H.King Sopwith Aircraft 1912-1920 (Putnam)
Insofar as it has achieved any distinction whatsoever in aeronautical history, the Cobham three-seat bomber has attained that status firstly by reason of the fact that it was the only multi-engined Sopwith to be built; second because it was the largest of the company's triplanes. Thus, in the absence of further aeronautical distinction, one may proffer a topographical note, for this aeroplane was named after the Surrey town not far removed either from Kingston-on-Thames, where it was built, nor from Brooklands, where it was flown. Town-names, indeed, distinguished its generally comparable near-contemporaries the Nieuport London, Avro Manchester, Boulton & Paul Bourges and de Havilland Oxford - not forgetting that far bigger triplane bomber the Bristol Braemar. (The Siddeley Sinaia one might be forgiven for forgetting, though like the Cobham Mk.II this had advanced-type Siddeley engines).
From the foregoing it must not be assumed that the Cobham was devoid of technical interest, and although that interest is largely concerned with the powerplant, the airframe and armament also invite some comment.
The triplane arrangement though it distinguished also the London and Braemar - was obviously a Sopwith speciality, and one that continued to be favoured, by Herbert Smith for instance, for the Mitsubishi torpedo-dropper that was completed in Japan in 1922. Inasmuch as the degree of stagger differed between the upper and the lower wings and the interplane struts were accordingly 'cranked' in side elevation the Cobham resembled both the Snark fighter and the Japanese machine just mentioned. However, whereas the Snark had positive stagger (though of different degrees) between the three sets of wings, the Cobham, in its Mk.II form at least, had noticeably back-staggered top wings - and even in the Mk.I form the centre wings were so set back from the others as to result in positive stagger for the 'upper storey' and neutral or negative stagger for the lower one.
The Cobham Mk.I had A.B.C. Dragonfly engines, was designed in the early summer of 1918, and was inspected in mock-up form in August of that year. The intended primary role was bombing over short and medium ranges, though an armed-reconnaissance capability was a secondary requirement. Internal bomb-stowage was to Sopwith no novelty; and on the Cobham this provision was made between the roots of the bottom-wing main spars. One quoted load of 750 lb seems reasonable; but in any case external storage of heavy-calibre bombs under the lower longerons would appear to have been compromised by the inverted-pyramid tubular structure which anchored the wide-track four-wheel landing gear to the fuselage (one pair of wheels beneath each engine). A fuselage access panel, beneath the root of the port centre wing, probably signifies vertical stowage - especially so as, at about the time when the Cobham was being designed, it was officially notified that: 'When a machine has to carry bombs vertically stowed - and this is by far the best method of stowage and can be employed for bombs of 50 lb., 65 lb., 100 lb., 112 lb., 230 lb. and 250 lb. - overhead girders must be fitted above the bomb cells to take the weight of the bombs.'
The study of a close-up photograph discloses another point of interest in the general area mentioned - this point concerning the roots of the bottom wings. These wings are apparently attached below the bottom longerons, and not directly to them; and this particular feature may have resulted from a decision - taken in October 1918 or thereabouts - to make the wings readily detachable. Some complications may thus have resulted, the cable-connected ailerons, for instance, being carried on all six wing panels.
Three Cobhams, H671-H673, were ordered, but in July 1919 work on the third was stopped. Although the intended engine for all three may well have been the A.B.C. Dragonfly - and thus powered the bomber was formally designated Cobham Mk.I - the chronic troubles that beset the Dragonfly caused the first machine, H671, to be fitted with high-compression Siddeley Pumas, and thus to be designated Cobham Mk.II. Though the Pumas had arrived at Kingston in November 1918, even at the end of March 1919 the Puma-powered Mk.II was incomplete, while H672 still awaited the Dragonfly engines shown in fine detail in the close-up photograph already mentioned (and dated almost exactly one year later - 4 March, 1920). Even in July 1919, Cobham H672 was still incomplete, though it is clear that a firm decision to fit Pumas in at least one airframe had been taken appreciably before the end of 1918, whereas the Cobham itself (as already remarked) was designed only in the early summer of that year. Therefore - now noting that the Dragonflies in H672 were ultimately installed with quite exceptional neatness, the nacelles being small in diameter for maximum propeller efficiency and the propellers themselves having large open-fronted spinners as, for instance, on one version of the Snapper and the Rainbow racer - close attention to the Siddeley powerplant is clearly warranted.
