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Supermarine Spitfire (late Merlin powered variants)

The British Supermarine Spitfire was facing several challenges by mid-1942. The debut of the formidable Focke-Wulf Fw 190 in late 1941 had caused real problems for RAF fighter squadrons flying the latest Spitfire Mk Vb. Fortunately Rolls-Royce engineers were already working on a new version of the Merlin which would take the Spitfire to new levels of performance. The Rolls-Royce Griffon engine was promising even greater performance increases. The Supermarine design team under Joseph Smith would succeed in keeping the Spitfire updated and improved through to the end of the Second World War and beyond.

This article presents a brief history of the Spitfire through its Two-Stage Rolls-Royce Merlin and also in its Griffon engine variants. Experimental Spitfires and drawing board projects derived from the basic design are also described.

Wing types

The Majority of the Spitfires from the Mk VIII on used four basic wing types, C through to E and a new wing designed for the Spitfire Mk 21. Apart from early Mk IXs converted from Vc airframes, the undercarriage indicator pins described in part one were not used on any of the later Spitfires.

C type

"Universal wing". The wing was structurally modified to reduce labour and manufacturing time plus allowing mixed armament options; A type, B type, or four 20 mm Hispano cannon.

  • 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 raked 2 inches (5.08 cm) forward, making the Spitfire more stable on the ground and reducing the likelihood of the aircraft tipping onto its nose.
  • The Hispano Mk IIs fitted in all cases were now belt fed from box magazines allowing for 120 rounds each (the "Chattellerault" system). The fairings over the Hispano barrels were shorter and there was usually a short rubber stub covering the outer cannon port.
  • The redesigned upper wing gun bay doors incorporated blisters to clear the cannon feed motors, and the lower wings no longer had the gun bay heating vents outboard of the gunbays.
  • The inner machine gun bays were moved outboard to between ribs 13 and 14.
  • The retractable landing lights were no longer fitted.
  • There were strong-points added outboard of the wheel-wells capable of taking a 250 lb (113 kg) bomb under each wing.
  • Several versions of the Spitfire, starting with the H.F Mk VIIs had extra 14.5 imp gallon integral fuel tanks added to the wing leading edges between the wing-root and the inboard cannon bay.

D Type

E type

Structurally unchanged from the C wing. The outer machine gun ports were eliminated, although the outer mg bays were retained and their access doors were devoid of empty shell case ports and shell deflectors. The inner gun bays allowed for two weapon fits
* Two 20 mm Hispano Mk II cannon with 120 rounds/gun in the outer bays.
* 2x .50 cal Browning M2 machine guns, with 250 rpg in the inner bays. Several C wing Spitfire L.F IXs of 485(NZ) Sqn were converted to carry the Hispanos and .50 Brownings just before D-Day.
or
* Four 20 mm Hispano cannon with 120 rounds/gun

Many Spitfires had their rounded wingtips replaced by shorter, squared off fairings to improve low-altitude performance and enhance the roll rate. These are sometimes referred to as "L.F" versions, e.g. L.F.IX. This designation referred to the low-altitude version of the Rolls-Royce Merlin engine and while many "L.F" Spitfires had the "clipped" wings, a number did not.

Beginning with the Mk. 21, the Spitfire had a new wing design armed with four 20 mm Hispano Mk II or V cannon.

Note

  • The Mark numbers did not necessarily indicate a chronological order; for example, the Mk IX was a stopgap measure brought into production before the marks VII and VIII.
  • 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.
  • Up until the end of 1942, the RAF always used Roman numerals for mark numbers. 1943 to 1948 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. Thus the Spitfire PR Mk XIX, for example became the PR 19 after 1948. This article adopts the convention of using Roman numerals for the marks I through XIX and Arabic numerals for the marks 21 through 24.
  • Type numbers (such as type 361) are the drawing board design numbers allocated by Supermarine.
  • All liquid capacities and measurements are in Imperial units.

With the development of the Merlin 61/63/66 and 70 series engines, with a two-stage, two-speed supercharger requiring an intercooler, several important modifications were made to the basic airframe and applied to all aircraft powered by these engines:

  • The longer Merlin 61 meant the nose forward of the engine/fuel tank bulkhead was 7 inches (17.8 cm) longer.
  • While early Mk IXs had the original size elevator horn mass-balances most had the enlarged version with the straightened leading edge.
  • The carburettor air intake on early to mid-production Spitfire IXs was a different shape to those of single-stage engined aircraft; they were shorter and had a wider air inlet.
  • Early Mk IXs had a teardrop shaped blister for a Coffman engine starter on the starboard side cowling, just behind the propeller. Although an improved electric starter replaced the Coffman unit, MK VIIs and the Mk VIIIs and Mk IXs with Merlin 63 and 66 series engines continued to carry this blister, which covered a cabin blower driveshaft.
  • Because the intercooler required a radiator, the radiator under the starboard wing was halved in size and the intercooler radiator housed alongside. Under the port wing a new radiator fairing housed a square oil cooler alongside of the other half-radiator unit.
  • A new 10 ft 9 in (3.27 m) diameter four bladed Rotol or de Havilland propeller was fitted, housed in a pointed spinner.
  • The exhaust units were changed to six "fishtail" stacks per side.
  • Under the nose, the three piece cowling was changed to a one piece layout. The oil tank was no longer a part of the cowling structure.
  • The Type C wing was fitted as standard. Some mid and most late production Spifire Mk IXs and all Mk XVI were fitted with the Type E wing. The teardrop shaped blisters covering the Hispano feed motors were reduced in size and more streamlined than those on the Mk VC.
  • Flush riveting for the fuselage was introduced in mid-1943
  • A streamlined round rear-view mirror replaced the rectangular version.
  • The small, teardrop shaped identification light behind the radio mast was removed.
  • A new rudder of greater area was fitted to Mk VII, Mk VIII, mid to late production Mk IXs and Mk XVIs. The new rudder had a pointed tip.

