The Rotary Aero-Engine from 1908 to 1918

The rotary aero engine is a special type of air-cooled radial engine, where the cylinders are arranged like the spokes of a wheel and turn around the crankshaft. The propeller is connected to the cylinders, while the crankshaft is fixed to the frame. The rotary aero engine, developed in 1908, set new standards of power and light weight within the aircraft industry. It was adopted by many pioneer aviators and widely used to set records of endurance, speed and height. Many aero engine manufacturers produced different models and variants of this type of engine, which was extensively used until the end of the First World War. The latest evolution of the rotary engine was the counter-rotary arrangement, which was devised and designed by the Siemens-Halske company. The distinctive feature of this type of engine was that the engine body (with cylinders and propeller) rotated in one direction while the crankshaft rotated in the opposite one. This result was obtained by using a bevel gear mechanism. However, rotaries were quickly and definitively replaced in 1918 by new kinds of conventional engine, which were developed in the same period by other manufacturers. The main features of rotary and counter-rotary aero engine and the performance limits that caused their decline will be described in this paper. The rotary engine will be compared with the conventional one in terms of power output, specific consumption, weight and inertia loads transferred to the frame.