Koolhoven F.K.51

The Koolhoven F.K.51 was a 1930s Dutch two-seat basic training biplane built by the Koolhoven Company.

Design and development
The Koolhoven F.K.51 was the winning design in a 1935 Dutch government contest for a new trainer. Designed by Frederick Koolhoven the prototype biplane trainer first flew on 25 May 1935. The aircraft was an equal-span biplane designed to use a variety of engines of 186 to 373 kW. It was a two-seater and had a fixed tailwheel undercarriage. The Royal Netherlands Air Force (LVA) ordered 25 aircraft in 1936 and 1937, powered by a 201 kW Armstrong Siddeley Cheetah V radial engine. A further 29 aircraft were later ordered with 261 kW Armstrong Siddeley Cheetah IX engine. The Dutch Naval Aviation Service ordered 29 aircraft each powered by a 335 kW Pratt & Whitney radials. The Royal Dutch East Indies Army bought 38 aircraft between 1936 and 1938 each powered by a 313 kW Wright Whirlwind. The Spanish Republican government ordered 28 F.K.51s, 11 with 298 kW Armstrong Siddeley Jaguar IVa radials and 17 aircraft (designated F.K.51bis) each powered by a 335 kW Wright R-975E Whirlwind radials. Production totaled at least 142 aircraft. Twenty-four fuselages of the F.K.51 were assembled at Aviolanda.

Operational history
While the majority of F.K.51s were employed as elementary trainers within the Netherlands or in reconnaissance roles by the Royal Netherlands Air Force in the Dutch East Indies, twenty-eight were clandestinely sold to the Republican government during the Spanish Civil War, all despite a Dutch embargo on the sale of arms to either side of that conflict. Some of those arriving in Spain were used as light bombers by the Republicans in the Cantabrian region of Spain.

The F.K.51s were in active use in Royal Dutch Flight Schools during 1939–1940 in the training of young Dutch pilots.

Some LVA F.K.51s were briefly used for reconnaissance duties following the German invasion of the Netherlands in May 1940, with several F.K.51s being destroyed on the ground by attacking Luftwaffe aircraft. The Dutch Naval Aviation Service's F.K.51s were used to carry neutrality patrols from January 1940, with most being damaged or destroyed during an attack on De Kooy Airfield on 10 May.

In December 1941, the Royal Dutch East Indies Army had 27 F.K.51s available for service, and on 5 December 1941, its flying schools were shut down, allowing the F.K.51s to form two reconnaissance squadrons based on Java.

Operators

 * Dutch Naval Aviation Service
 * Royal Netherlands Air Force
 * Royal Dutch East Indies Army
 * 🇪🇸 Spain
 * Spanish Republican Air Force
 * Spanish Air Force
 * Spanish Air Force
 * Spanish Air Force