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Aerodynamic Testing 211

One question is “are they accurate?” Several factors enter into the accu-
racy of wind-tunnel measurements. The most important factor affect-
ing the accuracy of measurements is the effect of wind-tunnel walls.
The wind tunnel introduces an artificial constraint, namely, walls.
The walls have two effects. The first is that they interfere with the
amount of air that can be pulled from above the model’s wings and
blocks the downwash on the bottom. This latter effect is just
The cost of military aircraft has
like ground effect and is called wall effects.
increased consistently since the
Another problem in a low-speed wind tunnel is that air will
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speed up in a constriction and the airplane model acts like a
A. At this rate, the entire defense
constriction in the venturi. In other words, the air accelerates
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as it moves around the model due to the blockage from the
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model. For obvious reasons, this is called the blockage effect.
Wall and blockage effects are illustrated in Figure 8.13.
Years of theoretical work have resulted in methods to correct for
wall and blockage effects. Unfortunately, wind-tunnel corrections are
not completely reliable, so wind-tunnel results must be backed up
with flight test. Normal wind-tunnel corrections amount to only a few
percent. However, the few percent can be extremely important when
trying to predict performance of the final airplane. It is not uncommon
for an airplane manufacturer to use the same wind tunnel for all tests
because the engineers acquire experience in estimating how a
particular wind-tunnel result will relate to actual flight data.
One of the most important pieces of data collected is called the drag
polar. This is a plot of lift vs. drag, as shown in Figure 8.14. At first
glance these data do not look very interesting, but look again. What

Fig. 8.13. Wall and blockage effect.