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Engines > Engine Construction > Flashcards

Engine Construction Flashcards

1
Q

What should an ideal intake provide

A

Equal air distribution
Minimum pressure loss in the duct

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2
Q

What are the 4 types of intakes and their uses

A

Pitot - passenger aircraft
Divided - fast jets (subsonic)
Variable throat - fast jets (supersonic)
Bellmouth - helicopters

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3
Q

What is the ideal speed for air entering the compressor

A

Mach 0.5

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4
Q

What are the 5 types of intake protection devices

A

Intake grilles
Ice shield
Momentum separator
Sand filter
Particle separator

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5
Q

How does a momentum separation device work

A

Force the air stream to make sharp changes in momentum, throwing water particles or other particles to the side due to their higher mass, leaving the air intake clean

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6
Q

What is the purpose of a compressor

A

To give a predetermined pressure rise throughout the compressor

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7
Q

What are the 2 types of compressor

A

Centrifugal
Axial

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8
Q

How much of the energy of the turbine is used to drive the compressor

A

2/3rds

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9
Q

How does a centrifugal compressor work

A

Uses diverging ducts to raise the pressure (and temp) centrifugal force causes the air to flow outwards along divergent vanes, accelerating the air

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10
Q

What is a diffuser section responsible for

A

Fitted to the impeller blades, takes the air and gives it another huge boost of pressure energy through diffuser vanes.

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11
Q

How much of the pressure rise occurs in the impeller and the diffuser

A

50:50

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12
Q

What are the advantages of using centrifugal compressors

A

Cheap & Simple
Low surge tendency
Efficient over wide speed range
Withstands icing and FOD
Easy to balance
Simple to maintain

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13
Q

What are the disadvantages of centrifugal compressors

A

Low compression ratio (14:1)
Large frontal area
High impeller tip speeds
No straight through airflow

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14
Q

What does a stage consist of

A

A set of rotors followed by a set of stators

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15
Q

How does a stator work

A

Air that flows through a rotor is accelerated and its pressure increased, it is then swept back onto stator blades, air is decelerated and its kinetic energy is turned into pressure. This process occurs for however many stages there are

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16
Q

What is the stagger angle

A

The twist along the blade from root to tip, ensures the same amount of work is done by the entire blade

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17
Q

What is the cascade effect

A

The making of a converging duct between the drum on which the rotors sit and the engine casing, to maintain air velocity

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18
Q

What are the advantages of axial compressors

A

Higher compression ratio
Straight airflow
Small frontal area
Capacity for further development

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19
Q

What are the disadvantages of axial compressors

A

Complex
Expensive
Will not easily accept ram air
Slow to accelerate
Susceptible to surge and stall

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20
Q

What is compressor stall

A

Breakdown of airflow over one or more stages of the compressor.
Can be caused by FOD, icing, overfuelling or flight manoeuvres

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21
Q

What is compressor surge

A

Compressor airflow delivery is less than combustion pressure ahead of it, causing air to flow backwards through the compressor

