Topic 05 - Air Systems, Starting, and Ignition Flashcards
air is bled off from various positions on the ______________ of a gas turbine engine and used to ________ the temperature of those internal parts of the engine which may suffer from ______________ heat
compressor, lower, excessive
bleed air is also utilized to ______________, which will increase the efficiency of both the compressor and the turbine
pressurize seals
air bled from the engine compressor is also used to service many aircraft systems which may include:
- air conditioning or pressurization
- hydraulic reservoir pressurization
- fuel tank pressurization
- domestic water tank pressurization
- thrust reverser actuation
- fuel heaters
- engine or airframe anti-icing
for use outside the engine itself, air is typically bled from two sources:
- the first source is an outlet on an early stage compressor, which supplies a continual low pressure bleed
- this continual low pressure bleed can be supplemented by a second source of air taken from the HP compressor through a control component called a high pressure bleed valve
during ____________ operation of the engine, the low pressure bleed is usually of sufficient pressure to maintain the air conditioning/pressurization system
high power
during ____________ operation, the low pressure bleed will fall and the high pressure bleed valve will open to ensure both an adequate pressure and sufficient airflow
low power
the high pressure bleed valve opens when the _____________ is selected because in this situation, the requirement is for hot air and lots of it, the higher the air pressure, the higher the air temperature
anti-icing
effects of bleeding air
- reduced thrust
- increase exhaust gas temperature
- increased engine RPM
- increased specific fuel consumption
when air is bled from the compressor, it reduces the ____________ of the engine and this will be indicated on the _____ or ______________ gauge
thrust, P7, Engine Pressure Ratio
educing the mass flow through the engine will reduce the amount of ________________ the combustion chamber and will have the effect of increasing the exhaust gas temperature
cooling air cooling
bleeding air from the compressor reduces the _______ on the turbine, which will have the effect of increasing engine RPM for a _____________
load, given fuel flow
air bled from the compressor is used to cool the turbine blades to seal both the ____________ and areas either side of the ______________
bearing chambers, turbine discs
when the bleed air has done its job, it is either _____________, or alternatively ejected into the ______________ at the highest possible pressure, thus achieving a small performance recovery
dumped overboard, main gas stream
if turbine blades and nozzle guide vanes are continuously cooled, then the temperature of their operating environment can exceed the _______________ of the material from which they are made
melting point
some modern turbofan engines use _________________ which works by controlling the temperature of a _________________ which is fitted closely around the turbine casing
active clearance control, tubular shroud
air, extracted from the ________________, flows axially through the _______________ between the HP compressor discs and the IP compressor drive shaft, which absorbs heat from the discs which assists in controlling the ________________ of the compressor blades.
IP compressor, annular passage, thermal growth
early gas turbine engines used low pressure compressor air in _____________
single pass internal cooling
the use of single pass internal cooling retained the temperature of the turbine blade below the critical level at which _______________ would occur
excessive creep
turbine blade cooling: when higher gas temperatures were required, passing _________________ compressor air through the blade as well as the _______________ air achieved a reasonable increase in the gas temperature before _____________ was experienced
high pressure, low pressure, blade failure
an additional increase in turbine gas temperature was attained by passing air through small holes in the _____________ and ______________ of the blade
leading, trailing edge
the film cooling (produced by passing air through small holes in the leading and trailing edge of the blade) created its own ______________ effect which protected the turbine blade from the onslaught of the hot gases coming from the combustion chamber
boundary layer
what was the optimum number of times air is allowed to pass through the blade?
five
turbine blade cooling: what is presently considered to be state of the art in turbine blade manufacture?
