Mitchell (centre left) in front of the completed S4. Biard is third from right. (Courtesy of Solent Sky Museum)
The S4 on approach.
While civilian pilots had abundant skill and were willing to risk the dangers of competition flying, their usual flying experience was in much slower machines. The case of Henri Biard was not untypical. The S4 was first flown by him on 25 August and, despite reporting slight wing tremors, he had to leave soon afterwards for the Schneider competition in America. When he resumed flying, influenza and damage to the floatplane resulted in its only just being ready for the trophy navigability trials on 23 October. Not only had his experience on the revolutionary S4 been very limited but, between his flying the Sea Lion III at a maximum speed of 175mph in 1923 and achieving 239mph in the new machine two years later, his day-to-day flying experience with Supermarine was with the Swan passenger amphibian, the Scarab reconnaissance amphibian, the Sparrow I light land plane, and the Southampton I flying boat, whose top speeds averaged out at something less than 100mph.
While 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 S4 was to set the design pattern for all future Schneider Trophy aircraft, and its clean cantilever flying surfaces were to be echoed by similar silhouettes in the none-too-distant Second World War.
Later, when Mitchell returned to the design of racing floatplanes, he turned from the wooden airframe of this 1925 aircraft to employ the metal structures that were also to become a feature of the future generations of fighter aircraft. It was his successes in the next three Schneider Trophy competitions that established his reputation outside the aircraft industry. But, when one considers the quantum shift from the Sea Lion of 1922 to the S4 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 S4. As E. Bazzocchi, of Aeronautica Macchi, said, ‘the real revolution of 1925 was the appearance of the Supermarine S4: its very clean design set the pattern for all subsequent Schneider racers’.
The S4 was a failure, but it had marked the emergence of a notably innovative designer dramatically pushing forward the frontiers of high speed flight. Indeed, Supermarine’s publicity in 1926 points out that the previously quoted top speed of over 226mph was later increased to an impressive 239mph.
SCHNEIDER TROPHY SUCCESSES
The Supermarine S6B winning the 1931 Schneider Trophy. (From a painting by the author)
THE HIGH SPEED DESIGNER CONFIRMED
After the failure of the revolutionary S4 in the 1925 Schneider Trophy, success in events not long afterwards led the shy young man who had joined his firm at the age of 21 to be honoured at Buckingham Palace, to give a talk on the BBC and to be elected a Fellow of the Royal Aeronautical Society. His solid, everyday work with Supermarine’s various amphibians and flying boats did not place his name in front of the general public; his involvement with the Schneider Trophy events of 1927–1931, however, was another matter.
The Schneider Trophy had begun in 1913 as a contest between the aero clubs of various nations, most of whose members were enthusiastic amateurs, as were their pilots. While the trophy continued to be organised by the clubs, the character and costs of the meetings had, by 1926, produced the first confrontation of government subsidised teams with well-organised military pilots and support staff. Equally, the demands of producing the sophisticated technology required of the modern winning entry was evidenced by the non-showing of Britain in that year, and by the complete or partial failure of all the leading aircraft which did compete, owing to lack of adequate development time.
While government assistance for a British entry had already been in evidence in 1925, increased support thereafter prompted Flight to comment that ‘Never in the history of British aviation have we tackled an international speed race in so thorough a manner.’ Mitchell was now able to profit from this even more determined support in the next two competitions, mainly thanks to Air Vice-Marshall W.G.H. Salmond, the Air Member for Supply & Research at the Air Ministry, and the Secretary for State for Air, Sir Samuel Hoare, allocating funds from the existing Air Ministry development budget.
Work was soon under way, involving careful appraisal by wind tunnel and tank testing of quarter scale models. Particular attention was to be paid to floats, flush-wing radiators and airscrews. Gloster, Supermarine and Shorts were asked to design machines capable of speeds not less than 265mph at 1,000ft, and Napier was responding by increasing the Lion engine’s compression ratio to 10:1, with a view to approaching the 900hp mark.
