Supermarine Spitfire variants powered by early model Rolls-Royce Merlin engines mostly utilised single-speed, single-stage superchargers. The British Supermarine Spitfire was the only Allied fighter aircraft of the Second World War to fight in front line service from the beginnings of the conflict, in September 1939, through to the end in August 1945. Post-war, the Spitfire's service career continued into the 1950s.Price 1991, p. 10. The basic airframe proved to be extremely adaptable, capable of taking far more powerful engines and far greater loads than its original role as a short-range interceptor had called for. This would lead to 19 marks of Spitfire and 52 sub-variants being produced throughout the Second World War, and beyond.McKinstry 2007, p. 6. The many changes were made in order to fulfil Royal Air Force requirements and to successfully engage in combat with ever-improving enemy aircraft.McKinstry 2007, pp. 5–6. With the death of the original designer, Reginald J. Mitchell, in June 1937, all variants of the Spitfire were designed by his successor, Joseph Smith, and a team of engineers and draftsmen.McKinstry 2007, pp. 67–69, 75.
Wing types The Spitfires with the single-stage Merlin engines used five different wing types, Type A, B, C, D and E wings, which had the same dimensions and plan but different internal arrangements of armament and fuel tanks.Barbic 1996, pp. 165–167. All Mk Is, IIs, and Vs and their derivatives had small, rectangular undercarriage indicator pins which projected at an angle from the upper wing surfaces when the undercarriage legs were locked down, providing a positive mechanical indication that the landing gears were indeed down and locked, since the pilot could not see the landing gear for himself.Air Ministry 1940, p. 7.Air Ministry 1941, p. 7. These were a backup to the indicator lights on the instrument panel, in case the lights malfunctioned or failed. All of these variants used Dunlop AH2061 tyres, mounted on alloy mainwheels which had five openings. The fixed, castering tailwheels used Dunlop AH2184 tyres.Morgan and Shacklady 2000, pp. 74, 172.
Starting with the Mk V, some Spitfires had their rounded elliptical wingtips "clipped" outboard of the ailerons, and replaced by shorter, squared-off fairings to improve low-altitude performance and enhance the roll rate, one area where the Mk V fell badly behind the rival Fw 190. Although these "clipped-wing" aircraft are popularly known as "LF" versions due to the fact that they were designated "LF" (i.e. Spitfire LF Mk V), the "L" actually refers to the different versions of the Rolls-Royce Merlin engines used, which were optimised for low-altitude performance with "cropped" supercharger impellers (Merlin 45M, 50M or 55M). While many "LF" Spitfires were given "clipped" wings to accompany the new engine variants, a number still retained the original wingtips.Price 1999, p. 135.
A type The A-type was the original wing design, with eight .303 calibre Browning machine guns, with 300 rounds per gun (rpg). The basic structure of the wing was unchanged until the C type in 1942. The one major alteration made to this wing, soon after production started, was the incorporation of a heating system for the gun bays, to prevent the guns from freezing up at high altitudes. To achieve this, open structures, such as wing ribs, surrounding the gun-bays were closed off, forming closed bays for each gun. Ducting was added, which drew heated air from the engine radiator and transferred it into the now-closed weapon-bays. Underwing vents, covered by streamlined triangular blisters just inboard of the wingtips, extracted the air, creating a negative pressure differential, and caused more heated air to be drawn in, ensuring a steady supply of heated air without any need for a mechanical blower. To keep cold air from blowing in via the muzzle openings in the leading edge, they were sealed with red fabric adhesive tape by ground crew while loading the ammunition trays. The guns fired through the tape, so they were no longer sealed after combat. Towards the end of 1940, the fabric covered ailerons were replaced by ones covered in light alloy. The A-type wing was only compatible with the 8 × .303 Browning machine gun armament suite.
B type The B-type wing was structurally an A type modified to carry one 20 mm Hispano cannon per wing, which replaced the two inner .303 machine guns. The retractable underwing landing lamp was repositioned. The cannon was installed in the innermost machine gun bay, next to the wheelwell, while the second gun was deleted altogether. The area where the inner guns' ammunition trays had been located, outboard of the cannon bay, was converted into a compartment to accommodate the 60-round drum magazine for the cannon. The upper and lower wing skins now incorporated blisters to clear the large ammunition drum (the underwing blisters came in two different shapes). The remaining outer pairs of .303's were not changed at all. Only one armament suite could be fitted to a Spitfire with the B-type wing: Two 20 mm Hispano Mk II cannon, each with a 60-round drum, and four .303 caliber Browning machine guns in the outer positions, with 350 rpg. Alloy-covered ailerons were standardised on this wing type.
C type The "C-type" wing, or "Universal wing" was a more heavily modified wing, redesigned to reduce labour and manufacturing time, as well as to allow different armament configuration options to be fitted in a single, universal wing structure which could accommodate several different armament layouts without serious structural modification or alteration (a similar kind of wing was developed for the Hawker Hurricane, allowing anything from 8 to 10 .303 Brownings, to four 20mm Hispanos, to twin anti-tank 40mm Vickers "S" guns and a pair of .303's to be fitted into the same wing). Without modification, the "universal" wing could accept "A", "B", "C" or "E-type" armament configuration, although the "E" type wasn't used until later marks of Spitfire. The "B" type, 2-cannon, 4-Browning layout was by far the most commonly seen on "C" wing Spitfires, until the Mk IX and the "E" wing. Differences in the "C" wings include a second cannon-bay added directly alongside the first, and the #3 Browning gun being moved slightly outboard from ribs 13 to 14, making the two outer machine guns closer together than in the older "B" type wing.
