AOPA Turbine Pilot Magazine - March 2025 - 88

RUDDER & WRENCH
Undisturbed airflow
Increased airspeed/reduced pressure
Upwash
Relatively high pressure
ultimately be directed downward from
the wing's trailing edge. This is accomplished
by an ingenious application of
the principles already discussed.
The low-pressure area above the
closely related and why an increase (or
decrease) of one results in a decrease (or
increase) of the other. This relationship
between airspeed and static pressure was
discovered by Daniel Bernoulli, an eighteenth-century
Swiss mathematician,
and is known as Bernoulli's Principle.
...
The figure above shows the airflow pattern
about a wing. Notice that the wing's cambered
(curved) upper surface is shaped
like the bottom half of a venturi tube. The
upper half of this imaginary tube is simply
the undisturbed, horizontal airflow at some
distance above the wing.
Notice what happens to the air flowing
over the wing's upper surface. As it
enters the venturi constriction formed
by the wing's camber, air accelerates just
the way it does when passing through a
conventional venturi tube. The result is
a corresponding decrease in pressure
along the upper surface of the wing.
This reduced air pressure is often and
erroneously called suction. The amount of
pressure reduction is actually quite small. A
fully loaded Cessna 177 Cardinal, for example,
has a gross weight of 2,500 pounds and
a wing area of 172.4 square feet. Dividing
the weight by the wing area results in the
Cardinal's wing loading of 14.5 pounds per
square foot. In other words, each square
foot of wing lifts 14.5 pounds. Since there
are 144 square inches in a square foot, we
see that each square inch of wing creates
only 0.1 pound (or less than 2 ounces) of lift.
It seems logical that the relatively
high-pressure air beneath the wing would
attempt to flow to the area of reduced pressure
above the wing. After all, this is what
happens in the free atmosphere. Air flows
from high pressure to low. But in the case
of an airplane, a wing separates the areas of
high and low pressure, and the wing rises
88
AOPA PILOT / March 2025
into the low-pressure area above it. (Some
of the relatively high-pressure air does curl
around the wingtip to " fill the low " above
the wing. This curling of air about the wingtip
breeds that hazard known as the wingtip
vortex and is responsible for induced drag.)
This explanation of lift could end at
this point but would leave the serious
student short of his destination.
Notice how the airflow in the figure
above completes its journey across the
wing. It flows not only rearward but also
Notice that the wing's
cambered (curved) upper
surface is shaped like the
bottom half of a venturi
tube. The upper half of this
imaginary tube is simply
the undisturbed, horizontal
airflow at some distance
above the wing.
downward. This action is called downwash.
Remember the lessons of Sir Isaac Newton
and the birds? When air (or anything else
for that matter) is deflected downward,
there must be an equal and opposite reaction.
This reaction to downwash is that
misunderstood force called lift. By adding
together the vertical component of force
with which each particle of air is deflected
downward, the total would exactly equal
the lift being created by the entire wing.
Additional lift can be created only by
increasing the downwash of air behind
the wing. Aerodynamicists are aware
of this and try to get as much air above
the wing as possible because all of it will
wing attracts the air approaching the
wing's leading edge. As a result, air from
ahead of the wing flows not only from
in front of the wing but also from below
it. This upwash increases the mass of air
flowing above the wing, and therefore,
the downwash behind it.
Parenthetically, there is a stagnation
point at the wing's leading edge above
which air flows above the wing and
below which it flows under the wing.
When the wing is flown at a large
angle of attack, a more highly constricted
venturi tube is created by the wing. This
further increases airspeed and reduces
static pressure above the wing. Greater
quantities of air are attracted over the
wing's leading edge and, as a result,
considerably more downwash is created.
Total lift is increased. (The reaction
produced by downwash is particularly
significant considering that each cubic
yard of sea-level air weighs 2 pounds.)
When air strikes the bottom of the wing
(particularly during flight at large angles of
attack), it creates greater downwash and
increases total wing lift. This deflection of
air from the bottom of the wing explains the
flight of a kite and the skimming of water
skis. Air (or water) is deflected downward,
causing an upward reaction. This is why it
can be said truthfully that-given enough
power-anything can be made to fly, even
the proverbial barn door.
Anyone who doesn't comprehend the
tremendous force created by aerodynamic
downwash has only to stand beneath a hovering
helicopter. This downward blast of air
is precisely what occurs during fixed-wing
flight. The rotors of a helicopter create lift
identically to the manner in which a fixedwing
creates lift. The only difference is that
helicopter wings rotate and create relative
wind without movement of the helicopter.
Fixed-wings encounter relative wind only
when the airplane is in motion.
We have learned much from the
birds, but we still watch them with envy.
There is always more to learn.
www.BarrySchiff.com
Downwash
http://www.BarrySchiff.com
AOPA Turbine Pilot Magazine - March 2025

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