Our model engines make their torque at much higher RPM than a Merlin, say. The prop was also geared on most warbirds, so they were only turning 1500 or so RPM (for 3000 crankshaft RPM). As such, they can use much larger diameters/more blades, relatively speaking. If we could get a 1.20 4 stroke to make 2 hp at 3000 rpm (instead of 12000) and then gear the prop 2:1, we could probably use close to scale props.