Hi, welcome to RCU. You sound like you're very handy. I don't know the plane or what size cells, but it sounds like they are too small or at least can't handle the current required by your set-up. Shjrinking or wrapping will make things worse since the heat is coming from inside the cells.

If you have any idea what amps the plane pulls or what the specific size cells are then a comparison may show that the cells are lower than required. I'm guessing the battery ran out very quickly? like 6 minutes or less. Perhaps propping down, gearing down, or dropping 1-2 cell will lower the current draw.