The term aero engine can, strictly speaking, be applied to any aircraft power unit, but it is normally applied to the specialized piston engines used in airplanes until they were largely succeeded by the JET engine.
The history of the development of the aero engine has been a struggle to combine high power, lightness and reliability. These qualities were also required in the automobile engine, which was developed at much the same time. But even the earliest aircraft demanded more power than the early automobile industry could produce.
When the Wright brothers came to search for an engine to put into their latest glider in 1903 they thought they could manage to fly with one of only 8 hp, provided it was not too heavy. They approached, without success, half a dozen makers of car engines. Eventually, they built their own engine and got 12 hp from it, but it was still relatively heavy at 15 lb (7 kg) to the horsepower. Thirty years later, engine designers were aiming at a power to weight ratio of over 1 hp/lb (2 hp/kg).
The Wrights’ first engine had four cylinders set in line like those of a small car engine. The year after their first flight, a five-cylinder motor designed by American engineer Charles Manly developed 50 hp at a ratio of 4 lb (1.8 kg) to the horsepower.
In the years leading up to World War I, the French led the field in aero design, producing several 50 hp and two 100 hp engines by 1908. But the best of these still only had a power to weight ratio of 3.7 lb (1.7 kg) to the hp.
Early engines were water cooled, with the cylinders arranged in line or in a V-formation as in an automobile. But in 1907 a new and highly successful type was introduced; the rotary engine. In this, the crankcase and cylinders revolved in one piece around a stationary crankshaft. The pistons were connected to a single pivot mounted off-center, so that they moved in and out as they revolved with their cylinders. The airscrew was connected directly to the front of the crankcase and turned with it.
This odd-sounding arrangement worked surprisingly well. It had fewer parts than a conventional engine, and since the cylinders moved rapidly around, they could be air cooled by fins mounted so as to take advantage of the draft. Both factors contributed to the lightness of the engine.
Rotary engines always had an odd number of cylinders. This reduced vibration, since there were never two pistons moving in exactly the same direction at the same time. The original 1907 Gnome engine had seven, and later types nine.