A rotary engine is an internal combustion engine where the pistons rotate around the crankshaft. In more conventional aircraft engines the pistons would be attached to the fuselage of the aircraft, and would drive a rotating crankshaft. The propeller would then be attached to the end of the crankshaft, either directly or via some gearing. In the rotary aircraft engine the crankshaft is firmly attached to the fuselage. The pistons would be arranged in a circle around that crankshaft, linked to form a piston block. The power from the cylinders would be used to spin that entire piston block. The propeller would then be attached directly to the front of the rotating pistons.
The rotary engine was developed to solve a number of problems with early engines. These engines worked at comparatively slow speeds (rpm). The slower piston speeds resulting in serious problems of vibration, which was solved by adding a heavy flywheel to the engine to smooth out these vibrations. In the rotary engine the pistons themselves act as the flywheel, reducing the weight of the engine.
The majority of First World War rotary aircraft engines were descended from a single cylinder static rotary engine developed by Uberursel, and known as the Gnom. In 1908 Louse and Laurent Sequin created the first Gnôme radial engine by arranging seven of these Gnom cylinders around a common crank shaft. The resulting engine, the Gnôme Omega No. 1 still exists, and is in the Smithsonian.
The Gnôme Omega No. 1 produced 50hp at 1,200rpm. It weighted 166.5lbs, giving it a power to weight ratio of 3.33lb per hp. In comparison the Wright Vertical 4 aircraft engine, an inline engine, produced in the same period, provided 36hp and weighted 180lbs, for a power to weight ratio of 5lb. The Gnôme rotary engine was licensed by a wide range of engine manufacturers around Europe (amongst them Uberursel, who then went on to produce the engines used in most early German fighter aircraft of the First World War).
The rotary engine had a number of advantages in 1914. They were less prone to overheating than other types of engines, as the cylinders were cooled as they rotated. Although they were not the most powerful engines of the day, they were much lighter than the alternatives. They were hard to stall, as the cylinders had a great deal of momentum.
However, they did have some serious flaws. Fuel was directly sprayed into the engine, eliminating the need for a carburettor. However, at the same time they lacked any throttle controls, so were always at full power. The only way to reduce engine power was to turn it off for short periods, a process known as “blipping”. The problem with this process was that when the engine restarted, the aircraft would often turn or dip.
Their most famous flaw was the gyroscopic effect they produced. As the cylinders rotated they imparted some of that spin to the aircraft itself. Rotary powered aircraft could turn quickly in the direction their engine rotated (the Sopwith Camel could turn right at great speed), but more slowly in the opposite direction. The same problem occurred on the ground, making rotary powered aircraft difficult to taxi under their own power.
At the start of the First World War the standard rotary engine was the 80hp Gnôme Lambda. The rotary engine was ideal for early fighter aircraft – compared to the often more powerful water-cooled engines it was lighter and more robust, lacking vulnerable cooling equipment. Rotary powered fighters dominated the skies over the Western Front into 1916, powering perhaps the most famous of all First World War fighters, the Sopwith Camel. However, by 1917 rotary engines had reached the upper limits of their performance. As engine speeds increased, more and more of the power produced went into to moving the cylinders and less into move the propellers. One solution was the counter rotary engine, produced by Siemens, but that appeared too late to have any significant impact on the war.
Rotary engines disappeared quickly after the end of the First World War. Although the engines themselves were cheap, they were expensive to run, using large amounts of fuel and lubrication. By the mid-1920s they had almost completely disappeared, replaced by more powerful air-cooled radial engines.