compressor rotor bearings.
The cases also improve and straighten the airflow into and out of the
compressor.
There are 2 types of compressors − the axial flow compressor and the
centrifugal flow compressor.
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Figure 3
HAM US/F
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01.04.2008
Compressor Components
03|Compr Comp|A,B1
Page 7
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MODULE 15
CENTRIFUGAL FLOW COMPRESSORS
Centrifugal flow compressors are usually found on small gas turbine engines
like APUs.
The main components of a centrifugal flow compressor are:
S the impeller,
S the diffuser and
S the compressor manifold.
The impeller and the diffuser have the same function as the rotor and stator on
the axial flow compressor.
The operation of this compressor is as follows:
As the impeller rotates, air is sucked in horizontally near to the centre of the
impeller.
The rotation pushes the air outwards by centrifugal force. This increases the
air velocity and therefore the energy of the air.
As the air flows through divergent ducts, which are formed by the impeller
vanes, some of the velocity increase is converted into static pressure.
The air then passes through the diffuser where the airflow velocity is
decreased and the pressure is further increased.
Centrifugal flow compressors can give pressure ratios of about 5:1.
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Figure 4
HAM US/F
SwD
01.04.2008
Centrifugal Flow Compressors
04|Centrif Fl Compr|A,B1
Page 9
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MODULE 15
Centrifugal Flow Compressors cont.
Some engines use multiple stages to give higher pressures.
The airflow enters the engine in the normal way and then passes through the
first stage.
The output from the first stage then moves via a manifold to the second stage
where the pressure is again increased.
It has been found that only 2 stages are efficient. This is because of the
pressure decrease that occurs when the airflow changes direction between the
stages.
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Figure 5
HAM US/F
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01.04.2008
Multiple Stage Centrifugal Compressor
05|Centrif Fl Compr|A,B1
Page 11
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MODULE 15
Centrifugal Flow Compressors cont.
Another option is the double entry type impeller.
Here 2 impellers are mounted face to face. This is why this type is also named
double face compressor.
It does not give higher pressures, but it has a higher airflow at a given diameter
because the air can enter from both sides. This saves weight and gives the
engine a smaller frontal area.
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Figure 6
HAM US/F
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01.04.2008
Double Face Compressor
06|Centrif Fl Compr|A,B1
Page 13
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Gas Turbine Engine
MODULE 15
AXIAL FLOW COMPRESSORS
Airflow in an axial flow compressor is along the horizontal axis of the
compressor.
You will remember that this airflow is made by the rotor blades.
You can see that the rotor blades and stator vanes are aerodynamically
shaped. This aerodynamic shape smoothes the airflow through the compressor
stages. Similar to the centrifugal compressor, the pressure increase is the
result of 2 actions.
Firstly the rotor increases the velocity and, therefore, the energy of the airflow
and then this energy increase is changed into static pressure. This is caused
by the diffuser−like shape of the channels between the stator vanes.
You can see that the channels between the rotor blades are also diffusers. This
means that the rotors not only increase the velocity of the airflow but also slow
it down a little to increase the static pressure.
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Figure 7
HAM US/F
SwD
01.04.2008
Axial Compressor Operation
07|Ax Fl Compr|A,B1
Page 15
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MODULE 15
Axial Flow Compressors cont.
Each stage of an axial flow compressor has a pressure ratio of about 1.3:1.
This means that if the air pressure enters the first stage with 10 psi, then the air
pressure is increased to 13 psi on the output side of that compressor stage.
This pressure now becomes the input pressure to the next stage.
To get the output pressure of the second stage, we multiply the pressure ratio
of 1.3 and get an answer of 16.9 psi. This is the input pressure for the third
stage and so on through the remaining compressor stages.
You can see that the pressure increase at each stage is very small. Therefore,
many stages are needed to raise the pressure to the high values necessary for
efficient combustion.
As the air pressure increases, its density also increases. This means that the
air needs less volume.
For this reason, the compressor cross section is gradually decreased. This
keeps a constant velocity of the airflow through the compressor.
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Figure 8
HAM US/F
SwD
01.04.2008
Multiple Stage Axial Compressor
08|Ax Fl Compr|A,B1
Page 17
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Gas Turbine Engine
MODULE 15
COMPARISON OF COMPRESSOR TYPES
Axial flow and centrifugal flow compressors have their own specific advantages
and disadvantages.
This makes each type applicable for different functions.
Some engines, like the one in the graphic, combine the 2 types.
Here is a list of advantages and disadvantages:
S At optimum compressor speed, axial flow compressors are more efficient
than centrifugal flow compressors.
S Centrifugal flow compressors have a good efficiency over a wide speed
range.
S Engines with axial flow compressors cause less drag than engines with
centrifugal flow compressors. This is because they have smaller frontal
areas.
S Engines with axial flow compressors can reach high total pressures by the
addition of many compressor stages.
S Centrifugal flow compressors have a very high pressure rise per stage.
S Centrifugal flow compressors are simpler to manufacture and lower in costs
compared to axial flow compressors.
S Centrifugal flow compressors have a lower weight than axial flow
compressors.
S Centrifugal flow compressors need less power for engine start than axial
flow compressors.
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Figure 9
HAM US/F
SwD
01.04.2008
Comparison of Compressor Types
09|Compar of Compr Types|A,B1
Page 19
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MODULE 15
COMPRESSOR OPERATION
NORMAL FLOW IN AXIAL COMPRESSORS
In the last lesson you learned the difference between centrifugal flow and axial
flow compressors.
The 2 compressor types have one thing in common. They are very sensitive to
changes in the airflow which passes through the compressor. This is because
the engine compressors have only 1 optimum operating condition for a given
pressure ratio, rotational speed and airflow. This condition is usually named the
design point.
At the design point the airflow in the compressor is perfectly matched.
On the graphic you can see the first stage of an axial compressor.
The airflow from the engine air inlet duct enters the rotor as shown. The arrows
show the direction of the airflow before it enters the rotor. The length of the
arrows represents the inlet velocity.
The rotor speed gives us a second parameter for the inlet airflow.
The arrow for the rotor speed points in the direction of rotation.
The length of this arrow represents the speed of the rotor blades.
For a smooth airflow into the rotor we need the correct relationship between
the inlet velocity and the rotor speed. This direction of the airflow is named the
resultant rotor inlet velocity.
You can see that the resultant rotor inlet velocity is a result of the air inlet
velocity and the rotor speed.
We can say that the airflow into the rotor is smooth as long as the angle of
attack to the rotor blades is small.
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Figure 10
HAM US/F
SwD
01.04.2008
Airflow Changes at Compressor Rotor Blades
01|Norm Fl in Ax Compr|A,B1
Page 21
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MODULE 15
Normal Flow in Axial Compressors cont.
We now follow the airflow through the rotor.
The shape of the rotor blades gives the direction for the airflow.
The rotor speed arrow at the outlet is the same as at the inlet but the resultant
outlet direction is different.
The rotor speed strongly deflects the outlet airflow. This outlet airflow is now
going in the correct direction for the inlet to the stator vanes.
The stator vanes decelerate the airflow and guide it to the next rotor stage of
the compressor.
The stator outlet velocity and the rotor speed again give the direction for the
next rotor blades. This airflow sequence continues through all the compressor
stages.
Remember that this condition is only optimum at one compressor speed.
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Figure 11
HAM US/F
SwD
01.04.2008
Airflow Changes at Compressor Stator Vanes
02|Norm Fl in Ax Compr|A,B1
Page 23
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Gas Turbine Engine
MODULE 15
COMPRESSOR STALL
The airflow in the axial compressor is smooth as long as the angle of attack on
the rotor blades and stator vanes is small.
The angle of attack changes if the rotor speed or the inlet velocity changes.
At a given rotor speed the angle of attack increases when the inlet air velocity
decreases. If this occurs, the airflow separates from the compressor airfoils
and causes a turbulent vortex.
This airflow separation is named compressor stall. Compressor stall changes
the proper airflow in the compressor. It causes the airflow to slow down, stop or
even reverse its direction. Stall can occur at some airfoils only. It is then weak
and almost unnoticeable. It can also occur at one or more compressor stages.
The engine then runs roughly and the rotor speed decreases slightly.
