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Part 4 Systems > BGT > Flashcards

BGT Flashcards

1
Q

How is thrust/propulsion generally provided?

What are the 2 types of this?

A

A suitable HEAT ENGINE - an INTERNAL COMBUSTION;

4 STROKE PISTON ENGINE and GAS TURBINES

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2
Q

What are the 3 main stages of evolution of the gas turbine?

A

TURBO JET or TURBO PROP;
TURBO FAN;
GEARED TURBO FAN

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3
Q

Explain how is Newton’s third law applies to the practical use of propulsion?
What are the similarities and differences between propeller and jet in this context?

A

AIR ACCELERATES passing through ENGINE;
FORCE required in ACCELERATION has EQUAL and OPPOSITE REACTION on apparatus producing acceleration;
Both propel using LARGE weight of air;
PROP has LARGE AIR SLIPSTREAM at LOW SPEED;
JET has SMALL jet of GAS at very HIGH SPEED

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4
Q

What was the earliest example of a jet reaction?

What is a modern day example?

A

120BC Hero’s engine toy;

Garden sprinkler

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5
Q

What is an incorrect assumption about jet propulsion?

A

Jet REACTION is the result of PRESSURE of the jet on the ATMOSPHERE

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6
Q

What is the thrust produced proportional to?

What relationship gives better efficiency?

A

MASS of AIR and its CHANGE in VELOCITY;

LARGE MASS and SMALL VELOCITY as it LOWERS the JET VELOCITY relative to ATMOSPHERE

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7
Q

What is another name for the Brayton cycle?
Why is it called this?
What are the stages of the cycle?

A

CONSTANT PRESSURE cycle;
PRESSURE is fairly CONSTANT across COMBUSTION section as VOLUME attempts to INCREASE, VELOCITY also INCREASES;
INTAKE, COMPRESSION, EXPANSION/POWER, EXHAUST

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8
Q

What is the most common intake for turbo jet and turbo fan?

What are the advantages of it?

A

FIXED GEOMETRY/CIRCULAR PITOT TYPE;

Fully use RAM EFFECT due to FORWARD SPEED and has MINIMUM LOSS of RAM PRESSURE with ALTITUDE CHANGE

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9
Q

What are the comparative advantages of centrifugal and axial flow compressors?

A

CENTRIFUGAL: MORE ROBUST, EASIER to DEVELOP and MANUFACTURE, favoured in SMALLER ENGINES for SIMPLICITY and ROBUSTNESS
AXIAL: CONSUMES MORE AIR for SAME frontal AREA therefore MORE THRUST, HIGHER PRESSURE RATIOS, INCREASE PRESSURE by ADDITIONAL STAGES

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10
Q

What is the principle of operation of centrifugal flow engine?

A

AIR RECEIVED at centre of IMPELLER in AXIAL DIRECTION;
IMPELLER ACCELERATES AIR OUTWARD through CENTRIFUGAL REACTION to its ROTATIONAL speed;
AIR EXPANDS in DIVERGENT DUCT/DIFFUSER;
BERNOULLIS is applied AIR SPREADS, SLOWS, and INCREASES in STATIC PRESSURE

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11
Q

What is the construction of an impeller?

A

FORGED DISC with INTEGRAL, RADIAL disposed VANES on ONE OR BOTH sides;
Forms CONVERGENT passages in conjunction with COMPRESSOR CASING;
Vanes may be SWEPT but usually STRAIGHT RADIAL for ease of manufacturing

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12
Q

What is the purpose of the diffuser?

What is the construction of the diffuser?

A

SLOW VELOCITY to INCREASE PRESSURE;
Can be INTEGRAL or SEPERATE;
Formed by VANES TANGENTIAL to IMPELLER;
Vanes DIVERGE to CONVERT KINETIC energy into PRESSURE energy;
INNER edges of VANES are in line with RESULTANT flow from IMPELLER

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13
Q

What is the principle of operation of the axial compressor?

A

ROTOR turned at HIGH SPEED by TURBINE so AIR is CONTINUOUSLY induced;
Air is ACCELERATED by ROTOR BLADES and swept REARWARD onto adjacent STATOR VANES;
Air is DECELERATED in STATOR passage and KINETIC energy is translated into PRESSURE energy

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14
Q

What is the purpose of inlet guide vanes?

