The Spitfire Site

A Tribute to Britain's Finest Fighter

Supermarine Spitfire Variants

Early Merlin-Powered Versions
Part 1

By Jeff Webb

Because the Spitfire was one of the few British aircraft produced before, during and after World War II it is an excellent example of an airframe which evolved and changed to fulfil several combat roles. The need to keep pace with enemy fighter aircraft meant that there was a need for engines which could also evolve to provide more power more efficiently at all altitudes. The Spitfire's partnership with the Rolls-Royce and the utilisation of their Merlin and Griffon engines was to result in the rapid development of increasingly more powerful engines with a range of superchargers and supercharger settings optimised for different altitude bands.

There were 24 marks of Spitfire, but also, as will be seen, many sub-variants within the marks. The entire Spitfire family may be divided by the generation of Rolls-Royce engines which powered the aircraft. Thus the first generation was powered by single-stage Merlins, from Merlin II to Merlin 50 and resulted in Spitfires Mks. I, II and V as the most prominent fighter variants. Two-stage Merlins (meaning the use of two-stage supercharger for increased altitude performance), from Merlin 61 to Merlin 70, provided the basis of mid-war development, Mks. VIII, IX and XVI being the most prolific versions of this family. Finally, the arrival of Rolls-Royce Griffon provided a basis for the final line of Spitfire development, exemplified by Mks. XII, XIV and their post-war derivatives.

This article describes the initial Spitfire line powered by single-stage supercharged Merlin engines.

It is notable that throughout the entire development process, which took place over twelve years, from 1935 through to 1948, there were no outstanding failures of the basic design: this is a real testament to the original genius of Reginald J Mitchell, his successor Joseph Smith and the design teams they led.

Official photograph of a Spitfire Mk. I from contemporary publication. Its shape was instantly recognizable already at the time of its service introduction, but the versatility and development potential of Mitchell's airframe was yet to come to light.
Note that the gun ports have been retouched to conceal the armament of the aircraft,

[Crown Copyright]

A few initial notes

No discussion about the various Spitfire marks can be complete without referring to the different wing types, and indeed such references are made liberally throughout this article. The single-stage Melin Spitfires used four different wing types, A through to D which had the same dimensions and plan but different internal arrangements of armament and fuel tanks. Readers are advised to refer to my previous article Concise Guide To Spitfire Wing Types for an overview of the basic wing types of the Spitfire and the differences between them. Here is but a brief summary.

• A type
  The original wing design, the basic structure of which was unchanged until the C type in 1942. The armament to be carried was 8x .303 Browning machine guns with 300 rpg.
   
• B type
  This was the A-type modified to carry a 20mm Hispano cannon. The alloy covered ailerons were standardised during the production of this wing type. One type of armament could be fitted, comprising two 20 mm drum-fed Hispano Mk II cannon with magazines carrying 60 rounds/gun plus four .303 Browning machine guns with 350 rounds each.
   
• C type, also known as "universal wing".
  This 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 Hispano Mk IIs fitted 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. Also, the undercarriage mountings were redesigned and the undercarriage legs raked 2 inches forward, making the Spitfire more stable on the ground and reducing the likelihood of the aircraft tipping onto it's nose.
   
• D type
  Unarmed long-range wing for reconnaissance versions with the D-shaped wing structure ahead of the main spar converted to integral fuel tanks capable of carrying 66 imp gallons.

Starting with the Mk. V some 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, eg: L.F Mk Vb. 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.

Also, readers should be advised that many Spitfires of one mark or variant may have been modified to another later in their career; for example, several of the first Mk Vbs were converted from Mk Ibs; the first Mk IXs were originally Mk Vcs, Mk. XVIs started their life as Mk. IXs and so on.

All liquid capacities and measurements quoted herein are given in Imperial units.

The First Generation: Spitfire Mk. I and its Derivatives

Spitfire Mk I (Supermarine type 300)

The Supermarine Woolston line started delivering the Spitfire Mk. I in late 1937 with front-line service commencing in August 1938. Over the next three years a large number of modifications were made, especially as a result of wartime experience.

The earliest Mk. Is were powered by the 1,030 hp (768 kW) Rolls Royce Merlin II engine driving an Aero-Products 10 ft 8 in (3.3 m) diameter two-blade wooden fixed-pitch propeller, weighing 83 lb (38 kg).

