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66 Cards in this Set
- Front
- Back
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What is the main limiting factor upon the maximum temperature in the combustion chamber? |
Materials the turbine is made from and the heat it can withstand |
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What is the first thing done to the air entering the combustion chamber? |
It must be decelerated from 500 ft./s to around 80 ft./s to prevent flameout |
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What is the ideal kerosene to air ratio in the combustion chamber? |
15:1 |
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How is the air entering the combustion chamber divided up? |
40% goes into the primary zone, 20 to the fuel nozzle and 20% to form the toroidal vortex. 40% is used for cooling and 20% is used for dilution |
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What are the two types of fuel spray used? |
Atomisation and pre-vaporisation |
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What is the negative of using pre-vaporisation fuel nozzles? |
The vaporisation tube must be heated for affective of vaporisation takes place meaning that you still need a atomised spray for initial combustion |
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Describe the three types of combustion chamber |
Multiple combustion chamber: individual chambers and air casing, flame tubes are all interconnected Tubo-annular/can-annular: A number of flame tubes fitted inside a common air casing. Annular: Single flame tube with an inner and outer casing, |
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What is the advantage of using and annular combustion chamber? |
For the same power output the lake for the chamber is only 75% of the tubo-annular, Interconnectors are not required therefore propagation of combustion is improved. Calling is reduced by 15% increasing combustion efficiency |
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Why is the reverse flow combustion chamber used? |
You can reduce the length of the engine significantly by wrapping the combustion chamber around components such as the turbines |
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What is the combustion efficiency of most gas turbine engines? |
100% at sea level and 98% at altitude |
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Where does pollution come from? And what are the four main types controlled? |
Incomplete combustion of fuel, unburnt hydrocarbons smoke and carbon monoxide and oxides of nitrogen |
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What principal conditions affect formation of pollutants? |
Pressure temperature in time |
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What are the different methods of coolingthe flame tube? |
Corrugated strip cooling, machine cooling ring, splash cooling strip and transpiration cooling. |
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I want speed turbine rotate and what temperature does it act between? |
Irritates and speeds over 1500 ft./s it may be exposed to temperatures between 150 and 1700°C and velocities of over 2500 ft./s |
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What is the compromise made between efficiency and weight of turbine blades? |
At higher speeds the sexual sickness and the weight must be increased disproportionately to maintain the same stress levels |
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What time are three types of turbine blade? |
Impulse, reaction and a combination of the two known as impulse reaction. |
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Explain how each type of turbine blade functions |
Impulse blades use convergent shapes to increase the gas velocity and reduce the pressure Reaction type out of the gas flow direction without changing the pressure |
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Why is it important to remove the swirl from the air passing to the exhaust system? |
Residual swirl reduces the efficiency of the exhaust system and causes jet pipe vibration |
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Why are turbine blades given a twist? |
The twist insures that the air does equal work decreasing the velocity and increasing the pressure meaning that pressure and velocity will be uniform on entering the exhaust system |
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Describe the operation and construction of nozzle guide vanes |
of aerofoil shape forming convergent ducts, hollow to allow for cooling |
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What is used to control turbine blade tip clearance? |
Active clearance control (ACC) for the compressor is used to call the turbine casing to the optimum temperature for tip clearance An abradable lining is also used |
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What is a jewel alloy disc or blisk used for? |
Used in small high powered helicopter engines as a ring of cast turbine blades bonded to the disk |
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Described turbo nozzle guidevanes |
Temperature is the main concern so I construction of nickel alloy coated in ceramic and cooling air is used |
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What is creep how is it related to turbines? |
Creep is the extension of material over time due to thermal stresses |
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What is TBC in regards to NGVs? |
TVC is thermal barrier coating and is a ceramic coating to increase the temperature tolerance of blades |
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Describe the three types of crystal structure used in turbine blades |
Equi-axed: crystals that lie in all directions Directionally solidify blades: Columns of crystals along the blade length Single crystal: The entire blade has grown for a single crystal |
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What temperatures can the exhaust system operate in? |
550 to 850°C and over 1500°C for afterburners |
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What is the function of the cone in the exhaust system? |
The cone prevents exhaust gases from passing over the face of the turbine Increase Passager between the exhaust totally out of wall |
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What is used an exhaust system to remove swirl from the gas stream? |
Turbine rear struts straighten out the gas flow before passing to the jet pipe |
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What kind of duct is the exhaust? |
Exhaust is a converging duct increasing the gas velocity but when velocity is reach over Mach 1 converging diverging doctors used expanding rapidly |
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What type of engine uses a mixer unit? |
A low bypass engine, this insures a thorough mixing of the two streams |
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How is exhausted in a high bypass engine? |
Exhausted separately the atmosphere or through aCommon integrated exhaust nozzle |
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What material are exhaust systems made from? |
Nickel alloy or titanium to shells the rest of the frame from the heat. Insulation blankets are also used along side ventilation air |
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How does the annular space function in the exhaust system? |
The space between skin layers induces a flow of air by ejector action as the hot gassed leave the exhaust |
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What unit is the measure of noice annoyance level? |
Perceived noice decibels PNdB taking into account pitch and pressure |
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Describe the three sources of engine noise |
