I'm running a saito82 14X4 on this particular setup...so i can only give a guess based on another plane i have. I have a run a 13X4 on my OS 55 AX and it did ok, but close to the ground it's too slow to pick up. You want to be able to pull out quick so you need a prop that will spin up fast. I have a topcap with a APC 12X4, and I have also thrown a APC 11.5X4. Both do equally as well on this topcap with the OS 55 AX. You just dont want to fly around full blast because it will spin these small props very fast. It just gives you alot of instant hump getting away from the ground . I think the 12X4 or 13X4 would be a decent starting place.