Many things ca affect the balance point of these airplanes and they are all variable. 95% of the paint you put on goes behind the CG so if you pile the paint on to make a slick finish it can make the plane tail heavy. Or you could have extra heavy wood in the fuselage and tail surfaces and or a light engine or all of the above. One of the best ways of balancing a tail heavy airplane like this is to make spacers to put between the firewall and engine since the engine is the heaviest thing ahead of the CG. I make these spacers the same shape as the firewall out of 1/4" plywood and epoxy them to the firewall. The spacers and glue add some weight but more importantly the move the mass of the engine forward.
Good luck,
Bob Nelson
Design Engineer/Customer Service
SIG MFG. CO.
Montezuma, Iowa
[email protected]