The Gnome 9-N engine was the first successful air-cooled rotary engine used extensively in airplanes during the 1909-1910 period. Numerous types of Gnome engines were designed and built, one of the most famous being the 165-hp 9-N "Monosoupape" (one valve) that was used during World War I primarily in the Nieuport 28.

Credits - U.S. Air Force Museum