If the stall becomes stronger it can affect all compressor stages.
This condition, named compressor surge, will be shown in more detail in the
next segment.
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Figure 12
HAM US/F
SwD
01.04.2008
Compressor Stall
03|Compr Stall|A,B1
Page 25
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MODULE 15
COMPRESSOR SURGE
Compressor surge is a very severe form of compressor stall.
It is generated as follows:
− a rapid decrease of airflow causes stall on some blades or stages.
− The stall causes a blockage in the airflow which leads to a stronger stall
in the subsequent stages.
− This causes low pressure zones in which the airflow comes to a stop and
reverses its direction.
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Figure 13
HAM US/F
SwD
01.04.2008
Compressor Surge
04|Compr Surge|A,B1
Page 27
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MODULE 15
Compressor Surge cont.
Surge can happen on all engine compressors. It can occur in the forward or in
the aft compressor stages.
Surge in the forward stages usually affects only a part of the compressor
blades. This is because of the bigger dimensions of the blades. The effect on
the compressor operation is not very dangerous.
Surge in the aft stages develops very rapidly over a large part of the
compressor. This behaviour is supported by the high pressure in the aft stages
and by the short compressor blades.
At a compressor surge in the aft stages, the airflow decreases rapidly and
causes a strong reverse flow. In the extreme, this reverse flow could be from
the combustion chamber back to the engine inlet.
During compressor surge the airflow collapses and builds up again at very
short intervals. It is usually very strong and comes with heavy vibrations and
loud banging noises.
The thrust decreases, the engine speed fluctuates and the exhaust gas
temperature increases.
Different methods are used to prevent these dangerous engine conditions.
They will be shown later in this lesson. First we will look at the reasons which
can cause compressor stall or surge.
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Figure 14
HAM US/F
SwD
01.04.2008
Fwd and Aft Compressor Surge
05|Compr Surge|A,B1
Page 29
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MODULE 15
REASONS FOR STALL AND SURGE
Compressor stall and surge is caused by problems in engine operation or by
damaged engine components.
All primary engine components can cause compressor stall if they are
damaged.
Dents or ice on the engine inlet cause turbulent or disrupted airflow which
decrease the inlet velocity.
Damaged rotor blades or stator vanes disturb the correct airflow and cause
stall. Even dirty compressor blades or stator vanes can cause stall.
Damaged or broken combustor components can cause a blockage in the
airflow and decrease the velocity in the compressor.
Damaged turbine components will decrease the airflow and dents in the jet
nozzle or broken objects in the exhaust system can also cause a blockage to
the airflow.
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Salt on Airfoils
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Inlet
Figure 15
HAM US/F
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Reasons for Stall and Surge
06|Reasons|A,B1
Page 31
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Reasons for Stall and Surge cont.
There are 2 operational reasons for compressor stall and surge.
The compressor can stall if the speed of the engine is too far below the design
speed and it can stall at incorrect acceleration or deceleration.
Remember that the efficiency of the compressor stages decrease if the
compressor operates below the design speed.
On a large axial compressor it is very difficult to match all the stages.
At very low speeds the efficiency of the forward compressor stages is much
better than at the aft stages. This means that the forward stages supply too
much air to the aft stages.
The air piles up in the aft stages, then slows down, comes to a stop and
reverses its direction.
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Figure 16
HAM US/F
SwD
01.04.2008
Compressor Stall due to Low Speed
07|Reasons|A,B1
Page 33
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Reasons for Stall and Surge cont.
Fast deceleration causes a similar problem on dual and triple spool engines.
This is because the rotor speed of the high pressure compressor decreases
faster than that of the low pressure compressor.
The low pressure compressor supplies too much airflow which cannot pass
through the high pressure compressor.
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Figure 17
HAM US/F
SwD
01.04.2008
Compressor Stall due to Fast Deceleration
08|Reasons|A,B1
Page 35
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Reasons for Stall and Surge cont.
Fast acceleration leads to too much fuel in the combustion chamber. This
increases the backpressure in the combustion chamber for a while.
The airflow through the compressor decreases and the compressor stalls.
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Figure 18
HAM US/F
SwD
01.04.2008
Compressor Stall due to fast Acceleration
09|Reasons|A,B1
Page 37
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MODULE 15
METHODS TO AVOID STALL AND SURGE
You will find 3 different design methods on aircraft engines to prevent
compressor stall and surge.
One method uses compressor bleed valves. Another method is the use of dual
or triple spool rotors instead of single spool rotors and the most modern
method uses variable compressor stator vanes.
Each of these methods can prevent stall on its own, but on most modern
engines you will find a combination of 2 or 3 methods.
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Figure 19
HAM US/F
SwD
01.04.2008
Methods to Avoid Stall and Surge
10|Methods|A,B1
Page 39
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MODULE 15
Methods to avoid Stall and Surge cont.
The use of compressor bleed valves is a simple and very effective method to
avoid compressor stall and surge. You will find bleed valves in the mid section
and sometimes in the aft section of the engine compressor.
They are open at low engine speeds so that the excessive airflow from the
forward stages can escape overboard.
This prevents an airflow blockage in the aft compressor stages, but they have
one big disadvantage: all the airflow, which is compressed first and then blown
overboard, causes a big loss in efficiency.
On large multistage axial compressors it is very difficult to match all stages for
all speeds of the engine. Bleed valves would be necessary at several positions
to guarantee safe operation.
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Figure 20
HAM US/F
SwD
01.04.2008
Avoid Stall and Surge by Compressor Bleed Valves
11|Methods|A,B1
Page 41
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Methods to avoid Stall and Surge cont.
A better method for these engines is to split the compressor rotor.
On the dual spool engine the forward part is named the low pressure
compressor or N1 compressor and the aft part is named the high pressure
compressor or N2 compressor.
You can see that each compressor rotor is driven by a separate turbine.
Another advantage of this multiple spool engine design is its excellent
acceleration capability. This is because of the smaller rotor mass of the split
rotors.
These advantages of a dual spool engine are even better on a triple spool
engine.
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Figure 21
HAM US/F
SwD
01.04.2008
Avoid Stall and Surge by Multiple Rotors
12|Methods|A,B1
Page 43
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MODULE 15
Methods to avoid Stall and Surge cont.
Variable stator vanes in the compressor are the most effective method to
prevent stall. They are moveable along the vertical axis.
You will find these variable stator vanes and variable inlet guide vanes in the
forward stages of the high pressure compressor.
The advantage of the variable stator vanes is that an optimum angle of attack
on the following rotor blades can be reached for every engine speed.
The compressor does not stall and it always operates at an optimum efficiency.
The disadvantage is that it needs a very complicated control mechanism.
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Figure 22
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01.04.2008
Avoid Stall and Surge with Variable Stator Vanes
13|Methods|A,B1
Page 45
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MODULE 15
COMPRESSOR CONTROL SYSTEM
SYSTEM ORGANIZATION
A compressor control system on a modern turbofan engine usually has a
variable stator vane system and a variable bleed valve system.
Additionally to these 2 subsystems some engines also have a compressor
bleed valve system for the HP compressor.
Each compressor control subsystem has control components, actuation
components and feedback components.
The control components are the control unit and several sensors on the
engine.
The control unit monitors the condition of the airflow in the engine compressors
and gives control signals to the actuators of the compressor control system.
The control components are either hydromechanical components or they are
electrical components on FADEC controlled engines.
The actuation components are mechanical components like actuators, rods,
linkages and many other transmissions.
The feedback components are either mechanical push−pull cables and
linkages or electrical position sensors like LVDTs or RVDTs. The feedback
components transmit the actual VBV or VSV position to the control unit.
On some turbofan engines you can find additional compressor control
components. These are HP compressor bleed valves. They are also controlled
by the engine control unit, but they are usually pneumatically operated valves
without a feedback system.
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01|System Org|B1
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VSV Actuator
VBV Position Sensor
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CIT Sensor
VSV Feedback
Cable
Control Unit
Figure 23
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01.04.2008
Feedback Components
01|System Org|B1
Page 47
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MODULE 15
System Organization cont.
On some turbofan engines you can find additional compressor control
components. These are HP compressor bleed valves. They are also controlled
by the engine control unit, but they are usually pneumatically operated valves
without a feedback system.
