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Name: Joel N.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: Thursday, November 28, 2002


Question:
In Bernoulli's principle and the theory of wing design, it says that the speed of the upper and lower air must make it to the end of the wing at the same time, thus the air on the curve travels faster. Why must the air on the bottom and top of the wing reach the end of the wing at the same time?


Replies:
The air must meet in order for the theory to work and thus produce lift-if the air does not meet then there is separation between the layers and the lift is lost.

M. Baldwin


Joel,

If the air above takes less time to get to the back, this causes a space to start to open up on top, near the back of the wing. This space has very little air and very little pressure. The low pressure pulls the air on top into the open space before it can get very big. The air on top is now flowing even faster. This effect stops when the air on top gets to the back of the wing just as fast as the air on the bottom. When the flow of any fluid starts to have an open space, fluid is pulled in from around it to balance things out. About the only situation I know of where this does not happen is in a swirl: a whirlpool or tornado.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


You may wish to consult:

http://www.howstuffworks.com/airplane3.htm

---unterman


Strictly, the two streams do not have to always meet up at the same time, but once the flow has stabilized they will. Picture the situation if they do not.

If it takes longer for the top airstream to reach the tail than for the bottom airstream to get there, then when a packet of air splits to go around the airfoil, the top part will still be on top of the airfoil when the bottom part is behind it. After this happens many times, there will be a large mass of air above the nose of the airfoil and a vacuum above its tail. The airfoil piles air on top, and carries it along as it moves. The air will accumulate at the nose of the airfoil faster than it can flow off the tail.

What is wrong with this picture? Basically, if there is high pressure at the nose of the foil and low pressure at the tail, air from the region of high pressure will flow to the region of low pressure. The greater the pressure drop, the faster the flow. So the air above the wing will move relatively quickly from front to back to maintain the smallest possible pressure difference between front and back. As a result, the air traveling the longest distance, over the top of the airfoil, moves faster.

Another way to picture it is that the air that accumulates at the nose of the airfoil will need to flow away fast enough to keep the accumulation from being more than just a momentary occurrence. There can be regions of higher and lower air pressure in different spots around the airfoil, but if the speed of the airfoil through the air is stable, the pressure zones should be too. That will only happen if air traveling around the top of the airfoil gets to the back at the same time as air traveling around the bottom.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois



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