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the stream of hot gases is intercepted
by a turbine, and used to drive a
rotor gearbox.
About 2/3 of the energy produced is
used simply to keep the engine
running. Most of the rest is used by
a power turbine for propulsion,
leaving enough energy to ensure the
gas falls out of the engine by itself,
so you don't need extra components
that will drain more energy.
The Inlet
Strictly speaking part of the airframe,
this is where air enters the system.
Its function is to convert ram-air
pressure (from forward movement)
into static pressure, ready for the
compressor.
The air travelling through the inlet
may well include other odds and
ends, like sand (in the desert) dust,
leaves, etc., especially in a helicopter,
when you will be in the lower parts
of the atmosphere anyway, and more
prone to foreign object damage. Fine
screens are used to combat this, but
they do restrict the airflow and have
an effect on your performance.
Another device is a particle separator,
which uses centrifugal force from
inlet air to create small swirls that
pick up small particles and drop
them into a sediment trap (that is,
rather like a vacuum cleaner). They
work with snow as well.
The Compressor
This is a rotating mass of impellers
or blades, designed to take vast
quantities of air, compress it (and
therefore heat it) for direction to the
combustor (below), so it's an air
pump, sometimes with the weight of
air delivered determined by the
engine RPM. That is, for any
specified RPM, the air volume will
be a definite amount. The
temperature rise across the
compressor could easily be 555° (as
on the Bell 407) and the
compression ratio nearly 10:1 for a
centrifugal compressor, and 25:1 for
an axial (which means more thrust
for the same frontal area).
The compressor can be centrifugal, or
axial, or both. As its name implies,
the centrifugal type uses impellers, as
used with water pumps, to fling air
outwards into channels leading to
the combustion chamber. The axial
compressor is essentially a series of
wheels in line with each other,
having fan blades around the outside
of each one. The blades used to be
attached separately, but now a
complete wheel is created, with
blades, out of one crystal. The air is
forced back into stationary stator
blades (or stator vanes), to alter the
characteristics of the flow – in fact,
the pressure is gradually increased as
it is forced into the smaller spaces
created by further blades
downstream. Each rotating wheel
with its set of stationary blades is a
stage, so several together (on the
same shaft) would constitute a
multistage compressor (the same
thinking applies to turbines, below).
A dual compressor, on the other hand,
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
Engines & Systems 53
turbine (in a helicopter, N1 is also
called Ng, and N2 is also called Np).
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
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