Capio777,

I still think you are missing the big picture. The condition for static longitudinal stability is that the Center of Gravity be forward of the Aerodynamic Center. Another way of saying this is that the derivative of the aircraft pitching moment about the center of gravity with respect to angle of attack be negative (dCM/dAlpha<0).

Your plots show the variation of section moment coefficient with angle of attack (dCm/dAlpha). These plots can be used to calculate the effective shift of the section Aerodynamic Center:

The change in moment coefficient resulting from a shift in reference point can be computed as follows:

(1) Cm = Cm_o + (Cl)*delta

where Cm_o is the moment coefficient about the original reference point, Cl is the section lift coefficient, and delta is the distance the reference point was shifted aft normalized by the chord. Taking the derivative with respect to angle of attack:

(2) dCm/dAlpha = dCm_o/dAlpha + delta*(dCl/dAlpha).

The Aerodynamic Center of a wing section is defined as the point where dCm/dAlpha = 0. Setting equation (2) equal to zero and solving for delta gives:

(3) delta = -dCm_o/dAlpha / dCl/dAlpha

For the plots you showed, the biggest difference between the two sections in dCm/dAlpha is less than 0.001 per degree. dCl/dAlpha is at least 0.10 per degree for almost all wing sections. This gives a delta of less then 0.01, or a shift of Aerodynamic Center between the two sections of less than 1% of the chord. Will this have an effect on longitudinal stability? Yes it will, but for an aircraft, it is an extremely small effect compared to the effect of a change in geometry such as that shown by KennyRoy above. Your thoughts?