BGT Final Final Flashcards
What is another name for the Brayton cycle?
Why is it called this?
What are the stages of the cycle?
CONSTANT PRESSURE cycle;
PRESSURE is fairly CONSTANT across COMBUSTION section as VOLUME attempts to INCREASE, VELOCITY also INCREASES;
INTAKE, COMPRESSION, EXPANSION/POWER, EXHAUST
Describe the process of the Brayton cycle using diagrams?
A-B: AIR is COMPRESSED from ATMOSPHERIC
B-C: HEAT added by introducing and BURNING FUEL at CONSTANT PRESSURE, INCREASING VOLUME slight DECREASE in PRESSURE
C-D: Gas from COMBUSTION EXPAND through TURBINE and JET PIPE to ATMOSPHERE, energy turned to MECHANICAL POWER by TURBINE and remainder on discharge provides PROPULSIVE JET
D-A: PRESSURE is AMBIENT AIR, VOLUME DECREASES to equal AMBIENT AIR
What are the differences of Brayton cycle to Otto cycle?
Brayton: Combustion at constant PRESSURE, cycle CONTINUOUS, elimates 3 IDLE STROKES therefore MORE POWER for engine SIZE, HEATS at constant PRESSURE (VOLUME INCREASES) so NO PEAKS or FLUCTUATION only requires LIGHT FABRICATION of COMBUSTION chamber and LOW OCTANE FUEL;
Otto: Combustion at constant VOLUME, cycle INTERMITTENT, PEAKS of PRESSURE in excess of 1000PSI requiring HEAVY CYLINDER CONSTRUCTION and HIGH OCTANE FUEL
What are the comparative advantages of centrifugal and axial flow compressors?
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
What is the purpose of inlet guide vanes?
STATIONARY VANES that DIRECT airflow into ROTOR at MOST DESIRABLE
What are rotor blades and their purpose?
What is the construction of rotor blades?
The FIRST COMPONENT in the COMPRESSION stage that MOVES AIR REARWARD through each stage;
AERODYNAMICALLY DESIGNED with varying ANGLE of INCIDENCE
What are stator blades and their purpose?
What is the construction of rotor blades?
Receive AIR at HIGH VELOCITY from ROTOR blades and act DIFFUSERS changing KINETIC energy into PRESSURE energy;
AEROFOIL shaped BLADES
What are the advantages of multi spool compressors?
OPERATIONAL FLEXIBILITY:
HIGH COMPRESSION RATIOS;
QUICK ACCELERATION;
Better CONTROL of STALL CHARACTERISTICS
What is a compressor stall?
What are the symptoms of it?
ABRUPT LOSS of EFFICIENCY of the AXIAL FLOW COMPRESSOR in turbine engine when AoA of COMPRESSOR BLADES becomes excessive, IMBALANCE between RPM and INLET VELOCITY;
NO SYMPTOMS however if about to SURGE may experience VIBRATION, and FLUCTUATIONS in TEMP, FUEL FLOW and THRUST INDICATIONS
What are the causes of a compressor stall?
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
What happens to airflow generally in a compressor stall?
What is the name and characteristics of a mild compressor stall?
WHat about a severe compressor stall?
Air flow SLOWS/STAGNATES/REVERSES;
TRANSIENT stall: NOT usually HARMFUL, CORRECT themselves after 1 OR 2 PULSATIONS;
HUNG stall: Significantly DECAY ENGINE PERFORMANCE, cause LOSS of POWER, DAMGE or FAIL ENGINE
Using disrupted airflow describe the process that leads to a compressor stall?
