The smallest commercially available 4-cycle for RC aircraft use was an OS FS-20, and in current production a Saito FA-30. They're OHV and use standard glow plugs, and they're approaching the smallest limit to combustion chamber size where you can fit practically large valves and a glow plug in there. I'd like to fly 4-cycle on models much too small for even these engines, models that normally call for 2-cycles displacing less than one CC. General rule of thumb is a 4-cycle aircraft engine is about 50% larger in displacement to a comparably strong 2-cycle, so we're looking at something in the 1.3-1.5cc range. Fuel wise, standard for engines that small is a rather hot blend. Base is methanol, castor oil mixed in for lubrication, nitromethane for extra kick and tunability. I go for a 35% nitro 20% castor blend on mine. Do you guys think it's even possible to build such an engine and get practical amounts of power out of it? 40-60 watts or so would be enough. I'm thinking a flathead would be a good start vis-a-vis getting glow plugs into it, but that also introduces some inefficiencies(particularly at high RPM) that won't help much.