Spitfire Mk III
It's no secret at my local club that I'm a bit of a Spitfire fanatic. The aircraft and it's variants make up 20% of my stash of over 300 kits – built and awaiting. While I've collected a number of references, until my recent acquisition of “Spitfire, the History” by Morgan & Shacklady they were sorely lacking. StH contains the entire history, as it was known in 1985, for every serial number of the Spitfire!
So my amateurish research now has some good references that allows me to dabble in some unique variants, one-offs and the usual mass quantities. Between each, and some nice help from the internet I can cross reference specifics and gain a somewhat high level of confidence I've got the subject right.
As an example, I'd recently decided to fill in between the major marks with some of the prototypes and small production examples. Specifically these are the Mk III, IV, VI, and XII. Now, those latter two were actually fielded as a combat aircraft, in about 100 machines each. The VI being the high altitude Merlin engined variant and the XII being the (mainly) low-altitude initial Griffon engined variant. Of the two Mk IV prototypes, one acted (effectively) as the Mk XII prototype and the other (as the later redesigned and redesignated Mk XX) the prototype for the Mk 20 series (21, 22 & 24).
But I'm digressing a bit. The Mk III was to be the ultimate Spitfire for 1941-42, two prototypes authorized in 1939 and delivered (first prototype) in early 1940, before France was invaded. Refined construction and design to get as clean an airframe as possible, plus the improved Merlin XX engine (later installed in the Hurricane II) with another 300 hp and better mid-high altitude performance. Supermarine was also working on designs for fighters that would (hopefully) follow the Spitfire in 1942, but that's a different story.
Flush rivets meant a smooth surface. The wings were clipped, two feet more than on subsequent marks, to reduce drag even more; the resulting short span ailerons were later seen on the Mk VII/VIII and Griffon wings. The wing was to eventually be the “universal” or “c” wing, installed on the second prototype; the first had the “a” wing. The tail wheel was now retractable. The main landing gear was fully enclosed, using the outer flap design from the original K5054 prototype.
The engine, with its single stage supercharger, required a 4 inch extension to the nose. This was accomplished by angling the firewall forward and extending the upper fuel tank cover by four inches, and lengthening the lower engine panels forward of the firewall.
Additionally, in order to get back some of the range lost by the higher performance engine, the fuselage fuel tank was increased in size using that additional space created by the forward angle of the firewall.
The new engine required a larger oil cooler and improved radiator. The former was of circular (not “D” shaped as one the Mk I/II) and was later implemented on the Mk V early in its production. The latter was completely redesigned and while it influenced the later Griffon radiator designs, was not implemented in production. It's unique, boundary layer design was only on the Mk III.
The Mk III was used to develop a new canopy/windscreen with the armor internal and integrated into the windscreen. This was later implemented on the Mk V production line. It was also tested with various props, ultimately resulting in the Rotol Jablo props later implemented on the Mk V.
As can be seen, much of the Mk III design improvements were implemented on later marks. A contemporary to the Mk V, it influenced the Mk V, but not so much as did the Me-109F. The first prototype Mk III was undergoing testing in late 1940 and into 1941, at about the same time as the Me-109F was being tested and fielded in France. The Me-109F outclassed the Mk I/II and an interim solution was needed until the Mk III could be delivered. A development of the Merlin XX engine, the Merlin 45, was mated to a Mk I airframe and tested in early 1941. My mid-1941 it was decided to shift Mk I/II production to this variant, as its performance easily matched the Me-109F. Shortly after this, the Mk III was canceled and all authorized production shifted to the Mk V in order to get the RAF fighter squadrons upgraded as quickly as possible.
The first Mk III prototype, N3297, was used from March 1940 to late 1941 and tested many refinements as described above; it's wing was replaced with a standard “a” wing in September 1940 as the short span wing was not liked by the RAF at the time. The second prototype, W3237, was used to develop the “c” or “universal” wing and was later upgraded to the Merlin 60 series engine ultimately used on the Mk VII/VIII/IX/XVI in production. Effectively W3237 was the prototype for these marks, although not documented as such.
Not all current publications agree on the Mk III details, some even insist there was only a single Mk III. But with some help from Spitfire researchers on the web I've got enough details to model this esoteric subject.
Modelling the Mk III
While only two were made, their configurations changed often during their lifetimes. So modeling the either of the Mk III's falls into the “it depends” category. W3237, in my mind's eye, would look very similar to a Mk V in its early configurations and more like a Mk VIII later in life. N3297 varied significantly over its life, early on it was very different from a Mk I/II or any other mark in appearance but later it began to look like a Mk Va after its wing was replaced in September. So to have a unique airframe in my collection yet have things I can point to that were in later marks, I decided to tackle N3297 as she (probably) looked in August 1940.
Spitfire Mk III, N3297, August 1940
I looked at 3 basic kits as my possible donors:
Italeri Mk IXc
Airfix Mk Vb
Airfix Mk Ia (new tool)
All donor kits will require a scratch built radiator and retractable tail wheel as these are so different from production kits. Also, the wing span will be shortened as well as the ailerons.
The Italeri kit is too short in the nose for a Mk IX, but too long by about the right amount for a Mk V making it just right in length for a Mk III! However the wing is all wrong as it's a “c” wing with two large symmetrical radiators. It also has a horrid fuselage that requires fairings from the sides to the wings, and the engine cover is not bulged enough for Merlin rocker covers. Lots of surgery required...
The Airfix Mk Vb is inexpensive and easily modified to the Mk III. It has all the right add-on parts for the later prop, oil cooler and canopy. Major issue is the wing, being the “b” wing but that's easily corrected. Surgery required is the the plug at the firewall.
The new tool Airfix Mk Ia is also inexpensive and just as easily modified as their Mk Vb kit. In it's favor is an already configured “a” wing. Additionally, the detail is much better; and the latest release (#2010) also includes the larger oil cooler as used on the Mk V.
I've decided to model the Mk III using the Airfix Mk Ia/IIa kit as the donor (#2010).
The Mk III looks much larger than the Fujimi and Airfix Griffon radiators simply because it is. These kit radiators seem a bit anemic now that I've fabricated the III radiator, however I must admit that my attempt may be inaccurate. I do not have drawings, only a hint at depth and some poor photos that give basic details, but not size. What I did was use the Airfix Ia radiator as a prototype, and as I fabricated I ensured the new radiator would fit length/width wise. 
