Page 24 - Understanding Flight

CH01_Anderson PFS2 7/25/01 8:55 AM Page 11

Basic Concepts 11

is measured (P in the figure). In the situation in the figure,
total Alexander Graham Bell was a
P is the same at both points A and B. In the language of founding member of the Aerial
total
pilots, this is also known as the Pitot pressure, and the figure Experiment Association (AEA),
illustrates a Pitot tube. which was organized to try to
The second pressure to consider is the static pressure be the first in flight. It was
(P ), which is measured perpendicular to the airflow disbanded in 1908 but had been
static
through a hole in the wall. This is the pressure most often partially successful with the
referred to when the air pressure is discussed in designs of Glenn Curtiss.
aerodynamics. In the figure the static pressure is higher at
point A than at point B.
The third pressure is the dynamic pressure (P ), which is the
dynamic
pressure due to the motion of the air, and is a pressure parallel to the
flow of air. The dynamic pressure is proportional to the kinetic energy
in the air. Thus the faster the air goes the higher the dynamic pressure.
This may seem a little complicated, so let us try to put it all together.
The total pressure P is the sum of the static and dynamic
total
pressures (P and P ). We have shown how to measure the
static dynamic
total, or Pitot, pressure and the static pressure. How is the dynamic
pressure of the air measured? Look at the setup in Figure 1.10.
Between the tube that measures the total pressure and the tube that
measures the static pressure there is placed a differential pressure
gauge. That is a gauge that measures the difference in pressure
between the two ports, which is the difference between total and
static pressure. Since static plus dynamic pressure is equal to total
pressure, this difference between total and static pressures is the
dynamic pressure.
If no energy is added to the air (by some mechanism such as a
propeller) the total pressure remains the same and an increase in
dynamic pressure causes a decrease in static pressure. So when one
reads that the pressure of air decreases because it is going faster, the
pressure referred to is the static pressure. But what if energy is added to
the air? In the right-hand corner of Figure 1.9 is the picture of a fan.
What has happened to the air pressures at point C? The fan is
accelerating the air, and thus has done work on the air. Thus the
dynamic pressure has increased. Since the air is not confined, the static
pressure is the same as in the surrounding environment and thus has