Fokker D.IX

The Fokker D.IX was a Dutch single seat, single engine fighter aircraft, the final, more powerful evolution of the Fokker D.VII World War I success, flown in 1921. The sole example was purchased by the US Army Air Service but judged not worth further development.

Design and development
The D.IX was the final development of the D.VII, an outstanding World War I fighter. It had a 300 hp (224 kW) Hispano-Suiza 8Fb (Type 42) water-cooled V-8, much more powerful than the standard D.VII's 160 hp (120 kW) Mercedes D III engine and than most other engines experimentally fitted to this airframe. Engine installation apart, the D.VII and D.IX were externally similar apart from their empennages.

The D.IX was a single bay biplane, its wings constructed in Fokker's established fashion with two box spars and plywood covering. The interplane struts were N-form; a second N-strut group leant out from the wing root to the upper wing forming, with other, shorter struts from the upper fuselage and nose, a kind of cabane structure to brace the centre of the upper wing. Ailerons were only fitted on the upper planes.

Its welded steel tube fuselage was flat sided and deep, with the single seat open cockpit well behind the trailing edge of the upper wing. The straight edged fin and rudder appeared small though the latter extended down to the keel. The strut braced tailplane was placed on top of the fuselage. The D.IX's fixed conventional undercarriage was of the single axle type, with the mainwheels on V-struts. This was much like that of the D.VII but the airfoil shaped axle fairing was expanded and used a fuel tank, an arrangement first tried out in the Fokker V.36. An extra strut from the rear of the tank to the central fuselage underside helped to support it.

The D.IX prototype first flew in 1921.

Operational history
The D.IX was sold to the US Army Air Service (UAAS) in 1922 and shipped to their experimental centre at McCook Field where they had earlier tried a variety of different engines in D.VIIs. They designated it the PW-6. In place of the original engine the US engineers fitted a local, licence built version of the Hispano Type 42, the 320 hp (238 kW) Wright M-2. They also fitted armament, two fixed 7.7 mm (0.303 in) machine guns in the upper engine cowling, firing through the propeller. Though it had a maximum speed about 20% greater than that of the standard D.VII and climbed twice as fast, the USAAS concluded it was not worth further development.