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Simple and effective concepts are the best. The rotary engine is a great example.

Used in airplanes,motorcycles, and cars; the rotary engine was a stable powerplant for vehicles around the world.

The idea is straightforward. No fixed rotating crankshaft as with the radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it.

They are easy to see in old aircraft because the crankshaft is fixed directly to an aircraft frame, and the propeller simply bolted onto the front of the cylinder block.

Basically a big gyroscope, the flywheel effect of the rotating engine mass could produce smooth power.

An interesting fact is that most rotary engines were built with an odd number of pistons.

Early used in aircraft, the rotary engine was used in most aircraft in the early 1900's. The were great air-cooled engines.

Because of the gyroscopic effect, it could sometimes make it more difficult to turn an aircraft. It was easy to go left, not so easy to go right.