Stan Hiller's earliest helo designs were coaxial and used relatively small 4-cylinder aircraft engines to drive the coax transmission, so it is indeed possible to power a small coaxial helicopter with a relatively small piston engine. Hiller also found that autorotating a coax main rotor blade setup was difficult and so his later designs used a more ...


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 ...


Those engines have different valve configurations, which mean they "breathe" differently. Better breathing supports more RPM's before the engine runs out of puff, so the most probable cause is the better breathing in the one that revs higher.

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