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The Supermarine Spitfire was one of the best fighter aircraft of its time. Unlike its counterpart, the Hawker Hurricane, it appeared to have immense room for future improvement. This would lead to 24 marks of Spitfire being produced throughout the Second World War in continuing efforts to keep up with the Luftwaffe and Royal Air Force requirements.
In total there were 24 marks of Spitfire and many sub-variants within each mark. This article presents a brief history of the Spitfire through all its variants. It should be noted that the numbering of the variants does not necessarily imply a chronological order; for example, the Mk. IX was a stop gap measure brought into production before the marks VII and VIII to address the urgent needs of the air force which could not wait for the more ambitious designs of the preceding marks. It is sometimes difficult to identify the variant of an individual Spitfire as many aircraft were built as one variant and later modified to be like another variant.
There is an apparent discrepancy in Spitfire numbering schemes in that sometimes Roman numerals are used and sometimes Arabic numerals are used. This is down to changes in RAF numbering schemes. Up until the end of 1942 the RAF would always use Roman numerals for mark numbers. From 1943 to 1948 was a transition period during which new aircraft entering service were given Arabic 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 marks I through XVI and Arabic numerals for the marks 17 through 24.
Wing types
Spitfire used five different wing types, designated "A" through "E," which overall had the same dimensions but different internal arrangements of armament and fuel tanks.
- A
- B
- Two 20 mm Hispano HS.404 cannons with 60 rounds/gun
- Four .303 inch Browning machine guns with 350 rounds/gun
- C - "universal wing" allowing armament consisting of either "A" type, "B" type, or four 20 mm Hispano cannon with 120 rounds each. Main landing gear was strengthened and moved 2 inches (5 cm) forward to reduce the tendency to "nose over" on landing. There was added provision for a 250 lb (113 kg) bomb under each wing. In practice, most aircraft carried:
- Two 20 mm Hispano HS.404 cannons with 120 rounds/gun
- Four .303 inch Browning machine guns with 350 rounds/gun
- D - long-range wing for reconnaissance versions with armament replaced by fuel tanks
- E - universal wing allowing two weapon fits
- Two 20 mm Hispano cannon with 280 rounds/gun
- 2x .50 inch (12.7 mm) Browning M2 machine guns, 500 rounds/gun
- or
- Four 20 mm Hispano cannon with 280 rounds/gun
Beginning with the Mk.21, the Spitfire had a new wing design armed with four 20 mm Hispano cannons.
Some Spitfires starting with the Mk.V had wingtips removed to improve low-altitude performance. These aircraft are sometimes incorrectly referred to as "LF" versions. The "LF" designation referred to the low-altitude version of the Rolls-Royce Merlin engine and while many "LF" Spitfires indeed had the "clipped" wings, a number did not.
Mk. I
Realizing that the initial order for 310 Spitfires was but the first of what was likely to be a long production run Vickers started construction of a huge new factory in Castle Bromwich to build Spitfires (in addition to their existing line in Woolston).
In 1938 their forward thinking paid off when the Air Ministry placed an order for an additional 1000 Spitfires from the new factory. It was followed in 1939 by an order for another 200 from the Woolston factory and, only a few months later, another 450. This brought the total to 2,160, making it one of the largest buys in history.
The Woolston line started delivering the Mk. I Spitfire in late 1937 with front-line service commencing in August 1938. The Mk. I was powered by the 1,030 hp (768 kW) Merlin Mk. II engine driving a two-blade wooden fixed pitch propeller. Only 77 had been completed before a three-bladed, two-position, metal propeller was substituted which greatly improved performance, along with bulged side panels to the canopy which improved the pilot's view behind. With these improvements the aircraft became the Mk. Ia.
Only a few units were equipped with the Spitfire by the opening of the war and the Hurricane would be the only fighter to see action in mainland Europe. By the opening of the Battle of Britain in July 1940, however, supplies had improved to the point where 19 squadrons were flying Spitfires, while another 27 were equipped with Hurricanes. By the end of the battle in October 565 Hurricanes and 352 Spitfires had been lost.
