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By the early 1920s the rotary aircraft engine was becoming obsolete, mainly because of an upper ceiling to its possible output torque, which was a fundamental consequence of the way the engine worked. It was also limited by its inherent restriction on breathing capacity due to the need for the fuel/air mixture to be aspirated through the hollow crankshaft and crankcase, which directly affected its volumetric efficiency. However, at the time it was a very efficient solution to the problems of power output, weight, and reliability.
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By the early 1920s the rotary aircraft engine was becoming obsolete, mainly because of an upper ceiling to its possible output torque, which was a fundamental consequence of the way the engine worked. It was also limited by its inherent restriction on breathing capacity due to the need for the fuel/air mixture to be aspirated through the hollow crankshaft and crankcase, which directly affected its volumetric efficiency. However, at the time it was a very efficient solution to the problems of power output, weight, and reliability.
Description
A rotary engine is essentially a standard Otto cycle engine, but instead of having a fixed cylinder block with rotating crankshaft as with a conventional radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to an aircraft frame, and the propeller simply bolted onto the front of the crankcase.
The rotation of the bulk of the engine's mass produced a powerful gyroscopic flywheel effect, which smoothed out the power delivery and reduced vibration. Vibration had been such a serious problem on conventional piston engine designs that heavy flywheels had to be added. Because the cylinders themselves functioned as a flywheel, rotary engines gained a substantial power-to-weight ratio advantage over more conventional engines. Another advantage was improved cooling, as the rotating cylinder block created its own fast-moving airflow, even when the aircraft was at rest.
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.
Like radial engines, rotaries were generally built with an odd number of cylinders (usually either 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.
Distinction between "Rotary" and "Radial" engines
Rotary and radial engines look strikingly similar when they are not running and can easily be confused, since both have cylinders arranged radially around a central crankshaft. Unlike the rotary engine, however, radial engines use a conventional rotating crankshaft in a fixed engine block.
Rotary engine control
It is often asserted that rotary engines had no carburettor and hence power could only be reduced by intermittently cutting the ignition using a "blip" switch, which grounded the magneto when pressed, shutting off power to the spark plugs and stopping ignition. However, rotaries did have a simple carburettor which combined a gasoline jet and a flap valve for throtting the air supply. Unlike modern carburettors, it could not keep the fuel/air ratio constant over a range of throttle openings; in use, a pilot would set the throttle to the desired setting (usually full open) then adjust the fuel/air mixture to suit using a separate "fine adjustment" lever that controlled the fuel valve.