The Merlin I to III series all relied on external electric power to start the engine; a well known sight on RAF fighter airfields was the "Trolley Acc" (trolley accumulator ) which was a set of powerful batteries which could be wheeled up to aircraft. The lead from the accumulator trolley was plugged into a small recess on the starboard side cowling of the Spitfire. 

Early on in the Spitfire's life it was found that at altitudes above about 15,000 ft (4572 m) any condensation could freeze in the guns causing unpredictable stoppages. The system of gun heating, was introduced on the 61st production Mk I.

Following complaints from pilots which required more headroom, a new form of "blown" canopy was manufactured and started replacing the original "straight" version in early 1939. This canopy improved headroom and enabled better vision laterally and to the rear.

A simplified design of pitot tube was introduced.

The manual hand-pump for operating the undercarriage was replaced by a motorised hydraulic system.

One of the earliest production Spitfires Mk. I displaying the characteristic initial features of this mark: two-blade propeller, unarmoured windscreen and fuel tank, "straight" cockpit hood and long "pole" antenna of the HF radio. All these elements, and more, were modified or replaced before the time of the Battle of Britain, leading to significant improvements in performance and combat-worthiness of the aircraft.

[Crown Copyright, via Jenny Scott]

At the outset of World War 2 the flash suspensors on the gun muzzles were removed and the practice of sealing the gun ports with fabric patches was instituted. The patches kept the gun barrels free of dirt and debris and allowed the hot air to heat the guns more efficiently. When the guns were fired the patches were shot through, and were always replaced by the ground-crew during rearming. At about the same time the original ring & bead sight was removed and replaced by Barr & Stroud reflection gunsight.

The Aero Products wooden propeller was replaced by a 350 lb (183 kg) de Havilland 9 ft 8 in diameter, three-bladed, two-position, metal propeller, which greatly improved take-off performance, maximum speed and the service ceiling. It also started the incremental weight increases which continued through the life of the airframe.

The "rod" aerial mast was replaced by a streamlined, tapered design.

From the 175th production aircraft, the Merlin Mk III, which had a "universal" propeller shaft able to take a constant-speed de Havilland or Rotol propeller, was fitted. Just before the Battle of Britain a de Havilland constant speed propeller, of the same diameter as the two-position unit, became available. Although this was a great deal heavier than the earlier types (500 lb (227 kg)) it gave another substantial improvement in take-off distance and climb rate.

A thick, laminated glass, bullet proof plate was fitted to the curved, one piece windscreen.

A 3mm thick cover of light alloy, capable of deflecting light machine gun rounds, was fitted over the top of the upper fuel tank. From about mid-1940, 73 pounds (33 kg) of armoured steel plating was provided in the form of head and back protection on the seat bulkhead and covering the forward face of the glycol header tank.

Rear view mirrors were added to the windscreen: two early "shrouded" designs were later replaced by a simplified, rectangular, adjustable type.

"Two step" rudder pedals, which were fitted to all frontline Spitfires just before the Battle of Britain: these allowed the pilot to lift his feet and legs higher during combat, improving his blackout threshold and allowing him to pull tighter sustained turns.

Starting in September 1940, an IFF (Identification friend or foe) transmitter was installed. This weighed about 40 lb (18 kg) and could be identified by wire aerials strung between the tailplane tips and rear fuselage. Although the added weight and the aerials reduced maximum speed by about 2 mph, it allowed the aircraft to be identified as "friendly" on radar.

At about the same time new VHF T/R Type 1133 radios started replacing the HF TR9 sets. These had first been fitted to Spitfires of 54 and 66 Squadrons in May 1940, but ensuing production delays meant that the bulk of Spitfires and Hurricanes weren't fitted for another five months. With the new sets, the pilots enjoyed a much clearer reception. The new installation meant that the wire running between the aerial mast and rudder could be removed, as could the triangular "prong" on the aerial mast.

Modifications to the Spitfires Mk. I in service were introduced incrementally, resulting in a variety of interim configurations at the unit level during the period of 1939-1940. Here, two Spitfires of No. 611 Squadron photographed in Autumn 1939 sport de Havilland three-blade two-pitch propellers, armoured windscreens and fuel tanks as well as bulged canopies, but they are still retaining the early form of radio masts.

[Crown Copyright]

Weight increases and aerodynamic changes led to later Spitfire Is having a lower maximum speed than the early production versions. This was more than offset by the improvements in take-off, climb rate and ceiling brought about by the two-position and constant speed propeller units.

