The Fokker S.IX was a military trainer aircraft produced in the Netherlands in the mid-1930s, designed at a Royal Netherlands Navy request for a machine to replace the obsolete Fokker S.IIIs then in service.
It was a conventional, single-bay biplane with staggered wings of unequal span braced with N-struts.
The pilot and instructor sat in tandem, open cockpits and the undercarriage was of fixed, tailskid type with divided main units.
The wing had a wooden structure, the fuselage one of welded steel tube, and the entire aircraft was fabric-covered.
The Navy approved the design and ordered 27 aircraft, later reducing this to 15.
The Royal Netherlands Army Aviation Group ordered 20 examples with a different engine, following this with an order for a second batch of 20.
None of these latter aircraft were delivered by the time of the German invasion of the Netherlands in 1940.
On 14 May that year, a few surviving Army S.IXs escaped to France alongside some S.IVs, but never flew again.
S.IX/1 – version with Armstrong Siddeley Genet Major engine for Army
S.IX/2 – version with Menasco Buccaneer engine for Navy