AIR SYSTEMS Flashcards
What subsystems fan engine air systems be divided into?
- Compressor control
- clearance control
- engine cooling
What is the purpose of the compressor control sub-system?
Prevents the compressors from stall or surge in all operating conditions
What does the clearance control system optimise?
Tip clearance between the rotor blades and stator casing to increase engine efficiency
What 3 subsystems are contained within the compressor control system?
- variable bleed valve
- variable stator vane
- HP compressor bleed valve
(Controlled by FADEC or hydro-mechanical fuel control unit)
How is the airflow into the high pressure compressor controlled?
VBV’S
What do VBV’S do?
Bleed some air off the LP compressor into the fan air discharge duct.
(Protects the LP compressor against stall and surge)
What do all modern HP compressors have to control the airflow into the compressor?
Several stages of Variable stator vanes
What are bleed valves at the HP compressor for?
Improve engine start and acceleration capabilities
Mid and aft stage of the HP compressor
What is the main purpose of the clearance control system?
Match the expansion of the stator case with expansion of the engine rotor
(Found in LP, HP turbines and HP compressor)
What are passive clearance control systems?
Use cooling airflow that cannot be switch off
Cannot optimise tip clearances for pal operating conditions
What is active clearance control?
Cooling air control valves. Control the amount of air used to cool.
(Controlled by FADEC or hydro-mechanical fuel control units)
What air do compressor clearance control systems use to heat the rotor?
Warm air. (Increasing expansion and reducing tip clearance)
What can the engine cooling system be divided into?
External cooling system
Internal air system
Where does the external cooling system supply air to?
Cavities between engine cowlings and engine cases.
Using ram air or fan air
What does the internal air system do?
Covers all airflow below the engine rotors and at bearing compartments.
(Using air from LP and HP compressor)
Internal cooling.
Where is some air from the internal air system used for sealing?
Some bearing compartments. (LP compressor bleed air)
Prevents oil escaping.
What is pressure balancing a method of?
Release axial loads on the thrust bearings in the engine.
Mostly internal airflow used
Where does the air for sealing bearing compartments exit?
Hollow N1 rotor shaft.
What does pressure balancing do?
Minimises the axial loads on the rotor bearing.
What does external cooling do?
Cools engine cases and all components in the fan and core compartment.
Prevents flammable vapours collecting in the engine.
What do NACA air intakes in the lower part of the fan cowls do?
Pick up ram air when the aircraft moves and ventilates the fan compartment.
How is the core compartment usually ventilated and cooled?
Fan air.
What is bowed rotor?
An engine can bend after engine shutdown due to expansion between the upper and lower engine zones.
What can a bowed rotor lead to?
A hung start or rotor wear.
Recommended there be a minimum waiting time between engine shutdown and engine start
How is an engine only cooled after shutdown ?
Convection.
Lower part gets colder than upper part. Warmer air leaves out the upper zones.
