NZCAA BGT Flashcards
Describe how gas undergoes changes in pressure, volume and temperature in accordance
with Boyle’s and Charles’ Laws.
Boyles law: At constant temp, pressure increases as volume decreases (inversely proportional). T=VP
Charle’s law: At constant pressure, as temp increases volume increases (directly proportional). P=V/T
Describe the Pressure-Temperature cylce
Pressure and temperature increase through the compressor, temperature increases at the start of combustion but slowly decreases, and pressure decreases as well. In the turbine stage, pressure drops and temperature drops.
Describe the changes to pressure, temperature and velocity of the gas flow as it passes
through each section of a turbine engine
Pressure: Increases through compression. Max pressure at end of compressor/diffuser. Pressure drops slightly through combustion. Drops significantly further during turbine stage.
Temperature: Increases through the compressor. Spikes during ignition. Decreases from there. Max temp is inside combustion chamber. Maximum skin temp is at NGV.
Velocity: Reduces during compression and most of combustion. During final stages of combustion increases. Increases through rotor blades, decreases through stats. Final increase at propelling nozzle.
Describe the comparative advantages of turbine engines versus piston engines for aircraft
propulsion
A turbine engine is much more efficient for the same weight, and have a better power to weight ratio
Describe how a turbo-prop works
The two designs, free-power and direct drive, use turbines in the exhaust to drive a reduction gearbox which then powers a propeller.
Describe the purpose, design and principles of operation of the engine inlet duct.
The inlet duct needs to provide air to the compressor and straighten the airflow to prevent compressor stall/surge.
Inlet ducts are either ‘pitot intakes’ (normal round) which are suitable for commercial operations, or ‘divided intakes’ which is typical of millitary aircraft.
Why are inlets generally shaped with divergent ducts?
To impart a pressure rise of the air as it is delivered to the compressor (Ram rise).
Describe the purpose of a compressor in a turbine engine
A compressor is designed to impart a pressure rise on the airflow and reduce velocity (at the diffuser)
Describe the basic principles of operation of centrifugal and axial flow compressors
Centrifugal compressors make use of an impeller which accelerates and compressors air (velocity and pressure increase), before entering a diffuser which converts the kinetic energy into pressure energy (velocity reduces pressure rises. The centrifugal compressor is simpler and easier to maintain but is not as efficient for the same frontal area.
Axial flow compressors make use of rotors which accelerate air onto a stator which converts this velocity to pressure. They are more expensive but can achieve the power and efficiency required.
Describe the compressor arrangements found on the various types of turbine engine.
Compressors are arranged in spools, typically either 2 or 3. The low pressure compressor (the front) is driven by the low pressure turbine (the last turbine), and has the slowest RPM. The intermediate and high pressure compressors are connected to the intermediate and high pressure turbines, and rotate at faster RPM (with the high pressure being the fastest).
Describe typical compressor pressure ratios for the various types and configuration of
turbine engine
Axial flow compressors can achieve a compressor ratio of 25:1, but each stage usually only achieves 1.2:1
Centrifugal compressors can achieve approximately 5:1 to 7:1
What are the two main variables that affect the AoA of the compressor blades?
Engine RPM and TAS.
Describe the factors that affect compression ratio
Air density, system leakage, number of spools, engine RPM
Describe the purpose of the impeller
Divergent passages that increase pressure AND velocity
Describe the purpose of inlet guide vanes (variable and fixed)
These are designed to present the air to the first stage rotors at the optimum angle of attack. VIGV can change depending on conditions.
Describe the purpose of the rotor blades
Rotor blades can either be impulse type or reaction type blades. They are designed to impart acceleration. Impulse blades will be located toward the front of the stream where the airflow has the most energy, and reaction type is required at the rear where they will be more efficient at extracting energy.
Describe the purpose of the stator blades
Stator blades increase pressure through the conversion of kinetic energy
Describe the purpose of the variable stator blades
Compared to fixed stator blades, maintain efficiency at different RPM.
Describe the purpose of diffusers
converty kinetic energy into pressure energy
Describe the purpose of bleed valves
Supply pnuematic systems and also encourage axial flow to prevent surge/stall.
State the reasons why axial flow compressors have a higher number of stages than
centrifugal compressors
While axial flow compressors can achieve a greater pressure rise overall, the pressure rise over each pair of stators and rotors is small.
State the reason for the decrease in size and increase in the number of compressor blades
towards the rear of an axial flow compressor
Each stage of stators and rotors must deal with different conditions, and there will be a specific blade angle that achieves the optimum reaction force.
State the reasons for and advantages of multiple spool compressors
This allows the seperate spools to operate at their design RPM for the conditions experienced at their stage.
Describe the common source of bleed air
Bleed air is typically ducted from the compressor outlet
Explain what is meant by compressor stall/compressor surge
Compressor stall occurs when the AoA is exceeded and airflow becomes disturbed. A surge is a flow reversal within the engine (likely the result of a stall)
Describe the symptoms of a stall/surge
Increasing fuel flow, fluctuating RPM, increasing turbine temperature, and a banging noise
Descrie a multi-can combustion system?
also known as multi-chamber, lots of little chambers which are independent. Good for maintenance because LAMEs can remove independent pieces.
Describe can-annular combustion
Lots of individual chambers that are interconnected to equalis pressure. Combines best features of annular and multi-can device.
Describe the annular combustion design
Ring of fire. Most efficient of all the combustion chambers. Bad for maintenance though.
Describe the reverse flow combustion chamber
Ideal for compact designs, most common on turbuprops and helicopters.
Describe the uses of primary, secondary and tertiary air flow through and/or around a
combustion chamber
20% of the airflow is delivered to the primary zone (via swirl vane and atomising nozzle), 10% through secondary zone to mix, an the 70% dilution/cooling air (tertiary air)
How do supersonic air intake ducts decelerate the airflow to a subsonic condition prior to compressor entry?