quintuple pass, multi-feed internal cooling with extensive film cooling
the nozzle guide vanes use a similar method of cooling to that which is used to cool the turbine blades but the one major difference is that only ___________________ compressor air is used
high pressure
heat conduction from the turbine blade to the turbine disc requires that the turbine discs are _________ and prevented from suffering _______________ from uncontrolled ___________________
cooled, thermal fatigue, expansion and contraction
the front and rear faces of each of the turbine discs are cooled by _______________ compressor air, the actual pressure in each disc cavity being controlled by _________________
high pressure, inter-stage seals
to prevent the leaking of oil or air into spaces where it should be excluded, several different types of seal are in use, where most of these seals work on the principle of the _______________
labyrinth
the _______________ consists of fins which rotate within an annulus of oil, or where the exterior of the seal is static, the annulus can consist of either a ________________ material or a _____________ structure
labyrinth seal, soft abradable, honeycomb
_________________ are used to either prevent or control leakage of air between sections of the engine which are operating at different pressures
inter-stage seals
the amount of pressure dropped across the seal depends upon the number of _______ over which the air must pass
fins
more pressure will be dropped by the air passing over the __________ before the low pressure zone, than will be dropped over the ___________ preceding the high pressure zone
five fins, two fins
if abradable materials were used in the manufacture of seals between two rotating shafts, there may be friction generated between the fins and the abradable material due to _____________ of the shaft which would create _______________ and the possibility of _______________
flexing, high temperature, shaft failure
an ____________________ can be used to provide the seal without risking friction buildup
inter-shaft hydraulic seal
the fins of the inter-shaft hydraulic seal rotate close within an __________ of oil, any deflection of the shaft will cause the fin or fins to __________ the oil, and the seal will be maintained without generating any undue friction or heat
annulus, enter
air used to seal bearing chambers is taken from the ______________ stages of the compressor through ______________ in the compressor ___________, and passed through communicating passages where it is required
intermediate, air transfer ports, rotor drum
when cooling and sealing has been carried out, how is the air disposed of?
- the high pressure air used for cooling is ejected into the exhaust system
- the low pressure air is fed out through its own dedicated vent pipe
three basic requirements which must be satisfied before a gas turbine engine can start
- compressor/turbine assembly must be rotated to get air into the combustion chambers
- fuel must be provided
- ignition must be provided to start the air/fuel mixture burning
there are other two requirements which satisfy servicing essentials:
- the necessity to motor over the engine with no igniters operating or ‘blowout’ or ‘motoring over’ cycle
- the need for the igniters to be operated independent of the start cycle
the two most common methods of rotating the high pressure compressor on modern civil aircraft are:
- electric starter motor
- air starter motor
the original means of starting gas turbine engines, but it has fallen out of favor because of its weight
electric starter motor
the electric starter motor is fixed in front of the _______________ and can also be attached to the __________________ which drives the compressor through the gearbox
compressor casing, engine accessory gearbox
the most popular starting system on large gas turbine engines which is light, simple to use, and very economical
air starter motor
air starter motor utilizes ____________ air as its power source
low pressure
air starter motor is attached to the _________________ of the engine
accessory gearbox
the sources of air available for engine start, in order of preference are:
- aircraft auxiliary power unit (APU)
- ground start unit
- cross bleed start
air from any of the sources is fed through an ____________________ to the starter motor’s air inlet where it is used to turn the ______________, then the air is subsequently exhausted to the ______________
electrically controlled air start valve, turbine rotor, atmosphere
the turbine rotor turns the ______________ which rotates the engine high pressure compressor shaft through a ___________________
reduction gear, ratchet clutch arrangement
a __________________ may also be included in the air starter motor system which can be used to de-energize the ___________________
flyweight cutout switch, air start valve
all gas turbine engines have two _________________ which each feed a separate ____________
high energy igniter units, igniter plug
high energy ignition systems have an output of approximately ______________
twelve joules
it may sometimes be necessary to help prevent engine _____________ to have the ignition system selected in circumstances other than engine starting which will cause the ___________ to erode, shortening its working life dramatically
flame out, igniter plug
to minimize erosion of the igniter plugs, some aircraft engines are fitted with a combination ignition system which includes a __________________, as well as the ______________________
low energy continuous selection, high energy starting selection
in a combination ignition system, the igniters are automatically deactivated at some point after ___________________
self sustaining speed
some aircraft have an ___________________ where if an aircraft stall is detected, the aircraft stall warning system will automatically select the __________________
automatic ignition system, continuous ignition system
the high energy ignition unit works on the principle of charging up a _______________ and then discharging it across the ___________________
very large capacitor, face of an igniter plug
what does HEIU stand for?
high energy ignition unit
the HEIU is mounted on the _____________________
side of the engine
two types of igniter plugs
- the older of these two types work in similar to a piston engine spark plug but has a much bigger spark gap
- the more modern version of the igniter plug is the surface discharge igniter plug
