What are advantages and disadvantages of Rotax and Lycoming engine designs?
The biggest difference between these two engine designs is that the rotax contains a reduction gearbox which allows it to turn faster and hence develop more power per cubic inch of displacement, and the lycoming is a direct drive engine limited to a rotating speed that maintains the  prop tips subsonic. This gives the rotax better power-to-weight ratio  but causes it to run hotter and shortens its time-between-overhauls. In addition, the gearbox itself requires overhauling which adds to the overhaul costs. In practical terms, air-cooled &amp; geared engines for GA aircraft have not proven as popular as direct-drive engines in similar applications; in fact it is common to replace geared engines with direct drive engines with larger displacement to improve reliability and reduce rebuild costs.