It was the French engineer Robert Esnault-Pelterie who invented the principle of a tractor propeller driven by a fixed or rotary star engine. This star-shaped architecture was widely used in aviation until the arrival of the turbojet engine. In 1911, the German engine manufacturer Oberursel Motoren bought the licence for the Omega and Lambda engines manufactured by the French company Gnome. The Oberursel rotary engines (photo) were produced in Germany and in Sweden, at Thulin. Oberursel engines from 80 to 160 hp equipped most of the Fokker monoplanes designed by Anthony Fokker during the First World War.