Page 233 - Understanding Flight

CH08_Anderson 7/25/01 9:02 AM Page 220

220 CHAPTER EIGHT

Gustave Eiffel is considered the concern because all the energy losses cause the air to be hot
father of applied aerodynamics. to start with. For the blowdown wind tunnels the air is often
In his later years he built wind heated before it reaches the venturi so that the test section
tunnels and designed airfoils. Of remains at a reasonable temperature. Vacuum wind tunnels
course, he is best known for have a problem that the room air is used and thus it is not
designing the Eiffel tower. practical to preheat the air. Therefore, the test section is very
cold. For example, a Mach 3 test section would be 274°F
( 170°C) if the air supply were at room temperature.

Hypersonic Testing
With the incredible power required for supersonic wind tunnels, how
can anyone expect to create hypersonic flow conditions, typically above
a Mach 5, in a test environment? The only effective method to do this
with a stationary model is with the blowdown method, lots of preheat-
ing of the air, and a very small test section. The key word in that last
sentence was stationary. Some hypersonic facilities actually use a com-
bustion gun, where gases combust in the breach to propel the model.
The problem with this technique is that the desired measurements must
be made on a nonstationary model, one that is moving very fast.
But there is another trick up an engineer’s sleeve. Hypersonic flight
implies that the Mach number is typically greater than Mach 5. Up to
this point we implicitly assumed that to achieve hypersonic
speeds we have to increase the speed in the test section or of
Many ideas that showed up in
the model. What if we were to decrease the speed of sound
WWII fighters originated in
instead? Sound speed differs for different gases. The speed of
airplanes specially designed to
sound decreases as the weight of the gas molecules increases.
compete in the Schneider Cup
So, instead of using air for our working gas, we could look for
races of the 1920s and 1930s.
a heavier gas, like carbon dioxide, although this will only
The Schneider Cup was the most
decrease the sound speed by 14 percent. The advantage of
prestigious air race in the world.
using an alternate gas is that the true speeds can be kept
reasonable, while the Mach number is fairly high.

Flight Testing

In this section we explain flight-testing techniques used to verify air-
plane performance as related to the concepts described previously.