THE SPITFIRE Mk 1B
Fighter equipment of the Royal Air Force and the Luftwaffe during the Battle of Britain consisted of the Spitfire and Hurricane. and Bfl09 and 110, the former pair with a battery of eight .303 Browning machine guns; the latter with a mixture of machine guns and cannon. The weight of cannon fire was devastating when a direct hit was achieved, and although the Air Ministry was aware that the British fighter armament was effective an improved, and heavier type, was needed for Luftwaffe aircraft were being equipped with heavier armour plate and this nullified, to some extent, the effect of concentrated machine gun strikes. Fighter Command was not at all disappointed with early results during the opening days of the Battle of Britain, and when offered the Hispano 20mm cannon was agreeable only if 60rpg could be guaranteed in drums. A second proposal was for four cannon with 150rpg or 6 X .5in machine guns.
In his original paper on the eight gun fighter Sqdn. Ldr. Sorley had not overlooked the heavier calibre gun for he had written: “The choice lay between the .303 gun, the .5in and a new 20mm Hispano-Suiza cannon, which was of great attraction to the French and other Continental countries. The .5in gun was newly developed and very heavy and was, in fact, a small cannon, and the Hispano was ‘super sensitive' to rigidity of mounting and was difficult to mount in aeroplane wings".
Progress had been made with the wing mounted cannon as can be witnessed by Mitchell's submission of the Supermarine Type 305 (In this submission reference was made to: "If production order for the Type 300 F3 7/34 is forthcoming, one set of wings can be fitted with four cannon guns to speed construction of a prototype to Specification F3 7/35") with four Oerlikon cannon in the wings to Air Ministry Specification F 37/35 in April 1936, full details of which appear in chapter three. Also, Supermarine were always aware of the possibilities of the cannon as can be seen by Beverley Shenstone’s report of the Paris Air Show in November 1936. Among other things he was looking for was examples of cannon armed aircraft. Part of his report relates: “One went to the exhibition hoping to see several types of cannon gun installation. Herein one was very disappointed. Where cannon guns were indicated in French machines they were mock ups, and very rough ones too. The installations were crude in the extreme, and one cannot think that they represented serious proposals. Wing installations seem to have died a natural death and the fuselage installation, either in or below, has taken its place. This change has, of course, been made possible by the use of twin engined machines”.
Previous to Shenstone's Paris trip a Mr. Orleans of Aero Engines Ltd, had visited Supermarine on 30 March to discuss installation problems associated with the Hispano 20mm cannon in the Spitfire, and he produced a drawing (S.K.1218) showing attachments points and other particulars. The gun was entering production in France following trials with it mounted above an aircraft's engine. No wing installation had been attempted. The maximum recoil force was 1100 lbs, and a 60 round cylindrical magazine had been developed and was also available in smaller sizes with fewer rounds. Hispano was of the opinion that a belt feed was not possible. The cannon. mounted on the engine, was held at four points and these positions could, with adjustment, be used for a wing installation. Recoil was absorbed by the front attachment and transmitted to the wing structure via a spring, the stored energy in the spring being expended in an adjacent dash pot.
The complete gun moved through a travel of 20mm when fired with the magazine remaining stationary relative to the structure. It would also function on its side and rate of fire. which could be pre-set. was between 520 and 720 rpm. Mr Orleans left Supermarine with the size o f the wing section under consideration and a request for further information regarding mounting.
Dowding was not impressed with the Hispano cannon for on 25 June 1938 he wrote to Sholto-Douglas at the Air Ministry saying he did not want nine squadrons of Defiants. He then commented: “There has been a lot of talk about the efficiency of the 20mm cannon and I have seen no proof that this gun will give decisive results. We ought to have carried out the most careful experiments to prove its value before we adopt it. lf this was not done I shall wake up in a year’s time and be told I am committed to have 15 squadrons of something with a 20mm cannon; whereas I can tell you now I do not want any and so perhaps save a large sum of money. I also want to be in the picture about the new single seat fighters".
Trials had taken place place at Shoeburyness on, among other obsolete airframes, the F 7/ 30 prototype K2890. Poor results were obtained with cannon shells exploding on contact with the outside skin with little effect to internal structure. Dowding thought that if cannon were to be specified for the Spitfire he would rather have a larger gun with a heavier missile and slower rate of fire, whereby one direct hit would destroy the target. He said: “Therefore, we should make a bold jump and start trials with a 37mm cannon“. But this was contrary to the current thinking of Sorley and Buchanan. who favoured the eight gun fighter.
Following upon the visit to Supermarine by Mr. Orleans, W. M. Hingston. of DTD. arrived at Woolston on 4 August 1938 with details of the Hispano cannon tests. during which it had functioned correctly in (a) an upright position. (b) raked up at 42° from vertical and (c) inverted. lt failed on its side. but as the magazine was unsupported it was thought it would perform correctly with the magazine when supported in the test rig at Boulton and Paul. Tests were also made on the effect of blast on wing leading edges. lt was considered practicable to replace the drum feed by an assisted belt, or hopper. to facilitate housing in the thin wing. but no work had been done on this.
