1
Q
How does thrust relate to drag in steady level flight? What type of engine provides this force?
A
- Thrust = Drag
- Force provided by internal combustion engine (heat engine)
2
Q
What are the two main types of internal combustion engines in aviation?
A
- 4 Stroke piston
- Gas turbine (Jet engine)
3
Q
What types of jet engines are used on fixed-wing vs rotary-wing aircraft?
A
- Fixed: Turbo-jet, Turbo-prop, Turbo-fan
- Rotary: Turbo-Shaft
4
Q
What does GTF stand for with respect to a type of gas turbine engine? What is it?
A
- Geared Turbo Fan
- It’s an engine where there is a reduction gearbox for the front fan.
5
Q
How is propulsion 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.
6
Q
What is the difference between the propeller and jet engine in terms of the acceleration of air?
A
- Propeller: Accelerates a large mass (slipstream) of air to a relatively little amount of speed.
Jet: Accelerates a smaller jet of air to a comparatively high speed.
7
Q
Where does the jet reaction stem from? What is the common misconception?
A
- Jet reaction occurs internally.
- Not with the exhaust reaction with the atmosphere.
8
Q
What is the resultant reaction (thrust) produced on the engine proportional to?
What relationship gives better efficiency and why?
A
- Mass of air accelerated.
- Change in the velocity of the air.
- Large mass of air and small velocity as it lowers the jet velocity in relation to the air, giving a higher propulsive efficiency.
9
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.
10
Q
What is the Brayton Engine Cycle defined as?
A
- Continuous thermodynamic cycle of the gas turbine engine.
11
Q
What is the Brayton Engine cycle widely known as? Why?
A
- Widely known as a constant pressure cycle.
- This is because, in the gas turbine engine, pressure is fairly constant across the combustion section as volume attempts to increase, thus increasing the gas velocity.
12
Q
What is the most common air intake for a turbo-jet or turbo-fan engine? What is the advantage of using this air intake?
A
- Most common one for engines flying at subsonic or low supersonic speeds is a short, pitot-type circular intake.
- This type of intake makes full use of the ram air effect due to the forward speed, and suffers minimum loss of pressure with respect to aircraft attitude.
13
Q
What are the comparative advantages of a centrifugal flow compressor compared to an axial-flow compressor?
A
- ) Easier to develop and manufacture.
- ) More robust compared to an axial flow compressor.
- ) Commonly used and favoured in smaller engines.
- ) Simplicity and ruggedness outweigh the disadvantages.
14
Q
What are the comparative advantages of an axial flow compressor compared to a centrifugal flow compressor? (7)
A
- ) Consumes far more air than the centrifugal flow compressor.
- ) Because it consumes more air, it provides greater thrust for the same frontal area.
- ) Can be designed to attain higher pressure ratios.
- ) The ability to increase the pressure ratio by the addition of extra stages has led to the adoption of this compressor in most engine designs.
- ) Improved engine efficiency.
- ) Improved SFC for a given thrust.
- ) Easier to maintain.
15
Q
What is the principle of operation of the centrifugal flow compressor? (2)
Does pressure rise in the impeller and diffuser? If so, what is the percentage of each?
What speed does the impeller required to be operated at? Why?
A
- Air arrives at the centre of the impeller in an axial direction and accelerates the air outward by centrifugal reaction due to its rotational speed. Air is continuously induced into the impeller.
- Air is then passed onto the diffuser section, where passages form divergent nozzles, causing air to spread, thus speed slows down and static pressure increases.
- Half of the pressure rises in the impeller and the other half rises in the diffuser.
- Maximising airflow and pressure rises through the compressor requires impellers to be operated at high tip speeds –> 1600 ft per second // 948 kts.
16
Q
What does the impeller consist of? (construction)
What type of vane(s) are usually incorporated in a turbine engine? Why?
A
- Consists of a forged disc with integral, radial disposed vanes on one or both sides forming convergent passages in conjunction with the compressor casing.
- Vanes may be swept back, but for ease of manufacture, straight radial vanes are usually employed.
