Supermarine Spitfire Variants – The Initial Merlin-Powered Line

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Supermarine Spitfire Variants

The Initial Merlin-Powered Line

Mk. Vb was the first production version of the Spitfire to use “clipped” wingtips as an option, reducing the wingspan to 32 ft 2 ...
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Mk. Vb was the first production version of the Spitfire to use “clipped” wingtips as an option, reducing the wingspan to 32 ft 2 in (9.8 m). The clipped wings increased the roll rate at lower altitudes.

A clipped-wing Spitfire Mk. Vb of No. 401 (Canadian) Squadron. Although this aircraft is a Mk. Vb, it features a later type canopy with integral windscreen usually associated with Mk. Vc’s.
[Library and Archives Canada]

Several different versions of the Merlin 45/50 family were used, including the Merlin 45M which had a smaller “cropped” supercharger impeller and boost increased to +18 lb. This engine produced 1,585 hp (1,182 kW) at 2,750 ft (838 m), increasing the L.F VB’s maximum rate of climb to 4270 ft/min (21.6 m/sec) at 3850 ft.

The tropical version of the Mk Vb could be identified by the large Vokes air filter fitted under the nose. This intake reduced the speed of the airflow to the supercharger, thus decreasing its efficiency and the performance of the aircraft. It was also fitted with desert survival gear behind the pilot’s seat. The Vb Trop was the first Spitfire variant to be used in large numbers overseas, playing an important role in wresting air superiority from the Luftwaffe over Malta.

Many Vb Trops were modified by RAF Depots in Egypt through replacing the Vokes filter with locally manufactured “Aboukir” filters, which were lighter and more streamlined. The new design also went some way to restoring the speed and “ram effect” of the airflow to the supercharger. Two designs of these filters can be identified in photos: one had a bulky, squared off filter housing while the other was more streamlined. These aircraft were usually fitted with the wide blade Rotol propeller and clipped wings.

The Vokes filter located under the nose of tropical Mk. Vs spoiled the graceful lines of the Spitfire in this area and had adverse effect on performance, but its use was a necessity in the Mediterranean theatre of operations. This is a Mk. Vc (Trop), with an external 45-gal slipper tank.
[US National Archives]

Spitfire Mk. Vc (Supermarine type 359)

As well as having most of the standard Mk V features this version had several important changes over the earlier Mk Vs, most of them modifications which were first used on the Mk III. First and foremost, the Type C “universal” wing was used. Most Vc wings had a large, bulged fairing on top of the wing to provide clearance for the ammunition feed motors of two Hispano cannon in each wing: because two cannon were seldom fitted these fairings were later reduced in size to more streamlined shapes.

The initial aim with the “C” wing was increasing the armament to four 20-mm Hispano cannon and indeed, initial production Spitfires Mk. Vc were delivered with this type of armament. Combat experience in the Mediterranean had shown that such heavy armament was impractical, and in time the production standard shifted towards two cannon and four machine guns.

The structure was re-stressed and strengthened. Also, the Spitfire Mk. Vc was the first Spitfire able to carry a range of specially designed “slipper” drop tanks which were fitted underneath the wing centre-section. Small hooks were fitted just forward of the inboard flaps; when the tank was released these hooks caught the trailing edge of the tank, swinging it clear of the fuselage.

A deeper radiator fairing was fitted under the starboard wing, and a larger oil cooler, with a deeper, kinked air outlet, to port.

The Mk. Vc introduced the new windscreen design first seen on the Mk III. Later, a number of Mk. Vbs were also fitted with this type of windscreen.

Late in the Mk. Vc production the elevator horn balances were increased in area. This modification followed the identical one carried on the Spitfire Mk. IX then in production to increase the lateral stability of the aircraft.

The bulk of the Mark Vc were sent overseas with and were used either in North Africa and the Mediterranean or in the Far East.

In total, Mk. V production was 6,487, consisting of 94 Mk Va, built by Supermarine, 3,911 Mk Vb, (776 by Supermarine, 2,995 Castle Bromwich and 140 Westland) and 2,467 Mk Vc, (478 Supermarine, 1,494 Castle Bromwich, 495 Westland) plus 15 PR Type Fs by Castle Bromwich.

With the advent of the superb Focke Wulf Fw 190 in August 1941 the Spitfire was, for the first time, truly outclassed hastening the development of the “interim” Mk. IX.

1944-production Spitfires Mk. Vc featured several modification common with the Mk. IX which by then was the main production variant of the aircraft. Among them are the individual exhaust stacks, modified elevators with enlarged horn balances and the “C” wing with smaller, streamlined cannon blisters.
[US National Archives]

Spitfire Mk. VI ( Supermarine type 350)

At the time that the Mk V was placed in production there were growing fears that the Luftwaffe were about to start mass producing very high flying bombers such as the Junkers Ju 86P, which could fly above the reach of most fighters of the time. It was decided that a new Spitfire variant would be required with improved high altitude performance.

The H.F Mk. VI differed from the Mk. Vb in the following ways:

For increased thrust at high altitudes it had a four-bladed Rotol propeller of 10 ft 9 in (3.27 m) diameter.
To counter the physiological problems encountered by pilots at high altitudes, it had a pressurised cabin with a pressure differential of only two pounds per square inch. To achieve this the forward and rear cockpit bulkheads were completely enclosed, with all control and electrical cables exiting though special rubber sealing grommets.

The side cockpit door was removed and replaced by a continuous fuselage skinning as on the starboard side of the cockpit.

The canopy was no longer a sliding unit: externally there were no slide rails. Once the pilot was in, the canopy was locked in place with four toggles and sealed with an inflatable rubber tube. It could be jettisoned by the pilot in an emergency.

Pressurisation was achieved by a compressor on the starboard side of the Merlin, fed by a long narrow intake below the starboard exhausts
The windscreen of production Mk VIs was the same as the type fitted to the Mark III and some Mk Vs with the added refinement of a hinged clear-view panel in the port quarter panel.

To help smooth out airflow around the wingtips the standard rounded tips were replaced by extended, pointed versions thus increasing the wingspan to 40 ft 2 in (12.2 m). Otherwise the wings were of Type B.

100 of the Mk VIs were built by Supermarine. Only two units, 124 Squadron and 616 Squadron, were fully equipped with this version, although several other units used them in small numbers as a stop-gap. The threat of a sustained high altitude campaign by the Luftwaffe didn’t materialise and, more often than not, the Spitfire VIs were used at lower altitudes where it was outperformed by conventional Spitfires. At high altitudes it was discovered that modified Spitfire Vs could perform almost as well as the Mk VI. At low levels, especially, pilots were often forced to fly with the canopy removed because the cockpit would get uncomfortably hot and they weren’t confident it would be possible to jettison the canopy in case of an emergency.

Spitfire P.R Mk. XIII

The PR Mk. XIII was an improvement on the earlier PR Type G with the same camera system but a new engine, the Merlin 32, which was specially rated for low-altitude flight. It carried a light armament of four .303 Browning machine guns. The first prototype Mk. XIII was tested in March 1943.

Twenty six Mk. XIIIs were converted from PR Type G, Mk. IIs or Mk. Vs. They were used for low level reconnaissance in preparation for the Normandy landings.

The prototype Spitfire Mk. XIII, featuring two vertical and one oblique camera installed in the area behind the cockpit. In other respects this mark was similar to the Mk. Va.
[Crown Copyright]

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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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