Brian, I think you may be right and I think definitions vary depending on the authors. It looks like the AC (wing AC) in this cg calculator is defined as the point on the center line corresponding to 25% of the MAC. I always believed the AC (aircraft AC) and NP were the same thing, that is, the point on the aircraft where a change in angle of attack produced no pitching moment. In the missile business we used center of pressure instead of neutral point. I find all disciplines have there own confusing lingo to guarantee we commoners need them to solve our problems.

Jim O