The Fokker D.IX was a Dutch single seat, single engine fighter aircraft, the final, more powerful evolution of the

Fokker D.VII The Fokker D.VII was a German World War I fighter aircraft designed by Reinhold Platz of the Fokker-Flugzeugwerke. Germany produced around 3,300 D.VII aircraft in the second half of 1918. In service with the ''Luftstreitkräfte'', the D.VII qu ...

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

success, flown in 1921. The sole example was purchased by the

US Army Air Service The United States Army Air Service (USAAS)Craven and Cate Vol. 1, p. 9 (also known as the ''"Air Service"'', ''"U.S. Air Service"'' and before its legislative establishment in 1920, the ''"Air Service, United States Army"'') was the aerial warf ...

but not developed further.

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 8F The Hispano-Suiza 8 was a water-cooled V8 SOHC aero engine introduced by Hispano-Suiza in 1914, and was the most commonly used liquid-cooled engine in the aircraft of the Entente Powers during the First World War. The original Hispano-Suiza 8A ...

b (Type 42) water-cooled V-8, much more powerful than the D.VII's original 160 hp (120 kW)

Mercedes D III The Mercedes D.III, or F1466 as it was known internally, was a six-cylinder SOHC valvetrain liquid-cooled inline aircraft engine built by Daimler and used on a wide variety of German aircraft during World War I. The initial versions were introd ...

engine and even most other engines experimentally fitted to this airframe. Engine installation apart, the D.VII and D.IX were externally similar apart from their

empennage The empennage ( or ), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow.Crane, Dale: ''Dictionary of Aeronautical Terms, third ed ...

s. The D.IX was a single bay biplane, its wings constructed in Fokker's established fashion with two box spars and fabric covering. The

interplane strut In aeronautics, bracing comprises additional structural members which stiffen the functional airframe to give it rigidity and strength under load. Bracing may be applied both internally and externally, and may take the form of strut, which act in ...

s were N-form while the cabane structure included two fully triangulated forward struts connecting to the forward spar, and a pair of single struts connecting to the rear spar.

Aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement arou ...

s were only fitted on the upper planes. Its welded steel tube fuselage was flat sided, with the single seat open cockpit behind the

trailing edge The trailing edge of an aerodynamic surface such as a wing is its rear edge, where the airflow separated by the leading edge meets.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 521. Aviation Supplies & Academics, 199 ...

of the upper wing. The straight edged

fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...

and

rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adverse yaw a ...

appeared small though the latter extended down to the keel. The strut braced

tailplane A tailplane, also known as a horizontal stabiliser, is a small lifting surface located on the tail (empennage) behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed-wing aircraft such as helicopters and gyroplan ...

was placed on top of the fuselage. The D.IX's fixed

conventional undercarriage Conventional landing gear, or tailwheel-type landing gear, is an aircraft undercarriage consisting of two main wheels forward of the center of gravity and a small wheel or skid to support the tail.Crane, Dale: ''Dictionary of Aeronautical Ter ...

was of the single axle type, with the mainwheels on V-struts. This was much like that of the D.VII but the

airfoil An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of an object whose motion through a gas is capable of generating significant lift, such as a wing, a sail, or the blades of propeller, rotor, or tur ...

shaped axle fairing was enlarged to contain a fuel tank, an arrangement first trialed 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

(USAAS) in 1922 and shipped to their experimental centre at

McCook Field McCook Field was an airfield and aviation experimentation station in Dayton, Ohio, United States. It was operated by the Aviation Section, U.S. Signal Corps and its successor the United States Army Air Service from 1917 to 1927. It was named f ...

where they had earlier tried a variety of different engines in D.VIIs. They designated it the PW-6. Instead of the original engine a locally built licensed version of the Hispano Type 42, the 320 hp (238 kW) Wright M-2 was fitted. They also fitted armament, two fixed 7.7 mm (0.303 in) machine guns in the upper

engine cowling A cowling is the removable covering of a vehicle's engine, most often found on automobiles, motorcycles, airplanes, and on outboard boat motors. On airplanes, cowlings are used to reduce drag and to cool the engine. On boats, cowlings are a cove ...

, firing through the

propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...

. 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.

Variants

;D.IX:Fokker factory designation ;PW-6:

(USAAS) designation indicating that it was the sixth water-cooled pursuit (fighter) tested.

Specifications

References

{{USAAS fighters D.IX 1920s Dutch fighter aircraft Aircraft first flown in 1921