The slope of the Cm curve is just an artifact of the program using 0.25c as a point of reference. Xfoil plots Cm relative to the 1/4 chord but that's not necessarily where the aerodynamic center is. To plot a moment you have to have a center point and thin airfoil theory says the AC is at 25%c so that's what most of the programs use but when your airfoil is more than a few % thick it's often not true. There is a simple formula to find the true aerodynamic center from the slope of the linear section. I posted it on HBA some time ago. I'll try to find it tomorrow. Anyway the point is that the only Cm value that matters is the Cm at zero lift (Cm0). As long as Cm0= some positive number and the static margin set to produce an equal negative moment the plane will be in trim.Ideally we want the CM to be positive at all times whilst maintaining the greatest lift and having the best area under the LD curve we can get.

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