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round twice.
Turbines
As mentioned above, the same
principles apply to jet engines as
reciprocating ones, only they're
applied in a different way. They also
use cheaper fuel, as compression is
not a factor in producing the power,
although avgas can sometimes be
mixed with jet fuel (see the Flight
Manual), at the expense of reduced
maintenance periods, as it doesn’t
the fuel pumps so well.
Turbine engines are discussed in this
book only in relation to helicopters,
because it is more likely that
somebody with a Private Pilot's
Licence is going to fly one.
Because so much air is used for
cooling, humidity has less of an
effect on jet engine performance.
The five basic parts of a jet engine
are the inlet, the compressor, combustor,
turbine and nozzle (the bit joining the
compressor to the combustor is
called the diffuser). They could be
combined or doubled back on
themselves in some engines,
particularly those in helicopters, to
save space, speaking of which,
remember that the engine is not
directly responsible for pushing the
machine forward, as it might be in a
jet aeroplane. Instead, it drives the
main rotor gearbox, which drives the
rotors – the disk formed by them is
what flies and pulls the rest of the
helicopter with it. In such
circumstances, the engine could even
be upside down, provided the
gearing could cope with it. In fact,
the PT6 has a "backwards" design,
with the compressor at the rear.
Another name for a jet engine is a
gas producer because, in a helicopter,
the stream of hot gases is intercepted
by a turbine, and used to drive a
rotor gearbox.
52 Canadian Private Pilot Studies
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 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. 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,
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