All Mk VII and Mk VIII Spitfires had the following changes:

  • The internal structure was strengthened and revised.
  • The ailerons were reduced in span by outboard of the outer hinges. There had been some instances of earlier models breaking up in the air in steep high speed dives, it was thought, because of aileron flutter.
  • 14 gallon (64 l) fuel tanks were fitted in each wing leading edge between the wingroots and the inner gun-bays.
  • An "Aero-Vee" filter was added to the carburettor air intake under the nose, identifiable by the long, streamlined fairing. This was also seen on mid to late Mk IX, and most P.R Mk X, P.R Mk XI and Mk XVI.
  • The main undercarriage legs, for the first time in the Spitfire's life, were fitted with forward facing torque links. In addition, the leg doors were slightly concave, allowing the undercarriage to sit lower in the wheel wells when retracted: this meant the upper wing skinning was free of the small bulge which had hitherto been necessary to clear the wheels. The wheels themselves were a reinforced "four spoke" pattern, replacing the "five spoke" pattern used since the first Mk Is. This revised undercarriage was also fitted on some mid to late Mk IXs and all Mk XVIs.
  • A retractable tailwheel was fitted, covered by small doors when in flight. This also applied to the P.R Mk Xs and most P.R Mk XIs.

Mk VII (type 351)

Like the Mk VI, the Mk VII was a high altitude pressurised variant, this time powered by the Merlin 70 series with two-stage, two speed superchargers. The cockpit was pressurised in a similar way to that of the Mk VI, although the details were slightly different. Other changes to the airframe are noted above. The Mk VII also had a Marshall manufactured compressor for the pressurised cockpit which was mounted on the right of the engine and drew its air through a long intake under the starboard exhaust stubs.

Extended, "pointed" wingtips were fitted to the Type C wings, increasing the wingspan to 40 ft 2 in (12.2 m). Because the threat from high altitude bombers never materialised many Mk VIIs later reverted to the normal, rounded wingtip.

Mk VIIs began to be fitted with a "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. The canopy was double-glazed and used rubber tubing to create a proper pressure seal against the fuselage. The canopy rails were bulkier than the standard Spitfire type.

In total, 140 Mk VIIs were built by Supermarine, the last of which used the Merlin 71 engine and reportedly had superb high altitude performance, with a service ceiling of . For instance, French ace Pierre Clostermann recalls in his book, The Big Show, the successful interception of a reconnaissance Messerschmitt Bf 109 by a Mk VII of 602 Squadron at over the British Home Fleet's base at Scapa Flow in early 1944.

Mk VIII (type 360)

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. When the "interim" Mk IX proved itself to be adequate for the RAF's needs it was decided to utilise the shadow factory at Castle Bromwich to produce that version only: the Mk VIII Spitfires were all built by Supermarine.

Apart from the lack of pressurisation, the Mk VIII differed little from the Mk VII. Some early production models had extended wingtips but the majority were fitted with the standard version. There were three sub-variants for low altitude (LF Mk VIII), medium altitude (F Mk VIII) and high altitude (HF Mk VIII) which were powered respectively by the Merlin 66, Merlin 63 and Merlin 70 engines.

According to Supermarine's Chief Test Pilot Jeffrey Quill:

"When I am asked which mark of Spitfire I consider the best from the flying point of view, I usually reply 'The Mark VIII with standard wingtips.' I hated the extended wingtips...They were of no practical value to the Mark VIII and simply reduced the aileron response and the rate of roll."
Performance was also increased with each modification. The F Mk VIII's top speed was at 25,000 ft (404 mph for the LF.Mk VIII at and for the HF.Mk VIII at 26,500 ft), with a service ceiling of 43,000 ft (41,500 ft for the LF Mk VIII and for the HF.Mk VIII). The two main tanks were given an extra 11 gallons for a total of 96 gallons and a self-sealing fuel tank with a capacity of 13.5 Imperial Gallons was added in each wing, allowing the fighter to fly for a maximum distance of with a full internal fuel load and with a full internal load and a 90 gallon drop tank. With a 170 gallon tank, the aeroplane could fly over .

A small number of Mk VIIIs were produced with four 20 mm cannon. A maximum external bomb load of 1,000 pounds (1 x bomb attached to the centre bombrack or 2 x bombs, one under each wing) could be carried. The thin wings, however, would not be fitted with bombs when armed with the quadruple cannon armament, nor could they accommodate an armament of four cannon with four machine guns.

A Mk VIII JF299 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. In trials, the new hood design was found to bring about great improvements to all-round visibility and, with several modifications, was standardised on later Spitfires.

This variant served almost exclusively overseas in the Mediterranean, with both the Desert Air Force and the USAAF, and in the Pacific, with the Royal Australian Air Force and in the China-Burma-India theatre.

After the Mk IX and Mk V, the Mk VIII was the third most numerous operational variant with 1,658 examples.

