I am not a begginer in nitro planes...however, I have just built a Ultimate 120 biwing. I can not use my standard Great Planes balance to find the CG. The balance is not tall enough to reach the upper wing where the instructions state the CG is 106mm from the center of the upper wing. Now what I have done was to cut a piece of PCV pipe into equal lengths. Drilled one hole in each end and mounted to wing plastic wing hold down bolts in each using epoxy. Then covered the top of the bolts with foam rubber. I then nailed the two PVC pipes to my work bench. I then placed the A/C on the points of foam covered bolts at the points marked 106mm. I placed a builders bubble on the fuselage between the upper wing braces. I then started to add weight to the nose. I am wanting someone to give advise as to my precept of the way this was done to determine the CG!!! I did arrive at a balance using the bubble with ( 6 ozs. ) of weight !!!

RC,
Here is a link to a device called the Vanessa for balancing planes, especially large ones.
Maybe this will help.
Good Luck,
KW_Counter

http://www.rcuniverse.com/forum/m_708060/tm.htm

It doesn't really matter how you arrive at the correct CG location, only that you are correct. What I mean is, make sure that the 106mm is exactly what they call out, and that the location on the wing is also correct, with the plane right side up, etc. You can use a commercial CG machine, the Vanessa, or you and a buddy's finger tips, as long as it balances where it should, you are OK.

Jesse