By employing devices which cause shockwaves
The primary function of air seals within a turbine engine is to
Pressurise labryinth and carbon seals
How is the demand by the pilot for more thrust handled mechanically in a turbine engine?
The power lever is connected to a fuel control unit which senses compressor inlet temperature, EGT, compressor outlet pressure, and engine RPM.
Is the lubrication of a high bypass turbofan engine typically low pressure/wet sump or high pressure/dry sump?
High pressure dry sump
What type of oil pump is common on turbine engines?
Gear type
The turbine temperature rising rapidly (unusually fast) towards the specified limit, is an indication of:
A hot start
Why are turbine engines often fitted with separate low and high energy ignition systems?
High energy igniters are used for abnormally adverse start conditions, such as high altitude or extreme cold.
why does a hot start occur and what are the symptoms?
Usually a faulty FCU, too much fuel resulting in rapid turbine temperature increase.
Why does a wet start occur and what are the symptoms?
Ignition problem: fuel is available but not enough spark., Symptoms: RPM stable at starting RPM, gase temp doesn’t rise
Why does a hung start occur and what are the symptoms?
Insufficient starter power to stabilise RPM. Symptoms: RPM not increasing.
Torching is usually the result of…
A wet start
What are the advantages in twin spool and triple spool engines?
Twin spool engines are lower cost and easier to maintain, but triple spool engines are shorter and lighter with better efficiency.
What are the purposes of the inlet?
Increase pressure and straighten airflow (reduce chance of stall)
Intake design is typically dictated by….
Climb and cruise speeds (because power is typically idle on descent)
The most positive indication of a engine light-up during start is….
Increase in EGT
How do bucket reversers function?
By blocker doors external to the exhaust duct moving to the rear of the exhaust to direct airflow forward
How do clamshell reversers function?
Through blocker doors inside the exhaust duct moving to blank off the exhaust thrust and directing it forward
The primary function of oil in a turbine engine is to…
prevent metal to metal contact
A combustion chambers liner and associated gas flow control is designed to…
separate and control gas flow to allow for cooling and to maintain a region of low velocity in the chamber
Describe how a coldstream reverse thrust system operates
Blocker doors in the bypass duct, direct ‘cold’ bypass air in a forwards direction.
a prominent indication of a stall in a gas turbine engine during start would be
banging noise, fluctuating RPM, rising EGT
What are the type main types of intake on a turbine engine?
Pitot and Divided. Pitot is used commercially, as the only disadvantage is disturbed flow at high AoA which should not be encountered. Divided is used on millitary aircraft, and the disadvantage is disturbed flow during yaw.
With respect to the combustion process, specifically the gases, how does a turbine engine differ to a piston engine
Turbine engine combustion occurs at near constant pressure (Charle’s Law), while piston occurs at constant volume (combined gas law)
What is the normal shape of the exhaust nozzle on a jet engine?
Convergent (further accelerate gas)
What type of oil pump is typically used on a turbine engine
gear type
Why is specific gravity considered when refuelling with AVTUR?
Because fuelling is done by volume, and to convert this to weight you need to use specific gravity.
What is the purpose of the nozzle diaphram (NGV)?
Maximise velocity out of the combustion chamber onto the turbine blade
Typically, the lubrication system is high/low pressure dry/wet sump
High pressure dry sump
How is RPM typically measured in a turbine engine?
Using a 3 phase AC generator and synchronous motor
How are turbine blades cooled in a turbine engine?
Compressor bleed air
During takeoff in a turbine powered aircraft, the correct technique would be to advance the throttles until
Maximum torque or max temp is reached, whichever occurs first
Does calorific value change with temperature?
No
Will the specific gravity of a fuel affect the range of an aircraft?
Yes, by volume an increased SG will inreased range (due to higher calorific value)
What percentage of energy is extracted by turbines in a turbofan engine?
75% (therefore 25% used for thrust)
What percentage if energy is extracted by the turbines in a turboprop?
90% (therefore 10% left as thrust)
What percentage of airflow is extracted by bleed air?
20%
How would you maintain constant thrust after turning the bleed air on?
Increase RPM (Which would increase SFC)
Describe the characteristics of AVTUR (compared to AVGAS)
Low viscosity, low volatility (high flashpoint), high calorific value, higher freezing point
Why is high volatility bad?
Volatility refers to the tendency to boil and cause vapour locks, and a fuel with high volatility (like AVGAS) will boil easily.
What is the purpose of scavenge pumps?
Used in different bearing chambers, because the different chambers will have different operating pressures.
What is a coarse strainer?
Fitted to between the oil tank and pump to protect the pumps by removing debris
What are scavenge filters
Fitted on each individual oil return line to collect debris
Wgat are the functions of air seals in a turbine engine?
To control airflow cooling by pressuring labyrinth and carbon seals.
How are bearing chambers normally cooled?
With lubricating oil
Do hydraulic seals allow a controlled flow of air across the seal?
No. (note that the other seal types do)
Is the High Energy ignition unit supplied by high or low voltage?
Low, however the system stores and collects ‘volts’ to release as a high voltage during ignition.
When would an igniter use high and low joules?
High joules during cold weather or high altitude relight. Low joules during continious operation (e.g. we weather takeoff)
Flameout is usually indicated by…
A rapid decrease in EPR, N1, turbine temp, and fuel flow
If an aircraft is placed in the ‘re-light’ envelope, is re-light assured?
Yes
For an airbone re-light, is the starter and ignition required?
Normally NOT the starter, but ignition IS required. Note that using the starter could result in damage
Equivalent horsepower is…
Shaft horsepower + residual exhaust thrust