At the start of the war the engine ran on the then-standard 87 octane aviation fuel. From March 1940 increasing quantities of 100 octane fuel, imported from the U.S, became available. This meant that during the defensive battles over Dunkirk the Spitfire Is benefited from an allowable increase in supercharger "boost" from +6 lbs to +12 lbs without damaging the engine. With the +12 lb "emergency boost" the Merlin III was able to generate 1,305 hp (973 kW) in a five minute burst. If the pilot resorted to emergency boost, he had to report this on landing and it had to be noted in the engine log book.

Combat experience showed that the fabric covered ailerons seemed to lock solid in high speed dives: this was caused by the fabric "ballooning" which, in turn, increased the control stick forces needed to move them. From November 1940 Supermarine started producing light-alloy covered ailerons as a which did not have this deficiency. Fighter Command ordered a crash programme to have all front line Mk Is and Mk IIs fitted.

No. 19 Squadron received several Spitfires Mk. I armed with two Hispano 20mm cannon during the Battle of Britain. These were known as the Mk. Ib, the eight machine gun Mk Is were retrospectively called the Mk.Ia. This early cannon installation was hampered by frequent jamming problems. In one engagement, only two of the 12 aircraft had been able to fire off all of their shells. Further cannon-armed Spitfires, with improvements to the cannon mounts were later issued to 92 Squadron and it was eventually realised that the best armament mix was an aircraft with two cannon and four machine guns. Most of these trial aircraft were later converted to the first of the Mk Vbs.

In all, 1,567 Mk Is were built (1,517 by Supermarine between May 1938 and March 1941, 50 by Westland, July to September 1941).

Foreign orders: Mk Is (Supermarine type 332, 335, 336 and 341)

The Type numbers 332, 335, 336 and 341 were given to versions of the Mk I which were to be modified to meet the requirements of Estonia, Greece, Portugal and Turkey respectively. Estonia's order was cancelled when the Soviet Union annexed the country early in 1940. The Greek and Portuguese orders were refused by the Foreign Office. The 59 aircraft for Turkey were approved, but after delivering 2 aircraft the Foreign Office put a halt to that too in May 1940.

The 208th production Spitfire I was sold to France and in June 1939 was delivered for evaluation.

Speed Spitfire (Supermarine type 323)

In late 1938, a standard Mk I, serial no. K9834 was taken off the production line and modified for an attempt on the World Speed Record.

A special "sprint" version of the Merlin II driving a Watts, coarse pitch, four bladed wood propeller was able to generate 2,160 hp (1,611 kW) for short periods. Smaller fuel tanks were fitted, in the expectation that any record attempts would be of short duration. The cooling system was redesigned to allow the liquid coolant to be boiled off over the duration of the flight. The wingspan was reduced to 33 ft 7 in (10.26 m), with the wingtips being rounded, and the radiator housing under the wing was increased in size. All military equipment was removed.

All panel lines were filled and smoothed over, all round headed rivets on the wing surfaces were replaced by flush rivets and an elongated "racing" windscreen was fitted. A tailskid replaced the tail wheel. Finally the "Speed Spitfire" was painted in a highly polished gloss Royal Blue and Silver finish. As it turned out, the finished aircraft actually weighed some 298 lb (135 kg) more than a standard 1938 vintage Spitfire.

Once the World Speed records were broken in quick succession, first by the Messerschmitt Bf 109 V13 and then by the Heinkel He 100 and Messerschmitt Me 209 it was decided that the Speed Spitfire needed a great deal more modification to even come close to the new speed records and the project lapsed.

At the outbreak of War the Speed Spitfire was modified to a hybrid PR Mk. II with the special Merlin II being replaced by a Merlin XII driving a variable pitch de Havilland propeller, and the racing windscreen replaced by a P.R wrap-around type. A conventional cooling system was restored. Nothing could be done about the reduced fuel capacity, and therefore K9834 could never be used as an operational aircraft. Flown as a liaison aircraft between airfields in Britain during the war, it was scrapped in June 1946.

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. It was soon found that modified Bristol Blenheims and Westland Lysanders were easy targets for German fighters and heavy losses were being incurred whenever these aircraft ventured over German territory.

