I played around with the Kyosho fan with a OS 18 car engine and it turned that fan around 26,000. The plane was very fast to 18 mph then just didn't go any faster as it was sucking to much air and then stalled the fan. It needs good smooth airflow into it. I would say a good 65 to 70mm 10-12 blade fan. Never really know till you play around with it. More rpm the better as most of these fans do not unload in the air like a prop plane does so you can pipe them for the rpm you want on the ground.