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Rotary Engines For Experimental Aircraft

Rotary Engines For Experimental Aircraft

The rotary engine was an early type of internal combustion aircraft engine, used mostly in the years shortly before and during World War I, in which the crankshaft remains stationary and the entire cylinder block revolves around it. It also saw use in a few early motorcycles and cars. 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.

Early in World War I, when aircraft were first being used for military purposes, it became apparent that existing inline engines were too heavy for the amount of power needed. Aircraft designers needed an engine that was lightweight, powerful, cheap, and easy to manufacture in large quantities. The rotary engine filled these goals. Rotary engines have all the cylinders in a circle around the crankcase like a radial engine (see below), but the difference is that the crankshaft is bolted to the airframe, and the propeller is bolted to the engine case. The entire engine rotates with the propeller, providing plenty of airflow for cooling regardless of the aircraft's forward speed. Some of these engines were a two-stroke design, giving them a high specific power and power-to-weight ratio. Unfortunately, the severe gyroscopic effects from the heavy rotating engine made the aircraft very difficult to fly. The engines also consumed large amounts of castor oil, spreading it all over the airframe and creating fumes which were nauseating to the pilots. The final death of the rotary engine came when engineers attempted to turn the engines ever faster to create more power and discovered that cooling drag increased exponentially with engine speed.


A rotary engine is a standard Otto cycle engine, but instead of having a fixed cylinder block with rotating crankshaft as with the 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 cylinder block.

The effect of rotating the bulk of the engine's mass was an inherent large 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 piston engines typically had a power-to-weight ratio advantage over more conventional engines.

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.

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.