Basic Gas Turbine Flashcards
How does thrust relate to drag in steady level flight? What type of engine can provide thrust force?
Thrust = Drag
Force provided by internal combustion engine (heat engine)
What are the two main types of internal combustion engines in aviation?
- 4 Stroke piston
- Gas turbine (Jet engine)
What types of jet engines are used on fixed wing vs rotary wing aircraft?
Fixed: Turbo-jet, Turbo-prop, Turbo-fan, Advanced Turboprop (more than 4 propellers)
Rotary: Turbo-Shaft
What does GTF stand for with respect to a type of gas turbine engine? What is it? What reduction ratio is achieved?
Geared turbo-fan
Engine where there is a reduction gearbox for the front fan
Fan speed is half that of the turbine
What is Newton’s third law of motion?
Every action (force) has an equal and opposite reaction (force)
How is forward force generated when air is accelerated through the engine?
- Atmospheric air is accelerated as it passes through the engine
- Force required to produce this action, has an equal and opposite reaction
- Reaction force accelerates the apparatus in the opposite direction to the air
What is the difference between the propeller and jet engine in terms of acceleration of air?
Prop: Accelerates a large mass (slipstream) of air a relatively little amount
Jet: Accelerates a smaller jet of air to a comparatively high speed
When is the first known action of a jet reaction ‘engine’?
A Hero’s toy in 120BC
Spins a thing with water in it due to 2 nozzles out the side for steam to escape
Where does the jet reaction stem from? What is the common misconception?
- Jet reaction occurs internally
- Not with the exhaust reaction with atmosphere
What is the resultant reaction (thrust) produced on the engine proportional to?
Mass of air accelerated
Change in the velocity of the air
What is Bernoulli’s principle?
The sum of potential energy (pressure) and kinetic energy (velocity) of a fluid flow remains constant.
C = P(s) + 1/2ρV^2
How do static and dynamic pressure vary through a C-D nozzle?
Inlet: High static, Low dynamic
Throat: Low static, High Dynamic
Outlet: High Static, Low Dynamic
Describe where each stage of the Bratton cycle occurs in a gas turbine?
Induction: Inlet of the engine
Compression: Compressor stages
Expansion/combustion: In the combustor/burner can
Exhaust: Through the turbines and exhaust nozzle
Describe the Bratton cycle considering volume and pressure?
Compressor: pressure increases and volume decreases
Combustor: Pressure decreases slightly, Volume increases due to pressure trying to increase
Turbine/Exhaust: Pressure decreases to ambient and Volume increases
Ambient air: Pressure remains ambient, volume decreases
What is the generic/ overview description of the Bratton cycle?
A constant pressure cycle where air is the working fluid of the engine
What is the most common air intake fitted to Turbo-jet or turbo-fan engines? Why?
Short - Pitot type circular
- Maximum benefit from ram air due to forward airspeed
- Minimum loss of ram pressure with altitude changes
What are the advantages of the centrifugal compressor? What settings is it preferred in?
More robust
Easier to develop and manufacture
Preferred in small engines where simplicity and ruggedness is required
What are the advantages of the axial flow compressor? (7)
- Consumes more air for same frontal area
- Attain higher pressure ratios
- Give more thrust for the same frontal area
- Can have additional stages to increase pressure ratios more
- Improved efficiency
- Improved SFC for a given thrust
- Easier to maintain
What is the principle of operation of the centrifugal flow compressor?
- Compressor receives air at the centre of impeller
- Accelerates air outwards by centrifugal reaction to its rotation
- Air allowed to expand through diffuser
- As volume increases, speed decreases and static pressure builds
Describe the centrifugal compressor impeller?
- Forged disc with integral, radial disposed vanes
- Vanes on one or both sides
- Convergent passages formed by the impeller and casing
What variations are there of the vanes of a centrifugal impeller?
- Swept back vanes
- Straight radial vanes (Easy manufacture)
What is the purpose of a diffuser in a centrifugal compressor?
- To slow the air creating a pressure rise
- To convert kinetic energy into pressure energy
How are the diffuser vanes integrated/ arranged into the centrifugal compressor?
Vanes are tangential to the impeller
Vane passages are in line with the direction of the resultant airflow from the impeller
What is the principle of operation of the axial flow compressor?
- Airflow and compression flow parallel to the rotational axis of the compressor
- Rotor is turned at high speed by the turbine drive shaft
- Air continuously induced into the compressor
- Air accelerated by rotors and swept back onto stators
- Air decelerated in next stator stage and converted into high pressure
- Stator passages are divergent
What is one stage of a compressor on the same spool?