A

STATIONARY VANES that DIRECT airflow into ROTOR at MOST DESIRABLE

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15
Q

What are rotor blades and their purpose?

What is the construction of rotor blades?

A

The FIRST COMPONENT in the COMPRESSION stage that MOVES AIR REARWARD through each stage;
AERODYNAMICALLY DESIGNED with varying ANGLE of INCIDENCE

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17
Q

What are stator blades and their purpose?

What is the construction of rotor blades?

A

Receive AIR at HIGH VELOCITY from ROTOR blades and act DIFFUSERS changing KINETIC energy into PRESSURE energy;
AEROFOIL shaped BLADES

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18
Q

What are the advantages of multi spool compressors?

A

OPERATIONAL FLEXIBILITY:
HIGH COMPRESSION RATIOS;
QUICK ACCELERATION;
Better CONTROL of STALL CHARACTERISTICS;

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19
Q

What are the advantages of multi spool compressors with changing density?

A

DECREASING DENSITY/INCREASING ALT allows LOW PRESSURE COMPRESSOR N1 to SPEED UP;
This allows RECOVERY of the PRESSURE LOSS from HIGH PRESSURE COMPRESSOR N2

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20
Q

What is a spool?
What is another term for it?
How is it numbered?

A

Any COMBINATION of TURBINE and COMPRESSOR;
GAS GENERATOR;
SINGLE SPOOL engine: Ng;
MULTI SPOOL engine: N1 is LOW PRESSURE COMPRESSOR, N2 and N3 is for INTERMEDIATE/HIGH PRESSURE COMPRESSOR

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21
Q

What is a compressor stall?

What are the symptoms of it?

A

ABRUPT LOSS of EFFICIENCY of the AXIAL FLOW COMPRESSOR in turbine engine when AoA of COMPRESSOR BLADES becomes excessive;
NO SYMPTOMS however if about to SURGE may experience VIBRATION, and FLUCTUATIONS in TEMP, FUEL FLOW and THRUST INDICATIONS

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22
Q

What are the causes of a compressor stall?

A

TURBULENT or DISRUPTED AIRFLOW to engine INLET - DECREASED GAS VELOCITY;
EXCESSIVE FUEL FLOW by abrupt engine ACCELERATIONS - INCREASED combustion back PRESSURE and DECREASED GAS VELOCITY;
EXCESSIVE LEAN fuel mixture by abrupt engine DECELERATION - DECREASED combust or back PRESSURE and INCREASED GAS VELOCITY;
CONTAMINATED or DAMAGED COMPRESSORS - DECREASED COMPRESSION and INCREASED GAS VELOCITY;
DAMAGED TURBINE COMPONENTS causing LOSS of POWER to COMPRESSOR and LOW COMPRESSION - DECREASED COMPRESSION and INCREASED GAS VELOCITY;
Engine operation ABOVE or BELOW designed RPM - INCORRECT RPM SETTING for PHASE of flight

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23
Q

What is the purpose of the combustion chamber?

A

BURNS FUEL supplied through NOZZLE with AIR from COMPRESSOR;
RELEASES HEAT in a way that AIR is EXPANDED and ACCELERATED to give SMOOTH STREAM of UNIFORMLY HEATED GAS at all CONDITIONS required

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24
Q

What is the air fuel ratio range during normal operation?
What ratio will kerosene efficiently burn at?
What is done to ensure this ratio?

A

45:1 and 130:1;
15:1 found in the PRIMARY COMBUSTION ZONE;
Using a FLAME TUBE;
20% of air mass is taken in by SNOUT or ENTRY SECTION

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25
Q

How is airflow divided around the combustor?

A

PRIMARY (air mixture): 20% - 12% flows AXIALLY through SWIRL VANES in area of FUEL ENTRY and 8% flows RADIALLY from FORWARD end of LINER
SECONDARY (cooling air): 80% - HALF provides COOLING over BOTH sides of the LINER PREVENTING the FLAME from CONTACTING METAL LINER and remaining TERTIARY joins at REAR of LINER and COOL MIX as it ENTERS TURBINE to acceptable TEMP

26
Q

What is the purpose of the turbines?