At this point the factories were at full production and the losses could easily be replaced (not so the pilots however). Production of the Hurricane as a front-line fighter was ramped down.
19 Squadron received several cannon-armed Spitfires known as the Mk. IB during the battle. The cannon's hitting power was recognised but jamming was a serious problem. Further cannon-armed Spitfires were nevertheless issued to 92 Squadron and it was eventually realised that the best mix was an aircraft with two cannon and four machine guns.
In all 1,583 of the original 2,160 Mk. Is were delivered before production switched to the updated Mk. II.
PR Mk. I Types - Early Reconnaissance Versions
Before the Second World War the conventional wisdom was to use converted bomber types for airborne photo reconnaissance. These bombers retained their defensive armament, which was vital since they were unable to avoid interception.
In 1939 Flying Officer Maurice Longbottom was among the first to suggest that airborne reconnaissance may be a task better suited to fast, small aircraft which would use their speed and high service ceiling to avoid detection and interception. He proposed the use of Spitfires with the armaments and radios removed and replaced with extra fuel and cameras.
As a result Spitfires were used for reconnaissance throughout the war. The original reconnaissance models were based on the Mk. I as follows:
- The PR Mk. IA had two F .24 cameras with 5 inch focal length lenses which could photograph a rectangular area below the aircraft. They were installed in the wing space vacated by the inboard guns and their ammunition containers as a stop-gap measure. It had been envisaged that much larger cameras would be installed in the fuselage immediately behind the pilot, but at the time the engineering capacity required to make this conversion to the Mk. I was not available.
- In the PR Mk. IB the camera lenses were upgraded to an 8 inch focal length, giving images up to a third larger in scale. It also had an extra fuel tank in the rear fuselage and was designated a medium range aircraft.
- The PR Mk. IC carried more fuel still and was the first photo reconnaissance aircraft to reach as far as Kiel. The extra fuel was carried in a tank behind the pilot and a blister tank under the port wing, which was counterbalanced by a camera installation on the starboard wing.
- The PR Mk. ID was the backbone of RAF photographic reconnaissance in 1941 and 1942. It carried so much fuel that it was nicknamed "the bowser". Early production models were very badly balanced and consequently difficult to fly. Despite these difficulties the type quickly proved its worth, photographing such long distance targets as Stettin, Marseilles, Trondheim and Toulon. Later models were better balanced, had the more powerful Merlin 45 engine as used by the Mk. V, and had heated cabins which were a great comfort to pilots on such long flights. A total of 229 Type Ds were built and the type was later re-designated the PR Mk. IV.
- The PR Mk. IE was built to address a requirement for oblique close-ups as opposed to high altitude vertical pictures. It is believed that only one Type E was built, N3117. It carried a single F .24 camera under each wing looking downwards at about 15 degrees below the horizontal. It proved most useful as it was able to photograph targets under weather conditions that would make high altitude photography impossible.
- The PR Mk. IF was a "super-long-range" version which entered service in July 1940. It was a useful enough improvement that nearly all existing Type Bs and Type Cs were eventually converted to the Type F standard. Operating from East Anglia it was just able to reach, photograph, and return from Berlin.
- The PR Mk. IG performed a similar role as the Type E before it. However the Type G carried the normal 8 gun fighter armament as otherwise the aircraft would have been very vulnerable to enemy fighters. It was designed to photograph its targets from just below the cloud base, wherever that happened to be.
Mk. II
With the end of the Battle of Britain the RAF gained some breathing room over the winter of 1940/41. They took this opportunity to work several additions into the production lines, creating the Type 329 Spitfire Mk. II.
Chief among the changes was the upgraded 1,175 hp (876 kW) Merlin XII engine. The added power boosted top speed by 15 knots (28 km/h), and improved climb rate somewhat. The climb rate would have been improved further if not for the addition of 75 lb (34 kg) of armour plating around the pilot.
