The Gnome Monosoupape 9N was a 9-cylinder rotary engine delivering about 160 hp (119 kW). Introduced in 1917, it represented one of the most powerful rotary engines built by Gnome, and it was widely used in the later stages of the First World War. The "Monosoupape" (French for single-valve) design simplified the valve system, reduced weight, and improved reliability compared with earlier Gnome types.

Aircraft using the Gnome Monosoupape 9N

• Nieuport 28 C.1 – France's first fighter delivered to the U.S. Air Service; the type entered combat in 1918 with the 94th and 95th Aero Squadrons.
• Sopwith Camel (late production versions) – some Camels were fitted with the 9N instead of Clerget or Bentley engines, giving excellent climbing ability but shorter endurance due to higher fuel consumption.
• Sopwith Snipe (prototypes) – the early test versions of the Snipe flew with the 9N before production settled on the Bentley BR.2.
• Avro 523 "Pike" – a British two-seat bomber prototype that flew with different rotary engines, including the 9N.
• Other experimental aircraft – the 9N was also tested in a variety of prototypes and small-scale projects where a high-power rotary was required.

Historical significance

The Gnome Monosoupape 9N was among the last and most powerful rotary engines to see active use. While it gave excellent performance in nimble fighters like the Nieuport 28, it also highlighted the limits of the rotary concept — high fuel consumption, torque effects, and poor efficiency compared with inline engines. By the end of the war, the 9N marked both the peak and the decline of rotary engine development.{{freeshipping_code_1}}{{payment}}