Although officially any aircraft with a universal wing is a "C" variant regardless of their armament configuration (excepting "E" types), in later popular (and incorrect) usage, "C" variants with the same armament as the "B" type are often called "B" variants, even though it's technically a "C", and structurally it is different than the older "B" wing Spitfire. The new wing now had two inner gun bays able to accommodate cannon, although one bay was usually left empty. The outer two .303 machine guns were also moved closer together than in "A" or "B" type wings, which is one of the primary visual aids to distinguishing an original "B" type from later universal wing "B types", along with the second pair of cannon ports, which are visible even though typically no weapons are fitted in the second cannon bay. In this case, the empty port is usually closed off with a rubber plug. The cannon mounts were also moved further back in the wing, so the barrel fairings of the cannon are shorter than on an original "B" Spitfire.
Few, if any, "C" Spitfires were built with the eight .303 Browning "A" armament configuration, which was viewed as obsolete by the time the Mk V went into combat. The "C" type wing was the first which could accommodate an all-cannon armament of four 20mm Hispanos; thus the "C" designation is popularly reserved for four-cannon Spitfires (i.e. Mk Vc). The "C" type armament was also rarely used (although not as rare as the "A" armament), due to the extra weight of the added cannon eroding performance and handling. Many Spitfires originally fitted with the "C" armament had the extra cannon removed in the field to save weight. In theory, a "Universal" wing Spitfire could carry an armament of four 20mm cannon and four .303 Brownings, but this would be heavily loading the aircraft indeed, and thus was never used. (An "E" type wing, usually seen in Mk IX and later versions, was similar to the "C" armament, but used a pair of American .50 caliber Browning AN/M2 heavy machine guns in place of the inner pair of Hispanos, giving an armament of two Hispanos and two .50cal Brownings.)
The Universal wing also took into account the fact that the Hispano cannon had been converted to belt-feed, a move which allowed each cannon to carry double the ammunition load, or 120 rpg in ammunition trays instead of 60 rpg as in the original drum-fed cannon of the Mk Vb. This also eliminated the need for under- and over-wing blisters to accommodate the large-diameter ammunition drum, instead only requiring a small blister to cover the electrical "Chattellerault" feed mechanism. The undercarriage mountings were redesigned and the undercarriage doors were bowed in cross section allowing the legs to sit lower in the wells, eliminating the upper-wing blisters over the wheel wells and landing gear pivot points. The stronger undercarriage legs were angled 2 inches (5.08 cm) forward when extended, making the Spitfire more stable on the ground and reducing the likelihood of the aircraft tipping onto its nose. In addition the retractable landing lights were no longer fitted. Structural hard-points were added outboard of the wheel-wells, allowing racks to be fitted under each wing which were capable of carrying one 250 lb (113 kg) bomb each.
D type The D type was modified for use on unarmed, long-range photo-reconnaissance versions of the Spitfire, such as the PR Mk Id (PR versions described in more detail below), with weapon bays left empty, and the relatively heavy-skinned leading edge wing structure ahead of the main spar converted to integral auxiliary fuel tanks capable of carrying Imperial gallons (300 L) in addition to that stored in the fuselage tanks ahead of the cockpit.Price 1999, p. 100. The filler caps were well outboard, between ribs 19 and 20 on the leading edge, because the wing tips were higher than the rest of the wing due to dihedral, and thus the only place a fuel filler cap could be located. An expansion and overflow vent was fitted to protect the tanks from bursting as the fuel vaporised and expanded from the hot sun beating directly on the thin metal wing skin (which doubled as the walls of the tank). This vent was fitted further outboard, between ribs 20 and 21, in the highest space in the tank, where vapor would naturally collect.Smallwood 1996, pp. 16–18.
Single-stage Merlin engine variants
Mark numbers, type numbers The Mark numbers did not necessarily indicate a chronological order; for example, the Mk IX was a stopgap measure brought into production before the Mks VII and VIII. In addition, some Spitfires of one mark or variant may have been modified to another; for example, several of the first Mk Vbs were converted from Mk Ibs; the first Mk IXs were originally Mk Vcs, converted, in some instances, by Rolls-Royce at their Hucknall facility.
Up until the end of 1942, the RAF always used Roman numerals for mark numbers. 1943–48 was a transition period during which new aircraft entering service were given Arabic numerals for mark numbers but older aircraft retained their Roman numerals. From 1948 onwards, Arabic numerals were used exclusively. This article adopts the convention of using Roman numerals for the Mks I-XVI and Arabic numerals for the Mks 17–24. Type numbers (e.g., Type 361) are the drawing board design numbers allocated by Supermarine.Morgan and Shacklady 2000, p. 593. Retrieved: 10 February 2008.
Prototype K5054 (Supermarine Type 300)