# How to calculate the location of C.G. A.C. & N.P. in static longitudinal stability?

We are in the conceptual stage of design and, using Raymer & Nicolai as references, I am doing a rough calculation of "power off" neutral point (N.P.) to find the optimum c.g. and a.c. positions. As seen in this photo, which is also reflected in Nicolai, when doing a static longitudinal stability analysis, the moments are summed assuming the c.g. is aft of the wing/body a.c. and the tail provides a negative moment (nose down) while the wing provides a positive moment. It assumes both tail and wing provide positive lift. I understand that in this configuration, the positive tail lift would shift the overall a.c. (neutral point) aft and the c.g. would just need to be in front of the neutral point for static stability.

However, what is confusing for me is that conventional stability usually involves placing the c.g. in front of the a.c. and n.p. so that the wing provides a negative moment (pitch down) and the tail stabilizes this by providing a negative lift (down force) to pitch the nose up. Why assume the c.g. is aft of the a.c. and the tail is providing positive lift? Why not sum the moments assuming the c.g. is in front of the a.c. for calculating neutral point?

Any help would be much appreciated, thanks!

• Look here: aviation.stackexchange.com/questions/47306/…. Does this help? – Peter Kämpf Mar 15 '18 at 23:56
• Kind of. I am still confused on why static stability in two respected textbooks is analyzed assuming a positive vertical tail lift and the c.g. behind the a.c. of the wing. I understand that theoretically that configuration can be stable, but why not analyze the moments and assume the c.g. is in front of the wing a.c. like in most conventional designs? – Jason Mar 16 '18 at 21:50