Supermarine Mk III - Angled firewall and enlarged internal fuel tanks

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Also posted on the Spitfire forum - I have been referred here by other another forum member I note that the Mk III had an angled firewall, and I believe re-designed and enlarged fuel tanks with the Merlin XX installation, and with the later Merlin 60 series installation. I believe that the Merlin Mk VII/VIII - and from those, the Mk XIV and later with the Griffon - were derived from the Mk III, but whilst the Mk XIV also had an angled firewall (same or different I don't know), the Mk VIII, with a Merlin, reverted to a vertical firewall, like the I/II/V/IX/XVI series. Why? Seems odd! I can't see any mention of this in the limited information on the Mk III I can find on the web. I have the Harleyford book on the Spitfire, but not the Morgan and Shacklady book which seems to be definitive. References to changes to engine, cowlings, intakes, radiators, wing, ailerons, undercarriage, windscreen, tailwheel etc. - all can be found in these sources. But not the angled firewall... Anyone have any ideas or references I can follow up? Many thanks John
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There is infos about the Mk.III in the latest issue of The Aviation Historian with profiles by Juanita Franzi.
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Canting the top of the firewall. I would suggest that it was to transfer spare usable volume from the engine bay to the fuel bay. The redesigned and very close fitting cowlings on the Mk III suggest that the oil tank has been relocated from the engine bay to the fuel/fluids bay...all pointers to the evolution of the Griffon engine installations. Of particular interest is the substantial reduction in wingspan over and above the standard clipped wing we see in the Mk V. Mark http://img.photobucket.com/albums/v634/Mark12/Mark12104/3-N3297PeterArnoldCollection001_zps36d73fff.jpg
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I see from the picture that it has the hinged extensions on the U/C spats; was this more common than I thought?
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Pointed quote:-I see from the picture that it has the hinged extensions on the U/C spats; was this more common than I thought? I have only ever seen this on the original prototype and this single Mk III. The Mk III was a comprehensive exercise in streamlining the airframe. Note the retracting tail wheel, the 'smooth' front windscreen and canopy interface, the 30'-6" wingspan...and the gloss finish camouflage suggesting this was second quarter 1940 latest. Mark

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Is it an angled firewall or simply an angled panel? My understanding is that all Spitfires had the same fuselage from firewall (or should that just be a frame number?) to rudder post. The Mk.III has a longer nose because of the gearbox on the two-speed Merlin XX. On the Hurricane this was allowed for by a cowling that was longer overall, but the panel lines remained vertical, which was also the choice for the later Merlin 60 series Spitfires. The important next step from the Mk.III was the application of its universal wing on the Mk.Vc, with the related wheel bay strengthening and raked-forward undercarriage.. The hinged wheel door extensions were not more common, but (as on other types eg Bf109K) seen on the Mk.III as a potential drag reduction that was not justified by the costs of production and maintenance in the field. The Mk.III was driven by three major design needs - higher speed, higher altitude performance, and a redesigned stronger wing/undercarriage. The use of what was seen as the RAF's standard Merlin (in mass production by Ford, and used on many other types) was part of this. The short wing unfortunately penalised the superior altitude performance given by the Merlin XX. It was then tried with the standard wing but by then the appearance of the Merlin 45 had given the better performance without the major changes and hence production delays that would have been required by the structural changes of the Mk.III. The Mk.III improvements, such as the integrated armoured windscreen, were later introduced on a more drip-feed basis. PS The enlarged main fuel tank on the Mk.VII/VIII was achieved by deepening the tank, at the cost of additional maintenance time and difficulty. It does seem a shame that this never made it to the main production lines, but again this would only have come at the cost of reduced production during the changeover.
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It does appear to be an angled firewall - and Peters notes make perfect sense; there is no provision for an oil filler in the cowling shown in that picture, so it must have been the earliest case of moving the oil tank to the fuel bay.
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This shot shows the angled firewall above the datum longeron to better effect and also the experimental 'smooth' front windscreen. http://img.photobucket.com/albums/v634/Mark12/Mark12105/3-N3297PeterArnoldCollection001a_zpsf991c924.jpg Later N3297 was fitted with a Dowty propeller and the first indication of what was to be the standard internal armoured windscreen introduced late on Mk V and all Mk IX. http://img.photobucket.com/albums/v634/Mark12/Mark12105/3-N3297PeterArnoldCollection002a_zps7c1b81e1.jpg Mark
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Of particular interest is the substantial reduction in wingspan over and above the standard clipped wing we see in the Mk V.
Peter, would you happen to know whether the MkIII wingtip was the same as that used on the Speed Spitfire, or was that unique?
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Pointed quote:- Peter, would you happen to know whether the Mk III wing tip was the same as that used on the Speed Spitfire, or was that unique? No, visually not the same, as seen in this shot of the plan-form. The wing span of the Speed Spitfire was 33'-8" suggesting just the rounding off of a basically standard wing tip. The Mk III main wing was shortened by one outboard rib with a corresponding shortening of the aileron. This would have made the chord at the wing/tip interface longer. Mark http://img.photobucket.com/albums/v634/Mark12/Mark12106/1-K9834SpeedSpitfirePeterArnoldCollection001a_zpsa334a364.jpg
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Yes indeed, quite different, I should have spotted that. Do you think that is just a super-fine surface finish or is it possible that the tip is an integral part of the wing, as on the prototype, rather than a separate bolt-on component?

