From Nighthawk to Spitfire

by John K Shelton

  A detailed account of the twenty-seven main versions which went into quantity production is provided in Appendix 2 and indicates just how far Mitchell’s basic design was capable of development, thanks to the dedicated wartime effort of Joe Smith and his design team. Meanwhile, drawings of their basic modifications will give an indication of these efforts.

  The drawing opposite shows first, Mitchell’s original semi-elliptical planform, followed by the clipped wing version for higher manoeuvrability at low level. The next illustration gives Smith’s most obvious departure from Mitchell’s original concept, the pointed wing version of the high-altitude Marks VI, VII, VIII and 21; finally, the fourth drawing shows the revised wing and tail surfaces, as seen in Marks 22 and 24.

  Perhaps a more graphic illustration of the continuous development of the wartime Spitfire can be seen in its changing side views, shown on page 172. Mitchell’s original design concept remained most clearly evident in the Merlin-engined fighter Mark I–II–V–IX line of development, in the PR Mark I–VII sequence and in the Seafire Mark I–II–III–XV series. The Griffon engine and the extra oil requirement in the longer range PR variants produced somewhat changed nose shapes, and increases in power also necessitated redesigns of the tail surfaces. The more powerful later Merlins had given rise to the simple change of a pointed and broader chord rudder, while the even more powerful Griffons produced a complete redesign of the unit. Another factor affecting a more fundamental redesign by other hands was the later availability of the teardrop canopy, which resulted in a cut down rear fuselage decking and a great improvement in rear view for the pilot.

  Four distinctive planforms.

  The p.169 comparison between the Mark I and the Seafire 47 gave further proof of how far Mitchell’s original concept had been capable of development but, setting aside the huge difference in load carrying between the two aircraft, it can be calculated that the latter’s 125 per cent increase in engine power had only produced a 25 per cent increase in speed. Thus, Mitchell’s propeller design could be seen to be reaching the limits of its potential. Nevertheless, one can surely agree with Rendall’s assessment of the Spitfire, that few aircraft straddled the transition between the two ages of aviation so comprehensively, let alone so elegantly – the biplane and the monoplane ages, the piston age and the jet age, the subsonic and the supersonic.

  The prototype and early Mark I with flat cockpit cover.

  An early mark with the more familiar domed cockpit cover.

  An early mark with the ‘beard’ engine cowling for the tropical air filter.

  A PR version with deeper engine cowling, symmetrical radiators, retractable tailwheel, and second version fin and rudder.

  The early Griffon-engined type with modified engine cowling, repositioned air intake, and third version fin and rudder.

  The final fighter version with balloon cockpit hood and lower rear fuselage decking, deeper radiators, and fourth version fin and rudder.

  The final Seafire version with contra-rotating propellers, further revised air intake position, and modified fourth rudder.

  APPENDIX 1

  PHOTOGRAPHS SHOWING THE DEVELOPMENT OF THE SPITFIRE

  One of the first Mark IIAs from Castle Bromwich. (Courtesy of E.B. Morgan)

  The Mark V, two cannon version, with clipped wings. (Courtesy of E.B. Morgan)

  The first of the three Mark V float conversions with early type tropical air filter. (Courtesy of E.B. Morgan)

  A HF VII with extended wing and second version fin and rudder. (Courtesy of E.B. Morgan)

  Mark XIIs of 41 Squadron. The redesigned cowling blisters for the Griffon engine are clearly evident on EB-B. (Courtesy of E.B. Morgan)

  The Mark XIV, showing the five-blade right hand propeller and slightly deeper radiators. (Courtesy of E.B. Morgan)

  The Mark 18, showing the camera aperture, the redesigned fin and rudder, lower fuselage decking and ‘tear-drop’ cockpit canopy. (Courtesy of E.B. Morgan)

  The Mark F22, showing the final revisions to flying surfaces and the fourth type fin.

