It doesn't look very draggy and is very Light. My wife's zero turn Mower is a V twin OHV two barrel Carburetter engine of 24.5 HP and it works Hard in Long grass and doesn't use a Lot of fuel. Also Air cooled Motors run at hotter More thermally efficient Temperatures That engine would be just loafing at 21 Hp. It depends on the L/D of the aeroplane and how good the Prop is at making thrust. Direct drive Means Maximum efficiency. Gears use more power than a chain, but nothing is best of all. IF it was a glider it would stay in the air with very little Power. Nev