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Figure 24
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HP Compressor Bleed Valves
02|System Org|B1
Page 49
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Gas Turbine Engine
MODULE 15
CONTROL COMPONENTS
We first look at the compressor control components of engines without a
FADEC system.
The main component is the engine control unit.
It is often named main engine control or just MEC because it is used for fuel
metering compressor control and active clearance control.
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Figure 25
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Compressor Control Schematic
03|Control Comp|B1
Page 51
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MODULE 15
Control Components cont.
For compressor control the MEC uses the core engine speed N2 and the
compressor inlet temperature CIT.
The MEC also gets information about the reverser operation. This is necessary
because thrust reverser operation increases the danger of compressor surge.
The N2 signal comes from the mechanical drive via the accessory gearbox.
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CIT Sensor
Figure 26
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01.04.2008
Compressor Control Components
04|Control Comp|B1
Page 53
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Gas Turbine Engine
MODULE 15
Control Components cont.
You usually find the CIT sensor in the fan frame between the LP and HP
compressor.
This sensor is usually a hydromechanical temperature sensor. It senses the air
temperature and converts the temperature into a proportional fuel pressure.
This fuel pressure is sensed by the MEC.
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Figure 27
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CIT Sensor
05|Control Comp|B1
Page 55
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Gas Turbine Engine
MODULE 15
Control Components cont.
On engines with a FADEC system the control unit for the compressor control is
the ECU (Engine Control Unit).
The ECU receives the N2, the CIT and the information about the reverser
condition and it also receives additional signals to improve the compressor
control. These signals are usually electric or pneumatic signals that differ from
engine type to type.
Some systems use the fan rotor speed N1, the engine inlet temperature T12
and mach numbers to improve the VBV control.
Other systems also use the HP compressor discharge pressure Ps3 as a surge
detection signal.
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Figure 28
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ECU Signals for Compressor Control
06|Control Comp|B1
Page 57
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MODULE 15
VSV SYSTEM ACTUATION COMPONENTS
The variable stator vanes operate by hydraulic power via a mechanical
transmission. There are one or two actuators for the VSV control mechanism
depending on the engine type.
The hydraulic fluid that operates the actuators is always fuel pressure from the
MEC or from the HMU.
You usually find the VSV actuators on each side of the HP compressor. They
move large actuator levers equipped with several small push rods.
The push rods are connected to actuation rings that surround the respective
compressor stage.
These so called unison rings move the individual VSVs via small lever arms.
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Figure 29
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01.04.2008
VSV System Actuation Components
07|VSV Sys Act Comp|B1
Page 59
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Gas Turbine Engine
MODULE 15
VSV System Actuation Components cont.
The VSV actuator is a simple hydraulic actuator with two fuel ports named rod
end port and head end port.
The VSV actuator has a double acting hydraulic piston that can travel the full
range between the closed stop and the open stop. It usually has a double seal
with a seal drain to prevent fuel leakage. Fuel leakage across the inner seal is
caught by the seal drain and drained to the engine drain mast via a drain line.
This drain line also drains away any leakage at the fuel line couplings.
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Figure 30
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VSV Actuator
08|VSV Sys Act Comp|B1
Page 61
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MODULE 15
VBV SYSTEM ACTUATION COMPONENTS
This segment shows 3 variable bleed valve systems that are used on modern
turbofan engines and describes their mechanical components.
One system uses flapper type variable bleed valves operated by hydraulic
actuators.
Another system that you can find on some CFM 56 engines also has the
flapper type variable bleed valves, but they are operated by a hydraulic motor.
The third system uses a bleed valve ring that opens and closes a
circumferential slot in the LPC stator case. This ring is also operated by
hydraulic actuators.
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Figure 31
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3 Bleed Valve Systems
09|VBV Sys Act Comp|B1
Page 63
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MODULE 15
VBV System Actuation Components cont.
The VBV system with hydraulic actuators is similar to the VSV system. It has 2
actuators, one on each side of the fan frame.
The actuators receive fuel pressure from the hydromechanical engine control
unit.
Via bellcranks the actuators rotate a unison ring that is connected with other
bellcranks to the individual variable bleed valves.
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Figure 32
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VBV System with Hydraulic Actuators
10|VBV Sys Act Comp|B1
Page 65
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MODULE 15
VBV System Actuation Components cont.
The VBV system with a hydraulic motor uses flexible drive shafts to operate
the VBVs.
The motor receives fuel pressure from the hydromechanical engine control unit
and rotates the flexible drive shafts.
Gearboxes on each VBV transmit the rotation to a small ballscrew actuator
which operates the variable bleed valve.
The ballscrew actuators translate the rotational movement from the drive shafts
into a linear movement for the VBV.
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Figure 33
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VBV with Hydraulic Motor & Gearboxes
11|VBV Sys Act Comp|B1
Page 67
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Gas Turbine Engine
MODULE 15
VBV System Actuation Components cont.
A system with a ring shaped bleed valve is very simple in design. It is used, for
example, on the V 2500 engine where it is called the booster stage bleed valve
system.
It has 2 hydraulic actuators, one on each side of the fan frame, to move the
bleed valve ring. The actuators move the ring via actuating rods.
The valve opens when the actuators push the ring forward so that air from the
last stage of the LP compressor can escape into the fan discharge duct.
The valve closes when the actuator pulls at the actuating rod.
Like the flapper type variable bleed valves, this booster stage bleed valve can
move in any position between fully closed and fully open.
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Figure 34
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Ring Shaped Bleed Valve
12|VBV Sys Act Comp|B1
Page 69
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Gas Turbine Engine
MODULE 15
FEEDBACK COMPONENTS
Feedback systems for VSVs and VBVs are either mechanical systems on
engines with hydromechanical control units, or they are electrical systems on
engines with FADEC.
Mechanical feedback systems usually have flexible push−pull feedback cables
that are connected between the control unit and the VSVs or VBVs. They are
similar for the VSVs and VBVs. As an example we take a closer look at the
VSV feedback cable.
On the engine in the photo the feedback cable is attached to the left hand VSV
actuator lever via a bellcrank. The VSV feedback cable is attached to the
actuator lever by an adjustable rod and a bellcrank. The conduit of this cable is
clamped to a bracket on the compressor case.
At the main engine control the conduit of the cable is attached to the housing
and the inner member is bolted to a lever arm. The attachment at the main
engine control is adjustable so that the feedback cable can be rigged to the
correct length.
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Figure 35
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01.04.2008
VSV Feedback Cable
13|Feedback Comp|B1
Page 71
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MODULE 15
Feedback Components cont.
Electrical feedback systems have position sensors to transmit the VSV or VBV
position to the engine control unit.
These sensors are either attached to the actuation mechanism or they are
integral components of the hydraulic actuators.
The electrical connector on the hydraulic actuator shows that it has an internal
position sensor.
Electrical position sensors are usually LVDTs or RVDTs that you have probably
seen before in other lessons.
They convert mechanical deflection into proportional electrical signals for the
ECU.
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Figure 36
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01.04.2008
Electrical VSV / VBV Feedback
14|Feedback Comp|B1
Page 73
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MODULE 15
Feedback Components cont.
You can also find separate LVDTs on some engines with hydromechanical
engine control units.
These engines are then operated on aircraft with a condition monitoring
system.
Via this condition monitoring system the maintenance personnel can get the
VSV and VBV position at any time on a printed report.
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Figure 37
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LVDTs
15|Feedback Comp|B1
Page 75
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MODULE 15
VSV / VBV SYSTEM OPERATION
In this segment you will see how the compressor control operates.
The airflow in the engine compressors changes when the rotational speed of
the compressors change. It also changes slightly when the temperature of the
air changes.
Therefore for compressor control, the engine rotor speed and the air
temperature are the most important parameters.
The variable stator vanes are used to provide an optimum angle of attack to
the compressor blades at all engine speeds. They ensure a stall free operation
at low engine speeds, a good acceleration performance and an optimum
pressure ratio at all engine speeds.
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Figure 38
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Compressor Control Parameters
16|VSV/VBV Sys Oper|B1
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MODULE 15
VSV / VBV System Operation cont.