Simply, how does it achieve this?

A

DRIVE the COMPRESSOR;
DRIVE the ACCESSORIES and SHAFT of TURBO PROP/SHAFT;
CONVERTS KINETIC and HEAT energy from the EXHAUST GAS to MECHANICAL WORK

27
Q

Specifically, how does the turbine produce mechanical work?

A

EXTRACTS ENRRGY from gas by REDUCING PRESSURE;
Achieved since the PRESSURE is CONVERTED to VELOCITY by CONVERGENT NOZZLES formed at TRAILING EDGE of STATOR and ROTOR BLADES;
Then TURBINES ROTATE

28
Q

What is the limiting factors of the turbines?

A

INLET TEMPERATURE is limited by construction MATERIALS, this is where GAS is HOTTEST;
Energy TRANSFER is NEVER 100% due to THERMODYNAMIC and MECHANICAL LOSS;
AERODYNAMIC LOSS in turbine BLADES, NOZZLE GUIDE VANES and GAS LEAKAGE over BLADE TIPS

29
Q

What is the principle of operation of the turbine nozzle?

A

Gas EXPANDS during COMBUSTION and forces into CONVERGENT DISCHARGE NOZZLES which ACCELERATE to SPEED of SOUND for the HAS TEMP;
At the same time, GAS is SPUN in DIRECTION of ROTATION of BLADES by NOZZLE GUIDE VANES;
On IMPACT with BLADES and REACTION through BLADES, ENERGY is ABSORBED and causes ROTATION of BLADE at HIGH SPEED;
This POWER drives the SHAFT and COMPRESSOR

30
Q

What are the inefficiencies experienced through the exhaust system?
How are these reduced?

A

Gas enters at 750-1200ft/sec which produces HIGH FRICTION LOSSES:
Speed is DECREASED to about 950ft/sec by DIFFUSION where passage AREA INCREASES between EXHAUST CONE and OUTER WALL of JET PIPE;
The CONE also PREVENTS flow across the REAR FACE of the TURBINE DISC
RESIDUAL SWIRL VELOCITY in stream;
TO REDUCE TURBINE REAR STRUTS in exhaust unit designed to STRAIGHTEN flow BEFORE gas enters JET PIPE

31
Q

What is process for creation of pressure thrust?

A

EXHAUST gas pass to ATMOSPHERE through a CONVERGENT PROPELLING NOZZLE INCREASING VELOCITY;
During MOST operations excluding low thrust (subsonic) EXIT VELOCITY is SPEED of SOUND to the EGT and NOZZLE is CHOKED;
When CHOKED, VELOCITY CANNOT INCREASE unless TEMP is INCREASED;
When UPSTREAM TOTAL PRESSURE is INCREASED ABOVE CHOKED value the STATIC pressure of GAS is ABOVE ATMOSPHERIC;
This PRESSURE DIFFERENCE is PRESSURE THRUST and is EFFECTIVE over NOZZLE EXIT area;
It is ADDITIONAL to thrust from MOMENTUM CHANGE of GAS stream

32
Q

How does the arrangement of a convergent-divergent exhaust duct change the thrust generated?

A

.

33
Q

What are the requirements of oils?

A 
LOW VOLATILITY;
ANTI FOAMING;
LOW LACQUER and COKE DEPOSITS;
HIGH FLASH POINT;
LOW POUR POINT;
FILM STRENGTH;
WIDE TEMPERATURE RANGE;
HIGH VISCOSITY INDEX
34
Q

Why are low volatility, anti foaming, low lacquer and coke deposits, and high flash point desirable qualities of oil?

A

LV: To MINIMISE EVAPORATION at ALTITUDE;
AF: For POSITIVE LUBRICATION;
LL+CD: Keeps SOLID PARTICLES MINIMAL
HFP: INCREASES the TEMP that FLAMMABLE VAPOURS will IGNITE when NEAR FLAME source

35
Q

Why are low pour point, film strength, wide temperature range, and high viscosity index desirable qualities of oil?

A

LPP: LOWEST TEMP oil will GRAVITY FLOW
FS: Excellent COHESION and ADHESION allowing OIL molecules to stick together under COMPRESSION and stick to surfaces under CENTRIFUGAL forces;
WTR: -60°F to +400°F
HVI: RETAINS VISCOSITY when HEATED

36
Q

What is the purpose and requirements of an oil tank?
Why is it required?
Where is it usually located?

