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On Oct 8, 11:38 am, Phil wrote:
First, I would like to point out that your post is interesting because it implies at first something which I disagree with, but then at the very end of the post, what you say is exactly true. Let me try to explain: If the airflow on top of the wing doesn't contribute to lift, then how can we explain the phenomenon of the wing stalling? When the wing stalls, it is the airflow over the top of the wing that detaches from the curve of the wing and becomes turbulent. This causes a radical loss of lift. To me, this indicates that the airflow over the top of the wing plays an essential role in providing lift. What I am saying is that Newton's law is not at play with downwash, not in the "uppper surface of wing pull down on molecules" sense. Yes, there is downwash. Yes, the camber of the wing will influence the net force exerted on the wing. Yes, there will be stalling, turbulence, etc. all these things will happen. The key here is that the air molecules that are above the wing cannot be pulled down by the wing more can they pull up on the wing. Those air molecules can only causes the lateral forces of friction (laminar drag), and a perpendicular downward force on the wing which aircraft designers obviously want to keep from happening. I know the Bernoulli effect has been invoked historically to (at least partially) explain the lift produced by the top surface of a wing. I think another way to look at it is the Coanda effect (http://en.wikipedia.org/wiki/Coand%C4%83_effect). The airflow tends to follow the curve of the top of the wing, and is displaced downward. As long as the air flow follows the curve faithfully, you have good lift. When the airflow detaches in a stall, you lose most of your lift. This top surface lift is combined with the downward displacement of air by the bottom of the wing. The wing is essentially throwing air downward using both the top and bottom surfaces. This is why a curved wing is a better lift producer than a simple flat wing. The top surface curve helps contribute to the lift. I agree that air is being thrown downward by the bottom surface. I do not think a top surfaces throws air downward. Even this Coanda effect says that contact, at least initially, is caused by a pressure differential. From your link above: "As a gas flows over an airfoil, the gas is drawn down to adhere to the airfoil by a combination of the greater pressure above the gas flow and the lower pressure below the flow caused by an evacuating effect of the flow itself, which as a result of shear, entrains the slow-moving fluid trapped between the flow and the down-stream end of the upper surface of the airfoil. The effect of a spoon apparently attracting a flow of water is caused by this effect as well, since the flow of water entrains gases to flow down along the stream, and these gases are then pulled, along with the flow of water, in towards the spoon, as a result of the pressure differential. Supersonic flows have a different response." "greater pressure above the gas flow and the lower pressure below the flow caused by an evacuating effect..." This is what I keep saying. I have been using the words "rarefication and rarefaction" and instead of "evacuating effect", but this is essentially what I mean. Now, how does the wing feel the lift? It feels high pressure on its bottom surface, and it feels low pressure on its upper surface. It is pushed up from below, and sucked up from above. That is how the airplane experiences the effects of the downward displacement of air. I agree with the downward force. I do not agree that there is a sucking force above, any more than I agree that there is a sucking force when a purpose sucks on a straw. Given that the bottom surfaces of the wing is already 14.7lbs/in^2, one simply needs to reduce the pressure above the wing to cause lift. This is what I tried to illustrate with my two-pieces-of-paper- superposed demonstration. But in many cases the bottom surface has even more than 14.7lbs/^2. -Le Chaud Lapin- |
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