A word about prop pitch and diameter. Lets say we have an 18x12 prop. The pitch to diameter ratio is .67 . Now another prop is a 24x 12 so now the ratio is .5 and finally a 32x12 is .38 ratio. If all props are spinning at 5000 rpm the airplane should fly the same speed. The issue is aerodynamic efficiency. The first prop has an efficiency of 77%, second 64% and the third is 53%. Yes super low ratio's have great STATIC thrust but as the airspeed increases the efficiency goes down dramatically. I hear people say a slow plane should have really low pitch. The Pietenpol air Camper recommended prop is a 76x 48 with a ratio of .63! That is a long way from the very low pitch's some modelers use. As Bob Boucher says "low ratio props are good for stirring paint" . He is an aerodynamics pHD who owns Astroflight. Now 3D was not popular in those days but please think about this when a prop is selected. Yes I am building a Pietenpol Sky Scout and it will have a Seidel ST-540 40cc radial. Prop will be an 18x12. -Tom