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Wings and Bernoulli Flow
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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Update: June 2012
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