John's Notes Flashcards
Gas turbine engine is what kind of cycle?
Gas Turbine Engine is the brayton cycle
meaning combustion occurs at constant pressure
What type of cycle is a piston engine?
Constant volume cycle
A turbojet engine has no _____
fan
Do fan blades change pitch like propeller blades?
No, fan blades are fixed pitch blades (always even number and typically 40)
What is ram ratio?
Ram ratio = compressor inlet pressure / ambient pressure
What is Ram temperature rise?
Ram temperature rise = compressor inlet temperature / ambient temperature
What is the simplest, most effective and most common type of inlet?
pitot type
What is a bifurcated intake?
2 inlets for the one engine
used on military aircraft
All subsonic ducts are ______ ducts
divergent
Dump and spill valves are used in which type of intake?
Moveable wedge intake
Bellmouth intake is gently ______ (divergent/convergent)
what is it used for?
convergent
helicopters and engine test cells
Compressor primary and secondary functions are
Primary = increase pressure
Secondary = cool engine hot parts, pressurise bearings, engine anti ice
Also supply customer service bleed air
CUstomer is airframe (pneumatics etc)
Compressor pressure ratio =
Outlet pressure / inlet pressure
What is the vaneless space in a compressor?
It is the space that exists between the impeller vane tips and the diffuser
What are the methods of securing compressor blades and why is a little bit of play given?
Fir tree and dove tail
To relieve stress and strains
Compressor stators and rotors are ________
while the overall compressor casing is _______
(convergent or divergent)
divergent
convergent
What is the compressor rise of an axial compressor vs centrifugal compressor?
1 or 2:1 per rotor/stator compressor but able to fit more on engine
Centrifugal is 10 or 15:1 but can only fit max 2 on engine
What is high aspect ratio means in relation to compressor blades?
Long and thin blades
What is a wide chord type compressor blade?
short and stubby
(type of aspect ratio)
There is always an ______ number of blades on a compressor for balancing reasons
even
Blade scatter is what?
Blades moving against each other
'’The blades , on moving, lean their shrouds onto the adjacent blades shroud’’
Compressor surge is what?
Surge is a reversal of airflow at the high pressure end
Another word for a mild stall is
transient stall
Another word for a severe stall is
Hung stall
The most efficient angle of attack of a compressor blade is _____ degrees
The AOA is based on what 2 vectors?
4
Engine RPM
Air Inlet velocity
Stoichometric ratio is what?
Stoichometric ratio is the ideal ratio to burn fuel in air and is 15:1, note it is by weight
15:1 represents
15 pounds of PRIMARY AIR and 1 pound of fuel
Name the 4 types of combustors?
- Multiple Combustion chambers
- Can-annular or tubo-annular combustion chambers
- Annular combustion chambers (most common)
- Annular reverse flow (turboprops)
There is only ever _____ (#) of igniters in a combustion section
There is fuel nozzles in every chamber however (2 sets of 10)
2
Primary air is what?
Primary air is that portion of air which mixes with the fuel for combustion purposes.
Tertiary air is what?
Tertiary air is air which acts as a cooling agent.
It takes no part in combustion
Secondary air is what?
Secondary air is that part of the air, which is metered into the combustion chamber to complete combustion
Swirl vanes do what and are located where?
Swirl vanes are located in the compressor section and act to slow down the air
Compressor blades are made of what material?
alluminium alloy, titanium or steel (cold section)
Turbine blades are made of what material?
Turbine casing is made of what material?
Turbine blades are made of nickel or nimonic based super alloy
Turbine casing is made of incanel
In general, describe Turbine nozzle guide vanes
Turbine nozzle guide vanes are convergent ducts that accelerate gases onto turbine blades
Internally cooled with compressor bleed air
(generally whether NGV’s are either straight or convergent is determined by if they are reaction or impulse, respectively)
Are turbine stators before or after the rotors?
Turbine stators are located in front of the rotors (turbine blades) and they are generally convergent increasing their velocity while reducing temp and press.
NOTE: Compressor stators are behind the compressor rotors(blades) and both are divergent to increase pressure
What is convection cooling in terms of Turbine blades?
Convection cooling is where compressor bleed air comes from the very bottom of the blade (fir tree area) and exits out the tip of the blade or at the trailing edge into the airflow.
