Keith, nice explanation re moving a pin from the LE to the TE of an example airfoil in order to explain AC. And of releasing a card at a slight down pitch attitude and seeing it tumble nose up (but isn't a card basically symmetrical airfoil, so no pitch moment?).
However the more I read from you and other sources, the more I realize I'll need to study even further to fully understand. Frankly, it seems to me it must be simply freaks of nature that (1) a cambered airfoil has a point along it's chord that the pitching moments are (almost nearly) constant as aoa in increased, and (2) that this point works out to be (almost exactly) at the quarter point, and (3) that a symmetrical airfoil will have upper and lower C of P's at the same point on the chord (ie, no pitch moment about the AC)
Thanks for your contributions
Hawk