A

To STORE appropriate quantity of OIL to ensure there is ADEQUATE SUPPLY under ALL OPERATING CONDITIONS and ALTITUDES over MAX RANGE;
Provide for LUBRICATION system DRAINING and REPLENISHMENT;
Must have an INDICATION method such as DIPSTICK/SIGHT GLASS/ELECTRONIC GAUGE;
Gas turbines usually are DRY SUMP so TANK is required;
MOUNTED on ENGINE or INTEGRAL unit in GEARBOX ASSEMBLY

37
Q

What is the air from the compressor used for other than cooling?
How is this achieved?

A

SEAL the BEARING HOUSING to PREVENT OIL LEAKING into engine main CASINGS or COMPRESSOR INLET;
DIRECTING AIR across BEARING OIL SEALS being INWARDS towards BEARING or OIL SUPPLY

38
Q

What are the methods of sealing used in gas turbine engines?

A 
LABYRINTH seals;
RING seals;
HYDRAULIC seals;
CARBON seals;
BRUSH seals
39
Q

What are labyrinth seals?

A

A FINNED ROTATING MEMBER with a STATIC BORE which is LINED with SOFT ABRADABLE MATERIAL, or HIGH TEMP HONEYCOMB structure

40
Q

What are ring seals?

A

A METAL RING which is HOUSED in a close FITTING GROOVE in the STATIC HOUSING

41
Q

What are hydraulic seals?

A

A SEAL FIN IMMERSED in ANNULUS of OIL which is created by CENTRIFUGAL FORCES, any DIFFERENCE in AIR PRESSURE IN and OUT of BEARING CHAMBER is compensated by DIFFERENCE in OIL LEVEL either side of FIN

42
Q

What are carbon seals?

A

A STATIC RING of CARBON which constantly RUBS against COLLAR on ROTATING SHAFT, SPRINGS used to maintain CONTACT between CARBON and COLLAR

43
Q

What is the purpose of the electronic engine controller?

A

Reads PARAMETERS of ENGINE;
Responds to PILOT POWER requirements INPUT by power LEVERS by SCHEDULING appropriate FUEL to achieve POWER given ATMOSPHERIC and ENGINE PARAMETERS;
Aims to achieve 15:1 AIR:FUEL MIX consistent with PISTON ENGINE RATIO while AUTOMATICALLY controlling FLOW to NEVER EXCEED turbine inlet TEMP

44
Q

Explain the principle of operation of the duplex type variant fuel control system?

A

.

45
Q

How is thrust indicated in the cockpit?
What does it measure?
When considering a fan engine what is measured differently?

A

THRUST METER or ENGINE PRESSURE RATIO GAUGE;
The RATIO of 2 or 3 PARAMETERS usually JET PIPE PRESSURE to INLET PRESSURE;
INTEGRATED TURBINE DISCHARGE/FAN OUTLET PRESSURE to COMPRESSOR INLET PRESSURE

46
Q

What is the purpose of the torque indicating system?
What is it proportional to?
How is it transmitted?

A

TORQUEMETER: Indicates POWER DEVELOPED by turbo prop/shaft;
Engine TORQUE or TURNING MOMENT proportional to HORSEPOWER;
Through the PROPELLER REDUCTION GEAR

47
Q

When is a torque indicating system required?

What is the importance of it?

A

TURBO PROP/SHAFT;

PRIMARY POWER and PERFORMANCE indication in cockpit

48
Q

What seperate system is required for starting a gas turbine?

What are the usual requirements of it?

A

IGNITION system - HIGH ENERGY;

Required to ensure engine SATISFACTORILY RELIGHTS at HIGH ALTITUDE and sometimes necessary for ANY CONDITION

49
Q

What is the procedure for starting a gas turbine engine?

