Rotary Engine

The rotary engine was an early type of internal-combustion engine, usually designed with an odd number of cylinders per row in a radial configuration, in which the crankshaft remained stationary and the entire cylinder block rotated around it. Its main application was in aviation, although it also saw use in a few early motorcycles and automobiles.

This type of engine was widely used as an alternative to conventional inline engines (straight or V) during World War I and the years immediately preceding that conflict. They have been described as "a very efficient solution to the problems of power output, weight, and reliability".

By the early 1920s, however, the inherent limitations of this type of engine had rendered it obsolete, with the power output increasingly going into overcoming the air-resistance of the spinning engine itself. The rotating mass of the engine also had a significant gyroscopic precession: depending on the type of aircraft, this produced stability and control problems, especially for inexperienced pilots. Another factor in the demise of the rotary was the fundamentally inefficient use of fuel and lubricating oil caused in part by the need for the fuel/air mixture to be aspirated through the hollow crankshaft and crankcase, as in a two-stroke engine.