I still have a old but good OS46SF H ringed heli engine ,want use it to flight airplane ,what will be the best propeller to fit?Thanks.

You will probably want to change the cylinder head from the heli type over to the aero type. Also, choose the prop according to the airplane, not the engine. I would think a 9"-11" diameter prop will work just fine but its going to really depend on the plane you're going to put the engine in.

I've seen people run a heli engine on a plane in the past. The head didn't seem to present a problem, but around here the temp is in the high 90s quite a bit during the summer, ha, ha. I would think most heli engines are made to rev up, so I would start with a 10X6 on that engine and work from there. Do you have a prop hub to fit that crankshaft?

APC 10.5 X 6 or 10 x 6 or 11.5 x 4
Jeff

Why remove the head?

The biggest issue is the short crankshaft on the heli engines.

the cranks arent much shorter in most cases, the os 46 sf engines love the 11x6 run great, of if you want more speed use a 10x7

The 46SFon my GP Escapade is very happy with a Master Airscrew 11X6 3 series. Very fast combo and Ihave not seen fit to try any others yet.
But I will.