17
Q
What is the purpose of the diffuser?
How is the diffuser constructed? (2)
Why are vane passages divergent?
A
- To slow down the speed of air and create a pressure rise.
- The diffuser assembly may be an integral part of the compressor casing, or a separate attached assembly.
- In each instance, it consists of a number of vanes formed tangentially to the impeller.
- The vane passages are divergent to convert the K.E. into pressure energy, and the inner edges of the vanes are in line with the direction of the resultant airflow from the impeller.
18
Q
Describe the principle of operation of the axial flow compressor? (3)
A
- Airflow and compression occur parallel to the rotational axis of the compressor (hence the name).
- During operation, a rotor is turned at a high speed by a turbine so that air is continuously induced into the compressor, which is then accelerated by rotating blades, and swept rearward onto an adjacent row of stator vanes.
- Air is then decelerated in the following stator passage and the K.E. is translated into pressure.
19
Q
What is the inlet guide vanes function?
A
They are stationary and their function is to direct airflow into the rotor at the most desirable angle.
20
Q
What is the purpose of the rotor blades?
Where are they located?
How are they constructed?
A
- Purpose is to move the airflow rearward through each stage.
- Rotor blades are the first component on a stage of compression.
- Constructed of an aerodynamic design, constructed with a varying angle of incidence or twist, similar to that of a propeller.
- Rotor blades are the rotating blades with the compressor.
21
Q
What is the purpose of the stator blades? (2)
How are they constructed? (2)
A
- Receive the air at high velocities, from the rotor blades, and act as a diffuser, changing K.E. to potential energy in the form of pressure.
- Correct the deflection given to the air by the rotor blades, and present the air at the correct angle to the next stage of rotor blades.
- Stator blades are aerofoil in shape and are stationary.
- Has a reducing cross-section of annulus as compressed air has a lower density.
22
Q
What are the advantages of a multi-spool compressor compared to a single spool? (3)
A
- Dual and triple-spool compressors were developed for operational flexibility.
- Operational flexibility provide high compression ratios, quick acceleration and better control of stall characteristics to the engine (because each spool controls itself).
- Improved efficiency which means that the fuel can be shed more accurately, meaning it can be converted into work better.
23
Q
What is the main advantage that a multi-spool compressor has as density reduces? (3)
A
- As altitude increases, air density allows the low-pressure compressor (N1) to speed up (as more air is fed into the compressor b/c of the reduced air density).
- This has the effect of recovering the subsequent pressure loss due to the rarified atmosphere through the high-pressure compressor (N2). N3 remains same optimum RPM
- This is applicable to any changes in atmospheric density.
24
Q
What is a spool? (also name the other term for it)
List the different types of spool for both single and multi spool engines (i.e. Ng etc)
A
- Combination of a compressor and a turbine, also called a gas generator.
- Single spool engine is Ng.
- Multi spool engines are designated as N1, N2 and N3.
- N1 being the low-pressure compressor.
- N2 or N3 being the HP compressor.
- For a three-spool configuration, N2 is the intermediate compressor and N3 is the HP compressor.
25
What is a compressor stall defined as?
The abrupt loss of the efficiency of the axial flow compressor in a turbine engine when the AoA of the compressor blades departs too far from the design intention i.e when compressor blade AoA's become excessive.
26
Are there any symptoms of an incoming compressor stall?
Typically no symptoms for a complete stall.
27
What are the possible symptoms of a surge?
| What are the possible symptoms of a compressor stall?
- If a surge is about to occur, vibrations may occur. ( A surge is when the engine demands a pressure rise from the compressor, which is higher than what the blades can sustain).
- Temperature, fuel flow, and thrust indication fluctuations.
For a compressor stall:
- High gas temp (high EGT).
- Load bang or an engine sneeze (engine sneeze is a fully blown stall).
28
When will a compressor stall typically occur?
On the ground, when the engine goes through to full power.
29
What are the different causes of a compressor stall?
1. ) Turbulent or disrupted airflow to the engine inlet (reduces gas velocity).
2. ) Excessive fuel flow caused by abrupt engine accelerations (reduces gas velocity as it increases combustion back pressure).
