It does come down to the fin design of the radiators. It is clear that the more open fin design on an air-cooled engine can provide faster airflow. A radiator system, using a larger cooler, more open fin spacing, etc. might be more efficient but would require more coolant and ducting to relocate the radiator. The front mounted smaller coolers are a compromise, but an acceptable one to provide a compact, lightweight installation that will work in different airframes. More suction is required at lower speeds to pull air through the coolers, less is required as airspeed build and pressure increase at the front. Hence the need for a cowl flap. The cowl flap, pulled up to almost closed during cruise, will regain the 5 kts of speed lost over an air-cooled engine. The latest ECU will reduce the fuel flows and the benefit of modern engines again become evident. Just gave a ride to an instructor this morning and he could not believe the nice feeling of flying behind this engine. Just like Van himself now chose the Rotax for the 12 and like the smooth operation of it, we have the same technology engine for the larger airplanes. And the efficiency does not have to be any worse than any other engine. In fact, at cruising speeds the auto engine is more efficient. The latest reports from Europe, by Hans that fly the engine in a classic French Jodel, have better speeds and efficiency than the original certificated Lycoming airplane. This is a testimony to the latest ECU and higher reduction drive gearing.
Jan Eggenfellner