O.Tapper Armstrong Whitworth Aircraft since 1913 (Putnam)

F.K.9

   The quadruplane which followed the F.K.8 was built in two forms, a prototype, which underwent considerable modification, and a production version of which, however, only a handful were actually built. The production model was certainly designated the F.K.10, and it is more than likely that the earlier version was called the F.K.9, although this nomenclature lacks positive confirmation. The quadruplane, which was probably built as a private venture, was designed to meet a requirement referred to in an official report as 'Spec. No.2c', for a two-seater fighter. At the time of its design the Sopwith Triplane was enjoying a considerable success; with its excellent manoeuvrability and high rate of climb, it had established a formidable reputation as a fighter, especially among enemy pilots. The prowess credited to the Sopwith, and to its equally successful antagonist, the Fokker Triplane, resulted in the production of a crop of triplanes, both friend and foe, most of which were, incidentally, without distinction. Koolhoven, perhaps, hoped to go one better on the principle that you cannot have too much of a good thing, and his new aircraft materialized with four heavily-staggered wings ranging from well below the fuselage to high above the pilot's head. The fuselage was comparatively slender, with pilot's seat situated immediately behind the rotary engine, from where his view, except rearwards and upwards, was virtually unobstructed. The observer's cockpit was placed immediately behind the third wing from the bottom, which itself was positioned just above the fuselage. This wing had no centre section and the leading edges protruded forward on either side of, and slightly above, the pilot's head. The wing structure was supported by plank-type centre-section and interplane struts with wire bracing, while the tailplane was adjustable and carried unbalanced elevators. There was a small fixed fin below the fuselage but none above, and the rudder was balanced. The undercarriage looked rather frail and consisted of two single wire-braced struts with a cross-axle and rubber-cord shock-absorbers. The wings of the F.K.9, as it first emerged, had no dihedral and the ailerons were set well in from the tapered wingtips. After flight trials the aircraft was modified with new wings whose ailerons extended to the wingtips and were rounded with no taper.
   The modified quadruplane, powered by a 110 hp Clerget engine, was tested at Upavon aerodrome, then the home of the Central Flying School, and was reported as being light to handle and easy to manoeuvre, but with a cockpit so cramped that the full movement of the control column, particularly from side to side, was not possible; in addition, the wheel for adjusting the tailplane trim was said to be inaccessible. The stability of the aircraft in all three senses was stated to be good, but the pilot complained of being smothered with oil and, not unnaturally, suggested that the cowling should be redesigned. During the trials it was found necessary to true-up the fuselage and undercarriage after every landing, however gentle, because of, in the words of the report, '... the absence of forward undercarriage struts ... so that on landing the full weight comes on the two side bracing wires of the nacelle'. The test figures showed that the top speed at ground level was just short of 100 mph, and that at 6,700 ft it had fallen off to 94 mph. It took 12 1/2 min to climb to 6,000 ft and the ceiling was about 13,000 ft, while the endurance was quoted as barely three hours. The report concluded that the performance was far below that called for in the specification.

F.K.9

   Dimensions: Span 27 ft 9 in (8.46 m); length 25 ft 10 in (7.87 m); height 11 ft 4 in (3.45 m); wing area 355 sq ft(32. 98 sq m).

   F.K.9
   110 hp Clerget
Max weight: 2,038lb (924 kg)
Empty weight: 1,226lb (556 kg)
Max speed
   Sea level: 100mph (161km/hr)
   3,000ft (914 m): -
   6,500ft (1,981 m): 94mph (151 km/hr)
   10,000 ft (3,048 m): 87mph (140 km/hr)
Climb to
   6,000ft (1,829 m): 12.5min
   6,500ft (1,981 m): -
   10,000ft (3,048 m): 25 min
Service ceiling: 13,000 ft (3.962 m)
Endurance: 3hr