On the diagram you can see the relation between the engine speed and the
VSV position. This curve is called the VSV schedule.
At high rotor speeds the VSVs are in the high speed position and at low speeds
they are in the low speed position.
Note that some engine manufacturers use the terms VSV open or VSV closed,
but as you can see, the VSV are never physically closed.
This VSV schedule applies for one air inlet temperature only. This means it
changes with the air temperature.
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VSV Schedule
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Figure 39
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VSV Schedule
17|VSV/VBV Sys Oper|B1
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MODULE 15
VSV / VBV System Operation cont.
The control of the variable bleed valves also depends on the engine speed and
on the air temperature, but the VBV movement is opposite to the VSV
movement.
You can easily see on the diagram that the VBVs are open when the VSVs are
closed and vice versa.
Open VBVs can become very critical during maintenance in the fan frame area
because pieces may drop through the valves into the engine.
The VBVs can be closed by a hand pump connected to the VBV actuators.
On some engines, however, the VBV position is not critical because either the
upper VBV area is protected by a screen or some FADEC systems close also
the VBVs at each engine shut down.
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Figure 40
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VBV Schedule
18|VSV/VBV Sys Oper|B1
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VSV / VBV System Operation cont.
Let us now look at the operation of a VSV control system. The VBV control is
similar and therefore will not be covered here.
The example below shows a simplified VSV system on a FADEC engine with
the engine control unit, the hydromechanical unit, one of the VSV actuators
and the mechanical linkage to the VSVs.
The ECU always calculates a VSV demand signal based on the actual
engine speed and compressor inlet temperature. This demand signal, for
example 10mV, is sent to the summation point in the ECU.
If the VSV actuator is in the correct position for the VSV demand, the signal
from the LVDT is equal to the demand signal. This results in a 0mV signal to
the torque motor of the VSV servo valve.
With the 0mV signal the torque motor moves the VSV servo valve to its
neutral position.
In this position the head end port and the rod end port to the VSV actuator
are closed.
If the engine speed increases, the ECU calculates a new higher demand
signal.
Compared with the feedback signal it now results in a torque motor signal
and the VSV pilot valve moves down.
Fuel pressure gets to the head end port of the VSV actuator and the VSVs
move towards open.
The VSVs stop when the feedback signal is equal to the demand signal and
the VSV servo valve returns to neutral.
The closing of the VSVs is similar.
The plunger of the VSV pilot valve moves up to supply fuel pressure to the rod
end port of the VSV actuator.
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Figure 41
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Operation of VSV Control System
19|VSV/VBV Sys Oper|B1
Page 83
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MODULE 15
VSV / VBV System Operation cont.
The VSV operation on an engine with a hydromechanical control unit uses
hydromechanical signals instead of electrical signals.
The engine speed signal and the CIT signal act on a 3−dimensional cam.
This 3−D cam that you already know from the fuel metering system also
operates the VSV servo valve.
The signal from a mechanical feedback cable returns the servo valve to neutral
when the VSVs have reached their demanded position.
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Figure 42
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VSV Operation / Hydromechanical
20|VSV/VBV Sys Oper|B1
Page 85
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Gas Turbine Engine
MODULE 15
HPC BLEED VALVE
HP compressor bleed valves are usually installed in the mid and aft stages of
the HP compressor.
The number of HP bleed valves on an engine differs from type to type. Some
engines have no HP bleed valves at all and others have up to 4 of them.
Because of the same reason, the valves are used differently. On some engines
HP compressor bleed valves are used for all operating conditions and on
others they are used for starting or acceleration and deceleration only.
There are also many different names for these valves. They are called start
bleed valves if they are used for engine starting only or they are called transient
bleed valves if they are used for acceleration and deceleration only.
Sometimes theses valves are also known as handling bleed valves.
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Figure 43
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HP Compressor Bleed Valves
21|HPC Bleed Valve|B1
Page 87
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MODULE 15
HPC Bleed Valve cont.
HP compressor bleed valves are usually pneumatically operated valves.
The pneumatic pressure comes from the last stages of the HP compressor.
On some engines they are also pneumatically controlled by a bleed valve
controller but on most modern engines they are electrically controlled by the
engine control unit via solenoid valves.
Below you can see a simplified HP compressor bleed valve system. The bleed
valves are usually spring-loaded open as shown.
The HP compressor bleed valve closes if the air pressure at this compressor
stage is strong enough to overcome the spring force.
It opens at any time when the compressor is likely to surge. This is controlled
by the ECU.
To control the HP bleed valves the ECU receives the rotor speed, the aircraft
altitude and the information about reverser operation.
With this information it calculates when to open or close the respective HP
compressor bleed valves.
To open the HP compressor bleed valves the ECU energizes the respective
solenoid valve.
When, for example, during acceleration, the solenoid is energized, PS3
pressure can reach the bleed valve and pushes it open. Excessive air in the
compressor escapes. This prevents compressor surges.
When the engine has reached a safe speed, the ECU de−energizes the
solenoid and the bleed valve closes.
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Figure 44
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01.04.2008
Open HP Compressor Bleed Valves
22|HPC Bleed Valve|B1
Page 89
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Gas Turbine Engine
MODULE 15
LP COMPRESSOR AND FAN MODULE
INTRODUCTION
Major modules are usually built up of individual sub−modules.
These sub−modules are not identical on all engines because they are defined
by the engine manufacturers. In this segment we take a closer look at 2
examples of fan modules.
The first example shows a CFM56 fan module. You can see that it is split up
into 4 sub-modules.
These are
S the fan rotor and booster module,
S the number 1 and 2 bearing module,
S the inlet gear box
S and the fan frame module.
Each of these main components will be discussed in detail in other segments.
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Figure 45
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01.04.2008
Fan Module
01|Intro|A,B1
Page 91
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MODULE 15
Introduction cont.
In the next example you can see individual components of the Pratt and
Whitney 4000 fan module.
Fan blades provide the initial compression and then pass the compressed air
to the engine primary and secondary gas paths.
The LPC/LPT coupling attaches and secures the LPC rotor to the LPT shaft.
This part is also called a forward shaft on other engines. The low pressure
compressor is driven by the LP turbine via the LPT shaft.
The fan stator cases give a flow path for the fan discharge air. They also
connect the engine with the nacelle.
The intermediate case is a major structural component of the engine. It also
has mounting locations for many engine components. On other engines this
part is also known as the fan frame.
The sub−modules are manufactured depending on engine type, size and
manufacturing requirements. This means that you do not find the same
sub−modules on all aircraft engines.
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Figure 46
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Fan Module Components
02|Intro|A,B1
Page 93
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MODULE 15
SPINNER CONE
The fan rotor is a typical sub−module of a main fan module.
The fan rotor can be generally divided into
S the spinner cone,
S the fan disc,
S the fan blades
S and the low pressure compressor rotor, which is also known as the booster
rotor.
The spinner cone is also known as the compressor inlet cone or nose cone. It
covers the end of the fan rotor disc and creates a smooth aerodynamic fairing
at the engine inlet.
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Figure 47
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Spinner Cone
03|Spinner Cone|A,B1
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Gas Turbine Engine
MODULE 15
Spinner Cone cont.
You can see that the spinner cone has either a curved front or a sharp tip.
These 2 design shapes minimize ice build−up on the outer surface of the
spinner cone.
On some engines the spinner cone is heated with warm air, which escapes
through anti−ice holes. This also helps to prevent ice build−up.
On other engines, like modern Rolls Royce engines or the V2500 engine, the
spinner cone has a soft tip. This soft tip causes a small imbalance during
operation which prevents large ice build−up.
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Figure 48
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Spinner Cone Ice Protection
04|Spinner Cone|A,B1
Page 97
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Gas Turbine Engine
MODULE 15
Spinner Cone cont.
On most spinner cones a spiral is drawn on the outer surface. Spirals or white
markings on the spinner cone are used to scare birds away.
The spinner cone can be designed as a single component as shown on the
graphic or it can be split into more parts as you will find on many other engines.
The cone is always attached to the front face of the fan disc.
Balance weights can be attached to the mounting bolts of the spinner cone or
they can be installed into threaded inserts at the outer surface of the spinner
cone.