  Seafire F46 with contra-rotating propellers and modified fin. (Courtesy of E.B. Morgan)

  APPENDIX 2

  MAIN VERSIONS OF THE SPITFIRE

  This book has been about Mitchell and his designs but, because of his early death, it was left to Smith and his team to fully realise the potential of his outstanding fighter. Chapter 9 has indicated the extraordinary extent of this development, but only a description of the twenty-nine main production versions can really do justice to the wartime effort that went into this continuous uprating. However, the detail of this work would seem best as an appendix as, even though it was his basic concept which had such potential, it was not Mitchell who had the oversight of it.

  Mitchell’s basic fighter was originally developed for high-altitude or low-altitude operation, and photo-reconnaissance work, indicated by the letters HF, LF and PR respectively (although these prefixes gradually disappeared as more versatile versions were developed). The carrier-based duties were allocated to the Seafires and there were also floatplanes and a two-seat trainer. Not all the variants went into service and, as we shall see, the mark numbers do not always indicate the actual chronology of their production.

  As will also become apparent, progressive increases in performance resulted, essentially, from Supermarine’s response to the remarkable supply of improved engines from Rolls-Royce, first Merlins and then Griffons.

  Mark I (1,550 Built)

  The Mark I was very little different in outline to Mitchell’s prototype; there was, however, an increase in flap angle, and the substitution of a tailwheel for the skid. By the time of the Battle of Britain, its two-bladed propeller was replaced with a three-blade, two-position type and a slab of bulletproof glass had been attached to the outside of the front windscreen; armour plating had been fitted behind the pilot’s seat and the straight-topped cockpit canopy was now dome-shaped, much to the relief of taller pilots. One particular irritant was the damage to pilots’ knuckles when operating the pump to raise the undercarriage; later Mark Is had a powered system.

  Control problems had also been encountered owing to the conventional practice of keeping the aileron light by using fabric covering. It was now discovered that this material tended to balloon out at speed and so the ailerons were given a metal skinning.

  An investigation into the need for low drag flush riveting of all the surface panels was also undertaken by gluing split-peas over the rivet heads to simulate the standard round-headed ones and by progressively removing them from different parts of the skinning. As a result, production Spitfires had standard rivets to the fuselage while the wing surfaces continued to be flush riveted.

  During the Battle of Britain, it was found that German bombers could often survive machine gun fire, thanks to their armour plating, and this led to the development of an alternative armament of two cannons and four machine guns. The Spitfires with the original eight-gun arrangement were thus designated ‘IA’ and the ones with the two cannons, ‘IB’. Unfortunately, the cannons had to be mounted on their sides in the Spitfire’s thin wing, which led to their jamming so frequently that the cannon version reverted to the eight machine gun version until the problem of stoppages could be solved.

  (For PR versions of the Mark I, see the photo-reconnaissance section which follows.)

  Mark II (921 Built)

  The Mark II was basically similar to the later production models of the Mark I; the essential change was the fitting of the more powerful Merlin XII engine with a Rotol constant speed propeller, giving an increase in speed of 15mph and an improved climb by nearly 500ft per minute. Some were available in time to take part in the later stages of the Battle of Britain.

  When the Mark II was withdrawn from front-line fighter squadrons, some were modified to operate in an air sea rescue role, by dropping dinghy and survival canisters to assist airmen until they were picked up by boat or seaplane. A small rack was also fitted to carry
smoke-marking bombs. This type was redesignated ‘ASR II’.

  Mark III

  The Mark III represented the first attempt to improve the type significantly, particularly by making the tailwheel retractable, clipping the wings and installing the Merlin XX with its two-stage supercharger. However, it was decided to give priority to upgrading the Hurricane and so the Mark III, which first flew in March 1940, never went beyond the prototype stage.

  Mark IV

  See the Mark XX and the photo-reconnaissance sections, below.

