ORIGINAL: gboulton
Couple of thoughts, having scratch built a bipe or two
First, hard to tell from the picture, but it LOOKS like you're measuring that 3.5" from out near the wing tip. Unless specified otherwise, it's fairly common to measure the CG from the CENTER of the top wing on most bipes. Not saying either way is 'right or wrong", just common, and possibly worth checking. Looks, by rough guess, that the 3.5" back in the center would be close to your 1.7" figure out near the tip.
**EDIT** Just re=read above, and saw you'd said both were straight wings...if that's the case, then clearly suggestion 1 above is moot...however, that top wing LOOKS to be swept back a bit in the pic. Could just be an illusion.
Thanks gboulton. Yup, they are both straight wings, here's a better pic taken while I was painting it.
Like you suggested, I measure at the center of the wing root. I just put a sticky note on the wingtip to show the CG in crossection.
Steve