Mk IX (type 361)

In the early months of 1942, with the clear superiority of the Focke Wulf Fw 190 over the Spitfire VB. there was much pressure to get Spitfires into production using the new two-stage supercharged Merlin 61 engine. In September 1941 the Spitfire Mk III prototype N3297 had been converted by Rolls-Royce at their Hucknall plant to take a Merlin 60, which had been specifically designed for use in the Wellington Mk VI high altitude bomber. The performance increase was described as a "quantum leap" over that of the Mk VB and another Spitfire airframe, R6700 was modified to take the new engine. Although design work on the Mk VII and VIII series was under way, these would take over a year to get into production and a counter to the Fw 190 was urgently needed. As a result the Air Ministry made the decision that Mk VC airframes should be converted to take the more powerful engine.

As a result many of the early IXs were converted Mk VCs which did not have any of the refinements which later appeared. These could be identified by the Type C wing with the large double blisters over the inner cannon bays and the identification light on the fuselage spine, behind the aerial mast. The elevator horns were also smaller in size than that of most Mk IXs which had larger horn balances.

Although the Mk IX's airframe did not have the aerodynamic and strength improvements, or the modified control surfaces of the Mk VII and VIII, the Mk IX still proved to to be an effective counter to the Focke Wulf Fw 190; production of the Mk IX ended in April 1945 and, in combination with the Mk XVI this "compromise" was produced in larger numbers than any other Spitfire variant.

In September 1942 the "High Altitude Flight" was set up at RAF Northolt to counter the threat of the high altitude Junkers Ju 86 R bombers. Two Spitfire XIs converted from VCs were stripped of armour, the .303 Brownings and other equipment and repainted in a lightweight PRU blue finish. One of them successfully intercepted a Ju 86R at over .

Production of the Mk IX finished at Supermarine in June 1943, with production continuing exclusively at the Castle Bromwich factory. Several major improvements were made to Mk IXs coming off the production line:

  • The Merlin 61 was phased out early in 1943 in favour of the Merlin 63. The new engine had increased power resulting from engine improvements and engine limitations of +18 lbs./sq.in and 3,000 rpm (5 minute combat). During the second half of 1943, production the Merlin 63 powered F. Mark IX was discontinued in favour of the Merlin 66 powered L.F. Mark IX.
  • Production of Merlin 66 powered L.F. Mk IXs, frequently referred to as the Spitfire IXB, initially ran in parallel with the Merlin 63 powered Marks. This version first went operational in March 1943 with the Biggin Hill Wing, comprised at the time of 611 and 341(Free French) Squadrons. This type was by far the most produced of the Spitfire IX variants, with over 4000 built. The maximum power of the Merlin 66 was 1,720 hp at 5,750 ft (1,283 kW at 1,752 m) and the maximum speed of the Spitfire LF IX was 404 mph at 21,000 ft (650 km/h at 6,401 m).
  • The H.F IX, with the specialised high altitude Merlin 70, entered service in the Spring of 1944. Serial listings show that the H.F Mk IX was produced in relatively limited numbers as and when they were required, with priority being given to versions rated for low and medium altitudes. Maximum power of the Merlin 70 was at 11,000 ft (1,275 kW at 3,353 m): maximum speed of the Spitfire H.F.IX was at 27,500 ft (669 km/h at 8,382 m). All-up weight was about 7,450 lbs (3,379 kg) irrespective of variant.
  • A new Mark II Gyro Gunsight was fitted. This gunsight calculated the correct angle of deflection to use when leading the target. Its introduction doubled the effectiveness of RAF gunnery and was a major factor in Allied air superiority.
  • The "E Type" wing was introduced. The .303 machine guns mounted in the outer wings were no longer fitted as most aircraft at that time had armour impenetrable by .303 bullets. The 20 mm Hispano cannon were moved outboard and a more effective .50 calibre Browning .50 cal M2/AN heavy machine gun, with 250 rounds per gun was added to the inner gun-bay. The improved armament was more effective for both air-to-air engagements and air-to-ground attacks.
  • Supplementing the range of "slipper" drop tanks introduced on the Mk V series, a new cylindrical drop tank, carrying 45 gallons, could be carried on the fuselage bomb rack used by most Mk IXs of the Second Tactical Air Force.
  • Late production Mk IXs were fitted with additional internal self-sealing fuel tanks in the rear fuselage: the upper tank carried 41 Gallons and the lower 34 Gallons. When both were full this enabled a ferry range of over , although they made the aircraft unstable in flight and only straight flight and gentle manoeuvres at low altitudes were recommended by the pilot's manual, and the pilot was also warned to avoid instrument flying whenever possible.
  • The cut-down rear fuselage and bubble canopy, trialled on a Spitfire Mk VIII, was incorporated into the model. With the cut down fuselage the rear fuel tanks were limited to a capacity of 66 Gallons. These were the rarest of the Mk IXs and many of them featured the "clipped" wings. The great majority of these saw postwar service with theSAAF, both in South Africa and in deployment in Korea during the 1950s.
  • Late production Mk IXs were fitted with a slightly bulged upper engine cowling, coinciding with the fitting of the Packard Merlin engines in the Mk XVI.
  • During early 1945, some Spitfire IXEs and XVIEs of 74 Squadron were fitted with a single RP-3 rocket under each wing. This was believed to be the only RAF Spitfire unit to use rockets operationally during the Second World War.

