Rotary Aircraft Engines

No, I don't mean Wankels, the kind of engines that you find in the RX- series of Mazdas.

Rotary engines were developed a few years after the turn of the 20th Century. In almost all piston aircraft engines, the propeller is attached, whether directly or by a gear train, to the crankshaft and the engine block is mounted to the airplane. In rotary engines, however, the crankshaft was mounted to the airplane and the propeller was fixed to the engine block.

"Jesus," you might exclaim. "That would mean the entire engine would be spinning around!"

Yes. It did.

That's not as insane as it sounds. It was easy to cool the engine, for the cylinders were spinning around. Another reason was that the art and science of balancing engines was poorly understood, the previous experience had been with steam engines, where the reciprocating parts were massive, rotated at relatively slow RPMs and were easily balanced. Piston engines, by comparison, have lots of parts whirring around at high RPMs and the early piston engines were prone to a lot of vibration.

Vibration shakes things apart, which is a Bad Thing when it comes to lightweight delicate structures, such as the early airplanes. The engine builders tried to solve that the only way they knew how, which was to add flywheels. Flywheels have to be heavy, which was not a good solution for airplanes that were already short of power. Going into World War I, virtually all aircraft engines produced under 100HP, or less than a Cessna 150s engine. By spinning the bulk of the engine around, the engine became its own flywheel.

There were, of course, many disadvantages. If you have ever held a spinning gyroscope and tried to turn it, you might recall that you could easily turn it one way and it was hard to turn it the other way. Airplanes with rotary engines were difficult to turn to the right and would turn to the left in an instant. The airplanes were, in no small part for this reason, difficult to fly. The Spad fighters, in particular, were notorious for killing their own pilots at a greater rate than did the Germans.

The engines also were low-speed engines, as the faster they turned, the more power had to be used to overcome the drag of the engines turning. This meant that because the engines turned relatively slowly, the propellers had to be both large and fat in order to eke out the maximum amount of thrust from the meager horsepower of the engines.

Rotary engines had no oil sump. Oil was mixed into the fuel in a total-loss lubrication system. Castor oil was the most widely used oil for rotaries, it spewed out of the exhausts in a fine mist. The pilots wore goggles to keep the oil out of their eyes and used those long scarves to wipe the goggles in flight. Castor oil was also used back then as a remedy against constipation, so you can imagine the result of having a fine mist of castor oil spewed in your face day in and day out. (The pilots coped with that by drinking lots of brandy.)

The most successful way to feed the fuel/oil mixture in was by introducing it through a hollow crankshaft, up through hollow connecting rods and directly into the cylinder. The engine only needed one valve per cylinder, which was spring-loaded to be open unless combustion pressure pushed it shut. Most of the engines did not have throttles, they were either running at full power or the electrical charge from the magneto was shorted out by means of a "blip switch" in order to momentarily reduce power. There were some engines that had throttles, but the pilots seemed to widely distrust them. Some other experiments were made with being able to only cut out a few of the cylinders to reduce power. But as fuel was always being fed to the engine, the engines were voracious and the airplanes did not have significant endurance.

Rotaries, as a result,were found in airplanes that needed to be as light as possible and as fast as possible, and for which long flights were not required. They were almost always found in fighters; bombers used larger liquid-cooled inline and v-engines.

The "blip switches" introduced their own problems. Remember that the engines were always being fed fuel; if a pilot held the blip switch down too long, the areas around the engine would become covered with fuel. When the switch was released and the engine fired, any flame coming out of the exhaust ports could (and sometimes did) set the airplane on fire. If the pilot held the blip switch down for too long on landing, the engine might spin down to the point that it wouldn't catch and if the airplane itself was going too slowly, there might not be sufficient airflow over the prop to turn it.

The rotaries did what they needed to, they produced enough horsepower for the airplanes at the time with as little a weight penalty as possible. But as the engines got larger to produce the horsepower for faster fighters, the drawbacks of the rotaries, especially the increased gyroscopic effect from the larger engines, consigned rotaries to the dustbin of aviation.