EXHAUSTS Flashcards
What aircraft is a clamshell door type reverser and bucket type used?
Older jet engines with low bypass ratio.
Clamshell- Pneumatically operated with bleed air from HP compressor)
Bucket- hydraulically operated
What do clamshell doors do?
Block normal flow path and use exit ducts and cascade vanes to direct gas in forward direction.
What is only deflected on turbofan engines for thrust reverse?
Cold fan air. (Fan reverser)
Why are reversers which use the hot core engine gas with the thrust reverse inefficient?
Hot environment needs excessive maintenance.
Commonly what type are most fan air reversers?
Cascade type with translating sleeves.
Pneumatically or hydraulically
What is a pivoting door reverser?
Four large reverser doors.
Hydraulically operated.
What is the common exhaust nozzles main advantage?
Reduces very high gas velocity’s of not gas by mixing both of the gas flows
(Higher propulsion efficiency)
How can we guarantee laminar vortex free and axially orientated exhaust gas flow?
The gases must expand completely in the exhaust nozzle.
How is exhaust airflow created?
Pressure ratio between the turbine discharge and ambient pressure.
(Higher engine speed higher pressure ratio)
What pressure ratio will convergent exhaust nozzles reach maximum airflow?
2:1
Why would a nozzle become choked?
Higher pressure ratios. In the smallest area of the nozzle the exhaust gas velocity equals the speed of sound.
Why would the exhaust jet burst into all directions?
If the pressure ratio is increased, airflow discharges at the exhaust with static pressure which is greater than ambient.
(Inefficient)
What ratio are convergent divergent nozzles build for?
More than 2:1
Higher gas velocity = Higher thrust
What aircraft are convergent divergent nozzles necessary for?
Aircraft which fly faster than the speed of sound .
Must be variable design as sensitive to RPM changes
Will high velocity gases increase or decrease propulsion efficiency?
Decrease
Cause turbulence with ambient air and loud exhaust noise
What exhaust do modern turbofan engines have?
Convergent, large enough to not choke.
What is the common unit used to express noise?
Effective perceived noise deciBel (EPNdB)
Sound pressure, pitch, duration
What are the two methods to adopted to reduce engine noise?
- selection of design
- methods to dampen interior and exterior noise sources
How can the sound of turbofan engines be further reduced?
Increasing the bypass
How can we reduce internal noise?
Reducing airflow disruption by optimising number of blades and canes in compressors and distance between components
What is the method adopted to reduce the noise of transmissions?
Airflow linings use noise absorbent materials.
Porous surfaces, supported by honeycomb structure transferring sound into heat energy
How can jet noise reduction be achieved?
Increasing the contact area of the exhaust gas with the atmosphere
How is high frequency noise absorbed into the atmosphere?
Quickly
What is a hush Kit?
Additional mixing of ambient air with exhaust gas before it enters the nozzle.
What is the deflection of gas stream during thrust reverse?
45 - 60 degrees
180 degrees most efficient but bad for engine
What airflow do modern turbofan engines use for thrust reverse?
Fan airflow as it produces most of the thrust.
Why can the engine compressor stall during thrust reverse?
Airflow through the engine is disturbed.
What speeds are thrust reversers most efficient?
High speeds.
What can happen at low speeds with the thrust reverser?
Engine can ingest it’s own reverse flow.
Causing compressor stall and high temp exhaust gases
How can some thrust reversers increase efficiency of brakes?
Reversed airflow can cause an air cushion below the wing decreasing load on main landing gear.
How are the disadvantages of thrust reverse minimised?
Reverse airflow is matched to engine position.
What is a problem with thrust reverse on a 4 engine aircraft?
Inner engine can ingest outer engines reverse airflow.
How is the direction of reverse flow controlled to be as safe as possible?
Cascade vanes.
Can an aircraft be deployed with a faulty thrust reverser?
Yes, must be deactivated properly.
What are the 3 main subsystems of the thrust reverser?
Control system
Actuation system
Airflow deflection system
What is the control system in the thrust reverser for?
Activate the thrust reverser.
Increase engine power for reverse thrust.
What is the reverser actuation system for on thrust reversers?
Either pneumatic or hydraulic systems.
Moves airflow deflection mechanism by signals from control system.
What is the airflow deflection system for on thrust reversers?
Directs airflow in optimum position for safe reverse thrust.
What is the reverser control system always protected by to make sure they do not deploy in flight?
Air to ground logic.
What is an advantage of using an alternate signal from the radio altimeter to allow deployment of thrust reversers?
On short runways can be deployed shortly before touchdown when altitude is less than 10 feet.
What is the main task of an exhaust on jet engines?
Accelerate the airflow coming from the turbine to efficiently make thrust.
What is the main task of an exhaust in a turbo prop engine?
Safely guide exhausts overboard.
What does the exhaust cone do?
Guides discharged airflow and prevents excess turbulence.
Also prevents reverse flow of the exhaust gases to the hub of the rear turbine.
Where are long ducted exhausts only found?
On older aircraft where engines are directly under the wing
What two exhaust flows do modern turbofan engines have?
Hot exhaust from primary
Colder air via secondary
What does a thrust reverser interlock system do?
Ensures engine power can only be increased once the reverser has been fully deployed.
On what thrust reversers are pneumatic reservoir actuation systems only used?
Cascade type thrust reversers with translating sleeves.
Air motors operating drive shafts, gear boxes and ball screw actuators
What do pneumatic reverser system use to lock the stowed reverser?
Brakes on air motors.
What locks the reverser door in the stowed position in hydraulic systems?
Hook of the latch mechanism.
What do reverse unlock messages indicate?
Reverser is not safe in stowed position (amber lights)
What does a green light indicate for thrust reversers?
Reverser is fully deployed and interlock system is released (full power can be applied)
