15.7 Flashcards
In simple terms, what does the exhaust do?
Guides gases from rear of turbine into atmosphere
The exhaust nozzle does what?
Accelerates exhaust gases which leave the engine
What does the exhaust cone do?
Guides the discharge flow and prevents excessive turbulence of gases
Additionally stops reverse flow
Advantage of common exhaust nozzle?
Reduces high gas velocities of hot gas
Disadvantage of common exhaust nozzles is what?
Increases the weight of the engine
What do mixers do?
Mix hot and cold gas flow to get higher propulsion properties
Disadvantage of mixers?
Add weight
Why must exhaust gases expand completely in exhaust nozzles?
To achieve optimum thrust from a given mass
The exhaust airflow is caused by the pressure ratio between what?
Turbine discharge and ambient temperature
Increasing pressure ratio eventually leads to what?
To a point where it is impossible to get more air through the smallest area of the nozzle
Convergent exhaust nozzles reach max airflow at pressure ratios of what?
Approx 2:1
The choked condition means what?
Means that in the smallest area of the nozzle, exhaust gas velocity equals speed of sound
Convergent divergent nozzles are used for pressure ratios of more than what?
2:1
C-D nozzles are only necessary for a/c that fly at what speed?
Faster than speed of sound
Convergent duct is used for what speed?
Sub sonic
Convergent divergent nozzles are used for what speeds?
Supersonic
What is the unit used to express noise?
Effective perceived noise decibel (EPNdB)
Noise is minimised how?
Reducing airflow disruption which causes turbulence
How is reduction of noise achieved?
Optimising numbers of blades and vanes in compressor
Noise absorbent materials
Noise absorbent materials convert sound energy to what?
Heat energy
When air leaves the engine, how does the exit nozzle size correlate to sound?
The larger the exit nozzle the lower the sound
Noise of exhaust jet decreases with what?
Decreasing outlet velocity
Achieved by high bypass ratio
What do thrust reversers do?
Produce the braking force by deflecting the exhaust gas stream to the opposite direction
What is the ideal deflection degree?
Why don’t we use this and what is the actual degree?
180 to completely reserve thrust
We only use 45-60 as anything more would cause major issues for engine
What type of reverser is installed on older jet engine?
Clamshell door type
How is a clamshell door type reverser operated?
Pneumatically operated by HP compressor
How is a bucket door type reverser operated?
Hydraulically operated
On modern turbofan engine with high bypass ratio, what type of air is deflected to give reverse thrust?
What is this called?
Only cold air
Called fan reverser
How are pivoting type doors actuated?
Hydraulically
What are some effects of thrust reverser operation?
-Engine compressor can stall as airflow in engine is disturbed by deployed reverser
-Most efficient at high speeds, at touchdown, reverser gas stream is deflected by ambient airflow
-at very low speeds, the engine can ingest its own reversed airflow
How are disadvantages minimised for thrust reverser operation?
The reversed airflow is matched to engine position
What is significant about thrust reversers on four engine aircraft?
The angle of reverse stream is more important as as reversed airflow of inner engines can be ingested by the outer engines
If distance between engines is large enough what effect does reverse airflow have on neighbouring engines?
No effect
Can faulty thrust reversers be released to service?
Yes
As they are just additional braking
What is used to activate thrust reversers and increase engine power for reverse thrust?
Control system
What is the reverser actuation system?
Made up of pneumatic and hydraulic components
Moves airflow deflection mechanism by signals from the reverser control system
What does the airflow deflection system do?
Directs airflow in the optimum direction for safe reverse thrust
How does the thrust reverser lever work?
When pulled a reverser control switch below control pedestal is activated
-used to start reverser operation and control direction of thrust reverser movement
Where is the thrust reverser lever located?
In cockpit
Thrust reverser lever is always protected by what?
Air ground logic
What does the air ground logic do?
Ensures thrust reversers aren’t deployed during flight
What is the thrust reverser interlock?
Sub system of reverser control system
Ensures engine power can only be increased after reverser is fully deployed
Where is the thrust reverser actuation system located?
Located on reversers with large blocker doors
All thrust reversers must have what to ensure reversers are kept safe in stowed position?
Locking system
When a reverser door is stowed, the hook of the latch mechanism does what?
Secures the blocker door in the stowed position
Where is airflow deflection system found?
Found in reverser cowling
What is the airflow deflection system made up of?
A fixed part
A movable part
Translating sleeve
Blocker doors move with movement of what?
Translating sleeve
The cascade vane segments are bolted together to what?
A fixed part of reverser cowling
Different angles of cascade segments?
Some deflect 45 degrees upwards
Some 45 degrees downwards
Some 90 degrees straight out of fan air duct
Some segments blocked
The case case segments do what to direction of airflow?
Direct it away from aircraft structure and to the ground
Reverse unlock messages indicate what?
What colour light?
Reverser is not safe in stowed position
Amber light
A green reverse light indicates what?
Reverser is fully deployed and throttle interlock is released so that engine power can be increase for reverse thrust
