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would have stages in tandem, but on
different shafts at different speeds,
to produce higher compression
ratios. The first in line would be the
low pressure compressor (N1), driven by
the low pressure turbine, which would
also be the slowest, via the low
pressure shaft, which rotates inside the
high pressure shaft, which performs
the same function for the high
pressure compressor (N2) and
turbine (in a helicopter, N1 is also
called Ng, and N2 is also called Np).
Airframes, Engines & Systems 53
The N2 shaft runs the opposite way
to N1, so the torques counteract and
cancel each other out, relieving stress
on the engine mounts.
The whole combination of shafts
and compressor is known as a spool.
The compression ratio is the difference
between the pressure of the air as it
comes out of the compressor and
the pressure at the engine inlet – it
should always be higher than the
back pressure from the turbine, or
the airflow through the engine could
go the wrong way.
Inlet Guide Vanes adjust air going into
the compressor, which are closed
when the engine is idling and fully
open at about 70% engine RPM.
Some engines have a small valve that
opens when the engine starts, to
correct the airflow so that the
compressor blades do not stall (a
compressor bleed) - for maximum
efficiency, and because engines have
to react quickly, you need to operate
as close to the stall as possible. At
low RPM, the engine is naturally is
not able to pump as much air, so you
need to "unload" it during start and
low power operations. A bleed air
system makes it see less restrictions
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.
The Turbine
This is where the hot air flows
through. As it spins, it also helps
54 Canadian Private Pilot Studies
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
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