The Koolhoven F.K.55 photographed prior to its single flight test.

The Koolhoven F.K.55 Single-seat Fighter Monoplane (1,000 h.p. Lorraine "Sterna" engine driving two airscrews in opposite directions).

The Koolhoven F.K.55 during ground trials at Welschap airfield.

A Dutch design that was much-studied at the Salon was the radical Koolhoven FK-55 fighter prototype. A single-seater, it was powered by a 12-cylinder Lorraine Petrel of 860hp (641kW) mounted in the centre-section and driving the two-unit, counterrotating, propellers via an extension shaft. Production FK-55s were to have been powered by a Lorraine Sterna of 1,200hp (895kW) and been fitted with a 20mm cannon in the propeller boss and four machine-guns in the wings. The FK-55 flew for the first and only time on June 30, 1938, for just a couple of minutes. It was fitted with fixed undercarriage for this flight, although a fully retractable unit was to be fitted to production examples.

The sensational Koolhoven fighter with its twin airscrews and Lorraine Petrel engine mounted behind the pilot.

The latest version of the Lorraine Sterna with extension shaft and oppositely rotating airscrews. This combination has been fitted in the Koolhoven F.K.55 fighter.

TWIN-SCREW. The Koolhoven single-seater fighter to be shown at the Paris Salon has two airscrews revolving in opposite directions.

The arrangement of the power plant and extension shaft of the F.K.55.

The method of driving the two airscrews of the Koolhoven (Lorraine Petrel).

A cooling "gill" on the New Koolhoven fighter.

The F.K.55 in its proposed definitive form with retractable undercarriage.