Cut-n-pasted from the SSE manual downloaded from the SIG website ...

<blockquote>Center Of Balance
Like all R/C models, the Somethin Extra was designed to be flown with the proper Center of Balance (or CG). As shown
on the plans, the CG falls exactly in the center of the 1/4" wing spar, or 3-1/2" back from the leading edge. As previously
mentioned, this design allows for a relatively huge shift in the location of the airborne battery pack. However, if for some
reason you find that your model is still either nose heavy or tail heavy, consider some of the following suggestions before
resorting to non-functional lead weight.</blockquote>