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210 CHAPTER EIGHT

Wind-Tunnel Data

What sort of data is collected in a wind tunnel and how is it used? The
most obvious purpose of a wind tunnel is to measure forces and
torques, which are twisting forces, on the airplane. Wind tunnels can
also be used to measure pressures and airflow patterns on parts of a
model. Much of this testing is highly specialized and complicated and
so it will not be discussed here. However, there are some interesting
things to learn about wind-tunnel testing and how it relates to the con-
cepts introduced in this book.

FORCES
First, we focus on lift, drag, and torques. A typical wind tunnel uses a
force and moment balance to measure these aerodynamic forces. Fig-
ure 8.12 gives an illustration of such a balance. Though the figure
shows only one gauge, in reality a force balance makes measurements
of all of the forces and torques on the model. In fact there can some-
times be more than one balance used, as can be seen in Figure 8.11.

Fig. 8.12. Wind-tunnel force balance.