M15.4 Flashcards
What does the rotor do in the compressor?
Sucks in air and pushes it to the outlet side of the rotor
What do stator vanes do?
Guide and slow the airflow to increase pressure
What comes first the rotor or the stator?
The rotor
What is the job of a inlet and outlet case?
To improve and straighten the airflow into the compressor
How many stages of centrifugal flow compressors are the most effective?
Two stages
What is meant by double entry type impeller (double face compressor)?
Two compressors face to face
Why is a double entry type impeller used on APUs?
Because it has a higher airflow at a smaller diameter which saves weight.
What are the channels between the rotor blades (in the stator stages) called?
Diffusers (increase static pressure)
What is the pressure ratio for each stage of an axial flow compressor?
1.3:1 (enters at 10psi leaves at 13psi)
Why does the compressor cross section decrease through out the compressor?
Because as the airflow gets compressed the pressure and density increases so the air needs less volume
What is the difference between axial flow compressors and centrifugal flow compressors?
Axial flow keeps it going in a straight line but a centrifugal pushes it out to the side
What is the design point of compressors?
The optimum operating condition
What is the resultant rotor inlet velocity?
The relationship between the inlet velocity and the rotor speed
What does compressor stall do to the airflow inside?
It can slows down, stop, or reverse its direction
What is compressor surge?
When there are multiple compressor stalls on the rotors of the compressors
What happens if there is a compressor surge in the first few stages of the compressor?
It usually affects only a part of the compressor blades due to there large diameter
How is compressor stall and surge caused?
By problems i engine operation or by damaged components
What happens if the air piles up in the compressors?
The air slows down too much and creates a stall or surge
What would fast acceleration lead to?
Too much fuel in the combustion chamber which increase back pressure
What is used to prevent stall and surge in a compressor?
- compressor bleed valves
- use dual or triple pool rotors
- variable compressor stator vanes
Where are compressor bleed valves found?
In the mid or aft section of the compressor
How do compressor bleed valves work?
They would open at low engine speeds for excessive airflow from the forward stages
What is the most effective method to prevent stall?
Variable stator vanes (they can change there angle of attack so it can always operate at optimum efficiency)
Where would variable stator vanes be found?
In the forward stages of the high pressure compressor
What is the main disadvantage of variable stator vanes?
They’re a very complicated control mechanism
What is the job of the fan blades?
To provide initial compression and then pass the compressed air to the engine primary and secondary gas paths
What is the job of the low pressure compressor and low pressure turbine couplings?
To secure them to the low pressure shaft
What is the job of a fan stator case?
To give a flow path for the fan discharge air
Why is the spinner cone curved or sharp?
Because they’re the best shapes to prevent ice build up
Why do some spinner cones have a soft tip?
To cause a small imbalance during operation which prevents large ice build up
Why is there a spiral on the spinner cone?
To show if it’s rotating and to share birds away
Where are balance eights attached?
To the mounting bolts of the spinner cone
What type of fitting do the fan blades use to sit in the fan rotor disc?
Dovetailed fitting
Why is there a loose fit between the blade root and blade seat?
So that sudden forces or torque loads do not exceed the maximum allowable stress
When flan blades are suddenly hit they deflect rear wards and forwards, whys this?
Because they can slide and cause friction which dampens the blade fluttering
Why is the shroud surface covered in a special coating?
To make sure they can slide on a dry surface which will leads to a breakage
Where is a fan shroud located?
Mid way up the blade
Why don’t wide chord fan blades have shrouds?
Because of the strength of the wide fan blade
What is special about the wide chord fan blade?
It has hollow sections in the blade
What is another name for low pressure compressor?
A Booster module
What are the two types of construction for compressor stator cases?
Closed stator case and a split stator case
Why are there multiple ring elements (flanges) on the outer surface of the case?
To help stiffen and prevent deformation
What are the lining elements on the inner surface of the fan case called and why are they used?
Acoustic panels and they have small holes to reduce noise
Why is an abrasive shroud used on the inner fan case?
To prevent damage to the fan blade tips when they come in contact at high speeds
What is the flan blade containment area made out of?
A ring of honeycomb construction covered in Kevlar
What are outlet guide vanes in the fan duct normally made out of?
Aluminium or composite
What main component can be found in between the low pressure compressor an high pressure compressor?
The fan frame (also known as intermediate case)
Where would you find the forward engine mount?
The aft side of the fan fram
why are the fan frame struts hollow?
To save weight
How is the moment of a fan blade worked out?
By using a special blade scale
Where would you find the moment weight on a fan blade?
On the blade root
What can be found on the blade root?
- a part number
- a serial number
- a moment weight
What is meant by computerised balancing?
When a computer works out the location of the fan blades for balancing reasons
What way round are most engine fan blades numbered?
The way of rotation
What are the methods to change a sinGle fan blade?
- Get the moment weight as close to the old blade as possible
- if the difference is too high add balance weights
- change the one blade and the opposite one
Where is the compressor outlet case found?
Between the last compressor and the combustion chamber
What is another name for the compressor outlet case?
The diffuser case
What is the older way of constructing a compressor rotor?
The use of discs and spacers which are put together using tie rods
What is the new way of constructing a compressor rotor?
By using multiple integral drums
Where is the rear drum bolted to?
The rear hub
Where is the CDP seal located and what does it do?
It’s located behind the last compressor stage and is used to prevent discharge of airflow from escaping into the bearing area
What are the three ways of connecting a fan blade to the fan disc?
- axial dovetail slot
- circumferential dovetail slot
- locking bolt slot
What forces does the front compressor case support?
Bending and torsion
What is used inside a high compression ratio?
Variable stator vanes in the first stages