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Spinner Cone Attachment
05|Spinner Cone|A,B1
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MODULE 15
FAN BLADE ATTACHMENT
The fan rotor disc is a very large disc made of titanium alloy. It is attached to
the forward rotor shaft and the rotor drum of the LPC.
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Fan Blades and Rotor Disk
06|Fan Blade Attachm|A,B1
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MODULE 15
Fan Blade Attachment cont.
The fan rotor disc helps to support the fan blades. The fan blades are mounted
individually in the fan rotor disc and the blade root is specially shaped to fit into
the the blade seat on the fan rotor disc.
A dovetailed fitting is used on most engines.
Fan blades are usually held in place by blade retainers or on other engines, the
fan blades are attached to the rotor disc by bolts or cylindrical pins.
Note that each mounting method makes sure that movement between the
blade root and blade seat is possible.
A loose fit is necessary so that sudden forces or torque loads do not exceed
the maximum allowable stress limits of the fan blades.
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Figure 51
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Fan Blade Attachment
07|Fan Blade Attachm|A,B1
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MODULE 15
FORCES AND LOADS ON ROTOR BLADES
Stress on rotor blades is frequently caused by forces such as centrifugal forces
and air loads. Air loads, which result from pressure and air resistance, are not
even and vary when the blades rotate.
These variations arise from disturbances in the inlet airflow. For example the
blade stress changes very rapidly if the blade passes through an area of
turbulent air.
If you look at the forces that act on a typical blade, you can see that there are
centrifugal forces, which push the blade firmly into its seat and air loads, which
bend the blade. The air loads act on the blades in the direction shown by the
arrows on the top view.
A loose blade attachment is a good method to keep the blade stress within
safe limits.
Sudden excessive loads can cause the blade to deflect in its seat, but the
centrifugal forces usually centre the blade again after the excessive loads are
gone.
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Figure 52
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Rotor Blade Forces and Loads
08|Forces&Loads|A,B1
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MODULE 15
Forces and Loads on Rotor Blades cont.
On long rotor blades you can often find another method to keep these forces
within safe limits. Long rotor blades have shrouds. The shrouds form a ring on
the fan rotor.
The shrouds on the rotor blades are sometimes called mid−span shrouds or
part−span shrouds depending on the engine manufacturer.
The shrouds help to keep the fan blade deflection within safe limits when very
heavy loads act upon them. For example heavy vibration, stones or birds.
At very high loads the blades deflect rearwards and forward.
They slide on the contact surfaces of the shrouds. This converts the energy of
the fluttering blade into friction therefore dampening the blade fluttering.
The shroud surface is covered with a special coating. This coating makes sure
that the blades can easily slide on each other. If they could not slide, because
of dry contact surfaces, the fan blades will overstress and the blades shrouds
would break.
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Figure 53
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Mid - Span Shrouds
09|Forces&Loads|A,B1
Page 107
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Gas Turbine Engine
MODULE 15
WIDE CHORD FAN BLADES
On modern gas turbine engines you often find so called wide chord fan blades.
These fan blades do not have shrouds because of their special design.
The blades are either
S hollow,
S made of titanium sheets which are bonded together,
S made of a bonded titanium honeycomb sandwich construction
S or they are made of a composite material with a titanium leading edge.
The main advantages of these wide chord fan blades are that they are more
efficient than conventional blades.
They can run with a lower speed which reduces noise and they are very
flexible.
The wide chord blade can withstand foreign object damage much better than
the conventional fan blade because of its high flexibility. This is because the
impact energy can be absorbed by the total blade span.
On a shrouded fan blade all the impact energy must be absorbed by the blade
tip.
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Figure 54
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Wide Chord Fan Blades
10|Wide Ch Fan Bld|A,B1
Page 109
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MODULE 15
LOW PRESSURE COMPRESSOR - ROTOR & STATOR
Another sub-module of the fan module is the low pressure compressor module.
It is also called the booster module.
The number of compressor stages in the low pressure compressor differ from
engine to engine, but the main components are usually the same.
They are
S the rotor drum,
S the stator case,
S the airflow splitter fairing
S and the interstage seals.
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Low Pressure Compressor Module
11|LPC R&S|A,B1
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MODULE 15
Low Pressure Compressor - Rotor & Stator cont.
The rotor drum of the low pressure compressor is attached to the fan disc. The
drum has dovetail slots around its circumference. These slots are for the rotor
blades of each stage.
Between the dovetail slots there are rings with sharp edges for the labyrinth
seal.
The labyrinth seal is made by the knife edges on the rotor drum and by the
opposing shrouds of the stator assembly.
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Figure 56
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LPC Rotor Drum
12|LPF R&S|A,B1
Page 113
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MODULE 15
Low Pressure Compressor - Rotor & Stator cont.
The shrouds of the stator assembly are either plain metal surfaces or they are
made up of individual segments.
These segments are filled with a soft abradable material where the knife edges
can cut in. This cut−in creates a very small gap between the rotor assembly
and the shroud and therefore improves the efficiency of the compressor.
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Figure 57
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LPC Stator Assembly and Shrouds
13|LPC R&S|A,B1
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MODULE 15
Low Pressure Compressor - Rotor & Stator cont.
The LPC stator case assembly encloses the LPC rotor and supports the stator
vanes of the low pressure compressor.
Usually the stator vanes are attached to the stator case in circumferential
dovetail slots or sometimes the vanes are bolted to the case.
The individual stator vanes are usually fitted together to make these vane
segments.
There are 2 different construction methods of a compressor stator case. You
either find a closed stator case or a split stator case.
At a closed stator case the stator vanes are fitted to the shrouds between the
rotor stages and the case segments for each stage are subsequently matched
during assembly.
On a split stator case all vanes are installed in 2 stator halves and bolted
together afterwards.
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Closed Stator Case
Split Stator Case
Figure 58
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LPC Stator Case and Vanes
14|LPC R&S|A,B1
Page 117
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MODULE 15
FAN CASE ASSEMBLY
The fan case assembly houses the fan rotor and the low pressure compressor
and it has the following main components.
S The fan case,
S the inner lining,
S an abradable shroud,
S the fan blade containment
S and the outlet guide vanes.
On modern turbofan engines the fan case has one or more ring elements,
which are bolted together.
Flanges on the outer surface of the case help to stiffen the case and prevent
deformation.
On small turbofan engines the fan case is usually made of a single piece.
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Figure 59
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Fan Case Main Components
15|Fan Case Assemb|A,B1
Page 119
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MODULE 15
Fan Case Assembly cont.
You will find lining elements on the inner surface of the fan case. These linings
provide a smooth surface for the airflow in the fan duct.
If you take a close look at these lining elements, you can see that they have a
very special design. There are many small holes on the surface of the lining.
The linings, which are often called acoustic panels, are found on the inner
surface of the fan case. The holes on this surface reduce the noise created by
the fan.
The outer surface of the acoustic linings is very thin to save weight. Note that
it can be easily damaged by FOD, tools or incorrect maintenance.
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Figure 60
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Acoustic Panels
16|Fan Case Assemb|A,B1
Page 121
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MODULE 15
Fan Case Assembly cont.
The shrouded area on the fan case has a special surface instead of the
acoustic panels. This surface is made of a soft material which prevents
damage to the fan blades if they come into contact with the shroud. The
shrouded area is known as the abradable shroud. The tips of the fan blade can
cut into the shroud if they are running at high speeds. This lets the fan blades
run with a very small tip clearance and with maximum efficiency.
Opposite to the abradable shroud on the outer wall of the fan case you usually
find a strengthening ring element, which is known as the fan blade
containment. This ring element is made of a honeycomb construction covered
by layers of Kevlar. Kevlar, as you probably know, is a very strong material
which is also used to make bullet proof vests. The fan blade containment
prevents a fan blade from penetrating the fan case if it breaks during high
speed operation.
On other engines there is an extra thick fan case in the area of the fan blades
for the same purpose.
The outlet guide vanes are installed in the fan duct behind the fan blades. They
are bolted to the fan case and serve as stator vanes for the fan stage. They are
usually made of aluminium or of composite material.