One stage is one set of rotors followed by one set of stators
What is the purpose of the inlet guide vanes?
To direct airflow into the first rotor at the most desirable angle
What shape and properties do compressor rotor blades have?
Aerodynamic design
Varying angle of incidence
Similar to a propeller
What is the purpose of the stators in an axial compressor? What effect do they have on the airflow properties?
To receive high velocity air from the rotor blades
To act as a diffuser
To convert kinetic energy into potential energy in the form of pressure
What shape and movement ability do axial compressor stators have?
Aerofoil shaped blades
They are stationary
What are the advantages of multi-spool axial compressors compares to single spool? (6)
- Operational flexibility
- Higher compression ratios
- Quick acceleration
- Better control of stall characteristics
- Improved efficiency
- Allows Low pressure compressor (N1) to speed up as air density reduces, while the N2 and N3 compressors remain at optimum RPM
How are different stages of a Gas generator designated on cockpit instruments when there are single or multiple spools in the engine?
Single spool: Ng
Multiple Spool: N1 low pressure compressor, N2 intermediate compressor, N3 High pressure compressor
What is a gas generator?
Any combo of a compressor and turbine, known as a spool
What is a compressor stall? What are the symptoms? (More detail on how each responds)
The abrupt loss of efficiency of the axial flow compressor when the AoA of the compressor blades becomes excessive
- Normally no symptoms/warnings of complete stall
- Some vibration
- High gas Temp,
- Fuel flow Indications fluctuate
- Thrust indications fluctuate/ Aircraft surges
- Engine sneeze/ loud bang
What are the causes of a compressor stall? (6) How do they help to stall the engine?
- Turbulent/disrupted inlet airflow (Reduce gas velocity)
- Excessive fuel flow due to engine accelerations (Reduced gas velocity, Increased combustion back pressure)
- Excessive lean mixture caused by abrupt deceleration (Increased gas velocity, Reduced combustion back pressure)
- Damaged or contaminated compressors (Increased gas velocity by reducing compression)
- Damaged turbine components, causing loss of power to compressor (increase gas velocity by reducing compression)
- Operation outside RPM envelop
How can a compressor stall cause sometimes be simplified to be?
An imbalance between the inlet velocity and the Compressor RPM
(Velocity and RPM combine to form vector that makes relative airflow and then it hits a propeller at an AoA)
What is the purpose of the combustion chamber? What must it be able to do?
Burning fuel supplied by nozzles with air supplied by the compressor.
Releasing heat energy
Expanding and accelerating air to give a smooth stream of uniformly heated gas for turbine operation
What overall mixture ratios can be found in the combustion section of the engine? What does the mixture ratio need to be to burn?
From 45:1 up to 130:1
Needs to be = 15:1 (15 air:1 fuel)
How is the air/fuel ratio decreased in the combustion chamber? What is the percentage that is needed?
- Only part of the air enters the primary combustion zone
- A flame tube meters airflow so only 20% of mass airflow is taken in the snout (entry)
How is the total air entering the combustion section divided?
Primary: 20%
Secondary: 80%
Primary Air:
12% flows axially through swirl vanes
8% enters radially
Secondary Air:
50% cooling blanketed over both sides of the liner
50% becomes tertiary air joining at rear liner and cools gas stream (20% of this will be burned)
What is the primary function of the turbine? What else does it do?
To drive the compressor
Can also drive accessories, shaft of turbo-prop or turbo-shaft engines
What is the principle of operation of the turbine?
- Converts kinetic energy and heat energy into mechanical work
- Extracts energy by reducing the pressure
- pressure reduced due to velocity increase at the convergent nozzles formed at trailing edge of the stator and rotor blades
How do temperature, velocity and pressure change in the turbine section of the engine?
Temp: decreases linearly
Pressure: initial rapid decrease, gradually reduces
Velocity: Increase and the alternating decrease/ increase
What operating conditions does the turbine have to endure?
The hottest gases in the engine, right at the front of the first turbine
What are the limiting factors of the turbine?
-Material construction to withstand high temperature
What causes losses in efficiency in the turbine?
Thermodynamic losses
Mechanical losses
Aerodynamic losses in turbine blades and nozzle guide vanes due to gas leakage over the blade tips
Describe the airflow through the nozzles and turbine blades after it leaves the combustion chamber? Speed? What shapes are the nozzles and turbine?
