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I pretty much understand the way a wing works and the different ways of explaining it, what I don´t understand is why the lift on a subsonic wing manifest itself at the 25% chord....

The 50% chord would seem more logical if You ask me....

If I was told that the lift IS NOT at the 50% chord and were told to make an (uneducated...;- ) guess as to where it then would be, I would probably guess at the 75% chord rather than the 25%....

As a plane like the Dash 7 which has VERY efficient flaps has its nose pressed down if the flaps are deployed at too high a speed, or, as a wing-shaped leaf would turn nose down at first....

I understand that the lift manifests itself at the 25% chord, I just haven't got a clue as to WHY....???

I just asked a kind of similar question about the lift-shift that occurs at supersonic speeds. I was thinking of asking this question together with that, but I guess it's more of a question of its own....

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Lift isn't at 25% of chord, the aerodynamic center is. Lift acts at the center of pressure, which depends on the wing's camber and angle of attack.

Broadly speaking, you can split lift into two constituents, one from camber and one from angle of attack. Deploying the flaps on the Dash-7 changes both camber and wing chord, and of course shifts the center of pressure back.

Changing the angle of attack will add a pressure change over chord which has its center at the quarter chord. The reason has to do with the strong local curvature near the leading edge which causes strong suction peaks. For a more mathematical explanation see the work of Birnbaum. Or read this answer.

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