Sunday, 19 March 2017

R.J. Mitchell’s Annus Mirabilis, 1925 : Part II – the S.4

As mentioned in my Blogpost 'Annus Mirabilis – Part I', the year 1925 marked R. J. Mitchell’s full emergence as a designer who had transcended current design precedents and who was now becoming a name to be reckoned with. After joining his aero firm in 1916, following an apprenticeship with a steam locomotive company, he became Chief Designer in 1919  (at the age of 24) and now justified his early appointment by producing not only the first standard naval reconnaissance aircraft since the end of World War I, the Southampton, but, even more startlingly, designed the racer which set the basic design configuration for virtually all the subsequent Schneider Trophy machines with his S.4.
     After Supermarine’s comprehensive defeat in the 1923 Schneider Trophy competition, Mitchell was now faced with the necessity of producing a racing machine which would have to be a significant departure from all the Supermarine aircraft which had preceded it. As with the Spitfire, however, his initial response was not especially original; indeed, his proposal, the Sea Urchin, still looked towards the flying-boat approach and might be regarded as, essentially, an improvement on the Italian Savoia S.51 racer, which came second to Mitchell’s Sea Lion II in 1922 but later went on the take the world speed record for seaplanes. (See my Blog: “Mitchell’s First Schneider Racers”). He proposed a similar sesquiplane approach and a high thrust line although it might be noted that his hull revealed somewhat similar styling to that of his Southampton, particularly in respect of the upswept rear hull. Additionally, the drag penalty of a high mounted engine was to be avoided by situating the engine in the hull and driving the propeller through bevel-geared shafting.

But the Sea Urchin proposal was not pursued because of serious doubts about the practicality of the propeller shaft gearing. Fortunately, as other nations were not sufficiently prepared for the 1924 Schneider Competition, the American hosts sportingly postponed the event and so allowed Mitchell time to comprehensively redesign an entry for the following year.
     When the secrecy surrounding its build was lifted, its sensational appearance was well summed up by Flight:
One may describe the Supermarine Napier S.4 as having been designed in an inspired moment. That the design is bold no one will deny, and the greatest credit is due to R.J. Mitchell for his courage in striking out on entirely new lines. It is little short of astonishing that he should have been able to break away from the types with which he had been connected, and not only abandon the flying boat type in favour of a twin float arrangement, but actually change from braced biplane to the pure cantilever wing of the S.4.
Supermarine S.4 (from a painting by the author)
The allocated Air Ministry serial number of the new machine was N197, although this was never carried and Supermarine referred to the new machine only as the S.4 – “S” presumably referring to Schneider and “4” indicating that it was the successor of the Mark III Sea Lion.
      It ought, perhaps, to be noted that the French speed record holder, the Bernard V-2 landplane, a year earlier, had displayed some features which might have prompted Mitchell’s new design: its Hispano-Suiza engine was a broad arrow design similar to the S.4’s Napier Lion engine and it was faired almost identically into the fuselage and wings; it also had cantilever flying surfaces, under-wing radiators and a similar pilot’s cockpit position. Nevertheless, when Harald Penrose of Westlands later wrote of “the startlingly novel and beautiful Supermarine S.4” he was at least reflecting the dramatic appearance of a revolutionary floatplane design and was surely right in responding to its fine lines  – in comparison, the Bernard had a much more clumsy appearance. 
      Supermarine’s Alan Clifton gave a more clinical “in-house” response and singled out the unique attachment of the floats: “It was an exceptionally clean design, with a central skeleton of steel tubing which included daring cantilevered float struts.” This central skeleton consisted of two sturdy “A” frames, with the engine mounting bolted to the front frame and the rear fuselage section fixed to the rear one; between these two frames, the wing centre section was placed and the floats were attached to the feet of the frames. This characteristically, and deceptively, simple structural arrangement was known in the works, less reverently, as “the clothes horse”. 
      Also, the cantilever wing proposal represented a bold departure from the earlier wire-braced company types – and, indeed, from almost all other aircraft of the time. In this case, stringers were rebated into the ribs and an early form of stressed skinning was achieved by sheeting the wing, top and bottom, with load-bearing plywood which decreased in thickness towards the tips.
     Thus it was that Mitchell felt able to take the, then, radical step of dispensing with struts and wire bracings for the wings and tail surfaces; he also did away with bracing wires for the floats, although they were fitted with two thin-section cross struts. An appreciation of the conceptual leap represented by the S.4 can be gained by a comparison of its forward-looking cantilevered structure with that of the previous Sea Lion which required thirty-three struts and forty-two bracing or control wires.

