Supermarine Spitfire Variants – The Initial Merlin-Powered Line

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Supermarine Spitfire Variants

The Initial Merlin-Powered Line

Spitfire Mk. II (Supermarine type 329) In the summer of 1939 an early Mk I K9788 was fitted with a new version of the ...
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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.

Second Generation: From Mk. III to Mk. V and its derivatives

Spitfire Mk. III (Supermarine type 330)

The Mk. III was the first attempt to improve the basic Spitfire design and introduced several features which were used on later marks. Powered by a Rolls Royce Merlin XX developing 1,390 hp (1,036 kW) the wingspan was reduced to 30 ft 6 in (9.3 m) and the area reduced to 220 square feet (20.4 sq m) while the overall length was increased to 30 ft 4 in (9.2 m). The strengthened main undercarriage was raked forward two inches, increasing ground stability and had flaps to fully enclose the wheels when retracted. The tail wheel was also made fully retractable. The windscreen was redesigned, with a built-in, internal laminated glass, bullet proof panel and optically flat, laminated glass quarter panels.

Only one Mk. III N3297 was built and first flown on 16 March 1940. Although the new Spitfire was developed to replace the earlier marks on the production lines, a decision to allocate the limited supplies of Merlin XX to the production of Hurricane Mk. II and Boulton Paul Defiant Mk. II to enable the latter types to remain combat-worthy meant that the Spitfire Mk. III lapsed. Priority then focused on the Mk. V series. Fortunately, many of the innovations tested on the Mk. III were later incorporated in the Mk. V production.

Later during its life N3297 became a power-plant development airframe, the wings were replaced with standard Type A and the aircraft was delivered to Rolls-Royce at Hucknall. A prototype Merlin 61 was installed, in effect making this aircraft (renumbered the ‘type 348’) the prototype Mk IX.

Spitfire Mk. V (Supermarine type 331)

Late in 1940, the RAF predicted that the advent of the pressurized Junkers Ju 86P bomber series over Britain would be the start of a new sustained high altitude bombing offensive by the Luftwaffe, in which case development was put in hand for a pressurised version of the Spitfire, with a new version of the Merlin (the Mk VI). It would take some time to develop the new fighter and with the advent of the new German Messerschmitt Bf 109F, an emergency stop-gap measure was needed as soon as possible: this was the Mk V.

The basic Spitfire Mk. V was a Mk. I with the new Merlin 45 series engine. This engine was a variant of the Merlin XX with the second stage of the supercharger omitted and a new single-speed single-stage supercharger design. It delivered 1,440 hp (1,074 kW) at take-off. Improvements to the carburettor also allowed the Spitfire to use zero gravity manoeuvres without any problems with fuel flow. Several Mk. I and Mk. II airframes were converted to Mk. V standard by Supermarine and started equipping fighter units from early 1941.

One of the earliest production Spitfires Mk. V, showing many features carried over from Mk. I production: early type exhausts, “A” wing and canopy. The aircraft has a de Havilland propeller. Only 94 Mk. Va’s were built and many of them were simple conversions of the Mk. I. Curiously, the aircraft is still equipped with HF radio with wire antenna.
[NASA]

The majority of the Mk Vs were built by Castle Bromwich. Initially, two variants of the Mk V were produced, with several sub-series:

  • Mk. Va
    Continued with the Type A wing with the eight .303 Brownings. This version could reach a top speed of 375 mph (603 km/h) at 20,800 ft, and could climb to 20,000 ft in 7.1 minutes. A total of 94 were built. One well-known Va was W3185 D-B flown by Douglas Bader when commanding the Tangmere Wing in 1941.
     
  • Mk Vb
    Starting with converted Spitfires Mk. Ib, the Mk. Vb became the main production version of this mark. Along with the new Merlin 45 series the B wing was fitted as standard.

As production progressed, many changes were incorporated, some of which which became standard on all later Spitfires. For example, the round section exhaust stacks were changed to a “fishtail” type, marginally increasing exhaust thrust. Later, some 1944-production Mk. Vs and Vb were fitted with six exhaust stacks per side, similar to those of Spitfire IXs and Seafire IIIs.

The features probably most often associated with the Spitfire Mk. V are represented in this photo of a mid-production Mk. Vb. Note the “B”-type wing, Rotol Jablo propeller with long spinner, fishtail exhausts and deepened oil cooler with circular air intake.
[US National Archives]

After some initial problems with the Mk I oil coolers a bigger oil cooler was fitted under the port wing. This could be recognised by a deeper housing with a circular air intake.

Many Spitfires in the Mk V and Mk IX family used main wheels which had a removable disc type hub fitted. This was used in an attempt to reinforce the wheel against the additional weight imposed by the carriage of drop tanks or bombs.

Two new “blown” cockpit canopies were introduced in an effort to further increase the pilot’s head-room and visibility.

A wide-bladed Rotol constant speed propeller of 10 ft 10 in (3.29 m) diameter was able to be fitted, superseding the de Havilland unit. The propeller blades were now made out of “Jablo” compressed wood and the spinner was longer and more pointed.

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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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