3. ) Excessively lean fuel mixture caused by abrupt engine deceleration (increases gas velocity as it reduces combustion back pressure).
4. ) Contaminated or damaged compressors ( increases gas velocity as it reduces compression).
5. ) Damaged turbine components, causing loss of power to the compression and low compression (increases gas velocity by reducing compression).
6. ) Engine operation above or below designed RPM (incorrect RPM setting for phase of flight).
30
What is the purpose of the combustion chamber?
| How must it be accomplished?
- Task to burn the fuel supplied through the fuel spray nozzles with the air supplied by the compressor and releasing the heat in such a manner that the air is expanded and accelerated to give a smooth stream of uniformly heated gas at all conditions required by the turbine.
- Must be accomplished in a manner where there is the least amount of loss in pressure, max heat released for the limited space available.
31
What is the overall fuel/air ratio of a combustion chamber in normal operation?
What ratio will kerosene burn fuel efficiently at?
How is this ratio achieved?
What percent of air is taken by the entry section?
- Between 45:1 to 130:1.
- Approximately or close to 15:1.
- Fuel must be burned with only part of the air entering the chamber --> primary combustion zone. This is achieved by means of a flame tube.
- Approximately 20% of the air mass flow is taken in the by the snout or entry section.
32
What percent of air is primary air?
| How is the primary airflow divided in the combustor?
- Primary air is 20%.
- Primary air is further divided --> 12% flows axially through swirl vanes in the area of fuel entry, the other 8% enters radially from the forward end of the liner, all of which supports combustion.
33
How is the secondary airflow divided in the combustor?
- Secondary air is 80%.
- Through the wall of the flame tube body, are a selected number of secondary holes, which a further 20% of the main flow of air passes into the primary zone.
- 40% is cooling air, which provides a cooling blanket over both sides of the liner, preventing the flame from contacting the metal surface of the liner.
- The remaining (tertiary) air joins the gases at the rear of the liner and cools the mixture as it enters the turbine to a temperature acceptable to the turbine components (20%)
34
What is the primary function of the turbine?
What are the other functions of the turbine?
Explain how does it achieve this?
- Primary function is to drive the compressor.
- Can also drive the accessories and the shaft of turbo-prop and turbo-shaft type engines.
- Done by converting kinetic and heat energy, from the heat gases, into mechanical work.
35
How does the turbine produce mechanical work?
- Turbine extracts energy from flowing gases by reducing the pressure.
- This occurs because the pressure is converted to velocity by the convergent nozzles formed at the trailing edge of the stator blades and rotor blades.
36
Where is the location at which the gases are at their hottest temperature? Why?
Where is the location at which the gases are at their greatest pressures?
- Due to the position of the turbine, immediately following combustion, the gases are at their hottest at the entry point of the turbine, i.e. compressor outlet.
- Compressor outlet.
37
What are the limiting factors of the turbine engine?
| Include what is the efficiency percentage of the turbine/combustor and how that percentage is distributed.
- Inlet temperature due to the material used.
- Aerodynamic losses in the turbine blades and nozzle guide vanes and gas leakage over the turbine blade tips.
- Efficiency of the turbine, i.e. energy transfer is never 100% due to thermodynamic and mechanical losses.
- Efficiency is between 90-98%. 98% nowadays. 2% of gas is carbon emission and the other 98% is used to convert heat energy.
38
What is the principle of operation of the nozzle? (3)
- When gas is expanded by the combustion process, it forces its way into the discharge nozzles of the turbine where, because of their convergent shape, it is accelerated to the speed of sound (2500 ft per second for the temp of gas).
- At the same time, gas flow is given a spin or whirl, in the direction of rotation of the turbine blades, by the nozzle guide vanes.
- On impact with the blades, and during the subsequent reaction through the blades, energy is absorbed, causing the turbine to rotate at high speed, and providing the power to drive the turbine and shaft and thus compressor.
39
At what speed does gas flow into the exhaust system?
Why is this a problem?
What is done to mitigate this problem?
