u/TP4297

Basically the title. From my understanding lift is generated all along the wing, but boundary layer air is moving slower than freestream air in all the diagrams ane theory I've read online. Is flow decelerated below the wing to keep the pressure difference, or is pressure lift not significant once flow crosses peak acceleration and slows down, and then lift due to flow pushed down takes over?

what dictates that there shall be no relative velocity in the boundary layer? further, what if slipping actually does occur in regular subsonic flow over a wing at sea level?

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I'm trying to understand boundary layer dynamics but online sources are putting me in a circular reasoning loop.

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"Why does flow slow down?"

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"Friction + adverse pressure gradient slows it down."

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"What causes the adverse pressure gradient?"

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"Flow slows down, reducing dynamic pressure and thus increasing static pressure due to Bernoulli's equation."

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So I wanna know what ACTUALLY is the chronology here. What causes the adverse pressure gradient?

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Here is my understanding so far (subsonic flow only):

•Velocity is measured in relative terms to the aircraft, so the foil itself is moving at zero velocity and freestream flow is moving backwards at negative of foil velocity

• as flow hits the wing, the air on top of the foil accelerates and thus speeds up

• Simultaneously boundary layer velocity is zero (it is moving at the same absolute velocity as the wing)

So now my questions are:

1.) What is the thickness of the boundary layer? Is it present on the entire wet area of the wing?

2.) if its relative velocity is zero, isn't the boundary layer air basically stuck to the wing? Like there is no change in that at all?

3.) Doesn't that also increase effective wing thickness because the incoming flow stream air will have to either treat it as such or compress itself to enter the boundary layer?

4.) Does it even actually exist or is it an imaginary interface between airflow and the solid lifting surfaces?

Please explain

Basically the question. I was under the understanding that flow accelerates upto a point because of curvature of wing, then velocity losses due to skin friction drag start cutting into and then slowing down airflow, increasing pressure gradually till the TE. Since lifting line theory assumes inviscid flow, how come the adverse pressure gradient is still there?