Page 144 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 131

Airplane Propulsion 131

thrust decreases with speed. This is an important difference between
the two types of engines and impacts the way the airplanes are flown.
In a turbine engine the energy of the combustion is transferred to
the exhaust, rather than a mechanical piston. Figure 5.9 shows how
one might conceptualize a turbine engine, starting from the rocket
motor in Figure 5.1. The rocket carries fuel and its own source of
oxygen. That is how it is able to operate in the vacuum of space. But
a turbine engine flies in the atmosphere where oxygen is plentiful. So
the rocket motor could be fitted with a compressor to supply high-
pressure air to the combustion chamber. To run the compressor, a
turbine fan has been placed in the exhaust to convert some of the
energy from the high-speed exhaust into mechanical work.
Conceptually, Figure 5.9 has the components of a jet engine,
though the implementation of the components is quite different in
practice. Figure 5.10 shows a more realistic drawing of a jet engine.
Basically it consists of a tube with an inlet (or a diffuser), followed by
a compressor for the air, a burner where the high-pressure air and fuel
are burned, and a turbine to power the compressor. At the exhaust
end of the turbine there is a nozzle to direct the exhaust to give thrust.
The three components, compressor, burner, and turbine, are the core
of the jet engine.

Air compressor Combustion High-velocity gas
chamber

Fuel

Air
Fig. 5.9. Conceptual idea of how to build a jet engine.