- Gas enters the exhaust system at velocities from 750 to 1200 ft per second.
- Problem because at high speeds, high friction losses occur, the speed of flow is decreased by diffusion.
- This is accomplished by having an increased passage area between the exhaust cone and the outer wall of the jet pipe.
40
What other function does the exhaust cone have?
What is the speed held down to at the exhaust unit outlet?
What are the additional losses caused by and how are they reduced?
- Prevents exhaust gases from flowing across the rear face of the turbine disc.
- It is usual to hold the velocity at the exhaust unit outlet to a Mach number of 0.5 i.e. 950 ft per second.
- Additional losses occur due to the residual swirl velocity in the gas stream from the turbine.
- To reduce those losses, the turbine rear struts in the exhaust unit are designed to straighten out the flow before the gases pass into the jet pipe.
41
Explain how pressure thrust is generated. Include:
What the speed of exhaust gases are at the exit and under what conditions?
When and where is the propelling nozzle choked?
What being choked means in terms of exhaust systems?
(7)
- The exhaust gases pass to atmosphere through the propelling nozzle, which is a convergent duct, increasing the gas velocity.
- The exit velocity of the exhaust gases is subsonic at low-thrust conditions only.
- During most operating conditions, the exit velocity reaches the speed of sound in relation to the EGT, and the propelling nozzle is said to be 'choked'.
- 'Choked' means that no further increase in velocity that can be obtained unless temperature is increased.
- As the upstream total pressure is increased above the value at which the propelling nozzle becomes 'choked', the static pressure of the gases at the exit increase above atmospheric pressure.
- This pressure difference across the propelling nozzle gives 'pressure thrust' and is effective over the nozzle exit area.
- This is additional thrust to that obtained due to the momentum change of the gas stream.
42
With a C-D nozzle, where is the throat located and where is the proper exit located?
What additional thrust (if any) does it produce?
- Convergent section exit is where the throat is located.
- The exit is now at the end of the divergent section.
- As gas leaves the restriction of the throat and flows into divergent section, it progressively increases in speed towards the exit.
- Reaction to this further increase in momentum is a pressure force, where a component of this force acting parallel to the longitudinal axis of the nozzles provides the extra thrust.
43
List the requirements of oils and for each oil requirement, and describe why they are desirable qualities of oil?
1. ) Low volatility: Minimise evaporation at high altitudes.
2. ) Anti-foaming quality: For more positive lubrication.
3. ) Low lacquer and coke deposits: Keep solid particle formation to a minimum.
4. ) High flashpoint: The temperature at which oil, when heated, gives off flammable vapours that will ignite if near a flame source.
5. ) Low pourpoint: The lowest temperature at which oil will flow due to gravity.
6. ) Film strength: Excellent qualities of cohesion and adhesion, a characteristic of oil molecules which allow the oil to stick together during compression loads and stick to the surface during centrifugal loads.
7. ) Wide temperature range: Be able to operate in variety of conditions, -60ºF to +400ºF.
8. ) High viscosity index: This indicates how well the oil will tend to retain its viscosity when heated to its operating temperature.
44
What type of oil is used in lubrication?
Synthetic anti-freeze oils.
45
What type of oil tank used in gas turbine engines?
| Where is the oil tank in a gas turbine engine located?
- Dry sump type, therefore an oil tank is required.
| - Mounted on the engine as a separate unit or integral within the gearbox assembly.
46
What is the function of the oil tank in a gas turbine engine?
What are the requirements of an oil tank? (2)
What is normally included in an oil tank?
- Function is to store an appropriate quantity of oil to ensure that the engine always has an adequate supply of oil under all operating conditions and attitudes over the max range likely to be flown.
- Must provide for lubrication system draining and replenishment.
- Must have means of determining system contents (dip stick, sight glass or electronic gauge within the cockpit) displaying the quantity as a percentage of full.
- Normally includes a de-aerating device to remove air from the returning oil.
47
What is the air from the compressor used for other cooling?
| How is this achieved?
- Seal the bearing houses, preventing the leakage of oil into the main engine casings or into the compressor inlet.