- Air forces its way onto the discharge nozzles
- Aerofoil shaped nozzles create convergent passage accelerating air to the speed of sound (2500fps)
- Air given a swirl in the direction of rotation of the turbine blades
- Air impacts (impulses) against the turbine blades and reaction will rotate the turbine at high speed
Describe the airflow through the exhaust system? What speed does it flow?
- Enters at relatively high velocity (750 - 1200fps)
- Air passes through diffuser to reduce the velocity to reduce friction losses
- Airflow exits the jet pipe at about M0.5 (950fps)
- Exhaust gasses pass into the propelling nozzle which is converging, thus increasing velocity
- During most operation airflow becomes sonic
- Pressure upstream of the throat increase and creates increased static pressure of the gas relative to atmospheric
What components are present in the exhaust system of the engine? What are their purpose?
- Diffuser: increase area between exhaust cone and jet pipe wall
- Exhaust cone: prevents exhaust gasses form flowing over rear face of turbine disc
- Rear struts: straighten airflow to reduce swirl from turbine and reduce losses
What effect is present due to the increased static pressure upstream of the throat where the airflow becomes sonic and atmospheric pressure?
Pressure thrust - acts over the area of the nozzle exit
What is it called when airflow cannot accelerate any further through the propelling nozzle?
Choked
What is the only way to increase the airflow exit velocity when the propelling nozzle is choked?
Increase the temperature and therefore the speed of sound of the air at the throat
With the exhaust section and propelling nozzle arrangement, where is the throat?
Exit/end of the convergent section of the propelling nozzle
What happens to velocity and pressure of the exhaust as it moves through the propelling nozzle?
Velocity increases and static pressure decreases
What happens to the airflow after it passes the throat in the divergence section? What additional thrust (if any) does it produce?
The velocity will increase
The reaction to the increased momentum is a pressure force acting on the inner wall of the nozzle. A component of this force acts along longitudinal axis producing more thrust
What are the requirements of the oils in a jet engine? (8)
Why does each requirement exist?
- Low volatility (to prevent evaporation at altitude)
- Anti foaming (for more positive lubrication
- Low lacquer and coke deposits (Keeps particle formation to min)
- High flash point (to prevent fire from vapour)
- Low pour point (Lower temp at which oil will flow in cold temps)
- Good film strength (so molecules stick together and stick to components under loads)
- Wide temp range
- High viscosity index (Means it maintains its viscosity properties well)
What are the components that must be incorporated into the oil tank?
- Sight glass/ dipstick/ gauge for measuring quantity
- De-aerating device to remove air from returning oil
What type of oil system are jet engines? Where is the tank usually located?
Dry sump
Tank can be integral within gearbox assembly or seperate
What are the requirements of the oil system?
- Store appropriate quantity of oil
- Supply engine with adequate supply of oil in all operating conditions
- Provide for draining and replenishing of oil
- Must have means to determine quantity of oil
How is air used to seal bearings? Where does this air come from?
Air is directed across bearing oil seals inwards to prevent oil from escaping into the main engine casing or compressor inlet
Air is bled off the compressor
What types of seals can be found on a jet engine?
- Labyrinth seal
- Ring seal
- Hydraulic seal
- Carbon seal
Describe a Labyrinth seal?
A finned rotating member with a static bore which is lined with soft arable material, or a high temp honeycomb structure
Describe a ring seal?
A metal ring which is housed in a close fitting grove in the static housing
Describe a hydraulic seal?
- Seal is formed by a fin in an annulus of oil which is created by centrifugal forces
- Difference in air pressure inside or outside the chamber compensated by a difference in oil level either side of the fin
Describe a carbon seal?
-Static ring of carbon which is constantly rubbing against a collar on a rotating shaft
What does the EEC do?
Electronic Engine controller
- Reads engine parameters and atmospheric conditions
- Responds to pilot power input requirements
- Schedules appropriate fuel flow to achieve demanded power
What will dictate/limit how much fuel the EEC demands?
- Achieving the ideal 15:1 air/fuel ratio
- Not exceeding turbine inlet temperature
Describe the dual manifold duplex fuel nozzle?
- Pressurising valve to regulate fuel flow to manifold
- As Fuel flow/pressure increases, pressurising valve moves to admit fuel to main manifold
- Primary fuel flows through large orifice to handle high fuel flows
- Secondary fuel flows through smaller secondary orifice at low fuel flows
- This provides combined fuel down primary and secondary manifolds
What are the advantages of the dual manifold duplex fuel nozzle over the single manifold duplex fuel nozzle?
- Better atomisation over wider flow range for same fuel pressure
- Efficient atomisation at low fuel flow during start and at high altitude
What is the valve in the duplex nozzle known as? Why?
