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Part 4 BGT Exam > BGT Exam > Flashcards

BGT Exam Flashcards

1
Q

how must an aircraft remain level in un accelerated flight

A

a thrust must be created that’s equal to and opposite in direction to the aircraft drag - provided by an internal combustion engine

  • this is based on Newton’s third law of motion
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2
Q

what are the two main types of internal combustion engines

A
  • four stroke piston engine

- gas turbine/jet engine

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3
Q

what are the different terms for gas turbine engines

A 
turbo jet
turbo prop
turbo shaft (rotorcraft) 
turbo fan 
geared turbo fan (improved turbo fan)
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4
Q

What is the principle of thrust creation on the gas turbine engine

A

Atmospheric air is accelerated as it passes through the engine

forced required to give the air acceleration has an equal effect in opposite direction (newton 3rd law) acting on the apparatus producing the acceleration.

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5
Q

through what methods are propulsion achieved on jet aircraft

A
  • accelerating a large air slipstream at comparatively low speed
  • small jet of gas at high speed.
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6
Q

when and how was the first example of jet propulsion shown

A

Hero’s jet engine toy in 120 BC

  • his toy showed how the momentum of steam issuing from jets impacts an equal and opposite reaction on the jets themselves.
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7
Q

is a jet reaction an external or internal phenomenon?

A

an internal phenomenon and DOES NOT result from the pressure of the jet on the atmosphere.

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8
Q

The resultant thrust on the engine is proportional to:

A

the weight of the air expelled by the engine.
AND
velocity change imparted to it.

eg. Giving a large mass of air a little extra velocity
or heavily accelerating a light mass of air

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9
Q

what is the preferred way to achieve most efficient propulsion

A

giving large mass of air a little extra velocity

achieved by lowering jet speed relative to the atmosphere

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10
Q

Bernoulli’s principle is stated as follows:

A

when fluid or gas is supplied at constant flow rate thru a duct, the sum of potential and kinetic energy is constant.

velocity pressure (ram air aka dynamic pressure) is inversely proportional to static pressure

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11
Q

how does Bernoulli’s act in diverging and converging ducts

A

if the air exits a converging duct, the pressure will be lower so velocity will be higher (increased pressure during compression)

if the air exits a diverging duct, pressure will be higher and velocity will be lower (increased velocity into turbine)

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12
Q

what is the gas turbine engine cycle called and it’s features

A

the constant pressure cycle aka The Brayton Cycle.

continuous thermodynamic cycle of gas turbine engine.

Called constant pressure because pressure is relatively constant across the compressor section as volume attempts to increase - increasing gas velocity.

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13
Q

What are the comparisons between the Brayton Cycle and the Otto Cycle.

A

air intake (suck)
air goes through compressor (squezze)
air is combusted with fuel (bang)
air produces thrust and exits the rear (blow)

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14
Q

what is the ideal intake for a turbojet/turbofan a/c

A

pitot type circular intake (conventional system)

makes best use of ram effect due to fwd speed and minimum loss of ram pressure

can be used in pods or in wings, but loses circularity in wings due to wing thickness

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15
Q

What are the functions of the duct

A
  • recover as much total pressure from free stream and deliver to the front of engine with minimum pressure diff.
  • uniformly deliver air with little turb and pressure variation
  • produce minimum drag on the aircraft
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16
Q

what is the purpose of intakes

A

to capture the mass flow of a uniform supply of air and deliver it to the compressor at the correct speed and turbulence free.

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17
Q

what are the different designs/constructions of inlets/intakes

A
  • fixed geometry/pitot type intake (conventional system)
  • divergent intake ( opposite reaction in divergent section is the thrust component)
  • supersonic fixed ( a big long C-D nozzle basically)
  • supersonic variable ( wedge inside the casing is lowered to increase speed to supersonic)
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18
Q

What are the advantages of centrifugal compressors

A
  • picks up air and accelerates it outwards
    • more robust
  • easier to manufacture and develop
  • favoured for smaller engines
  • used in combination with axial flow
  • simplicity and ruggedness
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19
Q

what are the advantages of axial flow compressors

A
  • each rotor stage accelerates air rearwards avoiding energy loss
  • consumes more air
    • higher pressure ratios
    • more thrust for same frontal area
    • increase pressure ratio by addition of stages
  • improved efficiency
  • improved SFC per unit thrust
    • easier to maintain
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20
Q

