Guys, just for my own edification, I drew the wing half-span of my Great Planes PT-19 and found the centroid and the MAC. The initial balance point, 4-15/16" from LE @ the fuse calculates to be 32% of the MAC, and the fore and aft CG range, 4-1/8" & 5-7/16" calculate to be 25% to 36% of the MAC. How do you determine what % of the MAC is the correct range on an airframe, whether it is a warbird or aerobat? 36% MAC sounds like an accident waiting to happen! Thanks for your help.

A size of the range is up to the taste of the designer and their abilities. There is no hard and fast rule for the size of the range as it's more of a bell curve sort of thing. A better pilot can deal with a wider range.

WHERE the center of the range is located is a bit easier to figure. It's related to the Tail Volume Coefficient. This is an equation that considers the wing and tail and the tail moment arm. The number that comes out of it is used to find the aircraft's horzontal neutral point and the CG is then chosen to be so much % ahead of that depending on how much stability the disigner desires.

So at first glance 36% may seem like a crash looking for a home but if the model has a large % tail area with a longer moment arm then it's fine. For example I have an RC old timer that has it's CG located at about 60% and it's actually STILL too stable. But I'm too lazy to modify the fuselage to let me put the electric motor battery pack back further. As it is it handles like a Gentle Lady with a heavy nose.

Thanks, Bruce. Apparently there is no easy answer. The plane does in fact have a long tail moment, and very large tail suraces. Sounds like I'll need to do some more research.

You might be interested in checking following link:
http://sky.prohosting.com/air2/cg_calc.htm
The NP position is based on the formula suggested by Alasdair:
NP = 25 + 40*Vbar (as a % of MAC)

Thanks, Adam. That's exactly what I'm looking for!