- Pressurisation and dump valve
- Dump valve will dump all fuel from the manifolds on shutdown to prevent vapour lock
- Also ensure definite cut-off of fuel at shutdown
What is EPR? What does it measure? How/ where is it measured?
Engine Pressure Ratio - measures thrust
- Measures ratio of inlet pressure to jet pipe pressure
- Fan engines may measure bypass/turbine outlet pressure (combined) compared to compressor inlet pressure
What is JTP? Where it it measured?
Jet thrust pressure
-Measures turbine discharge or jet pipe pressure
When is a torque indicating system required? What gauge displays this?
Indicates power developed on a Turbo-prop or Turbo-shaft on a torque meter
What is engine torque proportional to?
Torque turning moment is proportional to horsepower and is transmitted through the reduction gear.
What weight is given to torque indication in aircraft that require it to be measured?
It is the primary power and performance indication
How is a satisfactory ignition obtained in a jet engine? Why are they required?
- A high energy ignition system (HE)
- Required to ensure adequate relight at high altitude or for difficult ground starting conditions
What is the starting procedure for a gas turbine?
-Pre start checks
-Fuel pumps on
-Condition/ power lever to start position
-Select ignition to Start
-Ignition will normally release after a few seconds to On or Auto ignition position
-Monitor engine parameters
-Once engine self sustaining RPM starter and ignition will automatically switch off
-Move power lever to GND Idle
Turn off fuel pumps
What does placing the power/condition lever at start do?
Allow scheduling of fuel to occur at predetermined time
What does selecting the ignition to start do?
Provide continuous HE (high energy) ignition
When monitoring engine parameters what will a successful start be indicated by?
Steady increase in Temp and RPM Compressor speed (N1/N2/N3) and Torque within normal (green) range
What is a hot start? What could cause a hot start? What can be done to prevent one?
When for some reason engine temperature exceeds limits during start
Caused by:
-wet start
-fuel early or too much
Can be preventing by paying close attention to temperatures during the start and noticing if they are rising faster or higher than normal
What is a hung start? What may cause it?
When the ignition system activates but the engine cannot reach self sustaining RPM Caused by: -failure for engine to light off -failure to make power -insufficient fuel to sustain combustion
What signs are there of a hung start?
- RPM rise is slow
- Temperature rise is rapid
- Temperature rise does not occur at all
What could cause a tailpipe fire during start? Why?
Following a wet start
-because fuel may be pooling in the tailpipe, and with sufficient temp it may ignite fuel fumes
What actions should be taken in the event of a tailpipe fire during start?
- Fuel Selector: Off
- Starter: Keep cranking (to blow out the fire)
- Fire extinguisher: On (stop motoring engine prior to discharge
- Master switch: Off
- All other switches: Off
- Shutdown any other engines normally
- Report to engineering
What are the symptoms of a flameout during start? What should be done?
- Decrease turbine temp and RPM
- Engine will normally come to complete stop
-Secure engine and notify engineering
Can another start be attempted after flameout? In what circumstances?
If you re-read the restart checklist and realise you missed an item. If you performed the checklist accurately the first time, notify engineering
What engine anti-icing provisions may/may not be required for gas turbine engines?
- Some require no anti-ice due to not forming enough ice to be an issue
- Some turbo-props have oil reservoir in the prop reduction gearbox, providing some anti-ice capability
- Some may require a minimal amount of hot HP compressor airflow to provide anti-ice for the intake
How hot can an after burner flame be?
1700˚C
How is cooling of the jet pipe ensures when an afterburner is used?
- Fuel spray bars concentrated around axis fo jet pipe
- Allows some air to remain unburned around the walls of the jet pipe
- Provides a cooling barrier similar to that used in the combustor
Other than after burning, what is a way of augmenting thrust in a jet engine?
Water/ coolant injection
When coolant is sprayed in the engine, where can it be sprayed, and what the comparative advantages/ disadvantages?
Compressor inlet:
Combustion Chamber:
- More suitable for axial flow
- More even distribution
- Greater quantity of coolant can be used (more thrust)
What is SFC in relation to the jet engine? What units is it expressed in
Specific fuel consumption: The ratio of fuel consumption to thrust or shaft horsepower
Ib/hr per Ib thrust/SHP (Ib/hr/Ib thrust)
What is SFC determined by
Thermal and Propulsive efficiency of the engine
What is the most limiting factor in the power output of a gas turbine engine?
Turbine inlet temperature
gas temp at turbine entry
