Page 215 - Understanding Flight

CH08_Anderson 7/25/01 9:01 AM Page 202

202 CHAPTER EIGHT

Before delving into the purposes of wind tunnels, let us explore
some of the basic concepts behind the wind tunnel. First, we discuss
the subsonic venturi, which is often mistakenly used in descriptions
of flight.

Subsonic Wind Tunnels

A subsonic wind tunnel works like a venturi shown in Figure 8.1. A
venturi tube is the best example of Bernoulli’s theorem, which relates
speed and pressure in a tube and when no energy is added to the fluid
(see Appendix). As the air reaches the restriction in the tube, the
velocity increases. As discussed in Chapter 1, the increase in velocity
causes a reduction in the static pressure, measured perpendicular to
the direction of flow. Since the forces are low, the air density and tem-
perature remain essentially constant. As we will see later, this is not
true in the case of transonic venturis.

A WING IS NOT A HALF VENTURI
You might find it amusing to note that ground school courses
often introduce the venturi as an example of how wings fly.
The presentation includes moving one of the walls of the
venturi so far away as to not influence the other wall (see
Figure 8.2).What is left is a wall with a hump.The instructors
tell the students that because of Bernoulli’s principle this “half
venturi” has lift. But you now know that this is wrong.The
wall blocks the downwash, so there can be no lift. After
leaving the hump, the air is traveling at the same speed and in
the same direction as before the hump. As we know, if there is
no net change made to the airflow there cannot be lift. So,
what do you do when you see this on the FAA written exam?
Well, if you want to pass, you will have to give them the
answer they want to hear, even though it is wrong!

The simplest of all low-speed wind tunnels is a venturi tube. If you
have a household fan and cardboard, you can easily build a small
wind tunnel, such as shown in Figure 8.3. The fan is placed so that it
draws air through the wind tunnel. The model is placed in the venturi.
The cross section of the wind tunnel does not have to be round. In
fact, most wind tunnels have rectangular cross sections.