What are the functions of the compressor

A
  • supply sufficient air for combustion
  • increase air pressure
    SECONDARY: - supply bleed air
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21
Q

What is the centrifugal compressor principle of operation:

A
  • air arrives at centre of impeller (axial flow at this stage)
  • impeller accelerates air outward due to centrifugal reaction
  • air expands in diffuser (divergent duct)
  • Bernoulli’s principle acts (speed slows and pressure builds)
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22
Q

What is the PoO of the Impellor:

A
  • a forged disc with integral, radial disposed vanes on one or both sides forming convergent passages in conjunction with compressor casing
  • vanes may be swept back but are usually employed.
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23
Q

What is the PoO of diffusers:

A
  • purpose is to slow velocity of air creating a pressure rise
  • may be part of compressor casing or separately attached
  • consists of a number of vanes placed tangentially to the impellor
  • vane passages are divergent to convert Ek to pressure energy
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24
Q

What is the Axial flow compressor PoO:

A
  • airflow remains parallel to rotational axis of compressor
    • turning rotor induces air into compressor
    • rotors sweep air onto adjacent stators
  • stator passages are divergent
  • air accelerates thru stators
  • kinetic energy translated into pressure
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25
Q

What are the characteristics of guide vanes

A
  • stationary vanes ahead of first stage

- directs airflow into 1st rotor stage at most desirable angle (not considered a stage (part of inlet)

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26
Q

what are the characteristics of rotor blades

A
  • purpose is to move air rearward thru each stage (is a rotating unit within the compressor)
  • are designed aerodynamically and have an angle of incidence (twist) similar to that of a propeller.
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27
Q

What is the purpose of stator blades

A
  • to receive high velocity air and diffuse it, changing kinetic energy to potential as pressure (stationary unit within compressor)
  • aerofoil in shape
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28
Q

how are rotors and stators placed in the compressor

A

one rotor disc, followed by one set of stators

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29
Q

What are the advantages of Multi-spool compressors

A
  • operational flexibility
  • high compression ratios
  • control of stall
  • improved efficiency
  • As ALT increases, decreasing air density allows N1 to speed up which recovers pressure loss from N2.
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30
Q

What is a gas generator, how are spools distinguished?

A

any combination of a compressor and a turbine is a spool - combos are known as gas generators.

a single spool is NG

a multi spool is N1, N2 and N3 (according to number of compressors)

  • N1 is LP (always)
  • N2 is HP, (IP for 3 spool)
  • N3 is HP
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31
Q

What is a compressor stall?

A
  • abrupt loss of axial flow efficiency in a turbine engine when compressor blade AoA becomes excessive.

surge can occur if there’s aerodynamically induced vibration aswell as fluctuating temp, fuel flow and thrust indications.

32
Q

What is a positive and negative incidence compressor stall?

A

Positive incidence - low pressure stage at high speed

Negative incidence - high pressure stage at high speed

33
Q

what are the symptoms of compressor stall?

A
    • high gas temp
    • load bang
  • engine vibrations
  • loss of power
  • aircraft surge
  • engine sneeze (air goes back out thru intake)
34
Q

what are the causes of compressor stalls?

A

** turbulent or disrupted airflow to the engine inlet

** excessively rich fuel flow caused by abrupt engine accelerations (as combustor bp increases)

** excessively lean fuel mixture caused by abrupt engine decelerations (as combustor bp decreases)

** engine operation above or below design RPM (outside design envelope) (reduces compression)

  • contaminated or damaged compressors
  • damaged turbine components
35
Q

what are the purpose of bleed valves

A
  • bleeds air off during start up and spool up
36
Q

what is the purpose of the combustion chamber

A

burning the fuel supplied thru the fuel nozzles with air supplied by compressor, and releasing heat so that it expands and accelerates to give uniform heated gas

37
Q

What range can the fuel/air ratio of a combustion chamber vary? what are the conditions for kerosene to burn?

A
  • between 45:1 and 130:1
  • kerosene will only burn at close to 15:1 so can only be burnt with a smaller amount of air in the primary combustion zone through a flame tube. (20% of air mass flow taken by snout)
38
Q

what are the characteristics of the primary air path in the combustor

A

primary approx. 20% of air flows through

  • 20% is further split into 12% flowing thru axial swirl vanes and 8% flowing radially through forward end of liner.
39
Q

what are the characteristics of the secondary air path in the combustor

A

secondary approx. 80% of air flows through

  • 80% is halved into

cooling air covering both sides of liner (a further 20% is combusted in this process)
AND
the remaining air joins gases at rear of liner and cools mixture to acceptable temp for turbine components.

