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Part 4 Systems > Basic Gas Turbine > Flashcards

Basic Gas Turbine Flashcards

1
Q

How does thrust relate to drag in steady level flight? What type of engine can provide thrust force?

A

Thrust = Drag

Force provided by internal combustion engine (heat engine)

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2
Q

What are the two main types of internal combustion engines in aviation?

A
  • 4 Stroke piston

- Gas turbine (Jet engine)

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3
Q

What types of jet engines are used on fixed wing vs rotary wing aircraft?

A

Fixed: Turbo-jet, Turbo-prop, Turbo-fan, Advanced Turboprop (more than 4 propellers)
Rotary: Turbo-Shaft

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4
Q

What does GTF stand for with respect to a type of gas turbine engine? What is it? What reduction ratio is achieved?

A

Geared turbo-fan
Engine where there is a reduction gearbox for the front fan
Fan speed is half that of the turbine

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5
Q

What is Newton’s third law of motion?

A

Every action (force) has an equal and opposite reaction (force)

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6
Q

How is forward force generated when air is accelerated through the engine?

A
  • 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
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7
Q

What is the difference between the propeller and jet engine in terms of acceleration of air?

A

Prop: Accelerates a large mass (slipstream) of air a relatively little amount
Jet: Accelerates a smaller jet of air to a comparatively high speed

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8
Q

When is the first known action of a jet reaction ‘engine’?

A

A Hero’s toy in 120BC

Spins a thing with water in it due to 2 nozzles out the side for steam to escape

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9
Q

Where does the jet reaction stem from? What is the common misconception?

A
  • Jet reaction occurs internally

- Not with the exhaust reaction with atmosphere

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10
Q

What is the resultant reaction (thrust) produced on the engine proportional to?

A

Mass of air accelerated

Change in the velocity of the air

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11
Q

What is Bernoulli’s principle?

A

The sum of potential energy (pressure) and kinetic energy (velocity) of a fluid flow remains constant.
C = P(s) + 1/2ρV^2

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12
Q

How do static and dynamic pressure vary through a C-D nozzle?

A

Inlet: High static, Low dynamic
Throat: Low static, High Dynamic
Outlet: High Static, Low Dynamic

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13
Q

Describe where each stage of the Bratton cycle occurs in a gas turbine?

A

Induction: Inlet of the engine
Compression: Compressor stages
Expansion/combustion: In the combustor/burner can
Exhaust: Through the turbines and exhaust nozzle

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14
Q

Describe the Bratton cycle considering volume and pressure?

A

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

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15
Q

What is the generic/ overview description of the Bratton cycle?

A

A constant pressure cycle where air is the working fluid of the engine

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16
Q

What is the most common air intake fitted to Turbo-jet or turbo-fan engines? Why?

A

Short - Pitot type circular

  • Maximum benefit from ram air due to forward airspeed
  • Minimum loss of ram pressure with altitude changes
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17
Q

What are the advantages of the centrifugal compressor? What settings is it preferred in?

A

More robust
Easier to develop and manufacture

Preferred in small engines where simplicity and ruggedness is required

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18
Q

What are the advantages of the axial flow compressor? (7)

A
  • 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
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19
Q

What is the principle of operation of the centrifugal flow compressor?

A
  • 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
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20
Q

Describe the centrifugal compressor impeller?

A
  • Forged disc with integral, radial disposed vanes
  • Vanes on one or both sides
  • Convergent passages formed by the impeller and casing
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21
Q

What variations are there of the vanes of a centrifugal impeller?

A
  • Swept back vanes

- Straight radial vanes (Easy manufacture)

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22
Q

What is the purpose of a diffuser in a centrifugal compressor?

A
  • To slow the air creating a pressure rise

- To convert kinetic energy into pressure energy

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23
Q

How are the diffuser vanes integrated/ arranged into the centrifugal compressor?

A

Vanes are tangential to the impeller

Vane passages are in line with the direction of the resultant airflow from the impeller

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24
Q

What is the principle of operation of the axial flow compressor?

A
  • 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
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25
Q

What is one stage of a compressor on the same spool?

A

One stage is one set of rotors followed by one set of stators

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26
Q

What is the purpose of the inlet guide vanes?

A

To direct airflow into the first rotor at the most desirable angle

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27
Q

What shape and properties do compressor rotor blades have?