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22
Q

How can compressor stall be identified

A

Audible rumbling
Loss of rpm
High jet pipe temperature
Vibration

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23
Q

How can compressor surge be identified

A

Loud bangs accompanied by flames
Rise in JPT
Loss of rpm
Extensive engine damage

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24
Q

What are the combined effects of stall and surge

A

Reduction in engine life
Changes in material properties
Internal damage

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25
How can stall and surge be prevented
Inlet guide vanes Variable stator vanes Twin spool Compressor discharge valve Centrifugal compressor
26
What are inlet guide vanes
Vanes before the 1st stage rotors than can be set to the correct angle of attack and guide the air into the rotors to reduce the risk of stall
27
What are variable stator vanes
Similar to IGV, but operates over more stages of the compressor
28
What occurs to the AOA of the variable inlet as fuel pressure increases and decreases
Increase in fuel pressure - lower angle of attack Decrease in fuel pressure - higher angle of attack
29
How does a twin spool setup prevent stall and surge
LP and HP spools run at different speeds, more flexibility so stall and surge is less likely
30
What is a CDV
Compressor discharge valve A valve that bleeds air from the compressor to assist engine starting and helps prevent stall and surge during acceleration and deceleration
31
What is the risk of FOD to the compressors
Aero dynamic change to the blades Blockages Deterioration Malfunctions Engine failure
32
Why can a combustion chamber be built from light materials?
There is no peak pressure
33
What is the purpose of a combustion chamber
To give efficient stable operation over a range of altitudes and flight speeds, and reduce the temperature for the turbine
34
Why aren’t helicopter combustion chambers built to be relit upon failure?
Because helicopters will autorotate and have no need to be relit
35
What is the flame tube
Contains the flame during combustion, can be made of lightweight materials but must resist 1800-2000 degrees
36
What are flame tubes normally made out of
Nimonic alloy
37
How much air is used for combustion as a percentage compared to secondary uses
25%
38
What are the 2 types of combustion chamber in use
Annular Reverse flow
39
How does an ADV work
Consists of a simple spring and plate valve. When combustion chamber pressure falls off the the spring will open the drain and residual fuel will drain out of the chamber
40
What are the 2 types of burners
Atomisers - mist Vaporisers - gas
41
What are the 3 types of atomiser
Duplex Rotary atomiser Airspray nozzle
42
How does a duplex atomiser work
Uses 2 orifices to control the flow of fuel, giving it a wide range of usability, and provides efficient atomisation at lower flows required at high altitude
43
What are the advantages of vaporisers
Combustion is more easily controlled Less smoke Combustion zone is shorter
44
What are the advantages of an annular system
Structural Efficient Large total combustion area
45
What is the purpose of a turbine
Extract sufficient energy from the gas stream to drive the compressor and accessories, can also drive a propeller or rotor
46
At what speed can turbine tips spin
1500ft/sec
47
What is the standard Turbine Entry Temperature (TET)
700-1700 degrees
48
What are the 3 main parts of an engine turbine stage
Nozzle guide vanes (NGVs) Rotating turbine disc Turbine blades
49
What is the purpose of nozzle guide vanes
Decrease pressure Decrease temperature Massively increase velocity
50
Why are nozzle guide vanes usually hollow
To allow them to be cooled by intake air
51
What are turbine discs normally manufactured from
Nickel based alloys
52
What are NGVs normally manufactured from and why
Nickel based alloys, to resist high temperatures
53
What can be done to limit the temperature of the disc
Cooling air passed over both sides of the disc
54
What is the typical air/fuel ratio
60:1
55
How does a rotary atomiser work
Fuel is fed through the hollow compressor shaft, and centrifugal force as well as fuel pressure will provide the fuel as an atomised spray into the combustion chamber
56
How does an airspray nozzle work
Mixes a portion of air and fuel before presenting it into the combustion chamber. This avoids buildup of carbon and exhaust fumes
57
What 2 sections are turbine blades composed of?
Impulse - Root Reaction - Tip
58
How is a turbine blade attached to the disc
Fir-Tree root BLISK
59
What are the benefits of a Fir-Tree root
Extra serrations mean a greater surface area to resist against centrifugal forces Room for thermal expansion and contraction Allows the blade to bend slightly to resist fatigue
60
What happens to PVT as air passes over the nozzle guide vanes
Pressure decreased Temperature decreased Velocity increased
61
What must turbine blades be resistant to
Fatigue Thermal shock Corrosion Oxidisation High temperatures Centrifugal forces Rotational speeds High frequency gas flows
62
What is a BLISK attachment
Blades are permanently attached to the disk by being machined with the disk or by friction welding
63
What is blade creep
Blades growing in length while spinning at over their maximum speeds while red hot
64
What is the purpose of blade shrouding
To reduce the loss of gas through seepages across the blade tips during expansion and contraction
65
What are the 3 causes of blade creep during operational hours
High temperatures Centrifugal force Running hours
66
In what 3 stages is creep likely to occur?
Primary - New engine (during initial runs) Secondary - Normal service use Tertiary - Excessive temperatures and running hours
67
What maintenance is normally carried out on turbines
Turbine tip clearance check Bearing wear/failure Run down check Overheating check FOD damage check (compressor turbine exhaust) Corrosion/Erosion
68
In which areas are FOD checks completed when maintaining the turbines
Compressor Turbine Exhaust
69
What is the purpose of the exhaust cone
Protect the rear face of the turbine disc from overheating
70
What are thermocouples
Used to measure the average temperature in the jet pipe and provide pilot indication
71
What are 5 examples of different names for TET (Turbine entry temperature)
JPT - Jet pipe temperature EGT - Exhaust gas temperature TGT - Turbine gas temperature TIT - Turbine Inlet Temperature T4, T6
72
Why are temperatures on turbo prop and shaft engines lower than other engine types?
More energy has been extracted from the air to drive a propeller or MRGB
73
What is used to protect the aircraft structure from heat damage from the exhaust
Insulating blanket Air
74
What 3 things should be checked during exhaust maintenance
Cracking Corrosion Security of attachment
75
What is the formula for thrust
Mass airflow rate x (Velocity out - velocity in)
76
How can mass airflow be increased
Inject water into compressor Inject water into combustion gas flow
77
How can fan noise be reduced
Installing acoustic liners into engine cowlings or changing design of intake
78
What is a scarfed intake
Extended lower lip on intake to reflect noise away from ground
79
What are 6 examples of accessories that can be present in an engine system
Starter motor Fuel control unit Oil pumps Tacho generator Alternator/Generator Centrifugal breather
80
What are examples of non driven accessories in the gearbox
Scavenge filter Drain plugs Oil temperature bulbs Magnetic plugs
81
What are the 2 types of drives used for rotating components
Splined - Square teeth Serrated - triangle teeth
82
What is an internal gearbox made from
Aluminium
83
What is the external gearbox made from
Magnesium
84
Where can an external gearbox be found
Front centre or rear of engine
85
What is the normal speed of the accessory drives
5000-6000rpm
86
What is a hunting tooth used for
To evenly spread wear across all teeth on a gear
87
What 4 tasks is the fuel system used for
Combustion Starting Acceleration Stable running under varying conditions
88
What is it called when an exhaust has a converging duct on the end of it
Propelling nozzle
89
What shape is the Nacelle when it has been designed for noise suppression
Serrated

Engines (5 decks)

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