I don't think the prop is your problem. It's the shape of the loop. Take a look at a roller coaster loop, they're designed to maintain positive G's through out the loop. That's what you want, positive G's. The roller coaster loop is squished, tighter at the top.

I'd try pulling up gently and increasing your pull through the entire first half of the loop, then ease off as you pull around the back side. Should help.

Dave