P.R Mk IX (type 374) and F.R Mk IX

Pending development of a dedicated Merlin 61 PR Spitfire (the Mk XI) fifteen Mk IXs were taken off the production line and modified to carry two vertical cameras in the rear fuselage. A wrap-around PR type windscreen was fitted and a larger oil tank was installed under the nose. All armament was removed and a PRU Blue finish applied. It is believed no additional fuel tanks were installed, meaning that the PR IX relied on a slipper tank for extra range. One of these aircraft photographed the Moehne Dam after Operation Chastise.

F.R IXs were standard, armed Mk IXs modified with a single, port facing, oblique camera. These aircraft were used for low altitude "Dicing" missions in tactical support of army operations. 16 Squadron, which was a unit of the 2nd TAF, used several FR Mk IXs (painted a pale, "Camoutint" Pink, which provided excellent camouflage under cloud cover) to photograph the Arnhem area before Operation Market Garden.

Mk T. IX and TR. 9 (type 509)

Although there were wartime conversions of the Spitfire into a two-seat trainer including the one-off modification of a Mk VC by RAF no. 261 Squadron and a Mk IX converted for use as a trainer by the Soviets, the two-seat Spitfire trainer was primarily a postwar program. In 1946, a MK VIII (MT818) was the first Vickers-built trainer built as a demonstrator, but in 1948, 10 Spitfire T Mk IXs, were exported to India. In 1951, a further six TR 9 trainers were converted from the standard Mk IX to train pilots for the Irish Air Corps (IAC) Seafire fleet. The Spitfires provided transition training that included gunnery practice since the type was equipped with two .303 Browning machine guns, one in each outer wing bay. Most of the TR 9 aircraft passed to the ground technical training school at Baldonnel where they were used as instructional airframes for the training of aircraft engineers for the Air Corps. Four of the IAC aircraft survived and two went on to join the warbird fleet in the 1970s and later.

The Mk IX was the second most numerous variant of Spitfires produced. In total, 5656 were built, 561 by Supermarine and 5,095 by Castle Bromwich.

P.R Mk X and P.R Mk XI (types 362 and 365)

The Mk XI was a reconnaissance aircraft based on a combination of features from the marks VII, VIII and IX. The cameras, two vertically-mounted and sometimes an obliquely-mounted one, were installed in the fuselage behind the cockpit. The first Mk. XIs were built in November 1942 and production ended in 1944 when they were phased out in favour of the Mk XIX.

Physically the Mk XIs had a deeper nose fairing to accommodate a larger 14 gallon oil tank and it had the standard, unarmoured wrap-around PRU windscreen. "Booster" pumps for the wing tanks were fitted and covered by "teardrop" shaped fairings under the wings. In total 471 Mk XI were built by Supermarine. From April 1944 production ran concurrently with the P R Mk XIX before ending in December.

The Spitfire Mk X followed the Mk XI into production and was based on the reinforced Mk VII airframe with P.R Mk XI wings. It had a pressurised cabin and a Lobelle sliding hood, and gave similar performance. Only 16 Mk Xs were made, entering service in April 1944. All saw limited service in a high altitude reconnaissance role.

PR Mk XIII

The PR Mk XIII was an improved PR Type G with a single-stage Merlin engine and is described in Supermarine Spitfire variants part one

Mk XVI (type 361)

The Mk XVI was the same as the Mk IX in nearly all respects except for the engine, a Merlin 266. The Merlin 266 was the Merlin 66 and was built under licence in the USA by the Packard Motor Company. The "2" was added as a suffix in order to avoid confusion with the engines, as they required different tooling. All Mk XVI aircraft produced were of the Low-Altitude Fighter (LF) variety. This was not determined by the length of the wings (clipped wings were fitted to most LF Spitifres), but by the engine, which had been optimised for low-altitude operation. All production Mk XVIs had clipped wings for low altitude work. Many XVIs featured cut-down rear fuselages and were fitted with "bubble" canopies.

Because of a slightly taller intercooler and rearranged accessories on the Packard Merlins a new, bulged upper cowling was introduced and also appeared on late production IXs.

Armament for most Mk XVIs consisted of two 20 mm Hispano II cannon, each with 120 rounds and two .50 calibre Browning machine guns, each with 250 rounds. A single 500 lb (227 kg) bomb could be carried underneath the centre rack, and one 250 lb (114 kg) bomb could be slung under each wing. Some production aircraft had rear fuselage fuel tanks in addition to the main tank which allowed it to fly approximately as far as the Spitfire Mk VIII. Problems with the licence-built engines limited introduction to front-line squadrons for several months. A total of 1,054 Mk XVIs were built by Castle Bromwich.

Spitfire Floatplanes (types 355 & 359)

With the German invasion of Norway in April 1940 the RAF took an interest in the concept of using floatplane fighters in areas where airfields were not immediately available. To this end a Spitfire Mk I R6722 was taken in hand at the Woolston factory to be modified and mounted on Blackburn Roc floats. Tank tests were carried out at Farnborough, using a 1/7th scale model, it was found that the concept was basically sound, although the vertical tail surfaces would need to be enlarged to counter-balance the side area of the floats. The end of the Battle of Norway and the need for as many Spitfires as possible meant that R6772 was converted back to an ordinary fighter.

