Airco DH.2

In most common applications, the crankshaft was fixed solidly to the airframe, and the propeller was bolted to the front of the crankcase. There were three key factors that contributed to the success at the time:

--1. Smooth running – rotaries delivered power very smoothly (relative to the engine mounting point) there are no reciprocating parts, and the relatively large rotating mass of the crankcase and cylinders (as a unit) acted as a flywheel.

--2. Weight advantage – many conventional engines had to have heavy flywheels added to smooth out the power impulses and reduce vibration. Rotary engines gained a substantial power-to-weight ratio advantage by having no need for an added flywheel.

--3. 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. It is often asserted that rotary engines had no carburetor and hence power could only be reduced by intermittently cutting the ignition using a “blip” switch. This was literally true only of the Monosoupapetpe engine, which took the air supply in through the exhaust valve, and so could not be controlled via the crankcase inlet. Most rotaries however, had normal inlet valves, so that the fuel and lubricating oil was taken into the cylinders already mixed with air – as in a normal four-stroke engine.

Although a conventional carburetor, with the ability to keep the fuel/air ratio constant over a range of throttle settings, was precluded by the spinning crankcase; it was possible to adjust the air supply through a separate flap valve or “bloctube.” The pilot had to set the throttle to the desired setting and then adjust the fuel/air mixture using a separate lever that controlled the air supply.