Air Systems, Starting, and Ignition Flashcards
Air is bled off from various
positions on the compressor of
a gas turbine engine and used
to lower the temperature of
those internal parts of the
engine which may suffer from
excessive heat.
bleed air
The air bled from the
compressor which is used to
cool the internal components
of the engine is also utilized to
pressurize seals, which will
increase the efficiency of both
the compressor and the
turbine.
bleed air
Air bled from the engine
compressor is also used to
service many aircraft systems.
In a modern turbofan aircraft,
these may include:
- Air Conditioning /
Pressurization - Hydraulic Reservoir Pressurization
- Fuel Tank Pressurization
- Domestic Water Tank Pressurization
- Thrust Reverser Actuation
- Fuel Heaters
- Engine/Airframe Anti-Icing
For use outside the engine
itself, air is typically bled from
two sources:
- outlet on an early stage compressor
- HP compressor through a HP bleed valve.
During this operation
of the engine, the low pressure
bleed is usually of sufficient
pressure to maintain the air
conditioning/pressurization
system.
high power operation
During this 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 operation
This
opens when the anti-icing is
selected. In this situation, the
requirement is for hot air and
lots of it,
high pressure bleed valve
what is the requirement when the anti-icing is selected to open the high pressure bleed valve?
lots of hot air
what is the relationship between the pressure and temperature when the high pressure bleed valve opens?
directly proportional
When air is bled from the
compressor, it ________ the
thrust of the engine.
reduces
When air is bled from the
compressor, it reduces the
thrust of the engine. This will
be indicated on the
P7 or Engine Pressure Ratio gauge
Reducing the mass flow
through the engine will ________
the amount of cooling air
cooling the combustion
chamber. This will have the
effect of ________ the
exhaust gas temperature.
reduce; increasing
Increasing the mass flow
through the engine will ________
the amount of cooling air
cooling the combustion
chamber. This will have the
effect of ________ the
exhaust gas temperature.
increase; reducing
Bleeding air from the
compressor ________ the load
on the turbine, which will have
the effect of ________ engine
RPM for a given fuel flow.
reduces; increasing
The specific fuel consumption
will _______ whenever an
engine bleed is opened.
increase
used to cool the turbine
blades to seal both the bearing
chambers and areas either
side of the turbine discs.
air bled from the compressor
2 ways of disposing used air:
- dumped overboard
- ejected into the main gas stream at the highest possible pressure
There is a limit to the amount
of ________ which can be released
into the turbine from
combustion. This limit
imposed by the materials from
which the turbine blades and
nozzle guide vanes are
manufactured.
heat
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
these too must be cooled as they are also heated by conduction from the turbine blades.
turbine discs
works by controlling
the temperature of a tubular
shroud which is fitted closely
around the turbine casing.
active clearance control
Air, extracted from the IP
compressor, flows axially
through the annular passage
between the HP compressor
discs and the IP compressor
drive shaft. This air absorbs
heat from the discs which
assists in controlling the
thermal growth of the
compressor blades.
selective cooling
2 types of turbine cooling
- active clearance control
- selective cooling
The use of ________ retained the
temperature of the turbine
blade below the critical level at
which excessive creep would
occur.
single pass internal cooling
When higher gas
temperatures were required,
passing high pressure
compressor air through the
blade as well as the low
pressure air achieved a
reasonable ________ in the gas
temperature before blade
failure was experienced.
increase
created its own boundary layer
effect which protected the
turbine blade from the
onslaught of the hot gases
coming from the combustion
chamber.
film cooling which allows air to pass through small holes in the leading and trailing edge of the blade.