It will have been noted that the Puma engines as initially mentioned in this account were described as being of high-compression type, and it is therefore important to differentiate between this form of the Puma and the standard pattern. The latter - over 625 examples of which had been delivered by the Armistice - was officially described thus: 'This engine is of the stationary, water cooled type, with six vertical cylinders 145 mm. by 190 mm. It is rated at 240 h.p., but is capable of developing some 250 b.h.p. at the maximum permissible speed of 1500 r.p.m.’ The weight was given as approximately 625 lb, and though no compression ratio was quoted, the figure was in fact 4.9 to 1. (As many unkind things have been said about the Puma, it seems only fair to add that one man at least who knew it well declared that it was a great improvement on anything else of its size and capacity, and that it was very reliable as long as it was 'carefully handled and looked after'.).
During the latter part of 1918 a high-compression version of the Puma was under development, having a maximum power of 290 hp at 1,700 rpm, largely by virtue of the compression ratio now being raised from 4.9 to 1 to 5.4 to 1. For air-tests of this new version a D.H.9 was used in October 1918. In Sopwith Cobham H671 - the Mk.II, which was sent to Martlesham Heath in the summer of 1919 and was at Brooklands at that year's end two high-compression Pumas were installed in heavily louvred deep and slim nacelles mounted on the bottom wings and having frontal radiators. A Scarff ring-mounting for a Lewis gun was emplaced in the sharply rearward-sloping nose of the narrow fuselage, and there was a second mounting of the same type just abaft the wings.
That the fitting of high-compression Pumas in Cobham H671 is of more than passing interest is sure (even though similar engines were installed in the Avro Manchester, the Bristol Fighter and the D.H.9); and that it was no mere stop-gap measure to get a Cobham airborne in default of the Dragonfly is possible, for there was evidently a serious proposal to install in the Cobham (as in other British 'twins' and two-seaters) an engine called the Armstrong Whitworth Ricardo Patent R.H.A. And here it must be emphasised that 'Whitworth' is correct - not ‘Siddeley' as might be supposed. The 'R.H.A.' signified 'Ricardo-Halford-Armstrong', and the engine concerned was of twelve-cylinder vee form, fitted (to quote a contemporary statement) 'with a special supercharging device, by which means the engine is capable of maintaining 300 b.h.p. at 10,000 feet.' On another occasion it was claimed that, when full use was made of the supercharger at ground level, the output was 360 hp.
Before the earlier-quoted statement was made (correctly emphasising performance at height) an officially-issued publication was listed as having the title Supercharging as Applied to Aero Engines (Ricardo System); and though a 'Siddeley Puma R.A.' engine was one designation current, and though the Ricardo-Halford ‘Inverted Supercharger' engine was evidently tested at Farnborough in a D.H.4, there is still good reason to place the preceding facts on record in connection with Sopwith's one-and-only 'twin'. This belief is supported because two R.H.A. engines had been completed when the Armistice came, and four more were almost ready. Of the first pair, one was claimed to have been sent to Farnborough before the Armistice and to have been tested in a de Havilland airframe, though development was soon abandoned.
The Dragonfly-engined Cobham Mk.I (H672) was eventually to be air-tested, though not until the spring of 1920. This aircraft differed in tail design, the rudder (now having a lower horn balance, as well as the upper one as on the Mk.II machine H671) being extended well below the fuselage and necessitating a tall and elaborate tailskid. More basic were the changes in the setting of the triplane wings, as noted in the third paragraph of this present chapter.
C'obham Mk.I (Two A B C. Dragonfly) Span 54 ft (16.5 m); length 38 ft (11.6 m): height 13 ft (4 m). Maximum weight 6,300 lb (2,858 kg).
Cobham Mk.II (Two Siddeley high-compression Puma)
Data essentially as for Mk.I.