  Mark V (6,476 Built)

  The Mark V was designed as a stopgap aircraft in anticipation of a second Battle of Britain in 1941 and to counter the improved Messerschmitt Bf.109F which had greater speed and rate of climb than the Mark II. With the new Merlin 45, the Mark V now had a top speed of 376mph, an operating ceiling of 36,000ft, and was able to climb to 20,000ft in six minutes.

  This mark came out in three versions, now that the cannon jamming problem had been overcome. The VA retained the original eight machine gun layout; the VB had two 20mm cannon and four machine guns; and the VC had four cannons, a repositioned undercarriage, and could carry a 500lb bomb under the fuselage – thanks to the fitting of the stronger wing of the Mark III.

  As the strengthened ‘C’ wing also allowed for the ‘A’ and ‘B’ gun arrangement, it was known as the ‘universal’ wing and represented an important development, paving the way for the introduction of the ubiquitous Mark IX.

  There was also a float-equipped version, originally intended to operate from fjords, in response to the German invasion of Norway, and the Japanese entry into the war gave rise to a later consideration of the type, to be based where the terrain was unsuitable for the provision of airfields. Three Mark VB machines were specially equipped with floats, fittingly designed by Arthur Shirvall, who was responsible for those of the Supermarine Schneider Trophy winners. In view of its Schneider Trophy predecessors, it is not surprising that the test pilot found that ‘it was a most beautiful floatplane and all we had to do, predictably, was to increase the fin area to compensate for the float area ahead of the centre of gravity’. Thus, underfins, unique to the floatplane Spitfire, were fitted with a later addition of a fillet to the leading edge of the dorsal fin. These aircraft were sent in the end to Alexandria but never saw operational service.

  The Mark V was produced before America had joined the war and when Britain was heavily engaged in the Mediterranean and North African campaigns. It therefore appeared in larger numbers than any other Spitfire mark, remaining the main RAF fighter until the summer of 1942. The low-level (LF) Mark V continued in use into 1946.

  Mark VI (100 Built)

  The Mark VI signified the first serious move to improving the current ceiling of the Spitfire V. The necessary increase in wing area was achieved by the somewhat ad hoc expedient of adding pointed tips (which completely compromised the appearance of Mitchell’s original conception). The fitting of a pressurised cockpit also left much to be desired, as the hood had to be locked on before take-off and could not be opened in flight; and while it could be jettisoned in an emergency, the hood had to be completely removed for normal entrance and exit.

  This variant, equipped with a Merlin 47 engine and a four-blade propeller, was produced at the end of 1941, in response to the fear of Germany sending over high flying bombers, with a ceiling approaching 45,000ft. However, the German invasion of Russia caused these fears to recede and so this mark was not produced in large numbers.

  Mark VII (140 Built)

  The Mark VII represented, along with the Mark VIII, a significant step in the development of the Spitfire, thanks to the Merlin 61 with its two-stage, two speed supercharger. However, it was not produced immediately after Marks V and VI because substantial design changes were to be made – including the incorporation of fuel tanks in the wings, 11 gallons more fuel capacity in the fuselage and (finally) a retractable tailwheel. Also, the pressurised cockpit was now provided with a sliding hood, although with no hinged door in the side of the fuselage. The top speed was now 416mph.

  It began to appear in August 1942, and later aircraft were fitted with an extended fin and broader chord rudder. The HF version, with a Merlin 71, continued the use of the expanded span wing tips. However, the Mark IX was now being produced as another stopgap measure, and because of its improved performance no large production orders were placed for the Mark VII. Many of these variants were used for meteorological work.

  Mark VIII (1,658 Built)

  The Mark VIII, which appeared a year later, was the fourth most numerous variant, supplied with standard, clipped or extended wings for its many duties. Extra tanks in the wing roots now increased the fuel capacity to 124 gallons and a 250lb bomb could be carried under each wing. The Mark VIII was now tropicalised, without the bulky ‘beard’ air filter housing that had been fitted to the Mark V and it was used extensively in the Middle East, India, Burma and Australia.

