![]() The important thing to note is that the change in flow direction integrated from the stagnation point along the bottom edge is small (think as if the flow changes direction momentarily by 90 degrees, and then decreases to its final flow direction of alpha degrees off of freestream). After this initial acceleration, the flow follows the contour in a relatively straight streamline. The air impinging on the wing below the leading edge stagnation point but near the leading edge must accelerate away from the free stream flow line direction to follow the lower contour of the foil downwards. Both sections of the flow are accelerating equally around the foil and flowing along identical curves relative to free stream flow. The air flowing over the top and bottom is at the same speed and there is a positive(drag) pressure spike on the stagnation point of the leading edge, right in the middle (symmetric, alpha=0). Start with a symmetric airfoil at zero angle of attack. I will try to simplify, though it's a tough subject to do it with. However, potential flow theory has been used to create 'thin airfoil' theory which is relatively accurate within its defined assumptions. There is still debate about this topic, especially the mathematical derivation of how it works.
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