> But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back? I’

> But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back? I’m sure smarter people than I wrote the article, but the way I explain it to myself is that it’s a manifestation of the same basic force or effect where the lower pressure area needs to be filled somehow. Like how the wind blows, cyclones form, etc., except in this case giving the wing lift somehow ends up being part of the most efficient “fill the void” solution. Like how just behind a driving truck there’s an abrupt region of lower pressure; however, the air obviously doesn’t just keep on going straight forever but rushes in (incidentally, giving a boost to whomever happens to be tailgating). The gradual shape of the wing changes the scale of the effect, so that it happens constantly with tiny air ‘parcels’, each filling in the minuscule lower pressure region. (And, probably not unrelated to the fact that it’s intuitively unnatural for air to flow that way, lifting the wing a little apparently turns out to help even out that void most efficiently.) It’s interesting to ponder for sure.