Page 148 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 135

Airplane Propulsion 135

CENTRIFUGAL COMPRESSORS
Centrifugal compressors push the air out radially rather than along the
axis of the engine. An impeller, shown in Figure 5.14, adds energy by
accelerating the air radially. Impellers are popular on smaller engines
because a single impeller replaces several rows of blades in an axial-
flow compressor, making them much less expensive to build. There is,
of course, a drawback to this less expensive technique. The direction
of the air must be turned from heading radially, back to flowing along
the axis of the engine. This is the source of a significant energy loss.
This loss in efficiency is deemed unacceptable in larger jet engines.

MULTISTAGE COMPRESSORS
A compressor stage, made up of 10 to 12 rotor/stator stages or a sin-
gle impeller, can only do so much to compress the air. The solution

Fig. 5.14. An impeller connected to a shaft.