15.12 Air Systems Flashcards
What 3 subsystems can the engine air system be divided into?
1) the compressor control system
2) the clearance control system
3) the cooling system
What is the purpose of the compressor air control system?
It prevents the compressor from stalling or surging and increases the efficiency
What is the purpose of the clearance control system?
It increases the engine efficiency by reducing the space between the rotating turbine blades and the casing (tip clearance)
What type of system is the engine cooling system usually?
Passive
What controls the compressor air control system?
The F.A.D.E.C or the hydro-mechanical fuel control unit
What are the three subsystems of the compressor air control system?
1) the variable bleed valve system
2) the variable stator vane system
3) the HP compressor bleed valve system
What are VBV’s?
Variable bleed valves
What are the VBV’s responsible for?
Varying the airflow to the high pressure compressor by bleeding air from the LP compressor to the bypass duct which prevents surging and stalling of the LP compresssor
What are VSV’s and what are they for?
Variable stator vanes
They are used in the HP compressor during engine start and during acceleration so that the airflow through the HP compressor is not too large (optimising efficiency)
Some engines may use bleed valves at the HP compressor stage. If so, where would these be found?
At the mid or aft stages if the HP compressor.
Known as start bleed valves (as usually only used on startup)
What is the purpose of a tip clearance control system?
It matches the expansion of the stator cases and the expansion of the engine rotors.
Where are clearance control systems usually found?
The HP and LP turbines
Some engines also include the HP compressor
What is an air passive control system?
An air system that cannot be switched off
What is an air active clearance control system?
An air system that has cooling air control valves. These systems optimise efficiency of the engine
How are the active clearance control systems controlled?
By the FADEC system or Hydro-mechanical fuel control unit
What air does compressor clearance control systems use?
Warm air heating the rotors instead of cool air cooling the case
Why can’t you use cold air in the compressor clearance system?
The air available isn’t cold enough for case cooling
What two parts is the engine cooling system divided into?
- external cooling system
- internal cooling system
How are the components in the fan case cooled?
By using RAM air
What is the purpose of engine cooling?
It cools the engine and components making it more efficient and also prevents a build up of flammable gases
Where does the air come from for the internal cooling system?
The LP or HP compressor
What other purposes, other than cooling, does the internal air system have?
- air for sealing in bearing compartments
- pressure balancing
What is pressure balancing?
Using the internal air system of the engine to reduce axial thrust loading on the rotor bearings
Where does the air that is used for sealing in an engine escape?
Through the hollow N1 shaft
Which way does the pressure act on the…
- compressors?
- turbines?
Compressors = forward on the rotors Turbines = in the aft direction on the rotors
What type of intake is used for the air cooling of the fan case?
NACA air intake
When is there positive ventilation of the core compartment of an engine?
When the engine is operating; by using fan air
What is a bowed rotor?
The condition that can occur when an engine shuts down and cools at different rates
How does an engine cool after it has been shut down?
By convection (warm air rises)
If an engine has just been shut down, when can it be started again?
Depends on engine manufacturer - usually minimum waiting time between shut down and subsequent engine start
