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 (249 to 500 hp).
It was a two-seater and had a fixed tail wheel undercarriage.
The Royal Netherlands Air Force (LVA) ordered 25 aircraft in 1936 and 1937, powered by a 201 kW (270 hp) Armstrong Siddeley Cheetah V radial engine.
A further 29 aircraft were later ordered with 261 kW (350 hp) Armstrong Siddeley Cheetah IX engine.
The Dutch Naval Aviation Service ordered 29 aircraft each powered by 335 kW (449 hp) Pratt & Whitney radials.
The Royal Dutch East Indies Army bought 38 aircraft between 1936 and 1938 each powered by a 313 kW (420 hp) Wright Whirlwind.
The Spanish Republican government ordered 28 F.K.51s, 11 with 298 kW (400 hp) Armstrong Siddeley Jaguar IVa radials and 17 aircraft (designated F.K.51bis) each powered by 335 kW (449 hp) Wright R-975E Whirlwind radials.
Production totalled at least 142 aircraft.
Twenty-four fuselages of the F.K.51 was assembled at Aviolanda.
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.
The majority of F.K.51s were destroyed in May 1940 on the ground by attacking Luftwaffe aircraft.
142 aircraft were built in total.
7.85 m (25 ft 9 in)
9 m (29 ft 6 in)
2.8 m (9 ft 2 in)
27 m2 (290 sq ft)
990 kg (2,183 lb)
1,450 kg (3,197 lb)
Max take-off weight
1,600 kg (3,527 lb)
220 l (58 US gal; 48 imp gal)
1 × Armstrong Siddeley Cheetah IX,
7-cylinder, air-cooled radial piston engine,
261 kW (350 hp)
2-bladed fixed-pitch propeller
253 km/h (157 mph, 137 kn) at 2,200 m (7,218 ft)
232 km/h (144 mph; 125 kn) at sea level
229 km/h (142 mph, 124 kn) at 2,200 m (7,218 ft)
206 km/h (128 mph; 111 kn) at sea level
84 km/h (52 mph; 45 kn)
775 km (482 mi, 418 nmi)
6,300 m (20,700 ft)
Time to altitude
1,000 m (3,281 ft) in 2 minutes 24 seconds
2,000 m (6,562 ft) in 5 minutes 12 seconds
3,000 m (9,843 ft) in 8 minutes 36 seconds
53.8 kg/m2 (11.0 lb/sq ft)
0.1807 kW/kg (0.1099 hp/lb)
One or two 7.7 mm (0.303 in) Lewis gun(s) in the wing
Two 7.7 or 7.9 mm (0.303 or 0.311 in) machine guns, 1 in the wing and 1 for the observer.