Another well-known propulsion system is the gas turbine or jet engine.
There are several types of gas turbine engines, but the simplest ones are the
so-called turbojets.
These engines are shaped like a
cylinder containing several parts
inside, which rotates on a central
shaft.
An auxiliary electric motor is
needed to start the turbine engines.
The outside air enters the engine through the inlet into the compressor, which
consists of one set of fixed blades (stator) and another of rotating blades (rotor).
The air is then compressed at the compressor section and enters thereafter the
burner where the fuel/air mixture is ignited.
This creates a hot gas passing through the turbine and out the nozzle, which is
shaped to accelerate the hot exhaust.
The turbine uses the energy from the hot exhaust to rotate and since the turbine
is linked to the compressor by the central shaft, it will also keep the compressor
rotating, thus no longer needing the electric motor.
Normally the model aircraft turbines
use propane/butane gas along with
a glow plug to start the ignition and
rise the burner's temperature above
100oC before liquid fuel is injected
through small holes into the burner.
Once the combustion gets started,
the glow plug is no longer needed.
The combustion process may be controlled or stopped by regulating the amount
of the fuel available, the amount of oxygen available or the source of heat.
Unlike the conventional combustion/piston engines, the jet engines don't have a
natural limitation of the rpm.
This means that the rpm will keep rising as more fuel is fed to the engine until
the materials no longer withstand the high temperature and/or the high rpm and
will breakdown.
Therefore, an Electronic Control Unit (ECU) is required to limit the max fuel flow.
The max value is set by using an external device called Ground Support Unit
(GSU).
Since model aircraft powered by gas turbines usually fly very fast, with speeds
above 500Km/h (300mph), these type of engines are definitely not recommended
to beginners.
Besides, operating gas turbines also involves some risks.
So, it's highly recommended to read the BMFA Handbook regarding these engines.
For further info about the function of other turbine engine types such as Turbofan,
Turboprop and Ramjet, check here.
Some interesting books:
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