Tuesday, 7 March 2017

The Spitfire Wing – elliptical and thin.

The film about Mitchell, The First of the Few, shows the designer looking at seagulls, presumably for inspiration when contemplating his future fighter – had he been concerned with the sort of glider development going on in Germany at the time, a case might have been made out for his apparent interest in such birds. His keeping racing pigeons during his youth was evidence of an interest in flight, no doubt, but – as Jeffrey Quill observed – Mitchell had better things to do when designing aircraft than ‘looking at bloody seagulls’. [One might perhaps note that his first essay into fighter design, Type 224, had a gull wing but it was inverted and every effort was made to avoid the current aerodynamic problems of ‘flutter’!]
    Other assumptions that the legendary Spitfire emerged directly from his Schneider Trophy machines are also far from the truth – as any reference to the changing proposals around this time will show. Supermarine Drawing No. 300000 Sheet 2 shows how the Type 224 wingspan was reduced by nearly ten feet with an almost straight trailing edge and a swept back, straight leading edge with a rearwards sloping main spar; and Supermarine Drawing No. 300000 Sheet 11 also shows a continuance of the main spar positioning but a movement can be seen towards the more elliptically shaped wing that was to become, pre-eminently, the distinctive feature of the Spitfire:
Sheet 2
However, the much straighter leading edge of the eventual Spitfire wing shape was, particularly, a result of the advent of a new engine from Rolls-Royce which was to be significantly heavier than the one that had been previously considered and would therefore need a less swept-back wing, if fixed in the same position. By this time, the elliptical wing was coming to be regarded as the most efficient shape for the sorts of speeds and altitudes that were now being contemplated: as Shenstone said, “Aerodynamically it was the best for our purposes because the induced drag, that caused in producing lift, was lowest when this shape was used”.
   The Heinkel He.70 transport, which first flew in 1932 is no longer regarded as influencing the Spitfire wing design, but it did illustrate the growing appreciation that the elliptical wing was also a very efficient way of accommodating stress loads and hence of allowing a lighter structure. The enormous Kalinin K-7 was being built at the same time and embodied perhaps the ultimate symmetrical elliptical wing to support its 174 ft span and seven engines:
Kalinin K-7
but it is noteworthy that the generic elliptical wing shape had been considered by Mitchell in connection with his 185 ft Giant – projected before the Russian aircraft first flew [see my Blogpost: 'R.J. Mitchell's Giant]. And it is also worth mentioning that there was an earlier precedent for the elliptical wing approach – the two-seat light aircraft, the Baumer Sausewind, notable for its all cantilever structure as early as 1925. It was designed by the Gunter brothers before they joined Heinkel and produced the He.70 mentioned earlier. But, again, one finds that, in that same year, Mitchell had also produced something approaching an elliptical wing with his S.4. [see my Blogpost: 'R.J.Mitchell's Annus Mirabilis, 1925 - Part II] – which also featured startlingly new cantilever flying surfaces, but also required to withstand much greater loads than those of the German light aircraft.
     [The question arises: why did his S.5/6 of racers not follow this precedent? It may very well be that, with the short time available before the next  competition, the introduction of wing-surface radiators with the S.5 was sufficiently novel for Supermarine, without the added complexity of following a curving leading edge; and the move to metal wings with the S.6s was, again, perhaps enough to be going on with. In addition, the wire bracing reintroduced with these later aircraft made the strength/weight advantages of a more complex wing shape less obvious. It is, however, interesting to note that their unbraced cantilever tailplanes were perfectly elliptical in shape. See my Blogposts: 'The Flying Radiators'.]

The overseas aircraft mentioned above featured symmetrical ellipses particularly with reference to structural considerations. Other somewhat elliptical wing shapes closer to home, where speed was a foremost consideration, were familiar to the Supermarine design team, well before the Spitfire flew: both the Short Crusader, of 1927, and the Gloster VI, of 1929, employed shapes which approached the elliptical but for the narrower chord close to the fuselage – where the control surfaces were not involved:

                              Short Crusader                                                          Gloster VI

However, in the present context, the most intriguing shape was that of the Italian Piaggio Pc.7. 

Whilst it never progressed beyond its taxiing stage, the general arrangement of this rival Italian design would surely have been known to Mitchell and the elliptical wing shape modified by a straigher leading edge is uncannily similar to that which was developed for the Spitfire.  