A

PREFLIGHT CHECKS;
ELECTRIC FUEL PUMP(S) ON;
Place CONDITION/POWER LEVER in START position so FUEL flows at PREDETERMINED TIME;
IGNITION to START to get CONTINUOUS HE IGNITION for a few SECONDS then RELEASE and it returns to ON/AUTO IGNITION position;
Monitor engine PARAMETERS, TEMP and COMPRESSOR SPEED / ENGINE TORQUE, TEMP and RPM should STEADILY INCREASE;
Once SELF SUSTAINING RPM, STARTER and IGNITION will switch OFF;
Once engine made GROUND IDLE RPM shift CONDITION/POWER to GROUND IDLE position;
ELECTRIC PUMP OFF ensure CONSTANT FUEL PRESSURE/FLOW

50
Q

What causes a hot start to be carried out?

What can be done to see indications a possible hot start?

A

Engine TEMP during START CYCLE EXCEEDS stated LIMITS can sometimes be CAUSED by WET START;
Engine TEMP MONITORED closely during START as HOT START can be EXPECTED if TEMP RISES FASTER than usual

51
Q

What causes a hung start to be carried out?

What can be done to see indications a possible hung start?

A

FAILURE of engine to START, IGNITION system ACTIVATES but engine FAILS to reach SELF SUSTAINING RPM;
Can be engine DOESN’T LIGHT OFF, FAILS to make POWER, or INSUFFICIENT FUEL to SUSTAIN COMBUSTION;
Expected if RPM RISE is TOO SLOW, the TEMP RISES QUICKLY or FAILS to RISE

52
Q

When is there a risk of tailpipe fire during ground start?

A

Following a WET START as FUEL could be lying in PUDDLES in the TAILPIPE and if TEMP is adequate to IGNITE FUMES a FIRE could break out

53
Q

What is the immediate actions of a tailpipe fire during ground start?

A

FUEL SELECTOR OFF;
Keep CRANKING STARTER to BLOW OUT FIRE, but STOP PRIOR to SELECTING FIRE EXTINGUISHER as airflow oud BLOW AGENT away;
FIRE EXTINGUISHER ON as REQUIRED using aircraft SYSTEM or HAND HELD - CO2 preferably to AVOID engine CONTAMINATION;
MASTERS and ALL other SWITCHES OFF, SHUT DOWN any other already RUNNING ENGINES and SECURE aircraft;
REPORT to ENGINEERING

54
Q

What indicates a flameout has occurred?

What actions must be taken after a suspected flameout?

A

DROP in turbine TEMP and RPM;
Engine will usually come to COMPLETE STOP;
Before SECURING and ADVISING ENGINEERING check PRE-START checklist to ensure NOTHING was LEFT OUT if something was left out, REPEAT checklist and ATTEMPT START

55
Q

When might anti icing not be required?

What are some alternative methods of anti icing?

A

When ICE DOES NOT form in SUFFICIENT QUANTITY;
Some TURBO PROPS the OIL RESERVOIR is located within PROP REDUCTION GEARBOX providing some ANTI ICING CAPABILITY and only a MINIMUM of HOT AIRFLOW sourced from HP COMPRESSOR via BLEED VALVE is required to ANTI ICE INLET AREA

56
Q

How does the arrangement of the spray bars in the after burner affect the airflow?

A

Since TEMP is usually 1700°C or HIGHER, BARS are placed so FLAME is CONCENTRATED around AXIS of PIPE;
Only a PROPORTION of UNBURNT AIR is USED in COMBUSTION, the rest flows along the WALLS of JET PIPE to PROVIDE COOLING BARRIER

57
Q

What are the methods of injecting coolant into the airflow?

Which is more suitable for axial flow compressor engines? Why?

A

COOLANT sprayed DIRECTLY into COMPRESSOR INLET;
COOLANT INJECTED into COMBUSTION CHAMBER INLET;
Combustion chamber inlet since MORE EVEN DISTRIBUTION can be obtained and GREATER QUANTITY can be satisfactorily INJECTED

58
Q

What is SFC?
What is the unit for SFC?
What is it determined by?

A

The RATIO of FUEL CONSUMPTION to THRUST;
POUNDS of FUEL PER HOUR PER POUND of NET THRUST;
The THERMAL and PROPULSIVE EFFICIENCY of the engine

59
Q

What is the ultimate controlling factor limiting engine power output?

A

GAS TEMP at the TURBINE ENRTY

Part 4 Systems (4 decks)

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