The Mk. II was produced both in the IIA eight-gun and IIB cannon armed versions. Deliveries were very rapid, and they quickly replaced all remaining Mk. Is in service, which were then sent to training conversion units. The entire RAF had re-equipped with the new version by April 1941, and a total of 920 were built.
Mk. III and Mk. IV
With the Mk. II proving a match for the Luftwaffe fighters, the RAF asked Supermarine for much more ambitious upgrades to the basic design. Two proposals resulted.
The Mk. III was an airframe improvement, strengthening the design overall, adding additional covers and fillets over various openings, and the fitting of a retractable tail wheel. Combined with the improved Merlin XX engine, it was expected that the Mk. III would gain considerable airspeed and be able to fly at just over 400 knots (740 km/h).
The Mk. IV was much more radical. Although it was based on a similar airframe to the Mk. III, it also included the new Rolls-Royce Griffon engine with over 1,500 hp (1,100 kW) available. This extra power not only boosted the speed to over 420 knots (780 km/h), but allowed for a much heavier six-cannon armament. The Mk. IV appeared so promising that Mk. III was abandoned in its favour. Plans were made to have the new design reaching squadron service in October, becoming the standard RAF fighter by the start of 1942, but it was not to be. It turned out that the Mk. XII would be the first Griffon powered Spitfire to enter service.
As the Rolls-Royce Griffon began to replace the famous Merlin and speeds went up, it was discovered just how advanced the design of the Spitfire's wings were: with a safe Mach number of 0.83 and a maximum Mach number of 0.86, the Spitfire's wing was able to reach higher speeds without Mach-induced flutter than many much newer designs.
Mk. V
The Mk.V variant would be the most numerous of the type. Over six thousand were built.
Late in 1940 the Mk. II started meeting a new German aircraft in combat. Essentially a cleaned up version of the Messerschmitt Bf 109E that Spitfires and Hurricanes had bested the year before in the Battle of Britain, the new 109F or Franz (Friedrich) model was superior to the Mk. II Spitfire in many respects. Not only was it able to outperform the Mk. II Spitfire in speed and rate of climb, it also was able to out-turn it above about 18,000ft – something previously unheard of.
At this point the Mk. IV was not going to be ready in time to counter the new Franz. Meanwhile the Griffon was running into very serious production problems and it wasn't clear if it would ever be ready. As an emergency stop-gap measure was needed as soon as possible: this was the Mk. V.
The Mk. V was nothing more than a Mk. II with the newer Merlin 45 series engine. This engine delivered slightly more takeoff power at 1,440 hp (1,074 kW), but greatly increased the power available at higher altitudes due to a new single-speed single-stage supercharger design. While it was no Mk. IV, the Mk. V was able to hold its own with the 109Fs it was meeting.
Timing played an important part, as over the winter a serious problem in the tail structure of the Franz appeared, and all production was halted. The problem wasn't solved until the early spring, by which time the Mk. V had already started deliveries.
It would turn out that the problems with the Mk. IV's Griffon engine were as bad as some suspected, and it would be another two years before versions with that engine would enter service.
About 250 Mk.VCs were shipped to Australia for interception duties against Japanese air raids, by No. 1 Wing Royal Australian Air Force, which was based in Darwin from early 1943 onwards. The Mk.VCs received by the RAAF proved unreliable and — initially at least — had a relatively high loss rate. This was due to several factors, including pilot inexperience, engine overspeed due to the loss of oil from the propellor speed reduction unit (a problem resolved bythe use of a heavier grade of oil),* and the practice of draining glycol coolant before shipment, resulting in internal corrosion.
In total, 94 Mk. VAs (eight-gun), 3,923 Mk. VBs (cannon) and 2,447 Mk.VCs were built.
