I know a guy who did the same thing, but he got his to come back up just before he was about to hit the ground. The problem was he wasn't running in idle up when he did the loop. As he went inverted in the loop, he went negative pitch on the collective, but at the same time, putting his engine at idle, then when coming out of the loop, he had lost so much headspeed that it struggled to stay up as the engine tried to respool the blades back up. That guy, was me when i first started!

Oh yeah, make sure you have plenty of headspeed if you are doing the loop fast and hard.