A

Aerodynamic design
Varying angle of incidence

Similar to a propeller

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28
Q

What is the purpose of the stators in an axial compressor? What effect do they have on the airflow properties?

A

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

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29
Q

What shape and movement ability do axial compressor stators have?

A

Aerofoil shaped blades

They are stationary

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30
Q

What are the advantages of multi-spool axial compressors compares to single spool? (6)

A
  • 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
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31
Q

How are different stages of a Gas generator designated on cockpit instruments when there are single or multiple spools in the engine?

A

Single spool: Ng

Multiple Spool: N1 low pressure compressor, N2 intermediate compressor, N3 High pressure compressor

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32
Q

What is a gas generator?

A

Any combo of a compressor and turbine, known as a spool

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33
Q

What is a compressor stall? What are the symptoms? (More detail on how each responds)

A

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
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34
Q

What are the causes of a compressor stall? (6) How do they help to stall the engine?

A
  • 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
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35
Q

How can a compressor stall cause sometimes be simplified to be?

A

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)

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36
Q

What is the purpose of the combustion chamber? What must it be able to do?

A

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

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37
Q

What overall mixture ratios can be found in the combustion section of the engine? What does the mixture ratio need to be to burn?

A

From 45:1 up to 130:1

Needs to be = 15:1 (15 air:1 fuel)

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38
Q

How is the air/fuel ratio decreased in the combustion chamber? What is the percentage that is needed?

A
  • 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)

39
Q

How is the total air entering the combustion section divided?

A

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)

40
Q

What is the primary function of the turbine? What else does it do?

A

To drive the compressor

Can also drive accessories, shaft of turbo-prop or turbo-shaft engines

41
Q

What is the principle of operation of the turbine?

A
  • 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
42
Q

How do temperature, velocity and pressure change in the turbine section of the engine?

A

Temp: decreases linearly
Pressure: initial rapid decrease, gradually reduces
Velocity: Increase and the alternating decrease/ increase

43
Q

What operating conditions does the turbine have to endure?

A

The hottest gases in the engine, right at the front of the first turbine

44
Q

What are the limiting factors of the turbine?

A

-Material construction to withstand high temperature

45
Q

What causes losses in efficiency in the turbine?

A

Thermodynamic losses
Mechanical losses
Aerodynamic losses in turbine blades and nozzle guide vanes due to gas leakage over the blade tips

46
Q

Describe the airflow through the nozzles and turbine blades after it leaves the combustion chamber? Speed? What shapes are the nozzles and turbine?

A
  • 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
47
Q

Describe the airflow through the exhaust system? What speed does it flow?

A
  • 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
48
Q

What components are present in the exhaust system of the engine? What are their purpose?

A
  • 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
49
Q

What effect is present due to the increased static pressure upstream of the throat where the airflow becomes sonic and atmospheric pressure?

A

Pressure thrust - acts over the area of the nozzle exit

50
Q

What is it called when airflow cannot accelerate any further through the propelling nozzle?

A

Choked

51
Q

What is the only way to increase the airflow exit velocity when the propelling nozzle is choked?

A

Increase the temperature and therefore the speed of sound of the air at the throat

52
Q

With the exhaust section and propelling nozzle arrangement, where is the throat?

A

Exit/end of the convergent section of the propelling nozzle

53
Q

What happens to velocity and pressure of the exhaust as it moves through the propelling nozzle?

A

Velocity increases and static pressure decreases

54
Q

What happens to the airflow after it passes the throat in the divergence section? What additional thrust (if any) does it produce?

A

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

55
Q

What are the requirements of the oils in a jet engine? (8)

Why does each requirement exist?

A
  • 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)
56
Q

What are the components that must be incorporated into the oil tank?

A
  • Sight glass/ dipstick/ gauge for measuring quantity

- De-aerating device to remove air from returning oil

57
Q

What type of oil system are jet engines? Where is the tank usually located?

A

Dry sump

Tank can be integral within gearbox assembly or seperate

58
Q

What are the requirements of the oil system?

A
  • 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
59
Q

How is air used to seal bearings? Where does this air come from?

A

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

60
Q

What types of seals can be found on a jet engine?

A
  • Labyrinth seal
  • Ring seal
  • Hydraulic seal
  • Carbon seal
61
Q

Describe a Labyrinth seal?

A

A finned rotating member with a static bore which is lined with soft arable material, or a high temp honeycomb structure

62
Q

Describe a ring seal?