Exhaust jet: radiates rearwards cause by the mixing process of high-speed exhaust gases with surrounding air Turbine noise: white noise and discrete noise. White noise is the random background noise of air passing over the blades or turbulence in the air stream. Discrete noise is caused by the rotating blades as they pass through the wakes of the proceedings stationary veins. Compressor fan noise: large fans generate a lot of noise |
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What are the different types of noise suppression? |
absorption by acoustic lining Control of nozzle area Reduction of exhaust noise Fan duct cowling design |
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How does an acoustic lining function? |
Use of a porous face sheet and a honeycomb cavity section is always the appropriate parts of the noise spectrum |
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How does a variable nozzle intake function? |
The area of the hot stream to be reduced by approximately 50% |
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What is the difference between reversed thrust on a propeller aircraft and a turbo jet aircraft? |
On a turboprop aircraft reverse trust can be achieved by reversing the picture of the propeller I was on a turbo jet exhaust air must be vented forwards |
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What are the requirements for thrust reversal? |
Reasonable amount of thrust about 50% take off thrust Should not affect the normal working of the engine E.G.no increase in SFC Should not cause degree or hot air into the intake Hot gases should not impinge upon the nacelles, tyres, landing flaps, cabin windows Etc. Fire hazards must be avoided Weight complexity and cost kept to minimum Must not operate until required to do so ensuring that: Accidental selection of reverse to thrust is impossible No single failure in the operating system select reverse trust The thrust changing elements are biased away from the reverse position
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What are the three kinds of reverse thrust system used on turbojet? |
Clamshell doors Bucket doors Coldstream reverser system
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How does a Coldstream reverse system function? |
Actuated by an air motor, Translating Cowell moves rearward using the blocker doors to direct air from the bypass through the cascade veins pointing forward |
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What’s the difference between a clamshell door system and the bucket door system? |
The clamshell doors operate using pneumatic and are positioned in front of the mixer unit the bucket door system uses hydraulics and turns part of the exhaust cone into the doors |
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What are the safety features of reverse thrust? |
Can I be selected until engine throttle is brought to idle Mechanical lock prevents actuation unless selected Exhilaration forwards can only be obtained when reverse thrust lever is deselected and doors are closed And reversed roles going to be obtained when reverse thrust lever is selected and doors are opened Aircraft has to be on the groundAll very close when selecting reverse thrust |
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What happens if reverse thrust doors failed open? |
After 12 seconds a master caution is triggered no reverse trust is given |
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What is the bearing |
Any surface supports or is supported by another surface designed to produce minimum friction and maximum wear resistance |
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What speed can a shaft of the diameter of less than 1 1/2 inches run out without encountering serious vibration? |
25000rpm |
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Why are direct drive shaft not used |
Increased maintenance time as well as large internal gearboxes |
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When is an intermediate gearbox used? |
When it is not possible to directly align radio shafts and external gearboxes |
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What is the gearbox wrapped around the engine? |
To give a low frontal area to the airflow |
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What is a shear-neck and when is it used? |
It shears the drive shaft to prevent the unit from seizing and is not implemented in primary units as failure of these would cause the engine to shut down |
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As it provides the highest talk it should be positioned to give the shortest drive to the engine caught |
As it provides the highest talk it should be positioned to give the shortest drive to the engine caught |
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What is meant by hunting tooth ratios? |
This insures that each tooth of the gear does not engage with the same set of opposing teeth on the opposite gear spreading wear evenly |
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Why are helical gears used? |
The smooth running |
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How is the gearbox sealed? |
It is mainly sealed to prevent oil loss and contamination, manly using Labrinth seals or air blown seals (3psi) |
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What materials are the internal gearbox and external gearbox is made from? |
The internal gearbox is made from cast aluminium and the external gearbox is made from magnesium castings |
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What is the difference between a ball bearing and a roller bearing? |
Ballbearings can take both trust and radio loads where is roller bearings are used to support the shaft allowing axial movement |
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What must be ensured when installing an angular bearing? |
Installed the correct way round |
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How are the bearings in the bearing chamber lubricated? |
Pressurised oil is sprayed onto the bearings to lubricate and cool |
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Describe a squeeze film type bearing |
There is a small clearance between the outer race that is field of pressurised oil dampening radial motion and vibrations |
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Describe the five types of seals used in bearings |
Labrinth seals: A series of rotating cylindrical knife edge steps with precise clearances designed to control the pressure has airpasses over the knife edge Thread seals: the same as the Labrant seals but using a thread Carbon seals: assemblies using a mixture of carbon and graphite powder bound with viscous substances such as Coal tar. Held against the rotating seal by springs machine ground to a micro finish and should not be touched with bare hands Spring ring sale: when the shaft is not rotating see your clients tightly to the shaft expands on the centrifugal force Hydraulic seal: use a circular baffle ring that sits in the centre of the reservoir that is filled with oil held in position by centrifugal force causing a liquid seal |
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How many horsepower does that accessory drive system take away from the engine? |
400 to 500 hp |
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What is an issue when using centrifugal compressor is and reduction gearboxes? |
Space, when using such a fugal compressors internal gearboxes maybe located within a static nosecone more behind |
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What are the three different ways to mount onto driveshafts? |
Direct drive Stub shaft drive, using a splined drive allowing for actual movement Idler gear drive, Using streets burgers accommodating actual movement |
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What is the purpose of a radial driveshaft? |
The transmit drive from an internal gearbox to an accessory unit and also for engine starting purposes |