With the entry of Japan into the war the concept was revived in early 1942. A Spitfire V W3760 was fitted with a pair of floats 25 ft 7 in (7.8 m) long, mounted on cantilever legs. This aircraft was powered by a Merlin 45 driving a four bladed propeller of 11ft 3 in diameter (3.4 m). A Vokes filter was fitted to the carburettor air intake and under the tail an extra fin extension was added. Other changes included external lifting points forward of and behind the cockpit and a spin-recovery parachute with a rudder balance-horn guard. The Spitfire floatplane was first flown on 12 October 1942 by Jeffrey Quill. Soon afterwards the Vokes filter was replaced by an Aero-Vee filter, similar to that on later Merlin 61 series aircraft, which was extended to prevent water entry, and full Mk VB armament was installed. Two more VBs EP751 and EP754 were converted by Folland and all three floatplanes were transported to Egypt, arriving in October 1943. At the time it was thought that the floatplanes could operate from concealed bases in the Dodecanese Islands, disrupting supply lines to German outposts in the area which relied on resupply by transport aircraft. This scheme came to naught when a large number of German troops, backed by the Luftwaffe, took over the British held islands of Kos and Leros. No other role could be found for the floatplane Spitfires, which languished in Egypt, operating from the Great Bitter Lake. Specifications for the VB based floatplane included a maximum speed of 324 mph at 19,500 ft (521 km/h at 5,943 m), a maximum rate of climb of 2,450 ft/min at 15,500 ft (12.45 m/sec at 4,724 m) and an estimated service ceiling of 33,400 ft (10,180 m).

In the spring of 1944, with the prospect of use in the Pacific Theatre, a Spitfire IX MJ892 was converted to a floatplane. This used the same components as the earlier Mk VB conversions. Jeffrey Quill wrote:

"The Spitfire IX on floats was faster than the standard Hurricane. Its handling on the water was extremely good and its only unusual feature was a tendency to "tramp" from side to side on the floats, or to "waddle" a bit when at high speed in the plane."
Soon after testing started the idea of using floatplane fighters was dropped and MJ982 was converted back to a landplane.

Five aircraft were converted:

  • Mk I - R6722
  • F.Mk Vb - W3760
  • Mk V - EP751 and EP754
  • Mk IXb - MJ892

Griffon Engine Variants

Mk IV/XX (type 337)

On 4 December 1939, the Supermarine design staff produced a brochure which mooted the idea of converting the Spitfire to use the Rolls-Royce Griffon engine. A top speed of 423 mph (681 km/h) at 18,500 ft (5,639 m) was predicted. However, constant problems with the development of the Griffon meant that the decision to proceed with building a Spitfire with this engine didn't come to fruition until 1942, with the successful flight trials of the Mk IV.

The Griffon IIB which powered the Mk IV was a single-stage supercharged engine of 1,735hp (1,293 kW). Stronger main longerons were needed to cope with the weight of the Griffon and it required a bigger radiator and oil cooler, although it kept the asymmetric under-wing radiator layout of the single stage Merlin marks. The new engine had a lower thrust-line than the Merlin and was set with a minus ½ degree of downthrust. The lower thrust line and larger capacity of the new engine meant that the contours of the engine cowling were completely changed, with more prominent blisters over the cylinder heads, plus a third tear-drop shaped blister on the upper forward cowling to clear the magneto, and a deeper curve down to the spinner, which was much longer than previous types. The lower cowling lost its "pigeon-chested" appearance, with a shallower curve up to the spinner. A four blade Rotol propeller of 10 ft 5 in (3.1 m) was used. Apart from these differences the Mk IV airframe was closely related to that of the Merlin powered Mk III. One feature of the Griffon engine which was to catch a lot of pilots out was that the propeller rotated in the opposite direction to that of the Merlin; ie: to the right, from the pilot's perspective, rather than to the left. This meant that the powerful slipstream swung the Spitfire to the right on the ground, requiring the rudder to be deflected to the left during takeoff.

The Mark IV DP845 first flew on 27 November 1941. It had the full span C wing combined with a small tail unit and retractable tailwheel, and also had external bracket hinges under the wings, denoting the installation of braking flaps. These were soon removed and a mock up of a proposed six-cannon armament was fitted, three in each wing. The aircraft was soon renamed Mk XX, to avoid confusion with a renamed PR type, then it became the Mk XII.

Jeffrey Quill was Vickers Supermarine's chief test pilot and was instrumental in test flying and assessing every development of the Spitfire from the prototype K5045 to the last production Mk 24s, as well as some marks of Seafire. His assessment of the Mk IV/Mk XII prototype DP845 was;

Mk XII (type 366)

The Mk XII was the first Spitfire powered by a Griffon engine to go into service. The first of 100 Supermarine built production models started appearing in October 1942, and in total, two RAF squadrons were equipped with the model. Mk XIIs were manufactured from Mk VC and Mk VIII airframes: early production aircraft had the fixed tail wheels, five spoke mainwheels and small elevator balances. They had a single 85 gallon main fuel tank, giving a short range of little over on internal fuel. All were fitted with the larger, pointed tip rudder. The last 45 or so Mk XIIs, were based on Mk VIIIs with two wing fuel tanks, each containing a maximum fuel load of 14 gallons, and featured the larger horn balances, retractable tailwheel and undercarriage legs with torque-links, "dished" leg fairings and the reinforced four spoke wheels. The wheels were occasionaly fitted with disc-style covers. A later model IFF was fitted, replacing the aerials from the tailplane tip to fuselage with a rod aerial under the starboard wing. Another important feature of the Griffon engine Spitfires was the entirely flush-riveted finish which was progressively introduced on all Spitfires.

The single-stage Griffon engine (II or IV) gave the aircraft superb low and medium level performance, although the Mk XII's performance declined at higher altitudes: because of this all production aircraft had "clipped" wings. In comparative tests with a Mk IX it was faster at sea level, but above it had become slower. Handling, however, was considered to be better than previous Spitfire marks, and the clipped wings conferred excellent manoeuvrability through enhanced aileron response.

