Think of a pipe, which splits into two smaller pipes, which are then rejoined into a single pipe. However, one of the two pipes is longer than the other, and follows a wiggly path compared to the shorter straight pipe. Now pump air down those pipes. Is the air in the longer wiggly pipe going to travel faster because it has to further to go? Of course not. What an absurd suggestion.
A wing operates in a single body of air. Air moving along a pipe may not be the same body of air, as air in another pipe. Once it's not the same body of air, the rules of one body of air don't apply. But, let's consider the air in the wiggly pipe for a moment - when the air goes around the wiggle corners in the pipe, does the air on the outside of corner speed up and become less dense than the air slowing down on the inside of the same corner? 'Cause that
is the same body of air - the body of air in the one pipe?
Probably a simpler explanation is to forget the aerofoil for a minute and just imagine an inclined plane (flat plate) moving through the air. The air molecules under the plane get squashed together, increasing pressure. Behind the plane a void is created, until the air molecules above the plane rush in to fill that void, leaving an area of low pressure above the plane.
But if:
What I meant was, air molecules only know about the proximity of nearby molecules, and all basic physics can be explained on that basis. They certainly don't know or care where the trailing edge of the wing is, or how far they might have to travel in order to get there.
Why would the air molecules rush in to fill a void? What to they know?
Considering the flat plate + AoA as the mechanism by which lift is created (as opposed to the air having to speed across the upper surface to reconnect with the shorter path lower surface), How does an airfoil wing, operating at zero AoA create any lift? Wings don't suddenly stop lifting the aircraft when AoA is reduced to less than zero - In some cases, they gradually develop less lift as AoA reduces to zero, then maybe negative a degree or two. But the lift does not instantly become zero, when the molecules stop being squashed under the plane of a wing.
Consider the aerfoil of the wing of an American Champion Citabria, and Decathlon:
Wikipedia:
One of the major developments of the 8KCAB Decathlon over the 7KCAB Citabria is the Decathlon’s wing, which employs a semi-symmetrical
airfoil, as opposed to the Citabria’s flat-bottomed airfoil. This change gives the Decathlon better inverted flight and negative-g maneuver capabilities.
Why would the same type of plane (Citabria vs Decathlon) have different airfoils, if it were not an effort to extract the different positive vs negative lift? The Decathlon semi symmetrical airfoil is designed so the air
does have about the same distance to travel top over bottom, other than the difference induced by the AoA, and moved stagnation point. In the Decathlon, AoA is pretty important to lift, less so during upright flight of the Citabria. I have flown Citabrias with zero apparent AoA, and they still flew fine, supporting most, if not all of their weight by the wings....