Mk. VI
At the time that the Mk. V was placed in production there were growing fears that the Luftwaffe were about to start mass producing very high flying bombers such as the Junkers Ju 86, which could fly above the reach of most fighters of the time. It was decided that a new Spitfire variant would be required with improved high altitude performance.
The Mk. VI therefore contained two main refinements. For increased power at high altitudes, where the atmosphere is much thinner, it had a four bladed propeller. To counter the physiological problems encountered by pilots at high altitudes it had a pressurised cabin. It should be noted that the cabin is not like the fully pressurised cabin of a modern air liner; the pressure differential was only 2 pounds per square inch. The effect is to make 37,000 ft seem like 28,000 ft to the pilot, who would still have to wear his oxygen mask.
Despite these efforts the Mk. VI did not handle well at high altitude and was unpopular with operational squadrons.
Mk. VII
Like the Mk. VI, the Mk. VII was a high altitude pressurised variant. However it had a number of refinements. It had a new engine, the two-stage Merlin 61, and the airframe was strengthened accordingly to carry the extra burden. It carried extra fuel in two small wings tanks. Most dramatically, Mk. VIIs began to be fitted with the "Lobelle" type hood which opened by sliding backwards, as on non-pressurised versions of the Spitfire. This was a big improvement on the clamp down cockpit of the Mk. VI.
There had been some instances of earlier models breaking up in the air in steep high speed dives. It was thought that this may have been due to aileron flutter. To address this the Mk. VII had its ailerons reduced in span by 8½ inches to reduce the length of aileron outboard from its hinges. It was later concluded that the break-ups were actually due to longitudinal-instability, resulting from incorrect loading of the aircraft on the squadrons causing the centre of gravity to be outside the safe limits.
In total 140 Mk. VIIs were built, the last of which used the Merlin 71 engine and reportedly had superb high altitude performance with a service ceiling of 45,100 feet. For instance, French ace Pierre Clostermann recalls in his book The Big Show the successful interception of a reconnaissance Messerschmitt 109 with a Mk. VII from No. 602 Squadron RAF at 40,000 feet over the British Home Fleet's base at Scapa Flow in early 1944.
Mk. VIII
The Mk. VIII was an adaptation of the Mk. VII without the pressurised cabin, and was intended to become the main production model of the Spitfire. In fact, by June 1943, it had all but replaced the Mk. IX.
Apart from the lack of pressurization the Mk. VIII differed from the Mk. VII in few respects. Some early production models had extended wing spans but the majority did not. There were two sub-variants for low altitude and high altitude which were powered respectively by the Merlin 66 and Merlin 70 engines. The Spitfire Mk.VIII also featured a retractable tail wheel unit and a wide-chord rudder, sometimes nick-named the "Griffon rudder" due to the fact that this large control surface was often applied on Spitfires possessing Griffon engines causing more torque which could most simply be countered by this larger tail surface. Internally, the aircraft changed with the addition of wing-root fuel tanks allowing the fighter to fly for a maximum distance of 1,180 miles with a full internal and external fuel load.
Armament consisted of the regular "C wing" type standard. This armament type could include either four 20 mm cannon with 120 rounds per gun (rpg) or two 20 mm cannon with 120 rpg along with 4 X 0.303" machine guns with 350 rpg or 8 X 0.303" guns with 350 rpg. An external bomb load could accommodate 1,000 pounds.
A Mk. VIII was used to experiment with the use of a new cut-back rear fuselage and a "tear-drop" canopy. This was intended to aid pilot visibility; many Spitfire pilots who were shot down were done so by enemies who approached in the aircraft's blind spot, and so the Spitfire pilots never saw their killers. In trials the new hood design was found to be an enormous improvement to all round visibility, although there were some problems reported opening and closing the hood when the aircraft was travelling at speed and the hood was thought to be too claustrophobic.
