From Nighthawk to Spitfire

by John K Shelton

  While the F.7/34 agreement that Mitchell was working with referred to a four-gun ‘experimental’ fighter, the Air Ministry requirement F.10/35 had now been issued and it repeated an earlier F.5/34 call for at least six, and preferably eight, guns to ‘produce the maximum hitting power possible in the short time available for one attack’. One suspects that Supermarine and Vickers were looking well beyond their four-gun model and towards the F.5/34 requirement when the elliptical wing shape was finally decided upon. When Squadron Leader Ralph Sorley, in charge of the Operational Requirements section, asked if Mitchell could include four additional guns without trouble or delay, he received straightaway a quite positive response. Also, the elliptical wing, which allowed for the extra armament to be sited even further outboard, permitted their ammunition containers to be so positioned that, when emptied in action, they would not adversely alter the trim of the aircraft.

  The Piaggio P.7.

  The Heinkel He.112B.

  Spitfire.

  Mitchell’s modified elliptical design would, theoretically, have required the optimum main spar position to curve or at least slope backwards – as projected in the Supermarine No. 300000 drawings shown earlier. However, such an arrangement would have consequent constructional and weight/strength problems and so Mitchell typically selected a less complex arrangement whereby the main spar was set at right angles to the fuselage centre line. This structural consideration had the advantage of making it easier to align the wing ribs which were to be set at progressively decreasing angles of incidence as they approached the wing tips. It also was able to follow more closely the straighter wing leading edge mentioned earlier.

  The Thin Wing

  The first flights of the company’s Stranraer in 1934, with its thinner aerofoil, now clearly supported the developing view that the way forward for monoplanes was not represented by the thick, relatively lightly loaded wing of the Type 224 prototype. Ernest Mansbridge told Price that:

  Choosing the thick section wing was a mistake when we could have used a modified, thinner section as used on the S5 floatplane … We were still very concerned about possible flutter, having encountered that with the S4 seaplane. With the S5 and S6 we had braced wings, which made things easier. But the Type 224 was to be an unbraced monoplane, and there were not many of these about at the time.

  To a designer, considering the design of a wing much thinner than usual, the value of an elliptical shape was already appreciated, permitting, as it did, lightness with strength; but this concern was in order to support Mitchell’s pre-established aerodynamic concerns. As C.G. Grey had noted:

  An interesting point about those Curtiss biplane racers [of 1923–1926] was that the wings came almost to a knife-edge in front [producing an extremely low thickness/chord ratio of 6 per cent]. One of the American technical people told me at the time that they had come to the conclusion that, at the speed which these machines reached, the air was compressed so much in the front of the leading edge that it paid to cut it. I passed the information on to R.J. Mitchell of Supermarine’s who went into the idea quite deeply, and though he could not quite put a cutting edge on his Schneider Trophy monoplanes of 1927–1929 and 1931, he used the thinnest possible wings, and won every time.

  Harry Griffiths put the matter thus:

  He had one strong fetish, namely that for maximum performance the frontal area of an aircraft had to be as small as possible, hence … his insistence on the thin wing on the Spitfire against the advice of the experts at Farnborough.

  In this connection, Clifton’s later comments on Mitchell’s doubts about information derived from model testing deserve recording:

  I think that Mitchell decided to make the wing as thin as he did, and I wouldn’t like to be positive about this, but my recollection was that it was against some advice from the National Physical Laboratory in that case where wind tunnel tests, I believe, showed that there was no advantage in going below a thickness chord ratio of 15 per cent, whereas, the [Spitfire] wing was 13 per cent at the root and 6 per cent at the tip. I believe that this was due to the fact that at that time the question of the transition from laminar to turbulent flow in relation to the difference between model and full scale wasn’t understood and subsequently it was found that when you made proper allowance for that, there was an advantage, as the testing could be shown to prove, in going thinner.

