Page 147 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 134

134 CHAPTER FIVE

Today, commercial airplanes have flow compressors because the air continually moves along the
to turn back more often because axis of the engine. Each blade is basically a rotating wing, just
of inoperable toilets than like a propeller. There is one fundamental difference between
because of an engine failure. the compressor and the propeller. With the compressor, the
blades are in a duct and therefore the added energy results in
an increase in the pressure of the air rather than an increase in the
speed of the air. How pressure is produced, rather than speed, is kind
of interesting.
The axial compressor is made of rows of blades, made up of
rotating blades followed by stationary blades as shown in Figure
5.13. A typical row of rotating blades has 30 to 40 blades and is
called a rotor. Following each rotor is a stationary set of blades,
called a stator. The rotor’s job is to increase the energy of the air and
thus its pressure. The stator increases the pressure of the air further
by slowing it down from the speed at which it left the rotor. The
pressure increase across a single stage of rotor/stator is fairly low,
but multiple stages can produce fairly high compressions with high
efficiency.
It is not wise to try to increase the pressure too much across a single
stage because this increases the chances of the blades stalling just like
a wing that is trying to produce too much lift. The stall causes the flow
to reverse and is referred to as compressor stall. Rather than trying to
increase the pressure substantially across each stage, multiple stages
are used to decrease the pressure gain across each stage. The result is
that an entire compressor section of 10 to 12 stages may increase the
pressure by a factor of 10 or more.

Compressor Blades

Rotor

Stator (stationary)

Rotor

Stator

Fig. 5.13. A compressor consists of a rotor and a stator.