- Achieved by directing the air across the bearing oil seals, flow being inwards towards the bearing or oil supply, preventing the escape of any oil.
48
What are labyrinth seals? (made of)
| What are labyrinth seals used for?
- Comprises a finned rotating member with a static bore lined with a soft abradable material, or a high-temperature honeycomb structure.
- Widely used to retain oil in bearing chambers and as a metering device to control internal airflows.
49
What are ring seals? (made of)
| What are ring seals used for?
- Comprised a metal ring housed in a close-fitting groove in the static housing.
- Used for bearing chamber sealing, except in hot areas where oil degradation due to heat would lead to ring seizure within its housing.
50
What are hydraulic seals? (made of)
Formed by a seal immersed in an annulus of oil which has been created by centrifugal forces. Any difference in air pressure inside and outside of the bearing chamber is compensated by a difference in oil level on either side of the fin.
51
What are carbon seals? (made of)
- Consist of a static ring of carbon that constantly rubs against a collar on a rotating shaft.
- Several springs are used to maintain contact between the carbon and collar.
52
What is the function of the Electronic Engine Controller?
How does it achieve this function?
What is the purpose of the Electronic Engine Controller?
- Function is to read the parameters of the engine and respond to the pilot's power requirements input by the power lever(s)
- Schedules the appropriate fuel flow to achieve that power, given the atmospheric conditions and engine parameters are known.
53
What is the purpose of the Electronic Engine Controller?
Purpose to achieve the 15:1 mixture ratio of air/fuel, which is consistent with the piston engine ratio, while automatically controlling the fuel flow to ensure that the turbine inlet temperature is never exceeded.
54
How is thrust of an engine indicated in the cockpit?
What does it measure?
What are the ratios of a typical engine vs a fan engine?
- Thrustmeter, also known as an Engine Pressure Ratio gauge.
- Measures the ratio of two or three parameters.
- Ratio for typical engine is usually jet pipe pressure to inlet pressure. i.e. (Pt 1 which is inlet, Pt 4 to Pt 7 for jet pipe).
- Ratio for a fan engine is integrated turbine discharge and fan outlet pressure to compressor inlet pressure.
55
How was thrust monitored on earlier engines?
On an instrument called a Jet Thrust Pressure, it measured turbine discharge or jet pipe pressure.
56
What is the purpose of the torque indicating system?
What is the propeller torque proportional to?
What is it transmitted through?
- Indicate the power that is developed by a turbo-prop or turboshaft engine. Indicator used is a torquemeter.
- Proportional to the horsepower.
- Transmitted through the propeller reduction gear.
57
When is a torque indicating system required?
| Why is this important?
- Required for any torque producing engines, such as turbo-prop and turbo-shaft engines.
- For those engines, it is the primary power and performance indication in the cockpit.
58
When is a torque indicating system required?
| Why is this important?
- Required for any torque producing engines, such as turbo-prop and turbo-shaft engines.
- For those engines, it is the primary power and performance indication in the cockpit.
59
What separate system is required to start a gas turbine engine?
What is the name of the starting system of a gas turbine engine?
What are the requirements in order to start a gas turbine engine with regards to the system above?
- Ignition system.
- Provision must be made for ignition of the air/fuel mixture in the combustion system.
- Achieved with the use of a High-Energy (HE) ignition system.
- High-Energy systems are required to ensure that the engine obtains a satisfactory relight at high altitudes and is sometimes necessary for starting under any condition.
60
What is the typical starting procedure when starting a gas turbine engine? (6)
When will the ignition and start automatically switched off?
What indicates a successful start?
1. ) Turn electric fuel pump(s) on.
2. ) Place condition/power lever at start position, this allows the scheduling of fuel to occur at the pre-determined time.
3. ) Select ignition to start, this provides continuous HE ignition. It is usual that after a few seconds, the switch is released, as it is spring-loaded, and returns to the "on" or "auto-ignition" position.
4. ) Monitor engine parameters, Temp. compressor speed/engine torque. Once the engine has established a self-sustaining RPM, the starter and ignition will automatically switch off.