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Figure 61
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Abradable Shroud, Fan Blade Containment and OGV
17|Fan Case Assemb|A,B1
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MODULE 15
FAN FRAME
The next main component of the fan module between the low pressure
compressor and high pressure compressor is the fan frame.
On other engines this sub-module is also called the intermediate case.
The fan frame is one of the main structural components of the engine. It has 4
main sections:
S the outer case,
S some struts,
S an internal structure
S and a centre hub.
The centre hub supports the forward bearings of the engine rotors. It also
houses the internal gearbox on the forward end of the N2−rotor shaft. Around
the centre hub is the internal structure which divides the fan air duct from the
core engine duct.
In the internal structure of the fan frame you will often find variable bleed valves
(VBVs). These bleed valves are used to protect the LPC against surge. The
variable bleed valves bleed off excessive air from the low pressure compressor
into the fan discharge duct.
On the aft side of the fan frame you will also find the forward engine mount.
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Figure 62
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Fan Frame
18|Fan Frame|A,B1
Page 125
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MODULE 15
Fan Frame cont.
The outer surface of the fan frame case gives support for engine equipment
like ignition exciters, oil tanks or computers.
The fan frame struts connect the internal and external sections of the fan frame
together. They carry the loads from the engine bearings to the engine mount.
The struts are aerodynamically shaped in the areas where they reach through
the fan duct and through the core engine duct. This design reduces the drag of
airflow as much as possible.
The struts are always hollow to save weight. Another advantage of their design
is that all supply lines between the outer case and the inner bearing sump can
be routed through these hollow struts.
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Fan Frame and Struts
19|Fan Frame/A/B1
Page 127
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MODULE 15
MAINTENANCE PRACTICES
FAN BLADE MOMENT WEIGHTS
Imbalance to the fan rotor occurs if you remove material from a blade. Another
cause of imbalance is if you have to replace fan blades. You get an imbalance
because of the difference in moment weights between the fan blades.
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Fan Blade Imbalance
01|Fan Bld Moment Weight|A,B1
Page 129
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MODULE 15
Fan blade Moment Weights cont.
Imbalance to the fan rotor occurs if you remove material from a blade. Another
cause of imbalance is if you have to replace fan blades. You get an imbalance
because of the difference in moment weights between the fan blades.
A rotor can run free of vibration if the fan blades that are opposite to each other
create identical centrifugal forces.
The centrifugal forces, however, depend on 3 parameters. These are
S the rotor speed,
S the blade mass
S and the distance of the centre of gravity of the fan blade from the rotational
axis.
The moment weight is the product of the blade mass and the distance of the
centre of gravity from the rotational axis.
It is worked out by a special blade scale.
The moment weight changes with the blade mass and with the position of the
centre of gravity of the fan blade.
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Moment Weight Scale
Figure 65
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Moment Weight Calculation
02|Fan Bld Moment Weight|A,B1
Page 131
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MODULE 15
Fan Blade Moment Weights cont.
Fan blades have a different moment weight even if they have the same mass.
This is because the centre of gravity of each fan blade is different because of
manufacturing tolerances.
Note also that the moment weight can be given by different dimensions on
different types of engines.
You may find the moment weight given
S in gram inches
S or in cmg
S or in oz inches.
You may remember that fan blades cannot be manufactured to have the same
moment weight.
For example, you will find that the fan blades on a CFM56−3 engine have
moment weights that vary between 29001 gram inches and 33951 gram
inches. This means that you will usually not find 2 fan blades with the same
moment weight.
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Figure 66
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Different Moment Weight Dimensions
03|Fan Bld Mom Weight|A,B1
Page 133
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MODULE 15
Fan Blade Moment Weights cont.
You can usually see the moment weight at the blade root.
On the blade root you will find several numbers.
There is a part number which is the same for all the fan blades on this engine
type, a serial number which identifies each individual blade that has been
manufactured. And finally there is the moment weight.
Sometimes the moment weight is crossed out and another number is shown.
If the moment weight of a fan blade has changed because of blending, the
original weight is crossed out and the new moment weight is marked on the
root next to it.
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Figure 67
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Numbers on Fan Blade Root
04|Fan Bld Mom Weight|A,B1
Page 135
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MODULE 15
COMPUTERIZED FAN BLADE DISTRIBUTION
All the blades with their individual moment weights are generally matched on
the fan disc by computerized balancing.
The computer works out the best position for each blade on a fan disc.
A numbering system is used to place the fan blades on the disc.
Fan blade no.1 can usually be identified by an index mark on the fan disc or by
a so called offset hole.
From fan blade number 1 onwards the other fan blades are numbered as
specified by the engine manufacturer. Often the blades are numbered in the
direction of the rotation but on some engines they may also be numbered in the
opposite direction of the rotation.
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Blade No.1
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Fan Blade Numbering System
05|Comput Fan Bl Distr|A,B1
Page 137
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MODULE 15
Computerized Fan Blade Distribution cont.
We will now look at an example of how the fan blades are matched on a fan
rotor. All moment weights of the necessary 36 fan blades for a fan rotor are
given to a computer.
The computer works out the best position for the fan blades and prints a report
like the one shown in the table. The table tells you the position of each fan
blade on the fan disc.
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Figure 69
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Computerized Fan Blade Distribution
06|Comput Fan Bl Distr|A,B1
Page 139
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Gas Turbine Engine
MODULE 15
FAN BLADE CHANGE
If you have to change single fan blades, then 2 methods are proposed by the
engine manufacturers.
The best method is to replace the blade with a new blade that has a moment
weight which is as close as possible to the old one.
If the difference in moment weight between the old and new blade is small
enough (as specified by the manufacturer), then often no further balancing is
needed.
If the difference is larger than specified by the manufacturer, then you can use
balance weights.
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Figure 70
HAM US/F
SwD
01.04.2008
Fan Blade Replacement Methods
07|Fan Bl Change|A,B1
Page 141
EJAMF
Gas Turbine Engine
MODULE 15
Fan Blade Change cont.
Balance weights are used to balance the difference in the moment weight of
the old and new fan blades.
These balance weights are installed on the fan disc or in the spinner cone.
Many different balance weights can be installed into the spinner cone. This
means that on these engines you must put the spinner cone back in exactly the
same position as before. That is why an off−set mounting hole is used to find
the correct position.
If you must use balance screws, then you must calculate which screws to use.
The calculation method is explained in the relevant maintenance manuals.
The other method for changing single fan blades is to always change a pair of
blades with similar moment weights even if only one blade is damaged.
This means that you replace the damaged blade and also the blade which is
opposite to the damaged one.
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MODULE 15
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Figure 71
HAM US/F
SwD
01.04.2008
Balance Weights
08|Fan Bl Change|A,B1
Page 143
EJAMF
Gas Turbine Engine
MODULE 15
HIGH PRESSURE COMPRESSOR
INTRODUCTION
The graphic shows an example of a classic design of a high pressure
compressor on a single shaft jet engine. The main components of this
compressor are:
S a compressor inlet case,
S a compressor rotor with rotor blades,
S a compressor case with stator vanes
S and a compressor outlet case.
The compressor inlet case supports the bearing of the front end of the rotor.
The bearing is located at the hub of the inlet case. Several struts connect the
hub to the outer part of the inlet case.
The compressor rotor mainly has a drum which supports the rotor blades. The
drum is made up of several discs which are connected together.
There is a drive shaft from the turbine connected to the end of the rotor.
The compressor case is the structural connection between the compressor
inlet case and the compressor outlet case. It supports the stator vanes of the
compressor and forms the outer wall of the compressor gas path.
The compressor outlet case is between the last compressor stage and the
combustion section. The case supports the stator vanes of the last compressor
stage and the centre section of the outlet case also supports the rear bearing
of the compressor rotor.
The compressor outlet case is often named the diffuser case because the gas
path through the case is a divergent duct.
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EJAMF
Gas Turbine Engine
MODULE 15
Rotor Blade
Stator
Vane
Compressor
Outlet Case
Inlet
Case
Rotor
Drum
Strut
Drive
Shaft
from
Turbine
FOR TRAINING PURPOSES ONLY!
