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80%, due to various leakages and
losses, hence the need for
supercharging, above.
The momentum of the engine
(supplied by the flywheel) brings the
piston up again, to force the exhaust
gases out through the exhaust valve,
which has already opened, just as the
piston hit BDC:
Airframes, Engines & Systems 209
It closes as the piston gets to TDC,
after the fuel valve has opened early,
to allow some atmospheric pressure
in that will help push the exhaust
out, as the piston by this time is
starting to slow down, ready to
reverse direction.
There – that wasn't so bad, was it?
Note that, although there were four
cycles, the crankshaft only went
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, because
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, because it doesn’t lubricate
the fuel pumps so well.
Propellers are only efficient up to
certain speeds, below which jets use
too much fuel relative to the work
done—the airstream provided by a
propeller is of a large diameter and
moves around the same speed as the
aircraft it relates to. The output from
a jet engine, on the other hand, is
hot and thin, and very much faster,
which is why it’s fuel-inefficient (it’s
also noisy, and like driving a car in
permanent high gear. The noise
comes from hot air meeting the cold
air outside the aircraft too quickly).
At higher speeds, say above 350 kts,
jets are best only because propellers
don’t work at all, due to the prop
tips approaching supersonic speeds.
Of course, you could have a fast
airframe, like Concorde, but that just
introduces complexity. Officially,
piston engines are best below about
217 kts (250 mph), turboprops
between 217-390 kts (250-450 mph)
and jets above that.
Turboprops (or turbo shafts) are a good
solution, where the output of the
turbine drives a propeller shaft, but
your top speed is still limited, hence
the fanjet, or turbofan, which is simply
a low speed propeller (with lots of
blades) enclosed in a duct, so you
don’t waste power in the shape of air
that spills centrifugally out of the
sides. This not only gives better
thrust (actually, more than double),
but also provides cooling, and less
noise, because the temperature
differential between the gases and
the air is not so great. They are called
bypass engines because the bulk of
the air bypasses the core of the
engine – the bypass ratio is the
difference between the amount of air
accelerated only by the fan against
that passing through the engine. The
end result is a large, relatively slowmoving
column of cold air enclosing
a thin, hot and fast exhaust. Total
thrust is from the fan and the engine
(exam question). The back end of
the engine runs a lot hotter, though,
even white-hot in the early days.
Because so much air is used for
cooling, humidity has less of an
effect on performance of jets.
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
210 Canadian Professional Pilot Studies
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. In a turbojet, this would
be where the story stops, because
the thrust is used to propel the
machine forward directly. In a
helicopter, or a turboprop, the
stream of hot gases is intercepted by
a turbine, and used to drive a rotor
or propeller gearbox.
About 2/3 of the energy produced is
　
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