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Question:
I reviewed the q&a about the bernoulli principle and I am not satisfied. first, why must the air get less dense if it has to travel a greater distance? Why could not the portion of the air that goes over the airplane wing simple simply "get behind" the portion of air that goes under the wing? second, ass using that the density above the wing is less than the density below the win g, can one explain the lower pressure by saying that fewer molecules strike wing from the top than from the bottom?


Replies:
The answer to the second question is "Yes". The first will require some more thought... Hmmm. It appears the real reason the air is denser (or at least an understandable version of a reason) below the wing is the way the wing is tilted - the wing basically just compresses that air beneath it. The question of why the air that goes over the wing goes faster rather than getting left behind is (I think) actually quite complicated - you can imagine that if the wing had some kind of barrier sticking up in the middle, the air would get stuck behind that. The result of that is turbulence, and airplane wings are deliberately designed to avoid that kind of thing. In other words, the answer really depends on the detailed shape of the wing, which is deliberately designed to do the right thing (ie. make the air go faster smoothly, rather than falling behind and causing turbulence).

Arthur Smith

As stated in the previous response, wings are designed for LAMINAR flow, not TURBULENT flow. The lift of a wing occurs as long as the flow is both laminar and the path over the top is longer than the path under the bottom. What happens if the air over the top "gets behind" the air flowing under the wing?? Well, at the trailing edge of the wing you have a low density on top and the "correct d" density on the bottom. The net result is a pressure difference, resulting in flow from underneath the wing up over the top (from the backBACK side) -- i.e., turbulence!, rather than laminar flow.

gregory r bradburn


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