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CH05_Anderson 7/25/01 8:58 AM Page 137

Airplane Propulsion 137

It is in the burner that the energy is given to the air to produce
propulsion. But before the energetic exhaust can be allowed to
escape to produce propulsion there is some work for it to do.
Some of its energy must be extracted to power the compressor.
This is done by the turbine, which follows the burner.

Turbines
The turbine looks quite a bit like a single stage of a compres-
sor, only here the first set of blades that follow the burner are
fixed and do not rotate. These are called the turbine vanes. Fig. 5.16. An axial burner.
They are followed by a rotating set of turbine blades which (Photo courtesy of NASA.)
drive a shaft connected to the compressor. The arrangement is
illustrated in Figure 5.17. The purpose of the turbine vanes is to turn
the exhaust into the turbine blades. This allows for greater energy
transfer to the turbine blades.
A turbine is the reverse of the compressor. The air expands and
cools through each turbine stage, removing energy from the air. The
rotating turbine turns the shaft connected to the rotors or impeller in
the compressor. There must be as many turbine sections as there are
compressor sections. So a jet engine with two compressors, a low- and
high-pressure section, will have two turbines, each powering one of
the compressors with a separate shaft. The turbine/shaft/compressor
combination is referred to as a spool. Most large jet engines are two-
spool engines, meaning that they have a two-stage compressor driven
by a two-stage turbine. This was illustrated in Figure 5.15.

Fixed Turbine vanes

Rotating
Turbine blades

Fig. 5.17. The turbine with its vanes and blades.