At low altitude it was one of the fastest aircraft in the world; in one speed trial, held at Farnborough in July 1942 a prototype Mk. XII (DP845), piloted by Jeffrey Quill 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 dogfights with Spitfires of any type below . When the Mk XII was able to engage in combat it was a formidable fighter and several Fw 190s and Bf 109-Gs fell victim to it. The Mk XII's speed advantage at lower altitudes again became 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.

Mk XIV (type 379)

The first Griffon powered Spitfires suffered from poor high altitude performance due to having only a single stage supercharged engine. By 1943 Rolls-Royce engineers had developed 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 provided a substantial performance increase over the Mk IX 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 Mk XIV differed from the Mk XII in that the longer, two-stage supercharged Griffon 65, producing 2,050 hp (1,528 kW), was mounted 10 inches (25.4 cm) further forward: the top section of the engine bulkhead was angled forward, creating a distinctive change of angle to the upper cowling's rear edge.
  • A new five bladed Rotol propeller of in diameter was used, although one prototype JF321 was fitted with a six bladed contra rotating unit.
  • The cowling fasteners were new, flush fitting "Amal" type, and there were more of them.
  • The oil tank (which had been moved from the lower cowling location of the Merlin engine variants to forward of the fuselage fuel tanks) was increased in capacity from 6 to 10 gallons.
  • The Mk XIV adopted the twin radiator arrangement of the two stage Merlin variants, although the increased heat generated meant that all radiators were much bigger and the underwing housings were deeper than previous versions.
  • The "fishtail" design of exhaust stub gave way to ones of circular section.
  • To help balance the new engine the radio equipment was moved further back in the rear fuselage and the access hatch was moved from the left fuselage side to the right.
  • A new tail unit, with a taller, broader fin and a rudder of increased area was adopted.
  • The aerial mast was removed and replaced by a "whip" aerial further aft on the fuselage spine.

The first batch of aircraft to fly with the Griffon 60 series engines were six converted Mk VIIIs JF316 to JF321 which were designated Mk VIIIG. The first one of these was flown by Jeffrey Quill on 20 January 1943:

One prototype, JF321, was fitted and tested with a Rotol six bladed contra-rotating propeller unit; although this promised to eliminate the characteristic swing on take-off (caused by the propeller slipstream) the propeller unit was prone to failure. The pitch control mechanism controlled the pitch on the front propeller;

The contra-rotating unit was later used on production Seafire 46 and 47s. When the new fighter entered service with 610 Squadron in January 1944 it signified a new leap forward in the evolution of the Spitfire. Jeffrey Quill flew the first production aircraft, RB140 in October 1943:

It could climb to in just over minutes and its top speed, which was achieved at , was .

F Mk XIVs had a total of 112 gallons of fuel consisting of 85 gallons in two main tanks and a 12.75 Imp. gal fuel tank in each leading edge wing tank. In addition 30 gallon and 90 gallon drop tanks could be carried. The fighter's maximum range was just a little over on internal fuel since the new Griffon engine consumed much more fuel per hour than the original Merlin engine of earlier variants. By late 1944, Spitfire XIVs were fitted with an extra 75 gallons in two rear fuselage fuel tanks, extending the fighter's range to about on internal fuel. Mk XIVs with "tear-drop" canopies had 64 gallons. As a result, F Mk XIVs had a range that was increased to over .

Initially production rates were slow with only 341 being built until the end of 1944. 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. The Spitfire XIV was also famous for being able to catch the notorious German jet, the Messerschmitt 262.The Mk XIV was used by the 2nd Tactical Air Force as their main high-altitude air superiority fighter in northern Europe with seven squadrons operational by May 1945. Spitfire XVIs also began to arrive in the CBI theatre in June 1945, too late to operate against the Japanese.

FR Mk XIV

Late in 1944 a number of Mk XIVs were converted to have a single camera fitted, facing to port or starboard. To achieve this a new hatch, similar to the radio hatch on Merlin engine Spitfires, was installed. Otherwise this version of the FR Mk XVI was essentially the same as the standard aircraft. Later, purpose built conversions, also known as the F.R Mk XIV, had the cut down rear fuselage with its tear drop–shaped canopy, and a port facing fuselage camera, with an additional rear fuel tank of 34 gallons which extended the Spitfire's range to about on internal fuel. Because it was used mainly at low altitudes the FR Mk XIV had clipped wingtips.

In total, 957 Mk XIVs were built, over 430 of which were FR Mk XIVs. 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 XVII

The mark numbers XV and XVII (15 and 17) were reserved for the naval version, the Seafire, in an effort to reconcile the Spitfire numbering scheme with that of the Seafire.

Mk XVIII (Mk 18) (type 394)

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 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 guerrillas in the Malayan Emergency. The Royal Indian Air Force purchased 20 ex-RAF Mk 18s in 1947.

300 F and F.R Mk 18s before production ended in early 1946

Mk XIX (Mk 19) (types 389 and 390)

The Mk XIX was the last and most successful photographic reconnaissance variant of the Spitfire. It combined features of the Mk XI with the Griffon engine of the Mk XIV. After the first 25 (type 389s) 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. This version was the type 390.

The first Mk XIXs 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 sortie 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 (type 366)

Mark XX was given to the original Mk IV Griffon engine prototype DP845 to avoid confusion with the retitled Spitfire PR Mk IVs.

