A friend of mine will be building the TF Spitfire and will be using the RCV 90 engine. This engine is a four stroke with a 2:1 gear reduction. It is supposed to swing a 16x14 2-blade, 15.75x13 3-blade, or 15.5x11 4-blade prop at scale rpms. Check out their web site for more info. www.rcvengines.com