User talk:Nepgen, Johann

Hello Nepgen, Johann. Welcome to Wikipedia.

To answer your question, I left a ref in the article. However, I thought I'd explain it a little more directly. When dealing with an airplane wing in specific, there is a layer of air surrounding the wing called the boundary layer. This is the layer of air that changes rom zero speed (relative to the wing) to the speed of the free-stream air passing around the wing. At the wing's surface air is literally stuck to it, so relative to the wing it doesn't really move. Between the wing and the free-stream, the air gradually changes from zero speed to free-stream speed, so this is called laminar flow, because they air moves like sheets passing over each other at different speeds. As the Reynold's number grows, this boundary layer becomes smaller. Passing the critical Re would result in the total loss of the boundary layer, making even level flight impossible. The goal is to keep the turbulent flow out of the boundary layer.

The boundary layer actually makes the effective area of the wing larger and a slightly different shape than the actual wing. (Effectively, the wing is bigger and broader than it really is.)

Flow can be perfectly laminar right up until the moment the critical angle of attack is exceeded. At that point the boundary layer actually separates from the surface of the wing. Basically, the boundary layer jumps off the surface of the wing, creating a vacuum which is filled by turbulent air from the lower side of the wing "backwashing" around the control surfaces. With the boundary layer hanging off in mid air, the effective shape of the wing is drastically changed into one that is both no long aerodynamic but also generates no lift. So, while Re is critical in calculating the critical angle of attack, it is the actual flow separation from the wing that causes the stall, not turbulence within the boundary layer. I hope that helps explain. Zaereth (talk) 23:28, 27 November 2017 (UTC)