Wow, I've got a pt-19 with the same product engine and it will loop fine! I'm cheating though- using the ten pound test spider wire fishing lines... I went nuts and tried using 60 foot lines and that was a bad idea- but really funny to try!
It flew fine, but I had to keep moving my feet to get so the plane wasn't flying twoards me with all the bow in the lines! Don't use that 10 pound line, it's too weak- broke one on takeoff and had to wait out the tank with the plane doing real high circles above me...

A 6-4 prop is mandatory for really getting the plastic rtf cox to fly right, also at least 25percent nitro.