[Jeff Brooks]

I read this explanation somewhere on one of these threads, the engine output of the old rotary/radial engines is actually better than modern engines.

The old engines develop their horsepower at lower RPM, this means they have the torque to turn a larger prop. The larger prop has more thrust to push/pull the plane.

The modern engines develop their horsepower at twice the engine RPM so the prop they use must be smaller. A modern engine in a full scale replica usually has too small a prop to have the same performance as the original engine. so you end up with sluggish performance.

Building the aircraft just a bit smaller in scale is one way to make the relative prop size with a modern engine more compatable with the replica, which give it better performance.

If someone were to manufacture a modern engine that has the torque output of the old 1914-1920 engines, but the reliability of a modern engine, there might be more full scale replicas flying out there!