O.Tapper Armstrong Whitworth Aircraft since 1913 (Putnam)
The Koolhoven Multiplanes
In the early stages of the 1914--18 war, before the pattern of aerial warfare had developed, there was a school of thought in Great Britain which argued the merits of the 'flying battleship' or 'aerial destroyer'. This concept, which envisaged a large aeroplane with a multiplicity of guns having a wide field of fire, arose perhaps from the nation's deep-seated naval traditions. The proponents of the theory gave little regard to the virtues of speed or manoeuvrability, the idea apparently being that the aircraft would proceed in a dignified fashion, possibly in line ahead, firing broadsides at the enemy, who, it might be supposed, would adopt similar tactics. This may be extending the analogy too far, but the fact remains that considerable effort was expended in devising large fighter aeroplanes in which performance took second place to armament. Needless to say, the concept proved unsound, and it was the more adaptable fixed-gun fighter which dominated the scene where the battles were actually fought.
The large multi-seat-fighter notion certainly produced some odd-looking aircraft, with both Sopwith and Vickers trying their hand at the idea, but perhaps the strangest of all were the two Armstrong Whitworth triplanes produced to the designs of Frederick Koolhoven. The first of these featured two machine-gun nacelles mounted on the top surface of the middle wing, which was considerably longer than the other two. In order to provide the best possible field of fire for the two gunners, the nacelles projected well forward of the tractor propeller, which was situated but a few inches ahead of the wing leading edge. The pilot was placed behind the wings where his view in any direction, except upwards and backwards, must have been minimal. The engine was the new 250 hp Rolls-Royce twelve-cylinder unit which later became known as the Eagle. The undercarriage, which, like the rest of the aircraft, was highly unconventional, consisted of a single centrally placed shock-absorber strut terminating in two closely-spaced wheels, lateral stability being provided by a small single wheel under each wingtip. The tail was supported by a skid carried on long struts emanating from the underside of the fuselage at a point just aft of the wings. The whole aircraft seemed ill-balanced and gave the impression of frailty, and it is, perhaps, not surprising that Fairbairn-Crawford, the works manager, is on record as saying that he refused permission for it to be flown.
Subsequently, the design was re-vamped to conform to a requirement initiated by the War Office for a multi-seat escort fighter and Zeppelin destroyer. Using the same type of Rolls-Royce engine, the new triplane was larger than its predecessor and the span of all three wings was greater than before, with the overhang of the centre wing being less pronounced. A second bay was added to the wing structure, and the bracing appeared to be more substantial. The engine and propeller projected ahead of the wing in the conventional tractor position, and the pilot, again seated behind the wings, had a marginally better view but still not one calculated to arouse much enthusiasm. The undercarriage was short and carried a cross-axle with two pairs of wheels; the track was narrow and ground clearance for the lower wing was small. The two gun nacelles, this time attached to the underside of the middle wing, may have been designed to take the Davis gun. In April 1916 Armstrong Whitworth were supplied with two wooden mock-ups of the 6-pounder and the 2-pounder models for fitment to ‘... a large aeroplane now under construction for the War Office’, which can only have been the triplane. Four prototypes of the second triplane had been ordered in March 1916, but only No.7838 was built, it having by then become obvious that the large, ponderous fighting aeroplane was a mistake. Little is known about the test flights carried out by Peter Legh, but it seems they were somewhat perfunctory, with the performance failing to come up to expectations; in any case, interest in the project had already evaporated and the type was soon abandoned.
The place occupied by the triplanes in the F.K. series remains a mystery: both have been referred to as the F.K.12, but all the evidence points to the conclusion that this number is wrong. The comparative immaturity of the triplane designs would seem to indicate that they pre-dated the more workmanlike and more modem looking F.K.8 biplane and the subsequent quadruplanes which, for all their eccentricity, were more in accord with the designs of the later war years. More conclusive, perhaps, is the fact that both triplanes were designed before the adoption of the machine-gun interrupter gear, whereas the quadruplanes were clearly laid out with this type of armament in view. The true sequence of the F.K. numbers may never now be discovered, but the best guess is that the airship car, previously mentioned, which was an adaptation of the F.K.3 fuselage, was the F.K.4, with the two versions of the triplane following as the F.K.5 and the F.K.6.
