The rotary engine is an early type of internal combustion engine, usually designed with an odd number of cylinders per row in a radial configuration. The engine's crankshaft remained stationary in operation, while the entire crankcase and its attached cylinders rotated around it as a unit. 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. It has been described as "a very efficient solution to the problems of power output, weight, and reliability". By the early 1920s, the inherent limitations of this type of engine had rendered it obsolete. Description Distinction between "rotary" and "radial" engines A rotary engine is essentially a standard Otto cycle engine, with cylinders arranged radially around a central crankshaft just like a conventional radial engine, but instead of having a fixed cylinder block with rotating crankshaft, the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to the airframe, and the propeller was simply bolted to the front of the crankcase. This difference also has much impact on design (lubrication, ignition, fuel admission, cooling, etc.) and functioning (see below). The Musée de l'Air et de l'Espace in Paris has on display a special, "sectioned" working model of an engine with seven radially disposed cylinders. It alternates between rotary and radial modes to demonstrate the difference between the internal motions of the two types of engine. Arrangement Like "fixed" radial engines, rotaries were generally built with an odd number of cylinders (usually 5, 7 or 9), so that a consistent every-other-piston firing order could be maintained, to provide smooth running. Rotary engines with an even number of cylinders were mostly of the "two row" type. Most rotary engines were arranged with the cylinders pointing outwards from a single crankshaft, in the same general form as a radial, but there were also rotary boxer engines and even one-cylinder rotaries. Advantages and drawbacks Three key factors contributed to the rotary engine's success at the time: 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