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CH08_Anderson 7/25/01 9:02 AM Page 215

Aerodynamic Testing 215

other methods of visualization, but to discuss them in detail is beyond
the scope of this book.

TAIL ON AND TAIL OFF
If you visited a wind-tunnel test, there is a good chance you would see
an incomplete airplane. Probably the horizontal stabilizer will not be
installed. A normal wind-tunnel test will include many runs without
the horizontal stabilizer. There are two reasons for doing this. The first
is to be able to provide an estimate of the errors due to wall effects. The
horizontal stabilizer has its own upwash and downwash,
The sweep on the Boeing 727
which will enter into the measurements. By removing the hor-
wing is 32.5 degrees. United
izontal stabilizer, the corrections for the wing alone can be
Airlines liked 30 degrees, but
determined.
Boeing’s previous airplanes had
The second reason for removing the horizontal stabilizer is
35 degrees of sweep. So the
so the effect of the stabilizer on torques on the airplane in flight
engineers settled on 32.5
can be directly measured. As the name implies, a stabilizer is
degrees.
used to stabilize the airplane. The effect of the horizontal
stabilizer on the total torque on the airplane determines how
stable the airplane will be, how difficult it will be to fly, and where the
payload can be positioned, as was discussed in Chapter 4. In analyzing
flight characteristics there are many parameters that cannot be
determined theoretically. So the wind-tunnel data gives data that can
be used to fill in where theory cannot provide a clean answer.

Supersonic Venturis
In order to understand transonic and supersonic wind tunnels, it is
necessary to understand the transonic and supersonic venturi. Here
the compressibility of air cannot be ignored, and both the air density
and temperature change significantly. Let us first look at the flow of
air that is just below Mach 1 in a tube that decreases in diameter with
distance. As the diameter of the tube decreases, the velocity of
the air increases and the static pressure decreases. Since the
During the Gulf War, U.S.
forces are now quite high, as the air compresses both the den-
airplane loss rates were roughly
sity and temperature of the air increase. Because of this com-
the same as during normal
pressibility, the velocity and static pressure do not change as
training.
much as they would if the air were incompressible.