I have collected a couple older trainers and I can't track down the original plans. Once you have done modifications to a plane, what's the general rule about a balance point, front to back? The thickest part of the wing, and then trim it out and see how it flies? I bought one and there was much lead epoxied to the firewall and wondered once I added some tail weight in terms of new hardware and larger tail fins, and just wondering how to predict balance, when the plane is modified, or original plans are not available.

Your probably safe setting it between 25% and 28% back from the leading edge. Depending on the plane you could go up to 33% back, but your better off with a more forward CG to start with - more stable. If it displays tendencies of a nose heavy plane, then move the CG back slowly.

Hogflyer

In general the ballance point for your tipical trainer wing is 25%-30% of MAC (mean air chord). Meaning measure the chord of your wing from front to back, let's say it measures 10 inches. 25% - 30% would then be 2.5 - 3.0 inches from the leading edge. 25% would of course be your more cautious setting. If the glide slope is to steep at 25% then go ahead and move it back from there.

Helpful, but one big question: Does the measurement of the wing front to back include the ailerons? I added new larger ailerons. I also added new wing tips, so the "old" wing area is different, if you factor in the new ailerons and wingtips.

I have done it both ways, including the ailerons and excluding them and ended up with flyable models every time. I will sujest you exclude them for your first ballance point attempt as it will give you a more forward C/G. If you have oversized ailerons your C/G could be moved dangerously far back. Small 1 inch strip ailerons (common on trainers) would have little effect (.25 inch).

Check this out. I've used this too.

http://www.geistware.com/rcmodeling/cg_calc.htm#cg

Sorry about that last link... this is the one you need.

http://www.geistware.com/rcmodeling/cg_super_calc.htm