Page 219 - Understanding Flight

CH08_Anderson 7/25/01 9:01 AM Page 206

206 CHAPTER EIGHT

Fig. 8.7. The closed-circuit wind tunnel.

sophisticated wind tunnels have cooling vanes to take this heat out
and try to maintain a constant temperature. Other wind tunnels just
suffer in the heat. Some tunnels can get so hot that changes to the
wind-tunnel models must be performed with insulated gloves.
Transonic wind tunnels are designed to test aircraft at roughly Mach
0.8 to 0.85. These tunnels require much more power than their low-
speed counterparts. Since the power loss due to friction goes as the
airspeed cubed, much more heat is generated and must be removed.
Most wind tunnels have a single return, as shown in Figure 8.7.
There was a time when dual-return wind tunnels were popular. The
Kirsten wind tunnel at the University of Washington, Seattle is a dual-
return wind tunnel. The layout of this wind tunnel is shown in top
view in Figure 8.8. The advantage of such an arrangement is that two
smaller motors can be used rather than one large motor. There is also
an advantage of size with a dual-return wind tunnel. For reasons
beyond the scope of interest of this book, a dual-return wind tunnel
can support a larger test section while having a smaller footprint.
Smaller motors and a smaller footprint result in a lower cost of
construction. The disadvantage is that the two channels of air must
meet and become uniform by the time they reach the test section. This
causes additional technical difficulties. Figure 8.9 shows a model in the
test section of the Kirsten wind tunnel. The test section of this wind
tunnel has a cross section 8 ft high by 12 ft wide (2.4 m 3.6 m).
The NASA Ames 40 80 wind tunnel is the largest close-circuit wind
tunnel in the United States. In the jargon of wind tunnels a 40 80