Picture a tube or cylinder that holds a snugly fitting plug. The plug is free to move back and forth within this tube, pushed by pressure from hot gases. A rod is mounted to the moving plug; it connects to a crankshaft, causing this shaft to rotate rapidly. A propeller sits at the end of this shaft, spinning within the air. Here, in outline, is the piston engine, which powered all airplanes until the advent of jet engines.
Pistons in cylinders first saw use in steam engines. Scotland's James Watt crafted the first good ones during the 1770s. A century later, the German inventors Nicolas Otto and Gottlieb Daimler introduced gasoline as the fuel, burned directly within the cylinders. Such motors powered the earliest automobiles. They were lighter and more mobile than steam engines, more reliable, and easier to start.
Some single-piston gasoline engines entered service, but for use with airplanes, most such engines had a number of pistons, each shuttling back and forth within its own cylinder. Each piston also had a connecting rod, which pushed on a crank that was part of a crankshaft. This crankshaft drove the propeller.
Engines built for airplanes had to produce plenty of power while remaining light in weight. The first American plane builders—Wilbur and Orville Wright, Glenn Curtiss—used motors that resembled those of automobiles. They were heavy and complex because they used water-filled plumbing to stay cool.
A French engine of 1908, the "Gnome," introduced air cooling as a way to eliminate the plumbing and lighten the weight. It was known as a rotary engine. The Wright and Curtiss motors had been mounted firmly in supports, with the shaft and propeller spinning. Rotary engines reversed that, with the shaft being held tightly—and the engine spinning! The propeller was mounted to the rotating engine, which stayed cool by having its cylinders whirl within the open air.
During World War I, rotaries attained tremendous popularity. They were less