[bigiain]

In "toy planes" we (used to) use "clunk tanks", where the fuel pickup is a piece of flexible tube with a weight on the end, so no matter what orientation the airframe is in, the "sloshed fuel" and the fuel pickup end up in the same place (near enough). For aggressively aerobatic model planes (like control line combat wings) a pressurised fuel bladder was sometimes used (made, back in my childhood, we made these from baby pacifiers!)

Note: this is a couple or more decades old knowledge... Modern electric powered rc planes and builders don't have this problem to deal with.