Faith in this reliable engine was justified when it did, in fact, deliver 900hp in the ungeared version and 875hp in a geared one. Just as metal propellers were found to be superior to wooden ones as tip speeds increased, so it was considered that any extra weight or loss of engine rpm because of reduction gearing would be well offset by greater propeller efficiency.
In this respect, when the Air Ministry accepted the proposals of Supermarine, Gloster and Shorts, they were not only departing from the previous American and current Italian concentration of effort on one aircraft type, but were clearly further hedging their bets by supporting two Supermarine entries with geared engines and one with an ungeared unit, a geared Gloster machine and two ungeared ones, as well as a Short seaplane powered by a more standard air-cooled radial engine. This last aircraft was a considerable departure from the water-cooled, in-line type of engines which had produced the sleek, streamlined winners of the last three contests. As previous and current post-war RAF fighters were all powered by radial engines that were relatively light, there was a good reason for seeing what sort of performance the trophy competition might produce with the Crusader.
This Air Ministry order for seven machines was the largest ever given for a British Schneider Trophy entry and was only matched by Italy in the following year. Also, on 1 October, even before the January meeting to decide the date of the next competition, a High Speed Flight was formed, consisting of exceptional military personnel, to test and compete with the new aircraft, rather than relying on company pilots, as had previously been the case.
The S5
At Supermarine, Mitchell continued to place his faith in the newer monoplane approach and proceeded to strive for improvements on the S4 design. He was also content to use the reliable but powerful Napier Lion engine. Indeed, in the discussion that followed the 1925 Buchanan lecture to the Royal Aeronautical Society, he had remarked, ‘At one time I thought that the “Lion” engine was at a disadvantage with the American engines, but I have changed my views rather, and certainly consider the “Lion” is capable of winning the Schneider Cup.’
He must also have noted that the Curtiss CR-3 engine had been much more closely cowled than his S4 had been, thus allowing a slimmer fuselage. Accordingly, Mitchell (and Folland for Glosters) had consulted with Napiers, and the new Lion was designed whereby the engine’s frontal area was reduced by repositioning the magnetos and contouring the cam covers of the three cylinder engine banks to mate with the engine fairings fore and aft. Thus, Mitchell was able to reduce the cross section of his fuselage – so drastically, in fact, that the pilot’s cockpit was an extremely tight fit. The pilots sat on the floor of the machine, their legs almost horizontal and their shoulders coming up to and pressing against the underside of the cockpit coaming. The result was the slimmest fuselage of all the current and subsequent contenders.
Flight Lieutenant H.M. Schofield, one of the pilots of the RAF High Speed Flight which had just been created, described their visit to Supermarine ‘for a fitting’:
The method of reaching the seat was to squeeze in sideways and down as far as possible so that the shoulders were below the top fairing, then turn to face the front, and in my case it needed no ordinary effort to get my shoulders home. There were many sighs of relief from the watching design staff when the
last man had been ‘tried-in’, for it had been a near thing and it did look as though it was not going to be enough at times.
As there was insufficient room for the fuel tank in the fuselage, the starboard float was used. This expedient also had the advantage of giving the aircraft more stability in the air by lowering its centre of gravity. It would also help towards counteracting the torque of the engine which, during take-off, might be expected to cause the opposing float to dig in and swing the aircraft off line before it gained sufficient airspeed to be effectively governed by the control surfaces. Mitchell also offset the fuel-loaded starboard float an extra 8in from the centre line as an additional response to this expected problem.
However, the most telling improvement, apart from the more powerful engine promised by Napiers, was the estimated increase of about 24mph by the proposed change from the Lamblin type underwing radiators of the S4 to a system akin to that adopted by the Curtiss racers and, subsequently, by the Italian Macchi M39.