Include situation, symptoms (normal and extreme)
JET with 2 REAR MOUNTED ENGINES TURNS;
SIDESLIP causes LESS AIRFLOW and LOW INLET AIR VELOCITY;
INCREASE in EFFECTIVE AoA leading to possible NOISE/RPM fluctuation/EGT INCREASE;
Fixed by REDUCING POWER to correct proportion of RPM to INLET VELOCITY;
EXTREME caused by FUEL MALFUNCTION or foreign OBJECT INGESTION;
Leads to REVERSAL that put BENDING STRESS on REAR of compressor BLADES causing them to CONTACT STATOR VANES;
Can cause disintegration of ROTOR SYSTEM and complete ENGINE FAILURE
What is the purpose of the turbines?
Simply, how does it achieve this?
DRIVE the COMPRESSOR;
DRIVE the ACCESSORIES and SHAFT of TURBO PROP/SHAFT;
CONVERTS KINETIC and HEAT energy from the EXHAUST GAS to MECHANICAL WORK
Specifically, how does the turbine produce mechanical work?
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
List the turbine blade designs?
IMPULSE BLADE;
REACTION BLADE;
IMPULSE REACTION
What are the characteristics of impulse blade design?
STATOR VANE and ROTOR BLADE;
VANES form CONVERGENT DUCTS;
BLADES form STRAIGHT DUCTS;
ROTOR TURNS by IMPULSE as GASES IMPINGE on blades
What are the characteristics of reaction blade design?
STATOR VANE and ROTOR BLADE;
VANES form STRAIGHT DUCTS;
BLADES form CONVERGENT DUCTS;
CONVERGENING ROTOR TURNS by REACTION FORCE from EXPANSION/ACCELERATION of GAS
What are the characteristics of impulse-reaction blade design?
What is the most efficient set up?
STATOR VANE and ROTOR BLADE with COMPOUND CURVATURE;
TWIST distributes WORKLOAD EVENLY along length by keeping EXIT PRESSURE and VELOCITY uniform tip to base;
Accomplished by EXTRACTING different KINETIC ENERGY at VARIOUS blade STATIONS;
50% REACTION greatest at TIP, 50% IMPULSE greatest at ROOT
What is process for creation of pressure thrust?
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
What do convergent and divergent passage cause air to do individually?
Convergent: GAS ACCELERATES;
Divergent: GAS DECELERATES and PRESSURE INCREASES
What happens to the energy of gas when a convergent exhaust nozzle is used?
What can be used instead?
How does is this method more advantagous?
Energy is WASTED as GASES leaving do NOT EXPAND RAPIDLY enough to immediately achieve outside air PRESSURE;
If HIGH PRESSURE RATIO ENGINE use CONVERGENT-DIVERGENT nozzle to RECOVER WASTE energy;
Uses PRESSURE energy to INCREASE gas VELOCITY more therefore INCREASING THRUST
What are the requirements of oils?
LOW VOLATILITY; ANTI FOAMING; LOW LACQUER and COKE DEPOSITS; HIGH FLASH POINT; LOW POUR POINT; FILM STRENGTH; WIDE TEMPERATURE RANGE; HIGH VISCOSITY INDEX
Why are low volatility, anti foaming, low lacquer and coke deposits, and high flash point desirable qualities of oil?
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
Why are low pour point, film strength, wide temperature range, and high viscosity index desirable qualities of oil?
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
What is the purpose of magnetic plugs and chips in the oil system?
What is the principle of operation of each?
Fitted to RETURN SIDE of oil system to COLLECT FERROUS DEBRIS in RETURN OIL;
Magnetic plug: PERMANENT MAGNET inserted in oil, READILY REMOVED and INSPECTION good for EARLY WARNING of FAILURE;
Chip detector: MAGNETIC PLUGS that incorporate ELECTRONIC DEVICE that TRANSMITS signal to COCKPIT when there is SIGNIFICANT COLLECTIONS/SIZE of DEBRIS
What is the requirement of fuel systems?
Supply PRECISE amount of FUEL in ALL GROUND and AIR CONDITIONS;
FREE of DANGEROUS OPERATIONAL CHARACTERISTICS;
INCREASE and DECREASE POWER on command to obtain THRUST in ANY CONDITION
How is the fuel metering unit driven?
