Originally Posted by Andy2No
Could it be nose heavy? Or too much down thrust, maybe, or possibly both.
I've had that experience with a 200g triplane I made (there's a video somehwere). I just had way too much down thrust due to a high motor position, though the CG was about right. It scooted around on the floor at high speeds without lifting off, so I decided to chuck it - bad idea. It nose diived into the floor a few feet away. So, I figured out it was the thrust angle and pointed the motor up quite a bit (actually above horizontal). It then flew beautifully - straight into a basket ball net that hadn't been retracted into the ceiling. I've been meaning to fix it for a couple of years now.
Thanks for your suggestions. CG seems close to where my plan puts it and eyeballing the thrust line puts it pretty close to the datum line or slightly downwards. However, both suggestions are now on my suspect list.
I'll post after my next try.