RE: Sikorsky S-39 Build
I built a Martin China Clipper out of blue foam and the only thing I did was to Zerox several frame ststions along the hull to get the foam shape right.
It flies perfect on 4--400's.
ALL high wing planes are easy to CG and fly.
Make sure the first or only step is behind the CG and make sure the step edges are SHARP.
You want it to cause turbulance as the speed builds up.