I think were asking too much from a simple formula for getting a "ballpark" of where NP is located. I did a number of these calculations recently using the results to determine a range for the possible C.G locations. Basically, I consider this range to be the C.G. for a first flight and approximately the limit on how far back the final C.G. could be.

I used the results for figuring 4 things:
1) How long to make the nose of the plane
2) How much flexibility I need for mounting engine and battery
3) Determining the location of some parts (servos, receiver, fuel tank, landing gear etc), I had to move them forward
4) Deciding if I needed to loss weight in the rear of the plane, I did.


If you consider that the calculation only uses areas, you have to conclude it is only a rough estimate. To do this right you'd need to include Cl (lift coefficient) and Cm (pitching moment) of the wing, Cl of the tail, combined with cruise attitude, incidences of the wing and tail, and downwash at the tail. Probably the AeroEngineers can add to this list. This still only determines the NP for straight and level flight. As Dick notes, you may need to adjust your C.G. thinking for the type of flying you do.

Carl