Koolhoven FK-55 Experimental Fighter
Model Airplane News Cover Art for April, 1937
by Jo Kotula
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The Koolhoven F.K.55 was a Dutch fighter prototype of the 1930s. Designed as a high-performance aircraft, it never made it past the prototype stage.

The FK-55 was to be a single pilot monoplane fighter, in which the engine was placed behind the pilot, in the centre of the fuselage. Two counter-rotating propellers were driven by long shafts running from the engine. The main gun was to fire through the hollow propeller shaft. The craft was to have a retractable landing gears, something quite novel for the time. It had initially been proposed to do away with ailerons on the wings, and replace them with "slot-spoilers" for lateral control, however this idea was abandoned early in the project.

     

Koolhoven FK-55 Experimental Fighter
(left)The FK-55 in the wind tunnel
(middle) early patent by Dr. Koolhoven, Patent No. 1,679,038
(right) rubber-powered model incorporating contra-rotating props
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Of mixed metal and wood construction with wooden wings, the F.K.55's front fuselage was made of steel tube while wood formed the tail and rear fuselage. Its Lorraine Pétrel engine afforded just enough energy to take off and to stay in flight, and many designers thought that it should be upgraded to a supercharged Lorraine Sterna engine which would have afforded greater performance, however this never happened.

Here is an illustration from Popular mechanics that provides more technical details of the FK-55:

Koolhoven FK-55 Experimental Fighter
... in Popular Mechanics April, 1937
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With advanced materials and a bigger engine, these concepts were put into practice in the Bell P-39 Airacobra. When working, the centerline nose cannon was very useful to Russian pilots in destroying German tanks.

Our research turned up a truly amazing rubber-powered balsa model of the FK-55 that includes the contra-rotating propellors. Here is a video of the Koolhoven FK-55 rubber-powered model in action:

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