In August 1939, F/O 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 armament and radios removed and replaced with extra fuel and cameras. As a result of his representations two Spitfires, N3069 and N3071, were released by Fighter Command and sent to the Heston Flight, a highly secret reconnaissance unit under the command of Sidney Cotton. These two Spitfires were "Cottonised" by stripping out the armament and radio-transmitter, then, after filling the empty gun ports and all panel lines, the airframe was rubbed down to remove any imperfections. Coats of a special very pale blue-green called Camotint were applied and polished (this colour was later used by the RAF as a a basis for Sky (Type S)). It was the first in a new range of smooth, eggshell finish paints replacing the drag inducing matt finishes in use up until 1942.

Two two F.24 cameras with five inch lenses which could photograph a rectangular area below the aircraft were installed in the wing space vacated by the inboard guns and their ammunition containers. Heating equipment was installed on all P.R Spitfires to stop the cameras from freezing and the lenses from frosting over at altitude. These Spitfires, which later officially became the Spitfire Mk I P.R Type A, had a maximum speed 10 to 15 mph (16 to 24 km/h) higher than that of the fighter.

By January 1940, after several high-altitude missions by No. 2 Camouflage Unit (as Heston Flight was renamed in an effort to keep its genuine purpose secret) proved the concept. Further Spitfires were delivered and modified, becoming the first of the well established line of unarmed, high speed photo reconnaissance aircraft. This resulted in a proliferation of P.R. modifications, all conversions of existing Mk. I fighter airframes.

• Mk. I PR Type A
  The original conversions described above. Two built.
   
• Mk. I PR Type B
  In the Mk. I PR Type B the camera lenses were upgraded to an eight inch focal length, giving images up to a third larger in scale. This variant also had an extra fuel tank in the rear fuselage and was designated a medium range aircraft.
  
  It had been envisaged that much larger cameras would be installed in the fuselage immediately behind the pilot, but at the time RAF engineers believed this would upset the Spitfire's centre of gravity. Cotton was able to demonstrate that by removing lead weights, which had been installed in the extreme rear fuselage to balance the weight of the constant speed propeller units, it was possible to install cameras with longer focal-length lens in the fuselage.
   
• Mk. I PR Type C
  The Mk I PR Type C carried more fuel 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 in a blister tank under the port wing, which was counterbalanced by a camera installation in the starboard wing.
   
• Mk. I PR Type D
  The Mk I PR Type D was nicknamed "the Bowser." The D-shaped wing leading edges, ahead of the main spar, proved to be an ideal location for an integral tank . Accordingly, in early 1940, work started on converting the leading edges, between rib four through to rib 21, by sealing off the spar, outer ribs and all skin joins allowing 57 gallons of fuel to be carried in each wing. Because the work was of low priority, and with the urgent need for fighters, the first two hand-built prototypes of the PR Type Ds weren't available until October. In addition to the leading edge tanks these prototypes also had a 29 gallon tank in the rear fuselage. An additional 14 gallon oil tank was fitted in the port wing. The cameras, two vertically mounted F.24s with 8 inch or 20 inch lens or two vertically mounted F.8s with 20 inch lens, were located in the rear fuselage. With the full fuel load the c/g was so far back the aircraft was difficult to fly until the rear fuselage tank had been emptied. Despite these difficulties the type quickly proved its worth, photographing such long distance targets as Stettin, Marseilles, Trondheim and Toulon.
  
  Once the first two Type Ds, P9951 and P9952 had proven the concept the production aircraft were modified to increase the leading edge tank capacity to 66.5 gallons and by omitting the rear fuselage tank. These aircraft were better balanced and had the more powerful Merlin 45 engine as used by the Mk V, along with heated cockpits, which were a great comfort to pilots on such long flights.
  
  A total of 229 Type Ds were built. It was not only the first mass-produced reconnaissance variant of the Spitfire, but also provided a pattern on which all subsequent PR marks were based.

Spitfire PR Mk. IV with enlarged oil tank under the nose, "clean" wing leading edge with integral fuel tanks, de Havilland propeller, unarmoured windscreen, and sliding cockpit hood with teardrop observation blisters. Note also the lack of a radio mast and the camera port located just underneath the fuselage roundel.

[Library and Archives Canada]

• Mk. I PR Type E
  The Mk I PR Type E 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.
   