F.Manson British Bomber Since 1914 (Putnam)
Notable as being the only twin-engine Sopwith aeroplane ever built, the triplane Cobham's career was also bedevilled by its association with the ABC Dragonfly engines. Designed during the summer of 1918 to the Air Ministry Specification IV, as qualified in Specifications VI and VIII by virtue of variations in range and bomb load (all three of which were amalgamated in the Department of Research Type 3 Specification) - the Cobham began building in September that year.
Although the aircraft was designed as a three-seat medium bomber, capable of carrying three 250 lb bombs (stowed internally and suspended vertically), the greater part of the Cobham's life was preoccupied with attempts to come to terms with the thoroughly unreliable Dragonfly. The first of three prototypes ordered, H671, was completed at Brooklands in about December 1918 but, owing to delays in the delivery of the 320hp Dragonfly I engines, Herbert Smith (whose design the Cobham was) was instructed to make provision to install a pair of standard 240hp Siddeley Puma engines, so as to begin flight trials as quickly as possible. H671 was therefore termed the Cobham Mark II, and was flown in about April 1919. It was, however, never destined to receive Dragonfly engines as, some time in 1919, it suffered an accident and was undergoing repair at about the time that the first modified Dragonfly IA engines were starting delivery. It was therefore fitted with high-compression Pumas and underwent performance trials with these at Martlesham Heath in March 1920, and in November that year was delivered to the RAE, Farnborough, where it was last flown on 27 January 1921.
Neither of the other two Cobhams was flown during 1919; these, termed Mark Is, were both powered by 360hp Dragonfly IAs with redesigned cylinders and pistons. H672 and H673 were first flown in January and February 1920 respectively but, with the Sopwith company beginning to suffer serious financial difficulty, they were taken on Air Ministry charge and delivered to Martlesham Heath in February. Recurring engine failures caused them to be forwarded on to Farnborough to await a decision on the future of the Dragonfly and, when its development was abandoned in September, the Cobham Is were struck off Air Ministry charge.
The Cobham's airframe design underwent fairly extensive change when it was discovered that the Dragonfly was substantially over the weight originally notified to Sopwith; compared with the original works drawings of the aircraft, the Mark I prototypes had their engines set some fifteen inches further aft, with the plane of the cylinder centreline in line with the centre wing's leading edge. Moreover, the Mark Is featured changes in wing stagger, the top wings being rigged with positive stagger, and no stagger on the bottom wings; the Puma-powered Mark II featured slight sweepback and back stagger on the top wing. The Mark I was also found to require increased rudder area, this being extended below the fuselage sternpost, with horn balances at each end. This modification was said to have been demanded by Harry Hawker who, having had to land a Cobham with one dead engine, found the aircraft almost unmanageable and entirely devoid of rudder control.
Type: Twin-engine, three-crew, three-bay triplane medium bomber.
Air Ministry Specification: RAF Types IV, VI and VIII.
Manufacturer: The Sopwith Aviation Co Ltd, Kingston-upon-Thames, Surrey.
Powerplant: Mark I. Two 360hp ABC Dragonfly IA nine-cylinder, air-cooled, radial engines driving two-blade propellers. Mark II. Two 240hp Siddeley Puma six-cylinder, water-cooled, in-line engines.
Structure: Wire-braced wooden box-girder fuselage with ply and fabric covering; two spruce wing spars and fabric-covered wings.
Dimensions: Span, 54ft; length, 38ft; height, 13ft.
Performance: No records traced.
Armament: Single 0.303in Lewis machine guns in nose and midships positions with Scarff rings; bomb load, carried internally, said to be about 750 lb.
Prototypes: Three, H671-H673. H671, the Puma-powered Mark II, first flown about April 1919; H672 first flown, January 1920; H673 first flown, February 1920. No production.
H.King Armament of British Aircraft (Putnam)
Cobham. The Cobham triplane bomber was flown in 1920. This first and only twin-engined Sopwith carried its bombs internally in the fuselage between the roots of the main spar. A load of 750 lb has been mentioned, and this seems altogether reasonable. Two access panels were located in the side of the fuselage just below the centre wing. There was a Scarff ring-mounting for a Lewis gun in the nose and a similar installation on top of the fuselage just abaft the wings.