  At about that time, Hawkers had been advised by the National Physical Laboratory that their new wind tunnel results had shown no drag penalty with the thicker Hurricane wing. However, the laboratory scientists later found this advice to be incorrect – they attributed their earlier views to high wind tunnel turbulence, not appreciated at that time.

  It is known that, by the early 1930s, Mitchell had felt that wind tunnel tests with small models might not be very helpful and so one suspects that aesthetics and intuition had quite a lot to do with the final choice of the Spitfire wing shape. (One notes that Mitchell’s tail surfaces produced a more aesthetically pleasing general arrangement than those of the He.112B illustrated above.) Joe Smith was surely referring mainly to such considerations when he recorded the following description of Mitchell at the drawing board:

  He was an inveterate drawer on drawings, particularly general arrangements. He would modify the lines of an aircraft with the softest pencil he could find, and then re-modify over the top with progressively thicker lines, until one would be faced with a new outline of lines about three sixteenths of an inch thick. But the results were always worthwhile, and the centre of the line was usually accepted when the thing was redrawn.

  By way of a rejoinder, one is reminded of Mitchell’s forthright comment to Shenstone about the Spitfire wing: ‘I don’t give a bugger whether it’s elliptical or not, so long as it covers the guns.’ The designer is clearly acknowledging that various design compromises must shape the final outcome of a project, but one suspects he was also playing down his aesthetic concerns – Shenstone did say that the disclaimer was made ‘jokingly’.

  So far, little mention has been made of the input of Beverley Shenstone, who joined Supermarine at the end of 1931 and who soon became Mitchell’s chief aerodynamicist. Very substantial (and, it would appear, largely unsupported) claims have been made in a recent book for his influence upon the shape of the Spitfire wing – particularly the straight leading-edge component, the shape of the trailing edge and the aerofoil selection.

  One must certainly expect that the advice of this brilliant young man would not have been ignored. In Chapter 1 it has been reported how Mitchell used to call in the leaders of relevant sections and get them arguing among themselves. He would listen carefully, making sure that everyone had said what he wanted to, and then either make a decision or go home and sleep on it. Joe Smith has also indicated that he considered this quality contributed to Mitchell’s leadership: ‘In spite of being the unquestioned leader, he was always ready to listen to and to consider another point of view, or to modify his ideas to meet any technical criticism which he thought justified …’

  One might speculate that Mitchell felt confident to pursue the very thin wing, against the technical advice mentioned above, having been supported by detailed and persuasive theoretical submissions from Shenstone. And the very final shape of the Spitfire wing might also owe a great deal to the younger man’s views, as he had had direct experience of German aerodynamic theory that was well in advance of contemporary British practice. It would fit with Mitchell’s habitual management style that he soon recognised that the new man might well help Supermarine to progress beyond their already acknowledged lead in high speed design. We should give the chief designer credit for not being so flushed by his earlier Schneider Trophy successes that he did not appreciate what the younger man might contribute.

  On the other hand, it has been shown that Mitchell had been considering elliptical or thinner wings since the middle 1920s, and that gun and undercarriage housing (as well as aerodynamic arguments) would have been important considera
tions for the whole design team. Thus, recently reported diary entries by Shenstone about this period in his career are interesting:

  The elliptical wing was decided upon quite early on … The ellipse was simply the shape that allowed us the thinnest wing with sufficient room inside to carry the necessary structure and the things we wanted to cram in … Joe Smith, in charge of structural design, deserves credit for producing a wing that was both strong enough and stiff enough within the severe volumetric constraints.

  It is noteworthy that the chief aerodynamicist is generous in his praise of Smith’s structural input and perhaps he is too self-effacing about the importance of his own contributions, speaking impersonally about ‘our’ purposes and what ‘we’ wanted to achieve. Forty years later, at the Mitchell Memorial Symposium in Southampton, Shenstone again makes no special claims for his own input:

  I don’t think R.J. cared at all what the Germans were doing but he did care about the shape of wings, but he didn’t copy anything. I think all of us at the time realised that the thinnest wing can often be the best, whereas earlier, people were afraid of very thin wings in case they broke off. I think the essential thing is that Mitchell took advantage of everything he could which would improve his aircraft. Certainly Mitchell always did the thing which should be done.