  As with the Mark VII, the ailerons were modified and, with the Merlin 66 engine, resulted in an aircraft which was assessed by Jeffrey Quill: ‘I always thought the aeroplane which was the best from the pure text book handling point of view was the Mark VIII … With the standard wing-tip it was a really beautiful aeroplane’.

  Mark IX (5,653 Built)

  The Mark IX, like the Mark V, was planned as a short-term expedient, this time in response to the Focke-Wulf 190, which began to appear over northern France in late 1941, when it was found to out-climb, out-run and out-manoeuvre the Spitfire V.

  The Mark IX began to be produced in June 1942, and was essentially the earlier machine fitted with improved Merlins. The standard fighter had the Merlin 61 and 63, the low-altitude version had the Merlin 66, and the high-altitude aircraft the Merlin 70. These three versions were designated, respectively, F IX, LF IX, and HF IX.

  Lightened versions could now fly higher than the specially designed Mark VI, and one Mark IX is celebrated as engaging in the highest air battle of the Second World War – stripped of armour plating and with only two cannons, it pursued a Junkers Ju 86 bomber up to 43,000ft and damaged it before its guns jammed. This action took place, fittingly, over Southampton.

  The new mark’s top speed had now risen to 408mph and marked the point where the dedicated record-breaking S6B was overtaken in level flight by a fully equipped fighting machine. The new Merlins now required a four-bladed propeller to absorb the increased power, and the need for increased cooling resulted in symmetrical underwing radiator ducting for the first time.

  However, when the Mark IX was tested against a captured Fw.190 in July of the following year (Oberleutnant Armin Faber had conveniently landed it, a little lost, at RAF Pembrey Sands on 23 June 1942), the differences in performance were too small for comfort and the German’s rate of climb between 15,000 and 23,000ft was superior.

  Fortunately, Rolls-Royce were able to respond with the improved Merlin 66, and later, as with most aircraft after 1944, it was also fitted with a ‘teardrop’ canopy and lowered rear fuselage decking, giving it more speed and far better rearward visibility. Also, the supply of two 18 gallon fabric fuel cells in the wings and a 72 gallon tank in the rear fuselage produced an internal fuel capacity which was nearly twice that of the prototype Spitfire. With the addition of a 45 gallon drop tank, Jeffrey Quill flew a Mark IX from Salisbury Plain to the Moray Firth and back – the equivalent of East Anglia to Berlin and back – at less than 1,000ft.

  While the ‘C’ wing, with four machine guns and two cannons, was standard, an ‘E’ wing was introduced later and was equipped with the two cannons, together with two larger bore ½in Browning machine guns. Accordingly, two new designations were used: LF IXE and HF IXE.

  (There was also another floatplane conversion, in 1944, with the modified fin of the Mark V conversion, but no production orders materialised.)

  After the war, a 1941 proposal for a two-seat trainer version was revived and a Mark VIII Spitfire was converted by the expedient of moving the cockpi
t 13½ inches further forward, to allow for a separate instructor’s cockpit that was behind and above the front one. Twenty Mark IX aircraft were thus converted and sold abroad.

  The Mark IX, although intended as a stopgap type, was the second most numerous version.

  Marks X and XI

  See the photo-reconnaissance section, below.

  Griffon-engined Spitfires

  Rolls-Royce, before the outbreak of the war, had been developing a larger version of the Merlin engine, with a capacity of 36.7 litres instead of the 27 litres of its predecessor, to be known as the Griffon, and on 8 November 1939 N.E. Rowe of the Air Ministry suggested that it should be fitted to the Spitfire. Despite its much larger capacity, Rolls-Royce produced an engine which had a frontal area no more than 6 per cent greater and a length no more than 3in greater than its Merlin predecessors. Thus it became possible to adapt the Spitfire airframe to the enormously more powerful engine and to prolong the development of the Spitfire until the end of the war and after.