World War I - Rotary Aero Engines 

Rotary engines were an early type of internal combustion engines, designed with an odd number of cylinders in a radial configuration. These World War I rotary engines had a unique operating characteristic. The engine crankcase and cylinders would rotate as one unit, while the crankshaft remained stationary. Its main application was in aviation.
Various engine types were fitted in the Sopwith Camel, most common being the Clerget 9B/9Bf and later the Bentley BR1/BR2, but also the Gnome B2 Mono and the Le Rhône 9 series engines were used.