The second Mk XX, 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 (type 356)

In early 1942, it was evident that Spitfires powered by the new two-stage supercharged Griffon 61 engine would need a much stronger airframe and wings. The proposed new design was designated the Mk 21. However, at first its flight qualities showed a number of flaws that caused 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 of 11 ft (3.35 m), 7 (17.8 cm) inches greater than that fitted to the Mk XIV.
  • To ensure sufficient ground clearance for the new propeller, the undercarriage legs were lengthened by 4.5". To improve handling on the ground, the undercarriage legs were placed 7.75" 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 eight inches as they retracted, and extended the legs again when they were lowered. Bigger mainwheels of a strengthened four spoke pattern were fitted. New triangular fairing doors, fitted to the outer ends of the redesigned wheelwells, fully enclosed the wheels when the undercarriage was retracted.
  • A new wing structure was used, with thicker gauge light alloy skinning. The new wing exhibited a 47% increase in torsional stiffness, resulting in an increased theoretical aileron reversal speed of .
  • The ailerons were 5% larger, and were no longer of the "Frise" balanced type, instead being attached by continuous piano-hinges. They were extended by eight inches, meaning that with a straighter trailing edge, the wings were not the same elliptical shape as in previous Spitfires.
  • The armament was standardised to four cannon and no machine guns.

In other respects, the first production Mk 21s used the same basic airframe as the Mk XIV. The first true Mk 21 prototype, PP139 first flew in July 1943, with the first production aircraft LA187 flying on 15 March 1944. However the modifications over the Mk XIV left the aircraft over sensitive to trimming and LA201 exhibited poor performance in trials in late 1944 and early 1945. This led to a damning report from the Air Fighter Development Unit in which they wrote,

"...it must be emphasised that although the Spitfire 21 is not a dangerous aircraft to fly, pilots must be warned...in its present state it is not likely to prove a satisfactory fighter. No further attempts should be made to perpetuate the Spitfire family."

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. Jeffrey Quill was to write:

"The AFDU were quite right to criticise the handling of the Mark 21...Where they went terribly wrong was to recommend that all further development of the Spitfire family should cease. They were quite unqualified to make such a judgement and later events would prove them totally wrong."

After intensive test flying the most serious problems were solved by changing the gearing to the trim tabs and other subtle control modifications, with the result that 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 with 91 Squadron. 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 (type 356)

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 and a more powerful 24 volt electrical system in place of the 12 volt system of all earlier Spitfires. Most of the Mk 22s were built with further enlarged tail surfaces, similar to those of the Supermarine Spiteful. A total of 272 Mk 22s were built: 250 at Castle Bromwich and 27 by Supermarine at South Marston.

The Mk 22 was used by only one regular RAF unit, No. 73 Squadron RAF in the Middle East. However 12 squadrons of the Royal Auxiliary Air Force used the variant and continued to do so until March 1951.

Mk 23 (type 372)

The Mk 23 was to be a Mk 22 incorporating a revised wing design which featured an increase in incidence, lifting the leading edge by 2 inches. It was hoped that this wouldimprove the pilot's view over the nose in flight and increase the high speed and dive performance of the aircraft. The modified, handbuilt wing was first fitted to a Mk VIII JG204 which was tested from July 1944. However the tests were disappointing and after discussions at Supermarine it was decided to build a new prototype using the Mk 21 prototype PP139; the aircraft was designated F Mk 23 to be renamed the Supermarine Valiant. However once again the new wing gave less than perfect handling characteristics and so the Mk 23 was never built from the Mk 22 airframe as intended.

Mk 24 (type 356)

The final Spitfire variant, the Mk 24, was similar to the Mk 22 except that it had an increased fuel capacity over its predecessors, with two fuel tanks of 33 gallons (150 l) each installed in the rear fuselage. There were also zero-point fittings for rocket projectiles under the wings. All had the larger "Spiteful" tail units: modifications were also made to the trim tab gearings in order to perfect the F 24’s handling characteristics. Late production aircraft were built with short-barrelled, electrically fired Mark V Hispano cannon.

Performance was impressive – the F 24 achieved a maximum speed of , and could reach an altitude of in eight minutes, putting it on a par with the most advanced piston-engined fighters of the era.

Although designed primarily as a fighter-interceptor aircraft, the Spitfire proved its versatility in several different roles. In fighter configuration the F 24’s armament consisted of four short-barrelled 20 mm Hispano cannon – operational experience had proved that the hitting power of these larger weapons was necessary to overcome the thicker armoured plating encountered on enemy aircraft as the war progressed. The aircraft also served successfully in the fighter-bomber role, being capable of carrying one and two bombs, with rocket-projectile launch rails fitted as standard.

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.

Introduced into service in 1946, the F. Mk 24 differed greatly from the original Spitfire Mk I in many respects and undoubtedly brought the design to the peak of perfection, being twice as heavy, more than twice as powerful and exhibiting an increase in climb rate of 80% over the prototype aircraft, 'K5054'. These remarkable increases in performance arose chiefly from the introduction of the Rolls-Royce Griffon engine in place of the famous Merlin of earlier variants. Rated at , the twelve-cylinder Vee liquid cooled Griffon 61 engine featured a two-stage supercharger, giving the Spitfire the exceptional performance at high altitude that had been sometimes lacking in early marks.