 Streamlining was also achieved by mounting the newer Lamblin radiators horizontally on the underside of the wings; their fins and the oil cooling pipes on the underside of the fuselage were the only significant protuberances on the whole machine, with the coolant water being carried to and from the engine via piping buried in the underside of the wings. The control surfaces were activated also from within the structure via rods and torque tubes. Perhaps because approval to begin building had only been received on March 18, 1925, and also because of the move from flying-boat building, Supermarine subcontracted provision of the floats to Shorts.
    The pilot was situated low down and with a limited view and the high position of this wing also resulted in a blind spot ahead when taking off and landing. Biard claimed to have nearly collided with the liner Majestic on take-off – having not seen it at all [!] until the last minute – and when he came to land, he nearly hit a dredger. However, having survived the traumas of the first flight, the new machine went on to gain the World Speed Record for Seaplanes and the outright British Speed Record: 226.75 mph – nearly 40 mph more than the Curtiss CR-3 record established the previous October.

When the Supermarine team arrived at the proposed venue for the Schneider Trophy competition, Chesapeake Bay near Baltimore, on 5 October, the tented accommodation for the aircraft and for the workshops was found not to be ready. Finally, it was possible to begin erecting the aircraft on 12 October but six days elapsed before test flying was possible. Then a gale caused tents to collapse and a heavy pole fell across the tail unit of the S.4. It was repaired in time for the navigation tests on the 23rd of the month, but unfortunately, the success story of the S.4 then ended as it crashed into the bay following a steep turn which appeared, perhaps, to have caused a high speed stall; flutter or wing distortion was also suggested. Whatever the cause, it occurred at low level and Biard, the Supermarine test pilot, survived. 
     His own later account of the accident was that, as he came out of a turn at speed and dived down for a straight run, the control stick set up such violent side-to-side oscillations that he lost control. Penrose quotes a Flt Lt Linton Ragg of the Royal Aircraft Establishment at Farnborough as experiencing similar stick behaviour at about this time: ‘wing flutter had caused trying experiences, such as coming down with hand and knees badly bruised by the control column as it played hide-and-seek round the cockpit’. Biard’s description of side-to-side movement of the control column thus points particularly to aileron flutter and later remarks at Supermarine confirm this conclusion: Mitchell himself, concerned about the need to avoid overbalancing of the Spitfire ailerons in a dive, wrote “I believe this is the cause of several accidents involving ailerons” and Ernest Mansbridge, Mitchell’s stress man, explaining the thickness of the preceding Type 224 wing being due to caution, was more direct: “We were still very concerned about possible flutter, having encountered that with the S.4 seaplane”.

Whilst the American phase of the Schneider Trophy competitions had brought no luck to Supermarine, it can be seen as a most important milestone in Mitchell’s career. The S.4 was to set the design pattern for virtually all future Schneider Trophy winners and its clean cantilever flying surfaces were to be echoed by similar silhouettes in the none-too-distant World War II.  When one considers the quantum shift from the Sea Lion of 1922 to the S.4 of 1925, and the precedent that this latter aircraft set for the future, a special place should be reserved in British aviation history and in Mitchell’s design career for the ill-fated but beautiful S.4. – as E. Bazzocchi of Aeronautica Macchi said, ‘the real revolution of 1925 was the appearance of the Supermarine S.4: its very clean design set the pattern for all subsequent Schneider racers’.
N.B. Supermarine’s publicity in 1926 points out that the previously quoted top speed of over 226 mph was later increased to an impressive 239 mph. Despite the handicap of floats, this speed was only about 5 mph less than that of the 
Bernard landplane racer mentioned above.

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For reference sources, see my Blog: “Source Material and References. " An extended bibliography is included in my R.J.Mitchell at Supermarine; Schneider Trophy to Spitfire (details below) which also provides material for wider reading, grouped according to specific areas of interest. 

More information, photographs and three-view drawing of the S.4, as well as a full account of all Mitchell's completed designs and of the man behind them, will be available in a few weeks' time:

Advance Notice:  

R.J.Mitchell at Supermarine; Schneider Trophy to Spitfire.

This is a much expanded, completely up-to-date, second edition of my earlier work with 50% more photos, and 25% more text – 380 pp. instead of 250. There are general arrangement drawings of Mitchell’s 21 main aircraft types which flew, as well as 40 other drawings. There are 24 photographs, featuring or including Mitchell, as befits the first fully detailed and, I hope,  definitive account of the man and his work at Supermarine.

To obtain a copy at pre-publication prices, please enquire via the contact form in the sidebar. 

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