Bernoulli and Wings
I have read everywhere that the air along the curved top
part of an airfoil speeds up and creates lower pressure. But WHY
does the air speed up? How does the curved shape increase the air's
speed? And, does the air above and below the airfoil HAVE to meet up
at the back end?
When a significant distance above or below the wing, the air flow is the
same as if the wing were not there. As you get closer to the wing, the
effect becomes noticeable. If the air above the wing does not meet up
with the air at the back end, more air will move toward the wing front
than leaves the wing back. Some of the air molecules would have to
disappear as they passed over the wing. Passing over the curved surface
gives the air molecules some extra motion.
Consider two football players running down the field. They stay
aligned, passing each line on the field together. Arnold runs straight
down the field. Bob runs a curved path, moving to the left and then to
the right as he runs down the field. Although both would be described
as having run 100 yards, Bob had to take many more steps to get down the
field. If the players had speedometers on them, Bob's would read a
As for how this causes decreased pressure, consider the air just
starting to pass by the wing. The air already above the wing is moving
faster than the air already below the wing. The air above is moving
away from the air that is just reaching the wing. Therefore, the air
above does not push back as hard as does air below the wing. This in
turn allows the air that goes above the wing to speed up. This keeps
happening, so air above the wing keeps moving faster and at a lower
pressure than air in front of the wing or below the wing. The same
thing happens in reverse at the back of the wing. This is why the air
does not keep its low pressure after leaving the wing.
Dr. Ken Mellendorf
Illinois Central College
You cannot get away from the phenomenon.
Let us imagine that the air did not speed up over the top of the
wing. Because the air on top of the wing would have farther to go,
there would be missing air at the back on top of the wing. This
vacuum, being at lower pressure than that the surrounding air, would
suck in air to fill the void. Of course the only place to get that
air would be along the upper part of the wing! So, air atop the
wing would be forced to move faster as it rushed to the fill the vacuum.
Voila. Increased speed over the top of the wing.
Hope that helps. You might get a better idea of what happens by
moving an airfoil shape through water. Eddies form behind the wing
shape, but no holes in the water.
R. W. "Bob" Avakian
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
Yes, the air above the wing and the air below the wing must meet at the
trailing edge of the wing at the same time.
There are a lot of Why questions like that in Science, so you have a lot of
answers to discover during your lifetime.
Because the top part of the wing is curved, the air that flows over the top
of the wing has a farther distance to travel than the air that flows under
the wing in the same amount of time, so the air on top has to travel faster
to meet the bottom air at the trailing edge of the wing. And as you said,
the faster air on top of the wing creates a lower air pressure (Bernoulli's
principle) on top of the wing than below it, providing lift to the wing that
pulls the fuselage along with it.
Keep on asking questions.
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Update: June 2012