• Mk. I PR Type F
  The Mk I PR Type F was an interim "super-long-range" version which entered service in July 1940, pending the Type D. The Type F carried a 30 gallon fuel tank under each wing, plus a 29 gallon tank in the rear fuselage, as well as having an enlarged oil tank under the nose. 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 and photograph Berlin. 15 airframes of this type of these were based on the Mk V fighters.
   
• Mk. I PR Type G
  The Mk I PR Type G performed a similar role to the Type E. The Type G was much more closely related to its fighter ancestry, being fully armed with eight .303 Brownings and having the armoured windscreen. One oblique and two vertical cameras were installed in the fuselage, as well as a 29 gallon fuel tank just behind the pilot.

On all unarmed P.R conversions the gunsight was replaced by a small camera control box from which the pilot could turn the cameras on, control the time intervals between photos and set the number of exposures.

Also new to PR Spitfires were the "blown" canopies which incorporated large lateral teardrop shaped blisters, allowing the pilots a much clearer view to the rear and below, vital for sighting the cameras.

The profileration of PR sub-variants lead in 1941 to an introduction of new mark numbers, independent of those used for the fighter versions:

• The 'PR Type C' became the PR Mk. III
• The 'PR Type D' became the PR Mk. IV
• The 'PR Type E' became the PR Mk. V
• The 'PR Type F' became the PR Mk. VI
• The 'PR Type G' became the PR Mk. VII

Spitfire Mk. I PR Type G photographed in Canada after the war. At the time this variant was redesignated as PR Mk. VII.

[Pat Murphy coll.]

Spitfire Mk. II (Supermarine type 329)

In the summer of 1939 an early Mk I K9788 was fitted with a new version of the Merlin, the XII. With the success of the trials it was decided to use this version of the Merlin in the Mk. II which, it was decided, would be the first version to be produced exclusively by the huge new Nuffield "shadow" factory at Castle Bromwich.

Chief among the changes was the upgraded 1,175 hp (876 kW) Merlin XII engine. This engine included a Coffman cartridge starter instead of the electric system of the earlier Merlins and it required a small "teardrop" blister on the forward starboard cowling. The Merlin XII was cooled by a 70% to 30% water-ethylene glycol mix, rather than pure glycol used for earlier Merlin versions (an important improvement since at the time Glycol was both expensive and in limited supply - Ed.)

In early 1940, Spitfires Mk. I of 54 and 66 Squadrons were fitted with Rotol-manufactured wide-bladed propellers of 10 ft 9 in (3.27 m) diameter, recognisable by a bigger, more rounded spinner. Also here the decision was made that the new propeller would also be used exclusively by the Mk II. This engine/propeller combination increased top speed over the late Mk.I by about 6-7 mph below 17,000 feet, and improved the rate of climb. Due to all of the weight increases, maximum speed performance was still lower than that of early Mk.Is, but the Mk II's combat capability was far better. The Mk II was produced in IIa eight-gun and IIb cannon armed versions.

Spitfire Mk. IIa P7666 in the markings of No. 41 Squadron displays the most characteristic feature of this mark: Rotol constant-speed propeller with rounded spinner. It also serves as an example of late Mk. I and Mk. II production features - bulged canopy, armoured windscreen and fuel tank, modified pitot tube under port wing, new aerial mast and lack of wire antenna signifying the use of VHF radio.

[Crown Copyright]

Due to a shortage of Rotol units some late production Mk IIs were fitted with de Havilland propellers and reverted to using the smaller spinner.

After several production hold-ups the first 10 Mk. IIs were built in June 1940. Production rapidly improved and most front-line squadrons had re-equipped with the new version by April 1941.

A small number of Mk. IIs were converted to "Long Range" Spitfires in early 1941. These could be recognised by the fixed 40 gallon fuel tank which was fitted under the port wing. This tank produced considerable drag with which manoeuvrability was reduced, maximum speed was 26 mph (42 km/h) lower and the climb rate and service ceiling were also reduced. Several squadrons used this version to provide long-range bomber escort in the early stage of the Circus offensive.

A total of 921 Mk IIs were built, all by Castle Bromwich.

Spitfire ASR Mk. II (Supermarine type 375)

Once the Mk. II was taken out of frontline service, 50 of them were converted for air-sea rescue work, at first under the designation Mk IIc (type 375) but later referred to as the A.S.R Mk II. The Merlin XII was replaced by the Merlin XX, a "rescue pack" was fitted in the flare chute and smoke marker bombs were carried under the port wing.