Smooth running: Rotaries delivered power very smoothly because (relative to the engine mounting point) there are no reciprocating parts, and the relatively large rotating mass of the crankcase/cylinders (as a unit) acted as a flywheel. Improved cooling: when the engine was running the rotating crankcase/cylinder assembly created its own fast-moving cooling airflow, even with the aircraft at rest. Weight advantage: many conventional engines had to have heavy flywheels added to smooth out power impulses and reduce vibration.