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When using thin airfoil theory, there exist a point that the moment about this point does not vary with Cl, which is the aerodynamic center.

However for most of the real-world airfoils, Cm does vary with Cl, and Cm is measured relative to AC, which contradicts with the definition of aerodynamic center that moment is constant. So how is AC defined in the first place for a real-world airfoil?

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  • $\begingroup$ As you also said, the definition of aerodynamic center comes from the thin airfoil theory so it's not 100% valid for "real" airfoils. Data for $C_m$ is always plotted in respect to the quarter chord point by convention. Don't know if this answer your question $\endgroup$
    – sophit
    Jul 25, 2023 at 8:04
  • $\begingroup$ @sophit if that comment was intended to be an answer, please make it so. $\endgroup$
    – CGCampbell
    Jul 25, 2023 at 12:41
  • $\begingroup$ @CGCampbell: not sure this is what the OP asked, let's see what and if the OP says $\endgroup$
    – sophit
    Jul 25, 2023 at 12:50
  • $\begingroup$ @sophit that is exactly what I wanted. Thank you for the answer! $\endgroup$
    – SOAR1123
    Jul 25, 2023 at 17:06
  • $\begingroup$ Perfect 🤗 Then I'll convert it an answer $\endgroup$
    – sophit
    Jul 25, 2023 at 17:23

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As you also said, the definition of aerodynamic center comes from the thin-airfoil theory so it's not 100% valid for "real" airfoils.

Data for $C_m$ is anyway always plotted in respect to the quarter chord point by convention, which is the theoretical aerodynamic center from the thin-airfoil theory at subsonic speeds.

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