40
Q

what is the function of a turbine? how does it produce shaft power?

A
  • the function of the turbine is to drive the compressor but can also drive accessories and shaft or turbo prop or turbo shaft engines.
  • converts kinetic and heat energy into mechanical work to drive components
41
Q

what is the principle of shaft power creation in a turbine

A

** when gas reaches turbine entry it’s at it’s hottest

  • extracts energy from gases by lowering pressure
  • lowering pressure in converted to velocity by convergent nozzles at TE of stator and rotor blades.
42
Q

What are the limiting factors of turbines?

A
  • Inlet temp due to constriction materials

- energy transfer is never 100% efficient due to thermodynamic and mechanical losses (is about 90-98% efficient)

43
Q

What are the functions of the turbine nozzles in making the turbine rotate

A
  • when combustion expands the gas it’s forced into discharge nozzles and accelerated to speed of sound which is about 2500 m/s for the temp it’s at.
  • gas flow is spun and whirled at same time in direction of blade rotation by nozzle guide vanes
  • when gas makes impact with blades energy is absorbed causing turbine to rotate at high speed
44
Q

at what speed does gas from the turbine enter the exhaust system?

A

750-1200 ft per second

speed of flow is decreased due to high friction loss at those speeds through diffusion

45
Q

how is the gas flow diffused in the exhaust system

A

increasing passage area between exhaust cone and outer wall of jet pipe

46
Q

at what speed does gas flow exit the exhaust system?

A

about mach 0.5 / 950 ft per second

47
Q

what are additional losses caused by in the exhaust system, how are they reduced

A

residual swirl velocity in gas flow from turbine

reduced by turbine rear struts in exhaust system straighten out the gas flow

48
Q

describe how pressure thrust is formed

A
  • goes thru a propelling nozzle which is a conv duct increasing velocity
  • exit velocity reaches speed of sound in relation to EGT and nozzle is choked (no further speed increase available)
  • upstream total pressure rises above prop nozzle choking pressure and the static pressure at exit rises above atmospheric pressure.
  • pressure difference across nozzle is the pressure thrust which is additional thrust due to momentum change of gas flow.
49
Q

when is the exit velocity of a turbojet engine subsonic

A

subsonic at low thrust conditions only

50
Q

how is thrust created in the exhaust ducts

A

exhaust ducts are a C-D nozzle

  • sonic at throat and velocity increases in divergent section
  • increased velocity in divergent section creates increased momentum and pressure force acting on inner wall of nozzle which is extra thrust.
  • it’s the component of the force parallel to longitudinal axis of the nozzle
51
Q

what are all the requirements of oils

A

A. low volatility (minimise evaporation at high ALT)

B. anti-foaming quality (positive lubrication)

C. low lacquer and coke deposits (solid particle formation kept to a minimum)

D. high flash point

E. low pour point

F. film strength (particles stick under compression and centrifugal loads)

G. wide temp range (-60F to 400F)

H. high viscosity index (how well it retains it’s viscosity when heated up)

52
Q

what oils are used that meet all the requirements

A

synthetic anti-freeze oils

53
Q

where is the oil tank

A

mounted on engine as a separate unit or integral within the gearbox assembly.

54
Q

what is the function of the oil tank

A
  • 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 maximum range likely to be flown.
55
Q

what must the oil tank be able to do

A
  • provide for lubrication system draining and replenishment.
  • must determine system contents eg. dip stick, glass or gauge (gauge most common) as a percentage of full.
  • de-aerating device to remove air from returning oil.
56
Q

how are the bearing housings sealed

A

air from compressor is directed across the bearing oil seals, inwards towards the bearing/oil supply so no oil escapes

57
Q

what are the different types of seals and their functions

A

labyrinth seals - (finned rotating member with static bore lined w abradable material, or high temp honeycomb)

ring seals - (metal ring housed in close fitting glove in static housing)

hydraulic seals -fin in oil annulus, centrifugal forces. diff in pressure compensated by diff in oil either side)

carbon seals - (carbon ring rubs against collar on rotating shaft, springs maintain contact)