  At the same symposium, Clifton said, ‘Meanwhile Mitchell was moving on to … a very thin wing against expert advice … Mitchell was trying to put the thing together to get the maximum possible result.’ Shenstone was in the audience and one might have expected some gracious reference to the importance which has recently been claimed for him, but none is recorded. Also, in the published account of the meeting, C.F. Andrews submitted a letter he had received from Shenstone in which the latter emphasised their volumetric considerations:

  I do not think that the He.70 had much direct influence on [the] Spitfire’s elliptical wing. Various wing plan forms were sketched for [the] Spitfire, and the real down to earth reason for the elliptical wing was the fact that the elliptical taper is gradual near the fuselage and can be less than that for a straight taper wing, thus giving more space for retracted undercart, and in this case also for guns.

  I remember that I pointed out to Mitchell that the elliptical wing was optimum for induced drag, but he said he didn’t care whether it was elliptical or not as long as it had room for guns and undercart … the real advantage of the elliptical wing turned out to be its low induced drag at very high altitudes, such altitudes not having been considered during the design, but realised during the war … [See PR19, p.189. This last comment would seem to support the view that Mitchell was at least influenced by the aesthetics of the elliptical shape.]

  I think that Mitchell was fed up with wind tunnel tests after the F.7/30 [Type 224] disappointment. The only wind tunnel tests done on the Spitfire prototype were on a fairly large half model (sliced vertically stem to stern) for the sole purpose of studying the air flow through the underwing radiator.

  It remains a matter of conjecture as to what interpretation one should put on Shenstone’s statement that he ‘pointed out’ the advantage of the elliptical wing and the thin wing, or how far Mitchell had already made up his mind on these matters. Will we ever know exactly how the two minds met on this issue? We have seen that Mitchell had appreciated the value of the elliptical wing and of the thin wing before Shenstone joined Supermarine, but at least we can surely accept that Shenstone supplied detailed aerodynamic calculations which Mitchell took careful note of, and it may be that credit should be given to Shenstone for not trying to deservedly share the limelight with his famous chief designer.

  Beyond that, as we have seen, the Mitchell Symposium discussion does raise questions about the precise influence of Shenstone on the complex of considerations which led to the eventual shape of the Spitfire (its importance as a document in the Mitchell story seems hardly to have been noticed elsewhere). Perhaps one can do no better than quote from a Southampton Royal Aeronautical Society (RAeS) member’s summing up at this symposium:

  During the discussion, Mr Clifton was asked the origin of the elliptical wing form. No authoritative reason was put forward. I am inclined to think that it was the logical result of integrating aerodynamic and structural requirements. Comparing the F.7/30 development with the Spitfire, changes are evident which must have been consciously made during the project stage.

  The main spar, previously swept back, was set normal to the fuselage axis. The span, wing area and thickness to chord ratio were reduced. The straight tapered wing gave place to the elliptical form of lower aspect ratio. Thus the greater and more constant chord in the inner regions of the wing gave more space for the landing flap, undercarriage, radiator and gun installation, and provided sufficient thickness for a good structure.

  For optimum bending strength the spar should have been placed at 30 per cent chord but, as this would have encroached on installation space, the 25 per cent chord position was a better choice. This must have been intentional as it was also the aerodynamic datum for the varying incidence which was progressively reduced from root to tip. From the unswept spar at 25 per cent chord the familiar asymmetric ellipse naturally followed.

  The choice of a common aerodynamic and structural datum simplified work in the drawing office and hence manufacture. The unswept spar with the ribs at right angles was aerodynamically and structurally good, and simplified manufacture. The simple basic structure was the first step to low structural weight, for otherwise all the refined detail design would have been less effective …

  Considering these points as a whole, and remembering the lack of precise aerodynamic date in those days, so many imponderables could only have been resolved by R.J. Mitchell’s intuitive judgement; as wind tunnel work was limited to tests on spinning and the ducted radiator. As I see it, the elliptical wing is to the Spitfire as the ogee wing is to Concorde. They look right – and are right.