Drawing Board Projects

  • Supermarine 305: Designed to the same specification as the Boulton Paul Defiant and Hawker Hotspur. The basic Spitfire wings and undercarriage were mated to a new fuselage which provided room for a gunner and a remote control four gun turret (originally armed with .303 Brownings, later with Lewis machine guns.) Other modifications included a cooling system mounted in a chin radiator housing. There was no forward firing armament and dive brakes were added to the wings. Projected speed 315 mph (507 km/h) at 15,000 ft (4,572 m).
  • Supermarine 312: This was the basic Mk I Spitfire adapted to take four 20 mm Oerlikon cannon mounted in modified wings. The radiator and oil cooler were moved from under the wing to a duct under the fuselage. This was R J Mitchell's last design before his death in 1937.
  • Supermarine 324, 325 and 327: These were designs for twin-engined fighters which used the same construction techniques and the same elliptical wing planforms as the Spitfire. All three designs used tricycle undercarriages with the primary engines being Merlins with an alternative being the Bristol Taurus. The 324 and 325 were intended to carry an armament of 12 .303 calibre Brownings in groups of six in each outer wing while the 327 had the armament changed to six 20mm Hispanos mounted in the wingroots. The 324 and 327 used conventional tractor engines while the 325 used pusher engines.

See also

References

Footnotes

  • The Coffman starter was connected to the propeller reduction gearbox and used large shotgun-like cartridges loaded into a chamber to "kick-start" the engine. It was first used on the Spitfire Mk II and was also used by Napier Sabre powered aircraft.
  • After a series of accidents to Mk Vs in 1942 the A&AEE concluded that the break ups were 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.
  • Some of very early Mk IXs were converted by Rolls Royce at Hucknall with engine cowlings from Mk Vs extended with a new section welded on to the rear and extemporised blisters added, covering the intercooler which was mounted on the supercharger of the Merlin 61.
  • Based on figures taken from a trials report of the prototype taken at the Marine Aircraft Experimental Establishment at Helensburgh (near Glasgow) in 1943.

Citations

Bibliography

  • Bachelor, Len J. "Supermarine Spitfire (Griffons) Mks. XIV & XVIII". Aircraft in Profile, Volume 13. Windsor, Berkshire, UK: Profile Publications Ltd., 1973, pp. 265–288. ISBN 0-85383-022-3.
  • Bader, Douglas. Fight for the Sky: The Story of the Spitfire and Hurricane. London: Cassell Military Books, 2004. ISBN 0-30435-674-3.
  • Buttler, Tony. British Secret Projects: Fighters and Bombers 1935-1950. Hersham, Surrey, UK: Midland, 2004. ISBN 1-85780-179-2.
  • Caygill, Peter.Combat Legend; Spitfire Mks VI-F.24. Ramsbury, Marlbourough, Wiltshire, UK: The Crowood Press Ltd, 2004. ISBN 1-84037-400-4.
  • Cooke, Peter. "The Early Griffon Spitfires part 1: Article and scale drawings" Scale Models Magazine Vol. 9, No 109, October 1978. Hemel Hempstead, Hertfordshire, UK.
  • Cooke, Peter. "The Early Griffon Spitfires part 2: Article and scale drawings" Scale Models Magazine Vol. 9, No 110, November 1978. Hemel Hempstead, Hertfordshire, UK.
  • Dibbs, John and Tony Holmes. Spitfire: Flying Legend. Southampton UK: Osprey Publishing, 1997. ISBN 1-84176-005-6.
  • Jackson, Robert. "Spitfire: The History of Britain's Most Famous World War II Fighter." London, UK: Parragon Publishing, 2005. ISBN 0-75258-770-6.
  • Jane, Fred T., ed. "The Supermarine Spitfire." Jane’s Fighting Aircraft of World War II. London: Studio, 1946. ISBN 1-85170-493-0.
  • Laird, Malcolm and Steve Mackenzie. Spitfire the ANZACS; The RAF through Colonial Eyes. Wellington, NZ: Ventura Publications, 1997. ISBN 0-9583594-1-5.
  • McKinstry, Leo. Spitfire: Portrait of a Legend. London: John Murray, 2007. ISBN 0-71956-874-9.
  • Morgan, Eric B. and Edward Shacklady. Spitfire: The History. London: Key Publishing, 2000. ISBN 0-946219-48-6.
  • Palfrey, Brett R. and Christopher Whitehead. Supermarine Spitfire: History of a Legend. Royal Air Force (RAF). Retrieved: 27 December 2006.
  • Quill. Jeffrey. Spitfire: A Test Pilot’s Story. London: Arrow Books, 1983. ISBN 0-09-937020-4.
  • Price, Alfred. Late Marque Spitfire Aces 1942 - 1945. Oxford, UK: Osprey Publishing, 1995. ISBN 1-85532-575-6.
  • Price, Alfred. Spitfire Mark I/II Aces 1939-41. London: Osprey Aerospace, 1996. ISBN 1-85532-627-2.
  • Price, Alfred. The Spitfire Story: New edited edition. London: Weidenfeld Military, 1999. ISBN 1-85409-514-5.
  • Smallwood, Hugh. Spitfire in Blue. London: Osprey Aerospace, 1996. ISBN 1-85532-615-9.
  • Spick, Mike. Supermarine Spitfire. New York: Gallery Books, 1990. ISBN 0-8317-14034.
  • Thomas, Andrew.Griffon Spitfire Aces: Aircraft of the Aces 81. London: Osprey Aerospace, 2008. ISBN 1-84603-289-1.

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