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The counter rotary engine was developed by Siemens under the Siemens-Halske brand in an attempt to overcome some of the main limitations with rotary engines. The biggest problem with rotary engines, in which the cylinders rotated around a fixed crankshaft, was that they developed a gyroscopic effect, giving the aircraft a tendency to roll in direction of rotation. It was also difficult to increase the power of the rotary engine, as the faster the engine rotated, the more air-resistance had to be overcome by the cylinders.
Siemens responded by creating an engine where the cylinders, crankcase and propellers rotated counter-clockwise at 900rpm, while the crankshaft rotated clockwise at 900rpm. This produced at least the same power as a 1,800rpm conventional rotary engine, but with much less drag and almost no gyroscopic effect. The slower propeller speed was also more efficient.
This design did have some problems. It was more complex, with the crankshaft attached to the aircraft via a system of bevelled gears. The combination of more powerful cylinders with slower rotation produced engines that were more prone to overheating than conventional rotary engines.
Siemens produced two main counter-rotary engines during the First World War. The Sh.I was a nine-cylinder engine, capable of 110hp in normal use or 140hp when over-compressed. It was followed by the Sh.III, an eleven cylinder aircraft capable of 160hp in normal use and up to 240hp when over-compressed (for short periods at altitude). In comparison the most powerful Oberursal rotary engine, the UR.III, was a 14-cylinder engine capable of producing 145hp.
Unusually for a rotary engine, the Sh.III featured throttle control that could be used to slow it down as far as 350rpm. In contrast most rotary engines ran at either full power or not at all.
The Sh.III was used to power the excellent Siemens-Schuckert SSW D.III and SSW D.IV fighters, one of the best fighters of 1918, but one that arrived to late to make a real impact at the front.