P.Lewis The British Fighter since 1912 (Putnam)
The F.K.11 was not proceeded with but another design from Armstrong Whitworth, the three-seat F.K.5, also exhibited some equally remarkable features. It was built as the result of a War Office requirement for a multi-seat, long-range escort and anti-Zeppelin fighter, a specification to which Vickers and Sopwith also constructed prototypes.
Armstrong Whitworth proceeded to build the F.K.5 as a large triplane with its central planes of much greater span than those above and below. The 250 h.p. Rolls-Royce engine turned its propeller immediately in front of the leading edge and between the pair of gunners’ nacelles fitted to the centre wings. A revised version as the F.K.6 was built with a different fuselage, undercarriage and nacelles but, as with so many extremely unconventional designs, the machine was not a success.
F.Mason The British Fighter since 1912 (Putnam)
Armstrong Whitworth F.K.12
If the products of Sopwith and Vickers, in the search for an escort fighter, had appeared quaint, that of Frederick Koolhoven at Armstrong, Whitworth was nothing less than incongruous. Both the L.R.T.Tr and the F.B.11 had employed single nacelles in which to accommodate additional gunners, both selecting the top wing as a logical position from which to gain the widest field of fire. Koolhoven decided on two nacelles, and placed them at the front of the central wing of a large single-bay triplane, powered by a single 250hp Rolls-Royce Mark I.
When the F.K.12 first appeared towards the end of 1916, the central wing - with by far the greater span - was located well forward on the fuselage, so that the engine only just extended beyond the leading edge; the top and bottom wings were much smaller structures, carried on struts above and below the fuselage and rigged without stagger. The gunners’ nacelles were long structures mounted above the central wing, extending forward of the propeller. The undercarriage comprised a pair of main-wheels mounted on a single strut extending from beneath the engine and attached to the leading edge of the lower wing, and a small, sprung auxiliary wheel under each lower wingtip. A rear skid was carried on long pyramidal struts extending downwards from the fuselage immediately to the rear of the lower wing.
The first configuration was not considered successful and was followed by a no less extraordinary aircraft which was probably newly built, rather than an adaptation of the first. This retained the same arrangement of ‘short-long-short’ wings as previously but with a much deeper fuselage occupying the entire gap between the two lower wings. These two-bay wings were set further aft on the fuselage so that the engine extended further forward. The twin gunners’ nacelles, now mounted beneath the central wing, were much shorter so the gunners’s cockpits were behind and outboard of the propeller. (They were incidentally no more than two feet from the open ends of the big Rolls-Royce engine’s exhaust manifolds.)
The undercarriage now comprised two pairs of landing wheels mounted on heavy vertical members attached to the sides of the fuselage and were thus of very narrow track; a conventional tailskid was attached under the rear fuselage, and the wingtip balancing wheels were discarded in favour of hooped skids.The aircraft, No 7838, was flown by Lieut-Cdr Peter Legh, but no reliable records of the flight trials of the aircraft have been traced. A total of four F.K.12 prototypes was ordered by the War Office, but it is thought likely that the two aircraft described here were the only examples completed.
Type: Single-engine, three-seat, two-bay triplane escort fighter.
Manufacturer: Sir W G Armstrong, Whitworth & Co Ltd, Newcastle-upon-Tyne.
Powerplant: One 250hp Rolls-Royce Mk I twelve-cylinder liquid-cooled in-line engine driving four-blade propeller.
Armament: Two 0.303in Lewis machine guns on rocking-post mountings in nacelles carried on central wing.
Prototypes: Four ordered, Nos 7838-7841. It is not known how many were completed.
