The CG location isn't a do-or-die deal but you do need to start out in the safe range.

There is an excellent one-page CG computer application on the internet. You strap the wing on your model, whip out a yardstick and measure the sucker. It takes about 2 minutes. Then plug the measurements into the application and TAA DAAAA...... http://www.geistware.com/rcmodeling/cg_super_calc.htm The page is somewhat self explanatory.


Funny, but I've just been posting in a thread on the same subject on this forum. http://www.rcuniverse.com/forum/m_43...tm.htm#4318162 Some good info in that thread about the same issue.

I just used the application. The link is working fine.

I've been flying a H9 Ultra Stick 40. It doesn't seem to want to snap so I've been moving the CG back. When I assembled that ARF, I didn't bother to consult the booklet for it's recommended CG location. I've been doing this stuff since Civil War times and figured to go with what felt right to me. When the plane showed reluctance to snap, I still didn't look at the book. I just started moving the CG back.

Well, your question made me consider using that application to see where my CG was according to the formulas.

Turns out that the H9 UStick booklet suggests the CG be from 3.5" to 4" back from the LE. Turns out that the classical theory puts the CG range from 2.87" to 4.75" which has the suggested range dead in it's center. And turns out that I've got mine back to about the midrange for what H9 suggests.

The formula parameter for the online application suggests MARGIN values of 5%, 10%, 15%, and 20%. Taking H9's UStick as an example, the 10% and 15% margins would be their range and could be considered as the "safe" range.

With my UStick, the range of CGs covers almost two inches. The safe range is under an inch. Since the H9 UStick's wing chord is 12.5", that shows just how wide a range you can have the CG on such a model.