Page 231 - Understanding Flight

CH08_Anderson 7/25/01 9:02 AM Page 218

218 CHAPTER EIGHT

Test section

Supersonic
Mach 1
Mach 1

Subsonic chamber

Fig. 8.18. The supersonic wind tunnel.

into a region of lower pressure it accelerates, producing the
supersonic flow in the test section. After the test section the airflow
goes through a second venturi. Here the speed decreases until it
becomes Mach 1 at the throat. Since the air is going into a region of
higher pressure, as the channel opens up the flow slows down,
becoming subsonic again.
The supersonic wind tunnel has an additional source of
The Boeing 747-400 has 171 mi
power loss. In addition to the friction on the walls and the
(274 km) of wiring.
drag on the models, now there are losses associated with the
inevitable shock waves. All of these losses mean a lot of heat
is being generated. In order to run continuously, a supersonic wind
tunnel must have a large cooler, which is placed in the airflow in the
subsonic section.
The great amount of power required for supersonic wind tunnels
means there are very few continuous wind tunnels and they are not
very large. A 3 3 foot (1 1 m) test section is considered very
large and requires half a million horsepower (375 megawatts) to
operate at Mach 3. But there are other methods to test supersonic
aircraft.
One method is the “blowdown” supersonic wind tunnel depicted in
Figure 8.19. A huge tank is filled with high-pressure air and then
exhausted through a venturi. This kind of wind tunnel works quite
well but will only allow a few minutes of testing. However, a carefully
planned test can gather a tremendous amount of data in a very short
time. With this technique the energy required is generated and stored