The Supermarine S5 awaiting cowlings for its Napier Lion engine. Note the hinged cockpit cover to facilitate entry. (Courtesy of Solent Sky Museum)
The new radiators were to be made out of copper sheets, 8½in wide, with their outer surfaces formed to the contours of the upper and lower wing surfaces. Thus the outer sheeting, exposed to the cooling airflow, offered minimal additional drag. Corrugations on the inner surface of these radiators formed channels for the coolant and this was taken along troughs behind the rear wing spar, through the radiators and along the leading edge of the wing, and then pumped to a header tank behind the engine block. Attention was even paid to the effect of different paints on the effectiveness of the radiators. The engine oil was also cooled via tubes which ran along the outsides of the fuselage and up to a header tank behind the cockpit. (Not long after flying tests began, it was found necessary to increase the capacity of the tubing.)
In terms of structure, the Supermarine contender continued the move away from its S4 predecessor. In line with the company’s other developments at the time, the new machine was of mixed metal and wood construction, with the all-metal fuselage being a stressed-skin structure (which looked towards the Spitfire), while the flying surfaces were, like those of the S4, of wooden construction and ply-covered. It was designated ‘S5’ as it represented a complete redesign of the previous monoplane and also incorporated the new information gained from meticulous work at the National Physical Laboratory test facilities, which had been sponsored by the Air Ministry.
Mitchell had sent down three models for wind tunnel testing: one was a shoulder-wing design with wing roots cranked down and supported by streamlined struts from the floats; a second model had a low wing, similarly braced by struts; and the third configuration was an all-wire-braced proposal with a low wing position to give favourable bracing-wire angles.
Biard’s problems with forward vision during landing and take-off in the S4 no doubt influenced Mitchell’s considerations and, eventually, the flat, low wing position was chosen, particularly as it had been found that wires offered less resistance than struts. The wire bracing gave additional ‘belt and braces’ protection against a possible failure that a cantilever wing might experience, and a diagonal box spar was also fitted between the main spars to strengthen the wing torsionally against aileron loads.
The new wire bracing between the floats and from the floats to the bottom of the wing also allowed a wire ‘cage’ to be completed, as the wires from the upper fuselage to the top of the wings were fixed immediately above the float bracing attachment points. Mitchell was clearly guarding against any wing flexing which might have contributed to the S4 crash; and, whatever Mitchell’s private thoughts were about the need to step back from the revolutionary concept of the cantilevered S4, the pragmatic reversion to wire bracing also brought a further reduction of the weight and drag which had been represented by the sturdy float struts of the S4.
The balancing out of advantages and disadvantages attendant upon the wish to reduce frontal area and weight against the need to ensure adequate strength and pilot view, was set out by Mitchell after the race in his speech to the Royal Aeronautical Society in 1927:
(a) The primary object in lowering the wing on the fuselage was to improve the view of the pilot, which was never very good on the S4. The higher position of the wing no doubt gave a lower resistance due to fairing in the outside engine blocks and thus saving a certain mount of frontal area. A loss in speed of about 3mph is estimated from this alteration. This loss is more than balanced, however, by the importance of the improved view.
(b) The system of wire bracing of the wings to the fuselage and floats was adopted for a number of reasons. The unbraced wings and chassis of the S4 were very high in structure weight, and it was found very difficult to construct an unbraced wing sufficiently strong and rigid without making it very thick at the root, and thus increasing its resistance. The adoption of bracing was largely responsible for a reduction in structure weight of 45 per cent for the S4 to 36 per cent for the S5, with its corresponding reduction in resistance; also for the elimination of the two struts between the floats, and for the reduction in frontal area of the four main chassis struts. Against these must be set the addition of fourteen wires. It is not easy to estimate the final effect of a number of alterations of this nature, but from the analysis of the resistance of the two machines it is given on fairly good grounds that the overall effect was an appreciable saving in resistance, amounting to an increase in speed of approximately 5mph.
(c) The cross-sectional area of the fuselage has been reduced by about 35 per cent. This very large reduction was obtained through the redesign of the engine and the very closely fitting fuselage. This almost amounted to a duralumin skin in order to ensure that the very smallest amount of cross-sectional area was added. On several occasions during the construction of the fuselage the pilots were fitted, and much trouble was experienced through their being of varying dimensions … The reduction in body resistance was responsible for an increase in speed of approximately 11mph.
