Page 244 - Understanding Flight

App_Anderson 7/25/01 9:03 AM Page 231

Misapplications of Bernoulli’s Principle 231

Force

a b

Fig. A.2. Ping-Pong ball in a jet of air.

1
2
P ⁄2 v P
static total
Here P is the static pressure measured perpendicular to the flow.
static
The second term is referred to as the dynamic pressure where is the
density of the air and v is its speed. Thus, the dynamic pressure is a
measure of the kinetic energy of the air. P total is the total pressure. All
this was discussed in Chapter 1.
In a confined pipe the sum of the dynamic pressure and the static
pressure is a constant. If we know that constant, the static pressure
can be calculated from a knowledge of the air’s velocity. The same is
true for air that is given energy, say by a propeller or our breath. But
now we do not know the constant any more. In fact, kinetic energy
has been given to the air. Thus the dynamic pressure has increased,