This variant served mainly with in the Pacific Theatre of Operations beginning in 1943 with the Royal Australian Air Force. As an air force aircraft, its range left much to be desired as the aircraft could not be taken over long distances over sea as it could quickly run out of fuel causing the need for the pilot to ditch the aircraft over the ocean. Additionally - like most other air force fighters - this plane, if damaged and leaking fuel, could not retreat to a nearby carrier ship, but would have to fly all the way back to its base, further shortening its safe operational range. This caused the Spitfire Mk.VIII to have a higher loss rate in battle than other Allied fighters operating in the Pacific, which, for the most part, were Navy planes and could safely land on ships.
Nevertheless, the aircraft was a superb dogfighter and could outfly and outgun nearly every Japanese fighter it encountered. It was a great defense instrument and arguably outclassed all other planes operating in the Pacific as a defender of Allied territory. It was up to 50 mph faster than the commonly-fought Japanese Naval fighters and it was much more maneuverable than its American counterparts. It could even be seen as an integration between the American need for speed and the Japanese yearning for maneuverability. In the Mediterranean Theatre in Italy, the Mk.VIII fought superbly with the United States Army Air Force (USAAF), currently the United States Air Force. The 31st Fighter Group operated the fighter for some time until, in March of 1944, they had their aircraft replaced by - according to many 31st FG members - the "inferior" P-51 Mustang, needed for the growing necessity of long-range escort fighters.
Over 1,200 examples were pushed into service, resulting in the Mk.VIII being the third most numerous serving variant after the Mk.IX and Mk.V.
Mk. IX
In the early months of 1942 there was much pressure to get Spitfires into production using the new two-stage supercharged Merlin 61 engine. Although the Mk. VII and Mk. VIII variants were to use the new engine, they were not yet ready for production. To solve this problem the Mk. V was fitted with a Merlin 61 engine and the Mk. IX was born. Although the Mk. IX's airframe did not have the strength improvements of the Mk. VII and VIII that were really needed for the more powerful engine, the Mk. IX still proved to have vastly improved performance over the Mk. V.
Towards the end of September 1942 the Luftwaffe began launching high level bombing raids against England. Junkers Ju 86 R bombers flying at 40,000 feet were able to bomb England without impediment; the Spitfire Mk. VIs that tried to intercept them were unable to reach them. To counter the threat a pair of Spitfire IXs were stripped of everything not required for the role of high level interception, lightening them by 450 pounds each. On September 12, 1942 one of the aircraft successfully intercepted a Ju 86R above Southampton at 41,000 ft. The ensuing battle went up to 43,000 ft and was the highest air battle of the war. However the Spitfire did not perform well enough at that altitude to be decisive; whenever the pilot had a shot lined up it would slew and fall out of the sky. The bomber escaped safely with just one hit to its port wing, but having proven to be vulnerable to the RAF at high altitudes the Luftwaffe launched no further high altitude attacks against England.
In the summer 1944 several major improvements were made to Mk. IXs coming off the production line:
- They were fitted with the Mark II Gyro Gunsight. This gunsight calculated the correct angle of deflection to use when leading the target. Its introduction doubled the effectiveness of their gunnery and was a major factor in Allied air superiority.
- The E Type wing was introduced. It removed the .303 machine guns mounted in the outer wings as most aircraft at that time had armour impenetrable by .303 bullets. The 20 mm Hispano cannon were moved outboard and the more effective .50 calibre Browning heavy machine gun was introduced. The improved armament was more effective for both air-to-air engagements and air-to-ground attacks.
- As a result of its over-powerful engine and four bladed propeller, the Mk. IX had a tendency to swing to one side during takeoff. This was solved by fitting a larger rudder, giving the pilot better control during takeoff.
- The cut-back rear fuselage and bubble canopy, prototyped on a Spitfire Mk. VIII, was incorporated into the model.
- Extra internal fuel tanks were fitted, as the Spitfire had an embarrassingly short radius of action when operating over enemy territory. External wing tanks had been used as a temporary measure, but they increased drag and had to be jettisoned before entering combat.
