AOPA Turbine Pilot Magazine - March 2025 - 87

Almost every pilot is familiar with a
venturi tube, that hollow tube with the narrow
throat (facing page). The fascinating
workings of the venturi tube can be partially
explained using a flow of water. It's
easy to see that whatever amount of water
enters the inlet must come out the other
end. If this were not true, the water would
have to bunch up in the throat and become
compressed. But since water is incompressible,
this cannot happen. The same amount
of water, therefore, must pass the throat
as passes the inlet and outlet. Since there
is less space in the throat, the water there
must accelerate and travel more speedily.
A more visual example is when you partially
block the outlet of a garden hose with
your thumb or add a nozzle to the hose. In
either case, a venturi type constriction (or
throat) is created, and the water escapes
much more rapidly than it normally would.
What many do not appreciate is that
air and water behave similarly; both are
fluids. And since freeflowing subsonic
air is also considered incompressible, the
same thing happens to it when flowing
through a constriction: It accelerates.
Up to now, everything should seem
plausible. But now for the question
answered only in textbooks. Why does an
increase in airspeed within the throat of a
venturi tube cause a decrease in pressure?
Finding the answer requires a slight detour.
...
Air in motion-like any object-possesses
kinetic energy. A second form of
energy possessed by air is static pressure.
An inflated toy balloon is an excellent
example of static pressure (energy) being
stored. If air inside a balloon is allowed to
escape through the neck, the static pressure
converts to kinetic energy as the
air escapes.
Two important pieces of the lift puzzle
state that: (1) energy can neither be created
nor destroyed, and (2) air entering a venturi
tube consists of two significant forms
of energy: kinetic energy (of motion) and
static pressure.
The total amount of energy of air entering
a venturi tube is equal to the sum of
its kinetic energy and its static pressure.
As the airflow approaches the venturi's
throat, its velocity increases. This represents
an increase in kinetic energy. Yet
we learned earlier that energy cannot be
created. Has the law regarding the conservation
of energy failed? It has not.
What happens is that some of the
air's pressure energy is sacrificed (or converted)
into kinetic energy. In this manner,
the total energy content of the air remains
unchanged. This process of energy conversion
is identical to what happens when air
escapes from a balloon: air velocity increases
and air pressure decreases. Within the venturi
tube, static air pressure is sacrificed to
accelerate the airflow, resulting in reduced
pressure within the venturi's throat.
It should now be easier to understand
why airspeed and air pressure are so
87
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AOPA Turbine Pilot Magazine - March 2025

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