The rotary engine: brilliant and mad

For a few years either side of 1917, the standard fighter engine did something no engine has done since: the whole engine spun with the propeller, crankcase and all, while the crankshaft stood still. It was not a stunt — it was the most rational answer to a problem that briefly had no better one.

The problem: cooling without weight

A 1910s aero engine had to shed enormous heat with almost no weight budget. Water cooling worked — but a radiator, pump, plumbing and the water itself weighed as much as another engine, and one bullet anywhere in the circuit ended the flight. Air cooling was the obvious answer, except that a stationary air-cooled cylinder of the day, sitting behind a propeller at climbing speed, simply cooked. Fins helped; they were not enough at the power the war demanded.

The trick: spin the whole engine

The rotary's designers inverted the problem: if the airflow over the cylinders is too slow, move the cylinders. Bolt the crankshaft to the airframe, bolt the propeller to the crankcase, and let the entire star of cylinders whirl through the air at engine speed. Every cylinder now enjoys a permanent gale of cooling air, even with the aircraft standing still. As a bonus, the spinning mass of the engine is the flywheel, so the engine runs remarkably smoothly for its size — and the heaviest radiator in aviation is simply deleted.

The price: a gyroscope with wings

A hundred kilograms of engine spinning at 1,200 rpm is a serious gyroscope, and a gyroscope answers a push with a shove at ninety degrees. Yaw the aircraft right and the nose pitched down; yaw left and it reared up. The Dr.I and the Sopwith Camel would snap into right-hand turns and labour through left ones — lethal to novices, a weapon to veterans who attacked in right-hand spirals new opponents couldn't follow. Throttling was primitive too: many rotaries ran essentially flat out, slowed by cutting the ignition in bursts — the characteristic blip-blip-blip of a rotary landing.

Why it vanished

The rotary died of its own physics. Power meant more cylinders and higher rpm — but drag on the spinning cylinders grows with the cube of speed, so ever more of the engine's output went into stirring the air rather than pulling the aircraft. Past about 160 horsepower the arithmetic collapsed. The moment metallurgy and fin design let a stationary radial cool itself — helped, ironically, by lessons the rotary had taught — the layout was abandoned almost overnight. The static radial it gave way to powered aviation for the next thirty years; the rotary remains the only mainstream engine that solved its problems by refusing to sit still.