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by staying open until a certain
pressure ratio is obtained. Other
engines may use such a bleed to
prevent stalling when the throttle is
opened too suddenly.
A compressor stall reduces efficiency,
meaning less power. A cold stall only
affects a few blades or a small area
of the compressor, whereas a hot stall
involves it all, and may mean severe
damage caused by hot gases from
the combustor when the airflow
becomes reversed inside the engine.
There may or may not be a loud
noise to accompany this.
In fact, the compressor is an ideal
place from which to tap small
amounts of air (compressor bleed air)
for other purposes, such as cooling,
pressurising of oil systems or
operating anti-ice systems. However,
when doing this, the exhaust
temperature will tend to rise slightly.
For anti-ice systems, the bleed is
taken from the back end of the
compressor (that is, compressor
discharge air) that has already been
heated due to compression. It will
typically flow through the
compressor shell and hollow struts,
and the inlet guide vanes.
The Combustor
The air is divided into two streams
here, one for burning, and the other
for cooling. Once the flame is lit by
the spark igniter during engine start, it
stays that way till the engine is shut
down (you could say that the engine
is on fire all the time, and it's only
when the fire becomes uncontained
that it becomes an emergency). The
engine is spun initially by air from a
high pressure bottle or APU (or
even a V8 for the SR-71, or a
starter/generator in helicopters) and
the sparks ignited when the airflow
is high enough to keep the
temperatures down at the back end.
An auto relight system is designed to
restart the engine should it flame
out, providing a continuous spark
from the igniter all the time it is
switched on.
54 JAR Private Pilot Studies
The Turbine
This is where the hot air flows
through. As it spins, it also helps
drive the compressor, as it is directly
connected.
In a free turbine engine, used in
helicopters and some turboprops,
the exhaust goes through two
turbine stages, e.g. a compressor turbine,
and a power turbine. That is, there is
no direct connection between the
exhaust from the engine and the
gearbox it drives, which is important
for helicopters that need to
autorotate when the engine stops.
There is always a reduction gear
system to reduce the high RPM
coming from the power turbine
(rotor blades, for example, only go
round at about 300-400 RPM,
whereas a jet engine will be more like
33,000). A helicopter powered by a
free turbine may have a braking
system to keep the blades and
transmission stationary.
One of the most important
instruments in your cockpit is the
Turbine, or Exhaust Temperature gauge,
which shows the heat coming out of
the back end. It is particularly
important during starting because, if
the battery is too weak to spin the
engine properly, there will be less
airflow through it, and not as much
cooling available, leading to a hot
start and an expensive repair as the
back end melts. During flight, on hot
days, this temperature may well be
the limiting factor in the amount of
payload you can take, even if you
have lots of torque left.
Jet Fuel
Jet A, standard for commercial and
general aviation, is narrow-cut
kerosene, usually with no additives
apart from anti-icing chemicals. Jet
A1 has a different freezing point and
possibly something for dissipating
static, used for long haul flights
where the temperature gets very low.
Jet B is a wide-cut kerosene with
naphtha in, so it is lighter and has a
very low flash point (it's actually 2/3
diesel and 1/3 naphtha, but in
emergency you can swap the
naphtha for avgas to get pretty much
the same thing). It contains static
dissipators. Try not to mix Jet A and Jet
B - the mixture can ignite through
static in the right proportions, as Air
Canada found when they lost a DC-
8 on the ramp in the 70s. Static can
come simply from the movement of
fuel through the lines. Jet A weighs
about 5% more per litre than Jet B,
but it gives you a longer range, as
turbines work on the weight of the
fuel they burn, not the quantity. So,
if you load the same amount of fuel,
your machine will weigh more with
Jet A, but if you fill the tanks, you
will use fewer litres and less money.
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