The reason that planes stall at high alpha is that the leading edge demons get scared and let go when they can't see the ground anymore. Some of the demons are a little confused and they hold on backwards, causing drag. These little, invisible demons hold on to the leading and trailing edges of the aircraft and lift it into the air by flapping their wings (so, in a reductionist sense, lift is actually caused by feathers). This is simplified, but if you're up for it, thin airfoil theory, or maybe some XFOIL plots should give you some good insight into this one.Īccording to Shafer "Lift is caused by Lift Demons. The air being 'turned' by the leading edge is accelerating incredibly fast to change directions so quickly, creating a huge negative pressure and velocity increase. This produces a large leading edge pressure spike.
The flow slightly above the stagnation point also must accelerate, however because of the positive angle of attack, it must change direction from the free steam direction more dramatically (think 90 degrees up, then 90 degrees back to free stream, plus alpha degrees to get back down to the upper surface of the foil). 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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