The air makes a couple of passes through the blade to cool it
What is film/impingement cooling in terms of Turbine blades?
Film/impingement cooling is where compressor bleed air comes from the blade bottom (fir tree area) and passes through blade cooling it before being released through the leading edge of the blade forming a heat shield around it.
The more advanced version is called '’Transpiration’‘
What is the more advanced version of Film/impingement turbine blade cooling called?
'’Transpiration’‘
What is turbine blade creep?
Turbine blade creep is the elongation of turbine blades
It is divided into primary, secondary and tertiary creep
Single crystal construction is a turbine blade manufacturing method used to stave off _________
tertiary creep
Turbine blades are made out of what material?
Nimonic alloy, nickel based alloy
The future of turbine blade manufacture will be blades manufactured from reinforced _______
ceramics
Blisk is a combination of what?
A Blade and a Disk
both made as one
What are Reaction Turbine Blades?
Nozzle guide vanes are straight ducts that change the direction of the airflow onto the blades
The blades are convergent meaning they accelerate the airflow, reducing its pressure and temp
Describe Impulse Turbine Blades
Nozzle guide vanes or stator vanes are convergent to accelerate airflow onto blades
Turbine blades are straight ducts so velocity of airflow remains the same
These are usually found in pnuematic air starters
What are Impulse reaction turbine blades?
A combination of impulse and reaction turbine blades, varies with designer how much of each
What type of turbine blades are used with pnuematic air starters?
Pnuematic air starters = Impulse Turbine blades
Exhaust is a _____ duct
convergent
also called propelling nozzle
What is the only way to get more gas out of a ‘‘choked’’ nozzle?
To increase the gases temperature and thus its mach number
Afterburner of reheat is what?
Surplus oxgen
convergent-divergent duct always increasing velocity as starts supersonic
Area of nozzle opens up
As a basic rule, any sound over _____ dB is considered intense
Human earss can evalualte noise up to _____ dB
Anything over this is _______
100
120
damaging
What is a corrugated nozzle?
Cold air goes from fan duct across exhaust and mixes with hot air from core to get rid of thermal shock noise
Describe Multi-lobe nozzle?
splits exhaust into mulitple smaller exhausts increasing frequency of sound above what human ears can hear
The greatest reduction in noise level is achieved by the _____________
high by-pass ratio turbo-fan engine
Honeycomb structure in an engine is used for what?
noise supression
Name the hot thrust reversers
Name the cold thrust reversers
Hot = Bucket doors and clamshell doors
cold = translating sleeve and blocker doors
cascade vanes are incorporated into both
What is a meant by pour point of an oil?
The lowest temperature at which oil will gravity flow
Which has a lower freezing point:
Jet A1 or Jet A fuel
Jet A1
Explain flash point
Does fuel need a high or low flash point?
Flash point is the lowest temperature at which a liquid can give off vapor to form an ignitable mixture in air near the surface of the liquid. The lower the flash point, the easier it is to ignite the material
Fuel needs a high flash point
> _____ parts per million of free water (entrained water) is damaging and can cause engine flame out
30
If the amount of water detected in a fuel water test kit, the kit should turn ______
but in reality it will turn _______ _______
blue
greenish torquoise
Which type of water is dangerous in fuel?
Entrained (free) undissolved water or
dissolved water
Free water = entrained water or undissolved water and blobs of it in fuel are visible and problematic as if they freeze, they will block filters etc
Dissolved water is not a problem
Kerosene is ‘Hydroscopic’, explain
It absorbs moisture into itself, the warmer it is the more moisture it will absorb
What is meant by a ‘wet sump’ system?
Oil reservoir located at the lowest part of the engine as it uses gravity to return oil.
Oil reservoir is integrated into bottom of engine
What is meant by ‘‘dry sump’’ system?
The oil reservoir is located away from the engine near compressor or fan case (cold sections)
Needs scavenge return filters
Scavenge pumps are of atleast _____% capacity of the pressure system as ____ also comes back with oil in addition to oil expanding as hotter
150
air
Name and briefly describe the 3 types of oil pump?
- Gear pump (drive and driven pumps)
- Gerotor pump (male and female gears, cocentric, male has 1 less tooth)
- Vane pump (more tolerant of debris, prone to high rates of vane tip wear)
Filters are filtered _________ as greater surface area outside
Out-to-In
A micron is ________ of a meter
one millionth