The Mk. IX was the one of the most numerous variant of Spitfire produced. In total more than five thousand were built.
Mk. X and Mk. XI
The Mk. XI was a reconnaissance aircraft based on a combination of features from the marks VII, VIII and IX. The cameras were installed in the fuselage behind the cockpit and comprised of two vertical cameras and sometimes an oblique one. The first Mk. XIs were built in November 1942 and lasted until 1944 when they were phased out in favour of the Mk. 19.
The Spitfire Mk. X followed the Mk. XI into production and was nearly identical. It had a pressurised cabin and a Lobelle sliding hood, and gave similar performance. Only sixteen Mk. Xs were made, which saw limited service for high altitude reconnaissance.
Mk. XII
The Mk. XII was the first Spitfire powered by a Griffon engine to go into service. The first production models started appearing in October 1942 and in total two RAF squadrons were equipped with the model. The Griffon engine gave the aircraft superb low and medium level performance. In fact at low altitude it was one of the fastest aircraft in the world; in one speed trial a prototype Mk. XII (DP845) - shown left - raced ahead of a Hawker Typhoon and a captured Focke-Wulf Fw 190 to the amazement of the dignitaries present. However pilots found it difficult to exploit this advantage in combat as German pilots were reluctant to be drawn into dog fights with Spitfires of any type below 20,000 feet. The Mk. XIIs speed advantage was only really useful near the end of its front line service in Summer 1944, in which it shot down a respectable number of V-1 Flying Bombs. The Mk. XII variant was retired in September 1944.
PR Mk. XIII
The PR Mk. XIII was an improvement on the earlier PR Type G with the same camera system but a new engine, the Merlin 32, which was specially rated for low-altitude flight. It carried a light armament of four .303 Browning machine guns. The first prototype Mk. XIII was tested in March 1943.
Twenty six Mk. XIIIs were converted from either PR Type G, Mk. II or Mk. Vs. They were used for low level reconnaissance in preparation for the Normandy landings.
Mk. XIV
The first Griffon powered Spitfires suffered poor high altitude performance due to having only a single stage supercharged engine. By 1943 Rolls-Royce engineers had put together a new Griffon engine - the 61 series - with a two-stage supercharger. In the end it was a slightly modified engine, the 65 series, which was used in the Mk. XIV. The resulting aircraft was as great an improvement over the Mk. IX as the Mk. IX had been over the Mk. V. Although initially based on the Mk. VIII airframe, common improvements made in aircraft produced later included the cut-back fuselage and tear-drop canopies, and the E-Type wing with improved armament.
The first test of the aircraft was in intercepting V1 flying bombs, and the Mk. XIV was the most successful of all Spitfire marks in this role. Later it was used by the 2nd Tactical Air Force as their main high altitude air superiority fighter in northern Europe. In total 957 Mk. XIVs were built. After the war second hand Mk. XIVs were exported to a number of foreign air forces; 132 went to the Royal Belgian Air Force, 70 went to the Royal Indian Air Force and 30 of its reconnaissance variant went to the Royal Thai Air Force.
Mk. XV and Mk. 17
The XV and 17 marks were reserved for the naval version, the Seafire, in an effort to reconcile the Spitfire numbering scheme with that of the Seafire.
Mk. XVI
The Mk. XVI was identical to the Mk. IX in nearly all respects except for the engine, a Merlin 266. The Merlin 266 was a low-altitude version of the Merlin 66 and was built under license in the USA by the Packard Motor Company. All Mk.XVI aircraft produced were of the Low-Altitude Fighter (LF) variety. All production Mk.XVI's had clipped wings for low altitude work.
Mk. 18
The Mk. 18 was a refinement of the Mk. XIV. It was identical in most respects including engine (the Griffon 65) and cockpit enhancements, but it carried extra fuel and had a revised, stronger wing structure. Its handling was also nearly identical and so it was not put through any performance tests. Like the Mk. XIV before it there were fighter and fighter reconnaissance variants built.
