Supermarine Spitfire Variants – The Initial Merlin-Powered Line

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Supermarine Spitfire Variants

The Initial Merlin-Powered Line

Foreign orders: Mk Is (Supermarine type 332, 335, 336 and 341) The Type numbers 332, 335, 336 and 341 were given to versions of ...
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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.]

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6 Comments | Add New

By Jörn Dietrich  |  2010-07-07 at 05:42  |  permalink

Hi,

I greatly enjoy this site and the information published here. Well researched, well presented – excellent. However, I do wonder if the following statement regarding the replacement of the Merlin III with the Merlin 45 engine is 100% accurate:

“Improvements to the carburettor also allowed the Spitfire to use zero gravity manoeuvres without any problems with fuel flow.”

To my knowledge, the early variants of the Merlin 45 used a standard float type carburettor (SU), which soon got improved through the crude but effective Tilly orifice, and only later (late 1942?) got decent float type carburettors that had little/irrelevant difficulties with negative g’s and never received injection type carburettors, that had no difficulties with negative g’s.

Since this has been a point of major interest for me lately, could you please give more detail on that?

Thanks a lot, Jörn.

By James  |  2011-06-08 at 11:20  |  permalink

The fascinating thing about the Spitfire was it’s revolutionary construction, as a mass produced aircraft. Monocoque stressed skin construction was a rare concept in the late 30’s, compared to the conventional construction of a strong internal frame, with essentially a non load bearing outer cover. I wonder if Mitchell having worked on sea plane hulls at Supermarine, which would have also basically been the same, and seeing the strength of those flying boat hulls, had decided to adopt this type of structure for his fighter? The wing leading edge skins, seperate top and bottom, which when attached to the wing spars, formed the very strong leading edge “D” box, are amazing. They are one full span piece of sheet metal and look to me, to be amazingly complex in curve and shape. I would love to see how those were formed, and indeed, are formed today for re-builds?

By Editor  |  2011-06-08 at 13:35  |  permalink

@James: interesting point, but as a non-engineer I wonder just how novel the monocoque construction really was. How about the American high-speed airliners of the 1930s, or the Messerschmitt in Germany?

Boeing 247 first flew in 1933, Bf 108 appeared in Challenge 1934, both types being good examples of commercial applications.

Mitchell’s own S.5 built for the 1927 Scheider Trophy had semi-monocoque fuselage, too.

Best regards,
/Martin

By James  |  2011-06-09 at 11:30  |  permalink

Martin you’re quite right, had forgotten the Boeing, not to mention the DC2 and legendary DC3. I was thinking more in Mitchells back yard, in terms of RAF aircraft and RAF maintainance personnel. He may have been worried about his fighter getting chosen, when maintainance and mass production would be considered I wonder? Again you are also right about the S6. Also, an interesting point about the 109 prototype, which I think had flown quite some time before K5054?

By Editor  |  2011-06-09 at 14:06  |  permalink

@James: Just what I meant.

It would appear to me that all-metal monocoque construction was employed rather broadly at various places since the beginning of the 1930s, with some companies embracing the technology (Supermarine, Douglas starting with DC-2, Lockheed starting with Electra, Boeing 247, Messerschmitt starting with Bf 108). In contrast, some other successful companies of the time took a stance of waiting and seeing (Hawker, Junkers).

Regarding metal construction in general, I believe that the French Wibault acted as a leading star of all-metal aircraft production in the 1920s, together with – of course – Junkers of Germany. Both companies built their technology on corrugated sheet metal and so their own aircraft designs did not employ monocoque construction. However, Polish PZL which had acquired Wibault technology by license, was turning out all-metal monocoque fighters since the beginning of the 1930s.

At the same time, there can be no doubt that the technology had been new – so even though metal construction was presumably manageable at the drawing office level, there were problems of large-scale production. One of the reasons contributing to the well-publicised delays in the initial Spitfire production was just the lack of experienced metal workers in the Southampton area. Although not quite as publicised, there must have been similar problems among the RAF ground crew.

I’m sure that Mitchell was well aware of these problems, but made a knowing choice of maximising the performance of the new fighter at the expense of other parameters like cost or manufacturing simplicity. Luckily, his approach eventually proved to be right.

As a comment to your last point, obviously the Bf 109 first flew before the Spitfire, and we know that Mitchell had access to RAF intelligence sources so that he was informed about its existence. I remains unclear, however, how much detailed information about the Bf 109 was available in Britain at the time, and in any case, by the time the Supermarine was well advanced with their own design.

Thanks for picking up an interesting topic… 🙂

By Thomas Milo  |  2013-01-05 at 18:00  |  permalink

Spitfire PR IV serial x4492 seems to have a non-standard camouflage pattern below the cockpit. Do there exist more images of it to confirm this?

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