What combinations can the FMU operate under?
How is it controlled?
ENGINE DRIVEN;
MECHANICAL/HYDRAULIC/ELECTRICAL/PNEUMATIC;
HYDRO-MECHANICAL/HYDRO-PNEUMATIC/ELECTRO-HYDROMECHANICAL;
NOT DIRECTLY PILOT, from COMPUTER instead
Excluding the FMU what other control devices are included in the turbine engine?
What is their purpose?
PRESSURE CONTROL; FLOW CONTROL; PRESSURE RATIO CONTROL; ACCELERATION and SPEED CONTROL; Achieved by using a form of GOVERNOR; LIMIT ENGINE from EXCEEDING set PARAMETERS
What are fuel nozzles alternative name?
What is their purpose?
INJECTORS/DISTRIBUTORS;
ATOMISING/VAPOURISING FUEL to ensure RAPID BURNING
What is the construction of the simplex type and duplex type fuel nozzles?
Simplex: EARLY JETS, CHAMBER induces SWIRL into FUEL and FIXED AREA ATOMISING ORIFICE;
Duplex: PRIMARY and MAIN FLOW TUBE with 2 INDEPENDENT ORIFICES (1 much smaller), SMALL handles LOWER FLOW and LARGER handles HIGH FLOW as fuel PRESSURE INCREASE
What is bypass ratio?
What are the functions of the magnitude of bypass ratios?
What is a typical bypass ratio value? What are they reaching now?
What type of aircraft is it used on?
MASS AIRFLOW through BYPASS DUCT to the MASS AIRFLOW through PRIMARY GAS PATH in same TIMEFRAME;
SIZE of BYPASS DUCT, PRIMARY FLOW INTAKE and VELOCITY of air;
2:1 - 5:1 up to 30:1
SOME TURBOJET and ALL TURBOFAN
What is the principle of after burning?
What is it primarily used for?
Introducing and burning FUEL in EXHAUST GASES between TURBINE and PROPELLING NOZZLE of engine;
INCREASED TEMP results in INCREASED JET VELOCITY and INCREASED THRUST;
Improving TAKEOFF and CLIMB PERFORMANCE
Why can after burning realistically be achieved?
Approximately 1/3 of TOTAL AIR is COMPLETELY BURNT, so much of it is still AVAILABLE for further EXPANSION when MIXED and BURNT with more FUEL
How does the arrangement of the spray bars in the after burner affect the airflow?
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
What is the purpose of water cooling?
POWER output is proportional to MASS AIRFLOW and therefore AIR DENSITY;
WATER is used to COOL to INCREASE AIR DENSITY to RESTORE/INCREASE POWER produced/available
When an aircraft utilises injection cooling, when is water/methanol used?
Pure water: If COMPLETELY CONSUMED during TAKE OFF ROLL (B707);
Methanol added: NOT COMPLETELY CONSUMED during TAKEOFF ROLL so ANIT FREEZE qualities of methanol are used (TURBO PROP/HELICOPTER)
What are the methods of injecting coolant into the airflow?
Which is more suitable for axial flow compressor engines? Why?
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
What happens when spraying into compressor inlet?
What is the purpose of added methanol when spraying into the compressor inlet?
TEMP of COMPRESSOR INLET AIR and AIR DENSITY and THRUST are INCREASED;
If only WATER was injected TURBINE INLET TEMP would DECREASE;
With METHANOL, TURBINE INLET TEMP is RESTORED by it BURNING in COMBUSTION CHAMBER;
POWER is RESTORED WITHOUT adjusting FUEL FLOW
What happens when spraying into combustion chamber?
INCREASES MASS AIRFLOW through TURBINE, relative to COMPRESSOR;
PRESSURE and TEMP drop across TURBINE is REDUCED, INCREASING JET PIPE PRESSURE, INCREASING THRUST;
REDUCED TURBINE INLET TEMP due to injection allows scheduled INCREASE of FUEL FLOW