Despite Dowding's objections a letter arrived at Supermarine, addressed to Joseph Smith, on 20 December 1938, and he was instructed to prepare a scheme for a Spitfire to be equipped with one Hispano cannon under each wing. and also to produce drawings and a mock up*. Smith informed the Air Ministry that he was totally opposed to having exposed cannon under the wings and that he could design an installation which would mean fitting them on their sides in the wing with small blisters on the upper and lower wing surfaces. Pierson followed up Smith’s suggestions on 9 January I939 with a plan for four cannon in the wings, and the Air Ministry’s reply on the 24th of the month was that two were sufficient at the moment. Smith and Pierson had obviously resurrected the Type 305 fighter for both had included features of this design.
The fifth production Spitfire Mk I — K9791—was used as the mock-up trials aircraft and despite the need for a belt feed the 60 round drum was specified. This resulted. as forecast by Smith. in a blister above and below the wings to accommodate the magazine. ln order to speed trials and not wait for official approval of the mock up. work had proceeded with the installation of two Hispano cannon in Spitfire L1007. Vickers wrote to the Air Ministry on l9 January requesting delivery of the embodiment loan of guns and ammunition, but not until the end of March were they delivered. Weighing and C.G. determination took place at Eastleigh on I6 June and the aircraft had a tare of 4,589lb, and auw of 5,916.
First trials by Vickers took place the same month against a barrage balloon target and the following month. accompanied by Hurricane L1750 fitted with two cannon mounted under the wings, L1007 arrived at Martlesham Heath for further tests.
The Spitfire went next to AFDU at Northolt for service trials, which revealed that the gun was unreliable at low temperatures and heating would have to be provided. Early in January 1940 the aircraft was transferred to Drem for squadron trials and on the 13th. piloted by P.O. Proudman. it attacked and shot down a Heinkel III at 20.000 feet. Forty one rounds were expended before the guns jammed. L1007 was then transferred to Dishforth on 15 March. where it was found the deflector plates for the ejector chutes were partially to blame for gun stoppages. Squadron Leader J. G. Munro was to design a pair of successful replacements.
Dowding had. by now. accepted the idea of the cannon armed Spitfire and Supermarine was awarded a contract (980385/39) to select and convert 30 Spitfires to take the cannon wing. Lord Beaverbrook. then in complete control of aircraft production. was most impressed by the cannon Spitfire and in his impetuous manner authorised the production of 30 pairs of cannon wings. giving this order the utmost priority. Of the 30 Spitfires ordered 24 had been delivered by 16 August.
The second Spitfire to be converted to cannon armament was P9504. a MkI. and it was ready for squadron use by 4 April 1940. The first cannon Spitfire to reach an RAF squadron was R6261. going to No.19 in June for trials. and it was followed within days by R6770 and 6776. On I July the Air Ministry informed the CO of No.19 Squadron that the squadron was to be completely re-equipped with the cannon Spitfire. but when they were delivered they stayed on for only a few days at any one time. being rapidly returned to No.6 MU at Brize Norton for examination. The new Spitfires were not a great success for combat reports showed clearly the frustration of pilots who. having moved in for a kill. found the guns had jammed solid after a few rounds had been fired. Also. the aeroelasticity of the wing made it twist slightly in tight manoeuvres causing the ammunition drum to come into contact with the skin and jam. It did seem pointless to scramble a number of cannon Spitfires only to have stoppages at the vital moment. Dowding was blunt about the situation for in a letter to the Air Ministry on 15 July he wrote: “Two cannon Spitfire unreliable". The converted Mk ls carried the prefix designation CIG which, presumably, stood for cannon wing.
What was needed was a stop gap, a British compromise, and it appeared in the shape of a mixed armament Spitfire. P9504 was still at Manby with the new cannons installed but, what was fortuitous, it still had four of the original Browning machine gun mountings in the wings. lt was used to test a trial installation of two Hispano cannon and four Browning guns, which proved to be a great success, and within days a second Spitfire—X4257—had a wing, built from scratch, with the new armament and service trials began on 20 August. Five days later R6761, 6770, 6889, 6904 and 6919 were withdrawn from No.19 Squadron and modified to the same standards. They were soon followed by R6776 and 6833. All these R-serialled Spitfires carried the suffix CIG after the digits on the Supermarine works dockets, and X4257, together with a number of X-serialled Spitfires, had the suffix CMG which, presumably, indicated cannon/machine gun.
P9504 and R6770, together with X4257, were despatched to Boscombe Down for trials with the new armament, R6770 having modified spars because the cannons were still jamming in tight turns. R6904 was returned to Eastleigh for modifications to its cartridge ejection chutes. The cannon wing was now known as the ‘B’ and with it the Spitfire‘s tare weight was increased to 4893 lb and at take off 6385. The four cannon wing was given some consideration at that time but was turned down until the twin cannon aircraft was proven in service. Production of the Mk IB Spitfire was agreed with Supermarine, who guaranteed delivery of ten aircraft in October rising to 20 a month by November, plus the delivery of the first four cannon variant powered by a Merlin XX engine by the end of September. By this time, however, the urgency for the cannon installation had diminished for the German daylight raids were faltering.