The Mk. 18 missed the war. It was built up until early 1946 but it was not until January 1947 that an RAF squadron, No. 60 Squadron RAF which operated from RAF Seletar, Singapore, was re-equipped with the variant. Later other squadrons in the Far East and Middle East would receive them. Some 300 Mk. 18s were built but they saw little action apart from some involvement against guerillas in the Malayan Emergency. The Royal Indian Air Force purchased 20 ex-RAF Mk. 18s in 1947.
Mk. 19
The Mk. 19 was the last and greatest photographic reconnaissance variant of Spitfire. It combined features of the Mk. XI with the Griffon engine of the Mk. XIV. After the first 25 were produced later aircraft were also fitted with the pressurised cabin of the Mk. X and the fuel capacity was increased to 256 gallons, three and a half times that of the original Spitfire.
The first Mk. 19s entered service in May 1944 and by the end of the war the type had virtually replaced the earlier Mk. XI. A total of 225 were built with production ceasing in early 1946, but they were used in front-line RAF service until April 1954. In fact the last time a Mk. 19 was used to perform an operational act was in 1963 when one was used in battle trials against an English Electric Lightning to determine how best a Lightning should engage piston engined aircraft. This information was needed in case RAF Lightnings might have to engage P-51 Mustangs in the Indonesian conflict of the time.
Mk. XX
Only two handful of aircraft were ever designated Mk. XX and they were both prototypes for other marks. The first was DP845. Initially a Mk. IV, it was re-designated a Mk. XX shortly after its maiden flight on 27 November 1941 and was later refitted to be the prototype Mk. XII. The Mk. IV designation was taken over by the PR Type D.
The second Mk. 20, DP851, initially had a Griffon II engine and made its first flight in August 1942. In December it was refitted with a Griffon 61 and re-designated as a Mk. 21 initial prototype.
Mk. 21
In early 1942 it was evident that Spitfires powered by the new two-stage supercharged Griffon 61 engine would need a much stronger airframe, in particular with stiffer wings. The proposed new design was designated the Mk. 21. However its initial design had a number of flaws that caused considerable damage to the otherwise excellent Spitfire reputation.
Aside from the more powerful engine the Mk. 21 had several notable features:
- The propeller was changed to a five-bladed propeller with a diameter 7 inches greater than that fitted to the Mk. XIV.
- To ensure sufficient ground clearance for the new propeller the undercarriage legs were lengthened by 4½ inches. To improve handling on the ground the undercarriage legs were placed 7¾ inches further apart than before. These modifications presented a problem to the designers because the larger undercarriage did not have enough space in which to retract. They solved this problem with a system of levers that compressed the undercarriage legs by about 8 inches as they retracted, and extended the legs again when they were lowered.
- The armament was standardised to four cannon and no machine guns.
This report caused serious concern for Supermarine as their factory at Castle Bromwich had already been converted to produce Mk. 21s and more were coming off the production lines every day. Luckily the most serious problems were easily solved by changing the gearing to the trim tabs and other subtle control modifications, and the aircraft was cleared for instrument flying and low level flying in trials in March 1945.
It was January 1945 before Spitfire 21s became operational. They had little opportunity to engage the enemy before the war ended, but scored a rare success on 26 April 1945 when two Spitfire Mk. 21s shot up and claimed sunk a German midget submarine which they caught on the surface. With the end of the war most orders for the Mk. 21 were cancelled. Only 120 were completed.
Mk. 22
The Mk. 22 was identical to the Mk. 21 in all respects except for the fitting of a cut-back rear fuselage and tear-drop canopy. In previous marks these changes had not warranted a new mark designation. A total of 272 Mk. 22s were built.
The Mk. 22 was used by only one regular RAF unit, No. 73 Squadron RAF in the Middle East. However twelve squadrons of the Royal Auxiliary Air Force used the variant and continued to do so until March 1951.