Inlet Case
Hub with
Bearing
Rotor
Disc
Figure 72
HAM US/F
SwD
01.04.2008
Compressor
Rotor
Compressor Rear Compressor
Bearing
Case
Combustion System
Mounting Flange
HPC on Single Shaft Engine
01|Intro|A,B1
Page 145
EJAMF
Gas Turbine Engine
MODULE 15
Introduction cont.
On a twin spool engine the high pressure compressor is behind the low
pressure compressor. You can find the high pressure compressor inlet case
between the 2 compressors. It is known as the fan frame or the intermediate
case because of its position.
On modern high bypass engines the fan frame carries out the same function as
the compressor inlet case.
On modern engines the combustion case and the compressor outlet case are
combined in one component. The diffuser is part of the combustion case.
The rear bearing of the high pressure compressor is usually in the combustion
case between the compressor and turbine.
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HP COMPRESSOR
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MODULE 15
Fan Frame
Figure 73
HAM US/F
SwD
01.04.2008
HPC on Twin Spool Engine
02|Intro|A,B1
Page 147
EJAMF
Gas Turbine Engine
MODULE 15
ROTOR ASSEMBLY
A typical high pressure compressor rotor has
S a rotor drum,
S a forward hub,
S a rear hub
S and the rotor blades mounted on the drum.
There are 2 different construction methods for a compressor rotor.
In the older method, the disc type rotor is assembled from individual discs and
spacers.
These components and the rotor hubs are bolted together with bolts or long tie
rods as you can see in the graphic. Many tie rods are necessary to keep the
discs together.
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Figure 74
HAM US/F
SwD
01.04.2008
HPC Disc Type Rotor and Components
03|Rotor Assemb|A,B1
Page 149
EJAMF
Gas Turbine Engine
MODULE 15
Rotor Assembly cont.
Modern high pressure compressors have a larger number of stages ranging
from 9 up to 14 resulting in lots of individual rotor parts. The manufacturers aim
is to have as few parts to assemble as possible. So a design commonly used
on modern engines is of a rotor assembled from 2 or more integral drums.
In the example the compressor rotor has
S a forward drum,
S a centre drum
S and a rear drum.
Each of these drums is bolted to the adjacent drum and a separate disc. The
drums are connected to rotor discs by bolted joints. Note that the rear drum is
also bolted to the rear hub.
The forward hub is at the front end of the rotor. The rotor drums are made by
welding individual discs together at their circumference joint.
This forms a spacer between 2 discs as shown on the enlarged cross section
graphic.
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HP COMPRESSOR
Figure 75
HAM US/F
SwD
01.04.2008
HPC Rotor made of Discs and Drums
04|Rotor Assemb|A,B1
Page 151
EJAMF
Gas Turbine Engine
MODULE 15
Rotor Assembly cont.
On the outer surface of this compressor you can see groups of knife edges
between the blade rows. These knife edges are part of the seals formed by the
stator vanes and the compressor rotor. The knife edges form a labyrinth seal
with the seal surface on the stator vanes. This seal prevents reverse airflow
through the gap between the rotor and the stator.
Each HP compressor always has a CDP seal which is located behind the last
compressor stage. Here it is a separate disc with knife edges.
This seal prevents that the compressor discharge airflow can escape into the
bearing area below the combustion chamber.
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Figure 76
HAM US/F
SwD
01.04.2008
HPC Rotor Seals
05|Rotor Assemb|A,B1
Page 153
EJAMF
Gas Turbine Engine
MODULE 15
ROTOR BLADE FITTING
There are usually 3 different ways to fit rotor blades to a HP compressor rotor.
Two of these ways are used on this compressor rotor.
The first 2 stages on this compressor have axial dovetail slots in the rotor
discs. Rotor blades with dovetail shaped roots can be fitted into these slots.
The blades are held in place by blade retainers. This prevents axial movement.
The blade retainers are fitted into circumferential slots on the front face of the
disc.
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Figure 77
HAM US/F
SwD
01.04.2008
HPC Rotor Blades with axial Dovetail Roots
06|Rotor Bld Fitt|A,B1
Page 155
EJAMF
Gas Turbine Engine
MODULE 15
Rotor Blade Fitting cont.
On compressor stages with small rotor blades the discs have circumferential
dovetail slots. Each dovetail slot has 1 loading slot and 1 or 2 locking slots.
All the blades of 1 stage are inserted through a loading slot into a dovetail slot.
The blades are secured by a locking lug when all the blades for 1 stage have
been inserted.
The locking lug is held in position in a locking slot and kept in place by a screw.
The third attachment method uses bolts. The bolt is inserted through the disc
and into the blade root of each blade and locked by locking elements.
This kind of blade fitting is mainly used for the long blades at the first
compressor stages.
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HP COMPRESSOR
Figure 78
HAM US/F
SwD
01.04.2008
HPC Rotor Blades with circumf. Dovetail Slots
07|Rotor Blde Fitt|A,B1
Page 157
EJAMF
Gas Turbine Engine
MODULE 15
Rotor Blade Fitting cont.
The third attachment method uses bolts. The bolt is inserted through the disc
and into the blade root of each blade and locked by locking elements.
This kind of blade fitting is mainly used for the long blades at the first
compressor stages.
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Page 158
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HP COMPRESSOR
Figure 79
HAM US/F
SwD
01.04.2008
HPC Rotor Blade Attachment Methods
08|Rotor Blde Fitt|A,B1
Page 159
EJAMF
Gas Turbine Engine
MODULE 15
STATOR ASSEMBLY
The high pressure compressor case of a typical turbofan engine is split into a
front compressor case and a rear compressor case.
The front compressor case is usually made of 2 halves with a bolted
centreline joint. It supports the stator vanes in the first compressor stages.
Usually the rear compressor case is made of 2 halves, but it can also be made
as one single piece.
The rear compressor case is enclosed by the rear portion of the front
compressor case.
The front compressor case supports structural loads such as bending and
torsion so that the inner rear compressor case does not become deformed by
the loads.
This design principle allows closer clearances between the blade tips and the
compressor case.
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Rear Compressor
Case
Figure 80
HAM US/F
SwD
01.04.2008
HPC Cases
09|Stator Assemb|A,B1
Page 161
EJAMF
Gas Turbine Engine
MODULE 15
Stator Assembly cont.
Stator vanes can be grouped into segments or they can also be installed as
single vanes. They are usually installed in circumferential dovetail slots.
Usually the inner ends of the vane segments have an inner platform to
minimize vibrations caused by the airflow.
The vane segments have a seal surface for the labyrinth seal on the inner side
of the inner platform.
On some engines abradable linings are installed in the high pressure
compressor case.
The abradable linings are located on the inner compressor case wall opposite
the tips of the rotor blades.
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Figure 81
HAM US/F
SwD
01.04.2008
HPC Stator Vanes
10|Stator Assemb|A,B1
Page 163
EJAMF
Gas Turbine Engine
MODULE 15
Stator Assembly cont.
Often compressors with high compression ratios have variable stator vanes in
the first stages.
Variable stator vanes have a trunnion at each end. The outer trunnion fits into a
hole in the compressor case. The inner trunnion of the vane sits in a hole in the
inner shroud of the vanes.
The variable stator vanes are installed together with an actuation lever, a
bushing and some washers and are held in place by a nut.
To actuate the vanes, all the actuation levers of 1 compressor stage are
connected to the same actuation ring. So, all the VSVs move simultaneously.
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Variable Stator Vanes
Figure 82
HAM US/F
SwD
01.04.2008
Variable Stator Vanes
11|Stator Assemb|A,B1
Page 165
EJAMF
Gas Turbine Engine
MODULE 15
COMPRESSOR BLEED PICK - UPS
The HP compressor is the source of pressurized air for the pneumatic system
and also for engine related purposes.
For this task the compressor stator case has openings to supply the bleed air.
Bleed air is taken from different stages of the high pressure compressor.
The engine manufacturer selects the position of the bleed pick−ups on the HP
compressor depending on the pressure that is reached at each compressor
stage.
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Figure 83
HAM US/F
SwD
01.04.2008
HPC Bleed System
12|Compr Bl Pick−Ups|A,B1
Page 167
EJAMF
Gas Turbine Engine
MODULE 15
Compressor Bleed Pick - ups cont.
The bleed air is supplied through hollow stator vanes or through holes in the
outer platform of the stator vanes. The bleed pick−up holes are located around
the whole compressor case.