The floats were also reduced in frontal area by about 14 per cent. This was accomplished by using a much lower reserve buoyancy. The reserve buoyancy was 55 per cent for the ‘S4’ floats and 40 per cent. for the starboard float of the ‘S5’ [now being used for fuel tankage]. This figure is extremely low and called for very efficient lines.
The estimated increase in speed due to reduction in float resistance is 4mph. These reductions in resistance of fuselage and floats are due to lower cross-sectional areas and not to improvements in form.
(d) Wing-surface radiators were first fitted to the American machines in the 1925 race, and gave these machines a very big advantage in speed. The radiators added a certain amount of resistance to the machine due to their external corrugations increasing the area of exposed surface. As about 70 per cent. of the resistance of a high speed wing is skin friction, and the corrugations almost double the area of surface, it is reasonable to suppose that an increase of at least 30 per cent. of resistance is added to the wing. It is evident that a saving in resistance would result if radiators could be made with a flat outer surface, and that they would give no direct resistance to the machine. After much experimental work, radiators with a flat outer surface were produced. The chief difficulty experienced was in sufficiently strengthening and supporting the outer skin to enable it to stand the heavy air loads without making the radiators unduly heavy. The estimated increase of speed due to their use in place of Lamblin radiators used on the ‘S4’ is 24mph.
He could have also mentioned that, with the second S5 (which was to come first in the forthcoming contest), ‘the hundreds of tiny rivets all over the skin were now flush with the surface instead of projecting like a mass of wee knobs as they had done’. This reminiscence of Schofield looks forward to a similar concern with the Spitfire (mentioned on p.180).
The result of all these design considerations culminated in a machine which, when it went to Venice to compete in t
he 1927 Schneider Trophy, was seen by the Italians as a direct copy of their Macchi M39, which had won the previous year. While Mitchell, like other engineers, was perfectly willing to profit from the successful design solutions of others (see pp.119–20, his Dornier-inspired Air Yacht), the Italian criticism had not taken into account Mitchell’s trendsetting S4 of 1925, how long Mitchell had been contemplating his latest design, or how the wind tunnel tests had influenced his more pragmatic choice of this layout.
Nor did matters begin well for the British team as a whole. Bad weather prevented test flying until 10 September, only thirteen days before the required navigation tests were due. Then, on the following day, the Short Crusader crashed – it was found that the aileron controls had been crossed during re-rigging. The second batch of British planes arrived on 11 September, but bad weather again prevented test flying until the 21st, when it was found that the problem of fumes in the cockpits still needed attention. Both Supermarine and Gloster had discovered this problem before leaving for Venice but now one pilot, Flight Lieutenant S.M. Kinkead, was confined to his room all the next day. His machine also required attention as part of his spinner had come adrift, causing severe vibration.
The three permitted British entries were finalised as Flight Lieutenant S.N. Webster in the Supermarine N220, Flight Lieutenant O.E. Worsley in the Supermarine N219 and Kinkead in the better of the two Glosters. It was decided that the Supermarine N220 and the Gloster N223, with the unproven geared engines, were to fly flat-out in the expectation that Worsley, in the ungeared S5, would finish if, for any reason, the engines in the other two more complex machines failed.
Large crowds began to gather, and not just the locals who were strongly supporting the ‘local boy’, Captain Arturo Ferrarin. Major Mario de Bernardi was also a national favourite, having come first previously in the competition, and he was therefore well supported by those brought in by the Italian State Railway on special half-fare excursions. Unfortunately, a strong wind and a heavy swell made conditions too problematic for the sensitive floatplanes and the crowds had to return on 26 September, when conditions had improved. It was possible to commence the contest at 2.30 p.m. when Kinkead took off in the Gloster, followed by Webster and then de Bernardi. The new member of the Italian team, Captain Frederico Guazzetti, was next, then Worsley and, finally, Ferrarin.
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