If passing air through the
blade once lowers its
temperature, then passing the
air through more than once
will ________ it more.
lower
The
optimum number of times to
pass the air through the blade
was found to be
5
is presently
considered to be state of the
art in turbine blade
manufacture.
quintuple pass, multi-feed
internal cooling with extensive
film cooling
use a
similar method of cooling to
that which is used to cool the
turbine blades. The one major
difference is that only high
pressure compressor air is
used.
nozzle guide vanes
requirement for the turbine discs of heat conduction from the turbine blade to it
cooled and prevented from suffering thermal fatigue
The front and rear faces of
each of the turbine discs are
cooled by _______, the actual
pressure in each disc cavity
being controlled by ________
high pressure compressor air; inter-stage seals
prevents the leaking of oil or air into spaces where it should be excluded
sealing
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
soft abradable material or a
honeycomb structure.
labyrinth seal
are used to
either prevent or control
leakage of air between
sections of the engine which
are operating at different
pressures.
inter-stage seals
During operation, there is a
pressure drop across each fin
which results in a _______ from one side of the
seal to the other.
restricted flow of air
The amount of pressure
dropped across the seal
depends upon the _______ over which the air must
pass.
number of fins
which drops more pressure? (a) air passing over the five pins before the LP zone (b) over the two fins preceding the HP pressure zone
a
If abradable materials was
used in the manufacture of
seals between two rotating
shafts, there may be _______
generated between the fins
and the abradable material
due to flexing of the shaft. This
would create ________ and the
possibility of ________
friction; high temperature; shaft failure
can be used to provide the seal
without risking friction
buildup.
inter-shaft hydraulic seal
rotate close
within an annulus of oil, any
deflection of the shaft will
cause the fin or fins to enter
the oil, and the seal will be
maintained without
generating any undue friction of heat
fins of the inter-shaft hydraulic seal
Air used to seal bearing
chambers is taken from the
intermediate stages of the
compressor through ________ in the
compressor rotor drum, and
passed through
communicating passages
where it is required.
air transfer ports
When cooling and sealing has
been carried out, the air which
has been doing the job has to
be ________.
disposed of
The high
pressure air used for cooling is
ejected into the ________. The low pressure air is fed out through its own dedicated ________.
exhaust system; vent pipe
3 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.
2 other requirements which satisfy servicing essentials:
- Motoring over the engine with no igniters operating or ‘blowout’ or
‘motoring over’ cycle. - Igniters operated independently 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
It was the original means of
starting gas turbine engines,
but it has fallen out of favor
because of its weight.
electric starter motor
where can the electric starter motor be fixed?
- in front of the compressor casing
- attached to the engine accessory gearbox
It is the most popular starting
system on large gas turbine
engines. It is light, simple to
use, and very economical.
air starter motor
this method of obtaining engine rotation utilizes low pressure air as its power source
air starter motor
where is an air starter motor attached?
accessory gearbox of the engine
The sources of air available for
engine start, in order of
preference are:
- Aircraft Auxiliary Power Unit
(APU) - Ground Start Unit
- Cross Bleed Start
turns the
reduction gear which rotates
the engine high pressure
compressor shaft through a
ratchet clutch arrangement.
turbine rotor
may also be included in the system
which can be used to de-energize the air start valve.
flyweight cutout switch
All gas turbine engines have
two ________
which each feed a separate ________.
high energy igniter units; igniter plug
how many high energy igniter units do gas turbines have?
2
High energy ignition systems
have an output of
approximately
12 joules
It may sometimes be
necessary to help prevent
engine flame out to have the
ignition system selected in
circumstances other than
engine starting. Use of high energy ignition system on these occasions will cause the igniter plug to
erode, shortening its working life
To minimize erosion of the
igniter plugs, some aircraft
engines are fitted with a
________
which includes a low energy
continuous selection, as well as
the high energy starting
selection.
combination ignition system
automatically
deactivated at some point
after self sustaining speed.
igniters
If an aircraft stall is detected, the
aircraft stall warning system
will automatically select this
automatic ignition system
works on the principle of
charging up a very large
capacitor and then
discharging it across the face
of an igniter plug.
high energy ignition unit
this type of igniter plug work in similar to a piston engine spark plug but has a much bigger spark gap
older version
The more modern version of
the igniter plug is the ________.
surface discharge igniter plug