The first modified lB to go into action was R6889, which had rejoined No.19 Squadron on 3 October, and it was not an outstanding success. It was under powered and even with the promised Merlin lll engine had to be flown at maximum power to keep up with the Browning gun aircraft. The majority of converted Spitfires were never given an official designation and most of them had to be converted to Mk
VB standards with Merlin 45 at a later date. All Browning gun Spitfires were to be retrospectively designated Mk IA and the few cannon, and mixed armament aircraft, Mk IB. There were no true Mk lBs built on the production line. The actual cost of converting a set of ‘A’ wings to accept the Hispano gun armament was £379 16s. To convert a set of wings to the full ‘B’ wing standards was £640 11s. The cannon aircraft were converted under Mod 260 issued on 3-7-40.
The British version of the Hispano cannon was jointly developed with the French Air Ministry at the Chatelerault Arsenal until the fall of France in May 1940. Development was then transferred to the RAF Section of the Royal Small Arms Factory at Enfield. Series production was undertaken by four factories, one of which was the British Manufacturing and Research Co., a manufacturing subsidiary of Hispano Suiza.
ALTERNATIVE GUNS
Before leaving this history of the early cannon Spitfires mention must be made of other armament specified and tested during, and before, the same period. As long ago as July 1937 the Air Fighting Committee of the Air Ministry was considering a replacement of the .303 Browning machine gun and had issued a Memorandum on the 7th of that month for an ultra high speed gun capable of firing 2000rpm. The Memorandum called for trials with eight of the guns firing explosive ammunition and storage space for 4800 rounds. Full scale trials took place on 14 December 1937 with the Hungarian Gebauer gun and the effect was likened to a “welder's torch held against a stressed skin aircraft”. The new gun was intended to replace the Browning five years in the future. There were problems, of course, for increased rate of fire resulted in attendant barrel wear (Barrel life of the .5 in machine gun was said to be 7, 000 - 8,000 rounds) and increased weight. The suggestion was to drop the ultra high speed gun and concentrate on a gun with a higher muzzle velocity, which would provide an increased lethal range, but the additional velocity entailed a larger powder charge and cartridge. A compromise in the shape of an armour piercing .276in was suggested.
The Air Ministry took out insurance by asking for designs of two new .303in guns in March I938 as Browning replacements - the ultra-high-speed and the high muzzle velocity. A Blenheim airframe (K7154) was used for firing trials of the high velocity weapon on 9 December but penetration of vital, internal parts was insufficient. Blenheim K7041 was also used for the same weapon trials with better results when the gun was fired towards the target’s stern.
The Masden gun was also considered for the Spitfire as a replacement for the .303in Browning. This gun was the standard infantry model modified for remote, automatic control when installed in an aeroplane. The calibre was either 6.5, 8 or 11.35mm and the rate of fire increased from 450rpm to 1000 and 1200, and for this the special recoil spring had to be reinforced. The gun was belt fed and the rounds held together by steel links, this making the task of collecting the empty belt easy when compared to the standard gun belt. A 23mm Masden cannon was also considered and this, too, was belt fed with rounds connected with steel links, and it had a rate of fire of 400rpm. The Hispano company also produced a 23mm cannon, while Vickers had their .5in automatic gun, a 25.4mm, 37 and 40mm cannon. The first had a rate of fire of 450 to 650rpm; the second 100 and the last two 200. During the design/prototype stage of the Spitfire guns available were, Vickers 12.7mm, Hispano 20mm, Hispano 23mm, Vickers 25.4, Vickers 37mm, American Armament Co. 37mm, COW 37mm, Oerlikon 20mm.
Another new weapon to come to the Air Ministry’s attention was the American .5in Colt, examples of which arrived in England in June 1940. Drawings were ready the following month and trial installations commenced. and by the following August Supermarine were proceeding with a trial installation of six of the new guns, plus a second installation of six .5in and two 20mm Hispano cannons (with 120rpg), this being specified for the proposed Spitfire Mk Ill. ln the meantime the two 20mm cannon and four .303 Browning machine gun installation had been accepted and eventually developed into the “B” wing with the result that the Colt installations were put on a low priority on 9 December 1940 and the .5in machine gun eventually adopted several years later for the Spitfire Mk IX.
Before closing this chapter on Spitfire armament mention must be made of the work both BSA and Vickers completed on a version of the .5in Colt using a shaped charge. lt proved difficult to produce and was eventually abandoned. Also, the Air Ministry had listened to Dowding's idea for a larger weapon For the Spitfire and on 23 April 1940 gave approval for the development of a 13.2mm weapon based upon the French Hotchkiss gun. the new weapon being specified for the first Griffon engined Spitfire. the Mk IV. Supermarine also prepared a design study for a Spitfire armed with this gun with a Merlin engine as the Type 345.