This modified elliptical wing shape which Mitchell eventually settled on for both the engine weight reasons and the aerodynamic reasons, mentioned above, equally suited certain structural considerations. The perfectly symmetrical elliptical shape would theoretically have required the optimum main spar position to slope, if not curve, backwards, with consequent constructional and weight/strength problems – Mitchell, typically, selected a less complex arrangement whereby the main spar was set at right angles to the fuselage centre-line; this structural consideration distorted a perfect ellipse but had the advantage of making it easier to align the wing ribs which were already to be set at progressively differing angles of incidence as they approached the wing tips.
And, in this context, a return to Heinkel aircraft is called for – in respect of the He. 112B which was, in effect, a scaled-down version of the He.70 mentioned earlier and a contender for the contract which produced the Messerchmitt Bf 109.

It would appear that at least one German designer was coming to similar conclusions as Mitchell and so it is to be noted once again that Mitchell’s design predated that of the rival aircraft – which also embodied a move to a thin wing (see following). In fact, C. G. Grey had reported much earlier:
An interesting point about those Curtiss biplane racers [of 1923-26] was that the wings came almost to a knife-edge in front [producing an extremely low thickness/chord ratio of 6%]. 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 1927-9 and 1931, he used the thinnest possible wings, and won every time.

There had also been other moves at Supermarine towards the use of a much thinner wing: the first flights of the Stranraer (1934), with its thinner aerofoil, now clearly supported the view that the way forward was not represented by the thick, relatively lightly loaded wing of the Type 224 prototype [see my Blogpost: 'The Stranraer - R.J. Mitchell's last flying boat']. Ernest Mansbridge remembered:
Choosing the thick section wing was a mistake when we could have used a modified, thinner section as used on the S.5 floatplane … We were still very concerned about possible flutter, having encountered that with the S.4 seaplane. With the S.5 and S.6 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 strength factors of a much thinner wing than usual, the value of an elliptical shape was already appreciated, representing as it did an improvement in the strength/weight ratio but another important determinant upon the final shape of the wing was the decision to now move to a retractable undercarriage. The necessary arrangements for its housing in the thin wing meant that all the machine guns had to be placed much further outboard than might have been otherwise expected. In this respect, an elliptical form of wing was particularly attractive as it tapers towards the tip very slowly at first, so allowing the siting of guns in the necessarily wider positions.
   And whilst 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  Mitchell if he could include four additional guns without trouble or delay, he received straightaway a quite positive response.

It is generally 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 æsthetics and intuition had quite a lot to do with the final choice of the Spitfire wing shape; 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 worth while, and the centre of the line was usually accepted when the thing was re-drawn.
[By way of a rejoinder, one is reminded of Price’s report 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 the practical considerations of the professional engineer and the necessary design compromises must shape the final outcome of a project – but 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 soon became Mitchell’s Chief Aerodynamicist. Very substantial (largely unsupported) claims have been made in a recent book for his influence upon the shape of the Spitfire wing – particularly the straighter 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: one remembers Ernest Mansbridge’s description of Mitchell’s manner of dealing with a problem by calling in the leaders of the relevant groups and getting 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. And Joe Smith has indicated one of the qualities he considered 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 thus 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, among others. And the final shape of the Spitfire wing might also owe a great deal to the younger man’s views, who 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 mentioned above that he soon recognized that the new man might well help Supermarine to progress beyond their already acknowledged lead in high speed design; and 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 Shenstone might contribute.
    On the other hand, it has already been shown that Mitchell had been considering elliptical and thinner wings from the middle 1920s, and that gun and undercarriage housing (as well as aerodynamic arguments) had been also been important considerations – n.b. a reported diary entry by the Chief Aerodynamicist:
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 Shenstone 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 an RAeS (Southampton) Mitchell Memorial Symposium, 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 realized 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, A.N. 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.
In the RAeS published account, C.F. Andrews submitted a letter he had received from Shenstone in which he emphasized the volumetric considerations and also pointed out that some of the aerodynamic advantages of the elliptical wing were only perceived retrospectively: 
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 realized during the war. . .

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 (see Mansbridge above). Will we ever know exactly how the two minds met on this issue? It should meanwhile be noted that in G. Mitchell’s book, Clifton did write that "this shape was proposed by Beverley Shenstone” and that “he also advocated the thin wing which R J  adopted”. [But compare his comment at the Symposium.]
   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 almost legendary Chief Designer.  Beyond that, as we have seen, the Southampton 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. Perhaps one can do no better than quote from a Southampton RAeS member’s summing up at the 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% chord but, as  this would have encroached on installation space, the 25% 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% chord the familiar asymmetric ellipse naturally followed. The choice of a common aerodynamic and structural datum symplified work in the drawing office and hence manufacture. The unswept spar with the ribs at right angles was aerodynamically and stucturally 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.
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For reference sources, see my Blogpost: “Source Material and References" – an extended bibliography is included in my R.J.Mitchell at Supermarine; Schneider Trophy to Spitfire which also provides material for wider reading, grouped according to specific areas of interest. 


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