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Name: Dylan
Status: student
Age: N/A
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Question:
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?



Replies:
Dylan,

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 greater speed.

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
Physics Instructor
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
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology


Dylan

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.

Sincere regards,

Mike Stewart



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