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If you look at wing planforms of e.g. the Spitfire, you can see that the sweep measured at the 25% chord line is 0°. However, the leading edge is rounded/slightly swept and therefore induces spanwise flow across the wing. The problem also applies to trapezoidal wing planforms where the leading edge is swept back and the trailing edge is swept forward, but the overall sweep angle is still 0°.

For a wing with 0° sweep, shouldn't the leading edge always be perpendicular to the flow direction? Why is sweep measured at the 25% chord line and not at the leading edge? Has that something to do with the aerodynamic center?

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  • $\begingroup$ But couldn't you make the same argument for the trailing edge? I.e. shouldn't the trailing edge of a 0 deg sweep also be perpendicular? Because then you'd simply have a rectangular wing... Someone else can probably answer this better, but the CG is typically around 25% cord, and what you should really be interested in is the net, or mean aerodynamic effect of sweep, right? It would make sense to me that the net aero effect of sweep might be tied to a line along either the CG, or center of pressure. $\endgroup$
    – Michael Hall
    Jul 4, 2021 at 19:34

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Wing sweep is best measured at the average wing cord. And for subsonic aeroplanes the average wing cord is at 25%, because that is where the aerodynamic centre is located.

from prof. Gerlach's uni book

In the picture above we can see the linear forces acting on a wing, and the pitching moment $C_m$. We can choose any point on the wing cord to draw this vector diagram, but there is only one point where the pitching moment remains constant when lift increases, and that is the aerodynamic centre of the wing section.

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