Might be putting a VVRC 20cc gas engine in a Sig 4 Star 60 and wanted to know biggest diameter prop that it could take without without causing ground strike problems.

You can always go with slightly taller wheels and landing gear to put on larger props as well "and 3 bladed props". I have a Sig Senior with a 16-8 on there so it can be done !

Pretty simple, really. Just measure from your engine prop shaft to the ground. Unless I'm missing something?

i just did this a few minutes ago while putting the finishing touches to a repaired plane. I changed out the landing gear, already had the smallest prop I could use on the engine. When I put it on the ground and raised it to level I only had about a 1/4 inch clearance. Going from a set of 2 inch tires to 3.5 fixed the problem. Last $* 60 I built used a Saito 100 and I used a 14 inch prop but that was about as long as I could go without changing the tires. Been a while and it may have had more clearance then that?