  NAMING THE SPITFIRE

  When did R.J. Mitchell say that ‘Spitfire’ was ‘a bloody silly name’ for his fighter? Dr Alfred Price noted, via the logbook of test pilot George Pickering, that the earlier Type 224 was known as the Spitfire some time before July 1935 and so Mitchell’s remark might have been made about this time or earlier – when this fighter was so designated in a brief announcement by Supermarine in 1934:

  The ‘Spitfire’ is a single-seat day and night fighter monoplane built to the Air Ministry specification. It is a low-wing cantilever monoplane with the inner sections sloping down to the undercarriage enclosures. It has a Rolls-Royce ‘Goshawk’ steam-cooled engine with condensers built into the wing surfaces. Armament consists of four machine guns. No further details are available for publication.

  Thereafter, the revised project, following the disappointment of Type 224, was usually referred to in the works as ‘the fighter’ – after all, his most beautiful racer was only ever known as the ‘S4’. Gordon Mitchell’s book copies a Supermarine document of 29 February 1936 in which the soon-to-fly aircraft was referred to merely as the ‘Modified Single-seater Fighter K5054’ and he also noted that his father, ‘on occasions, erroneously referred in his diary to his machine as F.37/35’. (A significant confusion between Spec F.37/34 and the later F.10/35 requirement for an eight-gun machine? – see Sorley on p.152.)

  When the new design was officially named ‘Spitfire’ at the end of April 1936, it is just as possible that this was the time when Mitchell made the well-known comment – perhaps he did not want reminding of the disappointment of the first Spitfire or, with two years having elapsed, the original naming had faded from his mind, which was now looking towards the success of the new ‘fighter’. In 1936, Supermarine publicity reads:

  THE SUPERMARINE ‘SPITFIRE I’

  The ‘Spitfire’ is a single-seat day and night fighter monoplane in which much of the pioneer work done by the Supermarine Company in the design and construction of high speed seaplanes for the Schneider Trophy contests has been incorporated. [The company is si
lent about Type 224.] The latest technique developed by the company in flush riveted stressed-skin construction has been used, giving exceptional cleanliness and stiffness to wings and fuselage for a structure weight never before attained in this class of aircraft. The ‘Spitfire’ is fitted with a Rolls-Royce ‘Merlin’ engine, retractable undercarriage and split trailing edge flaps. It is claimed to be the fastest military aeroplane in the world.

  No further details of the machine are available for publication.

  It is interesting to note how, for the very first time when announcing an entirely new Supermarine design, the aircraft had been designated a Mark I. It might be that the company was merely wanting to avoid any further references to Type 224, and thus to draw a line under this less than successful machine but, in view of the many variants to be produced in the next nine years, one likes to think that the designation was prophetic.

  As well as the comment, ‘It’s the sort of bloody silly name they would give it’, Mitchell was also reported as saying that it could be called ‘Spit-Blood’ for all he cared. Mansbridge’s daughter has recorded that ‘Shrew’, ‘Shrike’ and even ‘Scarab’ had also been considered, and it is a matter of speculation as to whether our chief designer would have preferred any of these. But, by this time, the Aircraft Nomenclature Committee was no more and names were now selected, in discussion with the manufacturer, by the Air Member for Supply. Vickers’ suggested name for the new fighter, and accepted by the Air Ministry, was in all probability inspired by Ann McLean, the chairman’s daughter, who had habitually been referred to as ‘a right Spitfire’. For fighters, especially, words indicating speed and aggression were now being chosen (for example, ‘Fury’, ‘Gladiator’, ‘Gauntlet, ‘Whirlwind’ and ‘Hurricane’) and ‘Spitfire’ more or less fell into this general category.