Bleed manifolds collect the bleed air from the pick−up holes and supply it to the
respective consumers. On some engines the bleed manifolds are formed by
the outer compressor case. Bleed ducts are fitted on the manifolds.
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Figure 84
HAM US/F
SwD
01.04.2008
HPC Bleed Pick - Up Holes
13|Compr Bl Pick−Ups|A,B1
Page 169
EJ M15.04 B1 E
TABLE OF CONTENTS
M15.04 COMPRESSORS . . . . . . . . . . . . . . . . . . . .
1
COMPRESSOR TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPRESSOR COMPONENTS . . . . . . . . . . . . . . . . . . .
CENTRIFUGAL FLOW COMPRESSORS . . . . . . . . . . . .
AXIAL FLOW COMPRESSORS . . . . . . . . . . . . . . . . . . . .
COMPARISON OF COMPRESSOR TYPES . . . . . . . . . .
2
2
8
14
18
COMPRESSOR OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NORMAL FLOW IN AXIAL COMPRESSORS . . . . . . . . .
COMPRESSOR STALL . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPRESSOR SURGE . . . . . . . . . . . . . . . . . . . . . . . . . . .
REASONS FOR STALL AND SURGE . . . . . . . . . . . . . . .
METHODS TO AVOID STALL AND SURGE . . . . . . . . . .
20
20
24
26
30
38
COMPRESSOR CONTROL SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . .
SYSTEM ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . . . .
CONTROL COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . .
VSV SYSTEM ACTUATION COMPONENTS . . . . . . . . .
VBV SYSTEM ACTUATION COMPONENTS . . . . . . . . .
FEEDBACK COMPONENTS . . . . . . . . . . . . . . . . . . . . . . .
VSV / VBV SYSTEM OPERATION . . . . . . . . . . . . . . . . . .
HPC BLEED VALVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
46
50
58
62
70
76
86
LP COMPRESSOR AND FAN MODULE . . . . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SPINNER CONE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FAN BLADE ATTACHMENT . . . . . . . . . . . . . . . . . . . . . . . .
FORCES AND LOADS ON ROTOR BLADES . . . . . . . . .
WIDE CHORD FAN BLADES . . . . . . . . . . . . . . . . . . . . . . .
LOW PRESSURE COMPRESSOR - ROTOR & STATOR
FAN CASE ASSEMBLY . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FAN FRAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
90
94
100
104
108
110
118
124
MAINTENANCE PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FAN BLADE MOMENT WEIGHTS . . . . . . . . . . . . . . . . . .
COMPUTERIZED FAN BLADE DISTRIBUTION . . . . . .
FAN BLADE CHANGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
128
128
136
140
HIGH PRESSURE COMPRESSOR . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ROTOR ASSEMBLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ROTOR BLADE FITTING . . . . . . . . . . . . . . . . . . . . . . . . . .
STATOR ASSEMBLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPRESSOR BLEED PICK - UPS . . . . . . . . . . . . . . . .
144
144
148
154
160
166
Page i
EJ M15.04 B1 E
TABLE OF CONTENTS
Page ii
EJ M15.04 B1 E
TABLE OF FIGURES
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Axial Compressor Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IGVs and OGVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compressor Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Centrifugal Flow Compressors . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multiple Stage Centrifugal Compressor . . . . . . . . . . . . . . . . . . .
Double Face Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Axial Compressor Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multiple Stage Axial Compressor . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of Compressor Types . . . . . . . . . . . . . . . . . . . . . . .
Airflow Changes at Compressor Rotor Blades . . . . . . . . . . . .
Airflow Changes at Compressor Stator Vanes . . . . . . . . . . . .
Compressor Stall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compressor Surge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fwd and Aft Compressor Surge . . . . . . . . . . . . . . . . . . . . . . . .
Reasons for Stall and Surge . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compressor Stall due to Low Speed . . . . . . . . . . . . . . . . . . . .
Compressor Stall due to Fast Deceleration . . . . . . . . . . . . . . .
Compressor Stall due to fast Acceleration . . . . . . . . . . . . . . . .
Methods to Avoid Stall and Surge . . . . . . . . . . . . . . . . . . . . . . .
Avoid Stall and Surge by Compressor Bleed Valves . . . . . . .
Avoid Stall and Surge by Multiple Rotors . . . . . . . . . . . . . . . . .
Avoid Stall and Surge with Variable Stator Vanes . . . . . . . . .
Feedback Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HP Compressor Bleed Valves . . . . . . . . . . . . . . . . . . . . . . . . . .
Compressor Control Schematic . . . . . . . . . . . . . . . . . . . . . . . . .
Compressor Control Components . . . . . . . . . . . . . . . . . . . . . . .
CIT Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ECU Signals for Compressor Control . . . . . . . . . . . . . . . . . . . .
VSV System Actuation Components . . . . . . . . . . . . . . . . . . . .
VSV Actuator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Bleed Valve Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VBV System with Hydraulic Actuators . . . . . . . . . . . . . . . . . . .
VBV with Hydraulic Motor & Gearboxes . . . . . . . . . . . . . . . . .
Ring Shaped Bleed Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VSV Feedback Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
51
53
55
57
59
61
63
65
67
69
71
Figure
Figure
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Figure
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Figure
Figure
Figure
Figure
Figure
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Figure
Figure
Figure
Figure
Figure
Figure
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
Electrical VSV / VBV Feedback . . . . . . . . . . . . . . . . . . . . . . . . .
LVDTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compressor Control Parameters . . . . . . . . . . . . . . . . . . . . . . . .
VSV Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VBV Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operation of VSV Control System . . . . . . . . . . . . . . . . . . . . . .
VSV Operation / Hydromechanical . . . . . . . . . . . . . . . . . . . . . .
HP Compressor Bleed Valves . . . . . . . . . . . . . . . . . . . . . . . . . .
Open HP Compressor Bleed Valves . . . . . . . . . . . . . . . . . . . . .
Fan Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Module Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spinner Cone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spinner Cone Ice Protection . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spinner Cone Attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Blades and Rotor Disk . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Blade Attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rotor Blade Forces and Loads . . . . . . . . . . . . . . . . . . . . . . . . .
Mid - Span Shrouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wide Chord Fan Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Low Pressure Compressor Module . . . . . . . . . . . . . . . . . . . . . .
LPC Rotor Drum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LPC Stator Assembly and Shrouds . . . . . . . . . . . . . . . . . . . . .
LPC Stator Case and Vanes . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Case Main Components . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acoustic Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abradable Shroud, Fan Blade Containment and OGV . . . . .
Fan Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Frame and Struts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Blade Imbalance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Moment Weight Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Different Moment Weight Dimensions . . . . . . . . . . . . . . . . . . .
Numbers on Fan Blade Root . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fan Blade Numbering System . . . . . . . . . . . . . . . . . . . . . . . . . .
Computerized Fan Blade Distribution . . . . . . . . . . . . . . . . . . . .
Fan Blade Replacement Methods . . . . . . . . . . . . . . . . . . . . . . .
73
75
77
79
81
83
85
87
89
91
93
95
97
99
101
103
105
107
109
111
113
115
117
119
121
123
125
127
129
131
133
135
137
139
141
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EJ M15.04 B1 E
TABLE OF FIGURES
Figure
Figure
Figure
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Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
71
72
73
74
75
76
77
78
79
80
81
82
83
84
Balance Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC on Single Shaft Engine . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC on Twin Spool Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC Disc Type Rotor and Components . . . . . . . . . . . . . . . . . .
HPC Rotor made of Discs and Drums . . . . . . . . . . . . . . . . . . .
HPC Rotor Seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC Rotor Blades with axial Dovetail Roots . . . . . . . . . . . . . .
HPC Rotor Blades with circumf. Dovetail Slots . . . . . . . . . . . .
HPC Rotor Blade Attachment Methods . . . . . . . . . . . . . . . . . .
HPC Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC Stator Vanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variable Stator Vanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC Bleed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HPC Bleed Pick - Up Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
143
145
147
149
151
153
155
157
159
161
163
165
167
169
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EJ M15.04 B1 E
TABLE OF FIGURES
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EJ M15.04 B1 E
TABLE OF FIGURES
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