W.Green, G.Swanborough The Complete Book of Fighters
ARMSTRONG WHITWORTH F.K.6 UK
In 1915, Frederick Koolhoven, the chief designer of sir W G Armstrong Whitworth & Co Ltd, initiated work on a highly unorthodox three-seat triplane powered by a 250 hp Rolls-Royce 12-cylinder water-cooled engine. It was intended to accommodate two gunners each with a 0.303-in (7,7-mm) machine gun in shallow nacelles, mounted above the centre wing on each side of the fuselage, the gunners being seated ahead of the propeller plane of the tractor engine. Although a prototype was completed and allegedly designated F.K.5, this was never flown, being extensively damaged as a result of a ground loop during it first take-off attempt. The design was extensively revised early in 1916 to meet an RFC requirement for an airship interceptor and long-range escort fighter. The revised design is believed to have been designated F.K.6 (and certainly not F.K.12 as has sometimes been stated) and four examples were ordered, two of these being intended for the RNAS. In the event, only one F.K.6 was built. The gunners' nacelles were underslung on the central mainplane, armament remained two 0.303-in (7,7-mm) Lewis guns and the 250 hp Rolls-Royce engine was retained. It is believed that relatively limited flight testing was undertaken.
Span, 63 ft 0 in (18,89 m).
Length, 37 ft 0 3/4 in (11,29 m).
Height, 17ft 0 in (5.18 m).
J.Bruce British Aeroplanes 1914-1918 (Putnam)
Armstrong Whitworth F.K.12
THAT excellent aero-engine, the 250 h.p. Rolls-Royce which was later named Eagle, was originally designed and developed as a power unit for seaplanes. About the end of January, 1916, the Admiralty transferred a few of these engines to the War Office for experimental purposes; and the aircraft manufacturers of the day were invited to submit designs for multi-seat escort fighters embodying the Rolls-Royce engine. No doubt it was hoped to produce an aeroplane capable of defeating the Fokker monoplane, which was then coming into menacing prominence.
Several firms submitted designs to meet the specification, which demanded a maximum endurance of seven hours to enable the machine to be used for anti-Zeppelin work if need be; but only Armstrong Whitworth, Sopwith and Vickers were awarded contracts for the construction of prototypes. All three designs were highly unorthodox, and all provided accommodation for their gunners in unusual positions.
The Armstrong Whitworth design was the F.K.12, a big triplane which appeared in two different forms. The version which apparently came first was probably the most remarkable British aeroplane of the 1914-18 war. Its central mainplane was of much greater span than the top and bottom wings, and on it the fuselage was mounted almost symmetrically. There was virtually no forward-reaching nose on the fuselage; the airscrew revolved just in front of the leading edge of the central wing.
The undercarriage consisted of a central unit carrying two main wheels on a sprung leg, and a small sprung wheel under each wing tip. A long pylon just behind the trailing edge of the bottom wing was fitted with a skid which kept the tail of the fuselage clear of the ground.
The central mainplane carried two nacelles for gunners, each of whom apparently was to have had a Lewis gun on a rocking-post mounting. The gunners were in front of the tractor airscrew and had a fine field of fire in all forward directions.
The second F.K.12 had a similar wing configuration, but was a much more cumbersome machine. The wing span and area were increased, and the wings had two bays of interplane bracing. The fuselage was more conventional in appearance, but was deeper and filled the gap between middle and bottom mainplanes. The long nose must have interfered with the gunners’ view. The wing-mounted nacelles were retained, but were of different form and were underslung from the centre wing. The undercarriage consisted of two twin-wheel units under the fuselage. Serial numbers were allotted for four prototypes, but it is doubtful whether all were built.
The Armstrong Whitworth F.K.12 was tested by Peter Legh, but its performance fell short of expectations. Furthermore, in common with the Sopwith and Vickers types, it appeared at about the same time as an effective British interrupter gear for machine-guns. Development was abandoned in favour of more conventional designs which could be effectively armed.
Manufacturers: Sir W. G. Armstrong, Whitworth & Co., Ltd., Gosforth, Newcastle-on-Tyne.
Power: 250 h.p. Rolls-Royce.
Armament: Two Lewis machine-guns, one on a rocking-post mounting in each outboard nacelle.
Serial Numbers: 7838-7841.
H.King Armament of British Aircraft (Putnam)
F.K. 12. There were Sopwith and Vickers counterparts of this 1916 three-seat escort and anti-airship fighter triplane, all three having unconventional armament layouts to give a clear field of fire. The middle wing was attached to the top of the fuselage and carried two manned nacelles, each probably intended to have a Lewis gun on a pillar mounting. The guns would have commanded a wide field of fire in the forward hemisphere, being sited forward of the airscrew. Originally the gun-nacelles were above the middle wing; later they were underslung and of different form.