Mk. 23
The Mk. 23 was to be a Mk. 22 incorporating a revised wing design originally created for the Supermarine Spiteful. However when trialled on a Mk. VIII the new wing gave less than perfect handling characteristics and so the Mk. 23 was never built.
Mk. 24
The final Spitfire variant, the Mk. 24 was similar to the Mk. 22. It carried extra fuel, and had wing fittings for rocket projectiles. Some were built with shorter-barrelled Mark V Hispano cannon.
A total of 81 Mk. 24s were completed, 27 of which were conversions from Mk. 22s. The last Mk. 24 to be built was delivered in February 1948. They were used by only one RAF squadron, No. 80 Squadron RAF, until 1952. Some of the squadron's aircraft went to the Hong Kong Auxiliary Air Force where they were operated until 1955.
Comparison of Spitfire variants
| Supermarine Spitfire | ||||
|---|---|---|---|---|
| Mk.Ia | Mk.Vb | Mk.IXe LF | Mk.XIVe | |
| Wingspan | 36 ft 10 in (11.23 m) | 36 ft 10 in (11.23 m) | 36 ft 10 in (11.23 m) | 36 ft 10 in (11.23 m) |
| Wing area | 242.1 ft² (22.5 m²) | 242.1 ft² (22.5 m²) | 242.1 ft² (22.5 m²) | 242.1 ft² (22.5 m²) |
| Length | 29 ft 11 in (9.12 m) | 29 ft 11 in (9.12 m) | 31 ft 1 in (9.47 m) | 32 ft 8 in (9.96 m) |
| Height | 9 ft 10 in (3.02 m) | 11 ft 5 in (3.48 m) | 12 ft 8 in (3.86 m) | 12 ft 9 in (3.89 m) |
| Empty weight | 4,975 lb (2,257 kg) | 5,090 lb (2,309 kg) | 4,963 lb (2,251 kg) | 6,688 lb (3,034 kg) |
| Loaded weight | 6,186 lb (2,806 kg) | 6,770 lb (3,071 kg) | 7,370 lb (3,343 kg) | 10,258 lb (4,653 kg) |
| Engine | Rolls-Royce Merlin III | Rolls-Royce Merlin 45 | Rolls-Royce Merlin 66 | Rolls-Royce Griffon 65 |
| Power | 1,030 hp (770 kW) | 1,470 hp (1,096 kW) | 1,575 hp (1,175 kW) | 2,050 hp (1,530 kW) |
| Maximum speed | 363 mph (582 km/h) | 378 mph (605 km/h) | 401 mph (642 km/h) at altitude | 450 mph (721 km/h) |
| Rate of climb | 2,530 ft/min (12.9 m/s) | 2,665 ft/min (13.5 m/s) | 4,560 ft/min (23.2 m/s) | 4,580 ft/min (23.3 m/s) |
| Service ceiling | 32,000 ft (9,750 m) | 35,000 ft (11,300 m) | 41,500 ft (12,650 m) | 44,500 ft (13,560 m) |
| Wing loading | 26 lb/ft² (125 kg/m²) | 28 lb/ft² (137 kg/m²) | 30 lb/ft² (149 kg/m²) | 42 lb/ft² (207 kg/m²) |
| Power/mass | 0.17 hp/lb (0.28 kW/kg) | 0.22 hp/lb (0.36 kW/kg) | 0.21 hp/lb (0.35 kW/kg) | 0.20 hp/lb (0.33 kW/kg) |
| Combat range | 425 mi (680 km) on internal fuel | 470 mi (760 km) on internal fuel | 440 mi (700 km) on internal fuel | 460 mi (740 km) on internal fuel |
| Ferry range | 1,140 mi (1,835 km) | 855 mi (1,375 km) | ||
| Armament | ||||
British fighter aircraft 1930-1939 | Lists of aircraft variants
"Supermarine Spitfire variants"