I've been having some real issues with this as well. No matter what design I build, I cannot cancel out the flex and bend. It only happens after I try taking off, and it with larger planes, it happens on its own. I've watched Scott Manley build a spaceplane from scratch, and he didn't really reinforce anything. I'm thinking it's something to do with KSP's joints. I think I've heard Manley mention a joint strengthening mod, so that's what I'm gonna try next. I was hoping to enjoy B9, but I'm just been continually frustrated with even simple designs.