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Aerodynamic Testing 213

calculate local forces. These pressure measurements can also supply
validation data to numerical simulations.

FLOW VISUALIZATION
A third type of data collected in the wind tunnel is visualization data.
Unfortunately, in a normal wind-tunnel test there is very little to see.
You cannot see the wind flowing. Since wind-tunnel test sec-
The Boeing 747 was originally
tions are closed to prevent data corruption, you cannot even
designed to compete against
feel the wind. All you hear is lots of noise from the powerful
Lockheed’s C-5 transport. Boeing
fans. The only way to see what is happening to the air is to
lost the military contract but
have something visible to blow around.
went into production of the
The most common visualization tool is smoke. The
airplane as a commercial jet
problem with smoke is that in order to see details the wind
instead.
speed must be very slow. The very low speed can alter the
airflow enough to make the smoke results misleading. Another
problem with smoke is that in a closed-circuit wind tunnel the smoke
builds up after a while.
Another method is to use a mixture of clay with a fluid that
evaporates quickly. The clay is very fine and has the consistency of
talcum powder. This is painted on the model and then very quickly
the wind is turned on and the model positioned. Once the fluid
evaporates, the clay is left in a pattern of the surface flow, as shown in
Figure 8.15. Areas of flow separation are easy to spot as well as
patterns of the general flow around the airplane. In the figure one can
see that the last quarter of the wing is stalled. This method is very
effective but cannot be used if there are pressure taps, since the clay
will clog the holes.
Tufts of yarn are also useful. Very small tufts that minimize the
impact on the model can be glued to the model’s surface. The tufts
follow the direction of the airflow as seen in Figure 8.16 and can be
photographed for later analysis.
A fast-growing technique is the use of pressure-sensitive paints.
This is a paint that actually changes intensity and shades of a color
depending on the local pressure. So it is becoming possible to actually
see pressures on the surface of a model. This technology is still under
development but may become common in the future. There are many