Page 229 - Understanding Flight

CH08_Anderson 7/25/01 9:02 AM Page 216

216 CHAPTER EIGHT

If the tube decreases in size enough, the velocity of the air will
reach Mach 1. In a constricting tube, the air does not want to go faster
than Mach 1. In fact, in a restricting tube, the speed of the air will
always change in the direction of Mach 1. If the air before the
restriction is going faster than Mach 1, the dynamic pressure and
density will increase at the restriction, slowing the air down
until Mach 1 is reached. Once Mach 1 is reached, by either
In a restricting tube the speed
accelerating subsonic air or decelerating supersonic air, the
of the air will always change in
pressure will build up, moving the Mach 1 region forward into
the direction of Mach 1.
a smaller radius in the tube.
The net result is that the Mach 1 region moves to the smallest
restriction of the venturi called the throat. What happens after the throat
can be somewhat complicated but in general it depends on the pressure
downstream of the throat. If the pressure is the same as in the tube
before the venturi, the air returns to the initial conditions of velocity,
pressure, temperature, and density. If the pressure is lower, as in
expansion into a large low-pressure volume, the air will expand, causing
the velocity to continue to accelerate beyond Mach 1, as shown in Figure
8.17. This further expansion and acceleration results in a rapid decrease
in air temperature, density, and static pressure.
This expansion and acceleration rather than deceleration is
The most powerful rocket made
exactly what happens in the first part of the wing in transonic
was the Saturn V, which launched
flight. In subsonic flight the air decelerates and the pressure
the Apollo missions.
increases after the point of greatest curvature on the wing. In
transonic flight the air accelerates and the pressure continues

Mach 1
in throat

Fig. 8.17. The supersonic venturi.