58
Q

what does the EEC do

A

Electronic Engine Controller

  • reads engine parameters and responds to pilot power reqs input by power lever by scheduling appropriate fuel flow to achieve the power.
  • should achieve 15:1 ratio while controlling fuel flow so that TIT is never exceeded.
59
Q

how does the engine pressure ratio system work

A
  • shows the thrust of an engine
  • measures the ratio of jet pipe pressure to inlet pressure.
  • for a fan engine, may be ratio of turbine discharge and fan outlet pressures to compressor inlet pressure.
60
Q

how does the engine pressure ratio system work

A
  • shows the thrust of an engine
  • measures the ratio of jet pipe pressure (PT4 to PT7) to inlet pressure (PT1).
  • for a fan engine, may be ratio of turbine discharge and fan outlet pressures to compressor inlet pressure.
61
Q

How does the torque indicating system work

A

engine torque is used to indicate power produced by turbo-prop or turbo shaft engine and is shown on a torque meter.

engine torque is proportional to horsepower and is transmitted thru propeller reduction gear.

PRIMARY power and performance indication in cockpit

62
Q

What is a high energy ignition system and it’s requirement

A

gives provision for ignition of mixture in combustion system.

  • required to ensure that the engine will maintain satisfactory relight at high ALT and necessary for starting under any condition.
63
Q

Explain the gas turbine engine starting procedure

A
  1. Electric fuel pump on
  2. power lever at start position
  3. select ignition to start (continuous HE ignition). will go to ON or AUTO IGNITION after a few seconds
  4. Monitor engine parameters: temp and engine torque. (successful start if steady increase in temp and RPM)
  5. one at self sustaining RPM, starter and ignition will switch of automatically.
  6. Once ground idle RPM is reached, shift power lever to ground idle position
  7. turn electric fuel pump off and ensure fuel flow remains constant.
64
Q

What is self sustaining RPM

A
  • the speed at which the engine is capable of accelerating without the assistance of the starter.
  • is achieved when there’s sufficient airflow over turbine to allow turbine to drive the compressor. (must be accelerated to this speed quickly and smoothly)
65
Q

What is a hot start and it’s characteristics

A

occurs when the engine temp during the start cycle exceeds the stated limits.

if temp is rising quicker than usual, a hot start is expected.

wet starts can cause hot starts.

66
Q

What is a hung start and it’s characteristics

A

the failure of the engine to start - the ignition system activates but the engine fails to reach self sustaining RPM.

can be because of:

  • engine doesn’t light off
  • fails to make power
  • insufficient fuel to make combustion.

hung start is occurring if RPM is rising too slow, temp rises too fast or not at all.

67
Q

when is a tailpipe fire during start likely to happen

A

following a wet start the tailpipe fire risk is high as fuel may be lying in puddles in the tailpipe and if there is adequate temp to ignite fumes a fire could result.

68
Q

what actions should be taken after a tailpipe fire during start

A
  1. fuel selector OFF (power lever)
  2. starter - keep cranking to attempt to blow fire out (stop cranking prior to selecting fire extinguisher as airflow would blow extinguishing agent away from fire)
  3. Fire extinguisher ON
  4. Masters OFF all other switches OFF
  5. report to engineering
69
Q

What is an engine flameout and it’s characteristics

A

flameout will be noticed by

70
Q

What is an engine flameout and it’s characteristics

A
  • flameout will be noticed by a drop in turbine temp and RPM.
  • engine will come to a complete stop
  • check pre start checklist to make sure nothing’s been left. If nothing was then repeat checklist and attempt another start
71
Q

do all engines need anti icing devices installed

A

no because in some engines ice will not form in a sufficient quantity so devices are’nt needed

72
Q

how does the oil reservoir affect anti icing capability

A

it’s located inside the prop reduction gearbox and only a minimum of hot air flow is required to anti ice the inlet area. **(sourced from the HP compressor bleed valve)

73
Q

what is the function of spray bars

A
  • they are arranged around the axis of the pipe
  • results in only a proportion of unburnt air being used in combustion and remainder flows along jet pipe walls providing a cooling barrier
74
Q

what is the temp of the after burner flame

A

1700 C or higher

75
Q

what are the basic methods of injecting coolant into the airflow

A
  • coolant sprayed directly onto compressor inlet
  • injection of coolant into combustion chamber (more suitable for axial flow compressors)
    more even distrib. and more coolant can be injected
76
Q

what is SFC

A

the ratio of fuel consumption to thrust/SHP = SFC

expressed in pounds of fuel per hr per pound of thrust.

**determined by thermal and propulsive efficiency of the engine

77
Q

what is the ultimate controlling factor limiting engine power output

A

gas temp at turbine entry

Part 4 BGT Exam (1 decks)

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