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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 found on some
helicopters). 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 (they wash
out on a compressor). The air is
forced back into stationary stator
blades (or stator vanes), to alter the
characteristics of the flow – in fact,
Airframes, Engines & Systems 211
pressure is gradually increased as it is
forced into smaller spaces created by
further blades downstream.
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, pressure is
increased as it is forced into 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 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
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. In some
engines, N2 is the intermediate shaft
and N3 the high power. being the
smallest, N3 is the one started first.
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 – that for a
centrifugal compressor is 4:1. The
Engine Pressure Ratio (EPR), on the
other hand, is the difference between
air pressure coming out of the
turbine compared to that at the
compressor inlet. It is the measure of
　
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