15.7 Flashcards
On a jet engine what is the main function of the exhaust?
To generate thrust
What is the purpose of airflow in a turboprop engine?
To safely guide the exhaust gasses overboard
How does the APU reduce noice?
Using mufflers in the exhaust ducts
What does the exhaust cone do?
Collects the exhaust gas from the turbine and converts it to a solid flow
Why does the exhaust section direct hot gases rearwards?
To prevent turbulence and give the gas high exit velocity
What are the component of the exhaust?
Exhaust cone, Exhaust nozzle Tail pipe
What is the purpose of the 3 or 4 radial hollow struts?
The inner cone from the outer duct
What happens to the velocity and pressure of the gas after going through the exhaust cone?
Velocity decreases and pressure increases
Why does the velocity decrease and the pressure increase?
The other duct and inner cone has a divergent passage
What helps the hollow struts to support the inner cone from the outer duct?
Tie rods
How are the struts held in place?
Spot welded
What other function do the radial hollow struts perform? At what degree?
They straighten the swirling air and ensure it that would leave the turbine at 45 degrees.
What is the cone supported by?
Radial struts
What does the small hole located in the exit tip of the cone allow?
Cooling air to be circulated form the aft end of the cone (HP) to the turbine wheel
The tailpipe is contracted to ensure it is what?
Semi flexible
Why is a bellows arrangement constructed into tailpipe construction?
Allows movement, thermal expansion (reduces warping and stress) and maintenance.
What 2 methods of insulation protects the fuselage by reducing heat loss from exhaust system?
Insulation blankets and shrouds
What is the insulation blanket made up of?
Aluminium and fibreglass
What are the two types of exhaust nozzles?
Converging (Subsonic)
Converging diverging (supersonic)
Turbofan engines can be of what two types?
Ducted fan or unducted fan
What do ducted fan engines do?
The fan airflow is directed through closed ducts until it reaches the common exhaust nozzle where it mixes with the core airflow
How does the unducted fan engine work?
It has two nozzles, one for fan airflow and one for core airflow that flow out the exhaust separately from each other
What straightens the air as it come out of the turbine?
a converging nozzle
What happens to the exhaust air before it flows out of the exhaust?
Velocity increases which increases the thrust by 15%-20%
What is the percentage of thrust that turboprop produces?
10-15%
On a turboprop, what is the exhaust section known as?
A tailpipe
Where is the tailpipe mounted and what does it attach to?
In the nacelle and the forward end of the tailpipe is connected to the firewall.
What causes the air around the engine hot section to flow through the annular gap into the tailpipe?
Low pressure and high velocity
What shape is the forward end of the tailpipe?
Funnel shaped
What can happen if the area of the nozzle is too large?
Thrust can be wasted
What can happen if the area of the nozzle is too small?
The air can choke or stall
When is a variable area exhaust nozzle used?
When an afterburner or augmenter is used (because of increase mass flow)
What happens to the exhaust size when the afterburner is activated?
The open area increases
When does the exhaust area get smaller?
When the afterburner is not in function
What type of duct is formed in the first part of the exhaust nozzle?
Divergent
What type of duct Is formed in the second part of the exhaust nozzle where the flow is restricted by a smaller outlet opening?
Convergent
What does adjusting the area/diameter of the exhaust nozzle change?
The temperature of the air and its performance
At what Mach would you use a convergent-divergent duct?
Exceeding Mach 1
Advantage of convergent divergent nozzle?
High Mach numbers due to the high pressure ratio across the engine exhaust nozzle
What section of duct handles air at subsonic?
Convergent
What section of the duct handles the air at supersonic?
Divergent
Were does engine noise come from?
The fan, the compressor, and air discharge from the engine exhaust
Which source of noise is the loudest?
Exhaust discharge air
How is noise in the exhaust produced?
By air at high turbulent velocity moving through a quiet jet stream.
What influences the sound of the violent, turbulent mixing of exhaust gases with the atmosphere?
The shearing action caused by relative speeds between velocity and the atmosphere
As the velocity of the jet stream slows down, what happens to the mixing of atmosphere and turbulence?
It becomes coarser
What do engines that have low airflow but high thrust (due to high turbine discharge) have in terms of noise level?
They’re louder
Why are large engines quiter?
They have greater mass flow (At slower rates) (not little so fast)
How can engine noise be reduced by operating?
Operate the engine at a lower power with partial thrust
What does changing the noise pattern or the frequency of noise emitted by the jet nozzle do?
Makes the engine quieter
What do the corrugated perimeter type or multi tube type do?
It breaks up single, main jet exhaust into several jet streams
What are the two typical noise suppressors currently in use?
Corrugated perimeter type or multi tube type
What two effects does breaking the airstream up into smaller parts have?
Change in frequency may put the noise above audible range for human ear
High frequencies are more highly attenuated by atmospheric absorption (Fall off in intensity is greater and nose level is less at any given distance)
Where are acoustic linings on an engine?
Between the engine and the cowl
What do acoustic linings do?
Transfer acoustic sounds into heat
What do the linings consist of?
Porous skin supported by honeycomb and separations
How do thrust reversers manipulate airflow?
They deflect it in the opposite direction
On turbofan engines, what airflow is reversed and why?
Fan airflow because it has as high bypass ratio therefore the fan air creates most thrust
Where are thrust reversers only used?
On ground
What aircraft use a common exhaust nozzle?
Turbofan engines with a low bypass ratio
Where is the common thrust reversers sit?
In front or at the exhaust nozzle
What thrust reversers sit at the exhaust nozzle?
Bucket type
What type of thrust reversers sit in front of the exhaust nozzle?
Clamshell
When is a clamshell door-type and Bucket type reversers used?
On older jet engines with a low bypass ratio
How are clamshell door type reversers operated?
Pneumatically by HP bleed air
When clamshell doors rotate to their position, what do they use to block off the normal flow path and uncover exit ducts?
Cascade veins
How are bucket doors operated?
Hydraulically
On a turbofan engine with a high bypass ratio, what is only deflected to give reverse thrust?
Fan air (So it is called the fan reverser)
What type are fan reversers?
Cascade type (With a translating sleeve)
How are fan reversers operated?
Hydraulically (actuators) or pneumatically (drive motors)
What engines are pivoting door reversers used on?
Turbofan
How are the pivoting doors operated?
Hydraulically
In what direction do pivoting doors send the fan airflow?
Forward
Because of disturbed airflow from the thrust reverser, what can happen as a result?
The compressor can stall
Because thrust reversers are efficient at high speeds, what disadvantage does this cause?
The reversers gas stream gets nearer to the engine which means it can ingest its own airflow so either stall or get hot.
What ;act occurrence can damage the engine due to reverse thrust?
FOD as the airflow can swirl it in
How do you minimise the damage reverse airflow has on the engine?
Match the flow to the engine position compensated by the cascade vanes
When is the direction of airflow not matching the engines most important?
On 4 engine aircraft as you have twice the risk of the reversed air being ingested
When is the worry of reversed airflow being ingested settled?
When the engines are far from each other
What is an advantage of all the aircraft having the same cowlings?
Fewer spare parts are needed
What must you do when a thrust reverser is defective?
Deactivate them so they don’t unintentionally deploy.
What do thrust vectoring in military aircraft encourage?
Manoeuvring to change direction by relocating the direction of the exhaust nozzle.