A

A metal ring which is housed in a close fitting grove in the static housing

63
Q

Describe a hydraulic seal?

A
  • 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
64
Q

Describe a carbon seal?

A

-Static ring of carbon which is constantly rubbing against a collar on a rotating shaft

65
Q

What does the EEC do?

A

Electronic Engine controller

  • Reads engine parameters and atmospheric conditions
  • Responds to pilot power input requirements
  • Schedules appropriate fuel flow to achieve demanded power
66
Q

What will dictate/limit how much fuel the EEC demands?

A
  • Achieving the ideal 15:1 air/fuel ratio

- Not exceeding turbine inlet temperature

67
Q

Describe the dual manifold duplex fuel nozzle?

A
  • 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
68
Q

What are the advantages of the dual manifold duplex fuel nozzle over the single manifold duplex fuel nozzle?

A
  • Better atomisation over wider flow range for same fuel pressure
  • Efficient atomisation at low fuel flow during start and at high altitude
69
Q

What is the valve in the duplex nozzle known as? Why?

A
  • 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
70
Q

What is EPR? What does it measure? How/ where is it measured?

A

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
71
Q

What is JTP? Where it it measured?

A

Jet thrust pressure

-Measures turbine discharge or jet pipe pressure

72
Q

When is a torque indicating system required? What gauge displays this?

A

Indicates power developed on a Turbo-prop or Turbo-shaft on a torque meter

73
Q

What is engine torque proportional to?

A

Torque turning moment is proportional to horsepower and is transmitted through the reduction gear.

74
Q

What weight is given to torque indication in aircraft that require it to be measured?

A

It is the primary power and performance indication

75
Q

How is a satisfactory ignition obtained in a jet engine? Why are they required?

A
  • A high energy ignition system (HE)

- Required to ensure adequate relight at high altitude or for difficult ground starting conditions

76
Q

What is the starting procedure for a gas turbine?

A

-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

77
Q

What does placing the power/condition lever at start do?

A

Allow scheduling of fuel to occur at predetermined time

78
Q

What does selecting the ignition to start do?

A

Provide continuous HE (high energy) ignition

79
Q

When monitoring engine parameters what will a successful start be indicated by?

A 
Steady increase in Temp and RPM
Compressor speed (N1/N2/N3) and Torque within normal (green) range
80
Q

What is a hot start? What could cause a hot start? What can be done to prevent one?

A

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

81
Q

What is a hung start? What may cause it?

A 
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
82
Q

What signs are there of a hung start?

A
  • RPM rise is slow
  • Temperature rise is rapid
  • Temperature rise does not occur at all
83
Q

What could cause a tailpipe fire during start? Why?

A

Following a wet start

-because fuel may be pooling in the tailpipe, and with sufficient temp it may ignite fuel fumes

84
Q

What actions should be taken in the event of a tailpipe fire during start?

A
  • 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
85
Q

What are the symptoms of a flameout during start? What should be done?

A
  • Decrease turbine temp and RPM
  • Engine will normally come to complete stop

-Secure engine and notify engineering

86
Q

Can another start be attempted after flameout? In what circumstances?

A

If you re-read the restart checklist and realise you missed an item. If you performed the checklist accurately the first time, notify engineering

87
Q

What engine anti-icing provisions may/may not be required for gas turbine engines?

A
  • 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
88
Q

How hot can an after burner flame be?

A

1700˚C

89
Q

How is cooling of the jet pipe ensures when an afterburner is used?

A
  • 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
90
Q

Other than after burning, what is a way of augmenting thrust in a jet engine?

A

Water/ coolant injection

91
Q

When coolant is sprayed in the engine, where can it be sprayed, and what the comparative advantages/ disadvantages?

A

Compressor inlet:

Combustion Chamber:

  • More suitable for axial flow
  • More even distribution
  • Greater quantity of coolant can be used (more thrust)
92
Q

What is SFC in relation to the jet engine? What units is it expressed in

A

Specific fuel consumption: The ratio of fuel consumption to thrust or shaft horsepower
Ib/hr per Ib thrust/SHP (Ib/hr/Ib thrust)

93
Q

What is SFC determined by

A

Thermal and Propulsive efficiency of the engine

94
Q

What is the most limiting factor in the power output of a gas turbine engine?

A

Turbine inlet temperature

gas temp at turbine entry

Part 4 Systems (4 decks)

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