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There is infos about the Mk.III in the latest issue of The Aviation Historian with profiles by Juanita Franzi.
Yes, thanks. That was, in part, what prompted the question, which was an old query of mine. I have asked TAH if they have any more information.

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The enlarged main fuel tank on the Mk.VII/VIII was achieved by deepening the tank, at the cost of additional maintenance time and difficulty. It does seem a shame that this never made it to the main production lines, but again this would only have come at the cost of reduced production during the changeover.
Thanks for this - so not then a forward extension of the fuel tank

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Peters notes make perfect sense; there is no provision for an oil filler in the cowling shown in that picture, so it must have been the earliest case of moving the oil tank to the fuel bay.
Ah - that would probably be it. I see this is referred to in the documentation on the Mk XIV, but is not referenced on the Mk III documentation. It now makes sense - simpler to retain the oil tank provision as it was on the Merlin variants, but with the different engine mounts on the Griffon variants, use the method trialled on the MK III. Many thanks

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Many thanks Thanks to all for your replies - all now clearer. Are the photos shown in the posts from Morgan and Shacklady book? I don't have that in my collection, and I haven't seen most of these. Thanks again. Regards, John
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Yes indeed, quite different, I should have spotted that. Do you think that is just a super-fine surface finish or is it possible that the tip is an integral part of the wing, as on the prototype, rather than a separate bolt-on component?
Although the wing tip profile in plan-form is regular, in other views the transition from the end rib to this shortened tip looks a little bit 'lumpy' suggesting these were 'one off' bolted on tips to a standard wing. There is no obvious join line at the wing/tip interface and my assumption is that the whole area if not the whole aircraft has been 'filled', flatted and super finished to get that last 'mph'. Mark http://img.photobucket.com/albums/v634/Mark12/Mark12107/SpeedSpitfirePeterArnoldCollection002_zpsccf29d5b.jpg

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The Mark III fuselage, just as with the Hurricane II compared to the Mk.I, was 4" longer than the previous (and subsequent Mk. V & VI) fuselages. This led to the u/c being raked forward 2", simply by inserting a wedge between the pintle and the mainspar; this change was continued on the Vc and later Marks. With the extra weight "up front," it's entirely possible that the oil tank was moved aft solely so as to minimise its effect on the CoG. Modifications to the Merlin 45 air intake meant that it, too, was slightly longer, but this was circumvented by turning the carburettor controls through 180 degrees, which tucked them under the engine, meaning it could still fit into the original Mk.I space, leaving the cowlings untouched.
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Where to put the oil and the fuel. Designed basically as an interceptor, the Spitfire was always critical for fuel, compounded by increased demand as more powerful Merlin and later Griffon engines were introduced. On the first Griffon installation, the Mk XII, basically a Mk V with a 'single' stage engine, the oil tank was initially re-located behind the pilot. With the introduction of the two stage 'long' Griffon on the Mk XIV, space was freed up in the engine bay to permit the firewall to be canted forward and install the oil tank partially in this space in conjunction with further fuel installed in the wing leading edges. Image:- Geoff Spinks http://img.photobucket.com/albums/v634/Mark12/Mark12108/18-SM969DX06May081stenginerunGeoffSpinks154a_zps7241de7a.jpg The Seafire XV/XVII, basically developed from the Mk XII with upright firewall, reversed this oil tank, biting in to the top fuel tank but compensated by wing leading edge tanks. http://img.photobucket.com/albums/v634/Mark12/Mark12108/17-SX300-SX336-01-001a_zpsecaf90c4.jpg Mark

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Just a further question on the subject, I understand that there was a second Mk111 built. What I'd like to know was this one built to full Mk 111 spec. as per N3297? Thanks in advance.

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I believe that only one example flew with the short wing. STH is a little imprecise: it describes W3237 as having the fuselage mods and the Merlin XX, going on to discuss weight differences between the A, B and universal wings, apparently as referring (but not necessarily fitted) to this airframe. There would seem to have been little point in having another short wing aircraft.

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Ok thanks Graham for that gen.