5. ) In some cases, once the engine has reached ground idle RPM, it's usual to move the condition/power lever to a ground idle position.
6. ) Turn electric fuel pumps off, ensure fuel pressure/fuel flow remains constant.
- A successful start is indicated by a steady increase in both temperature and RPM.
61
When does a hot start occur//What is a hot start?
What is done to help mitigate a hot start from occurring?
What can you expect with an impending hot start?
Can a wet start cause a hot start?
- When engine temperature during the start cycle exceeds limits.
- To mitigate: engine temperatures are monitored closely during the start.
- An impending hot start can be expected if the temperature is seen to be rising quicker than usual.
- Yes.
62
What is a hung start?
What are the causes of a hung start?
When do you know a hung start is occurring?
- Failure of the engine to start, ignition system activates, but the engine fails to reach self-sustaining RPM.
- Engine doesn't light off // Fails to make power // Insufficient fuel to sustain combustion.
- A hung start is occurring if the RPM is noted to be slow // temperature could rise quickly or fail to rise at all, depending on the cause of the hung start.
63
When is there a risk of a tailpipe fire occurring during ground start? Why?
- Following a wet start, the risk of a tailpipe fire is high.
- As fuel could be lying in puddles in the tailpipe, and if the temperature is high enough to ignite the fumes, a fire could result.
64
What are the actions to be taken in the event of a tailpipe fire?
1. ) Fuel selector: OFF (Power/condition lever).
2. ) Starter: Keep winding to try blow the fire out. It is usual to stop the starter before using the fire extinguisher as the airflow could blow the extinguishing agent away from the fire.
3. ) Fire extinguisher: ON, as required, using aircraft system or hand-held, CO2 is most preferred.
4. ) Master switch: OFF, all other switches OFF. Shut down any other engines if necessary.
5. ) Report to engineering.
65
What indicates a flameout has occurred?
| What should you do in the event of a flameout? (3)
- Drop in temperature and RPM.
- The engine will normally come to a full stop, so secure the engine and notify engineering.
- Before notifying engineering, it is advisable to check the pre-start checklist to ensure nothing was left out.
- If something was left out, repeat the checklist and attempt another start.
66
Do all aircraft require anti-icing? Why/Why not?
- No
- Because icing is not a problem for the engine and because ice does not form in sufficient quantity, and no engine anti-icing provisions are necessary.
67
What is an alternative method of anti-icing? (3)
| Where is hot air sourced from?
- On some turbo-props, the oil reservoir is located within the propeller reduction gearbox, providing some anti-icing capability.
- Only a minimum amount of hot airflow is required to anti-ice the inlet area.
- This hot air is sourced from the HP compressor via the bleed valve.
68
How are the spray bars arranged?
Why are they arranged this way?
What does this arrangement result in?
- Sprays bars are usually arranged so that the flame is concentrated around the axis of the pipe.
- Because the AB temperature is 1700ºC
- This results in only a proportion of the unburned air being used in combustion, the remainder is allowed to flow along the walls of the jet pipe to provide a cooling barrier like in the combustor.
69
What are the two methods of injecting coolant into the airflow?
Which one is preferred (and for what type of compressor) and why?
1. ) Some engines have the coolants sprayed directly into the compressor inlet.
2. ) Other engines have the coolants injected into the combustion chamber inlet.
- The latter is preferred for axial flow compressors
- This is because it allows for more even distribution and allows a greater quantity of coolant that can be satisfactorily injected.
70
What is specific fuel consumption (SFC)?
What is the SFC unit?
What is the SFC determined by?
If the fuel consumption is high and the thrust is low, what is the SFC?
- In the interest of fuel economy and aircraft range, it is the ratio between fuel consumption to thrust or SHP, and this ratio should be as low as possible.
- lbs/hr/lbt
- Determined by thermal and propulsive efficiency of the engine.
- SFC will be high.
71
What is the ultimate controlling factor limiting the engine power output?
Gas temperature at the turbine entry.
Systems Part 4
flashcards
Decks in class (6)
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