The Flying Radiators – Part II, R. J. Mitchell's S.6B

With the possibility of a third win in 1931 and, therefore, the outright capture of the Schneider Trophy, Supermarine and Rolls-Royce began discussions with the Air Ministry predicting an increase of 25 mph over the Schneider course, assuming that the S.6 machines would be loaned back for modifications and that they would be piloted by High Speed Flight pilots. The estimate of the cost of a successful defence of the Trophy in 1931 was considered to be in the region of £100,000, and involved the production of two new and improved machines and engines to support them but, in view of the worsening economic climate, the government declined any help whatsoever.

      The response, especially in aviation circles was outrage but to no avail. Fortunately, the formidable and extremely wealthy Lady Houston was approached and she promised the required sum to sponsor Britain's entry and, incidentally, to embarrass the Labour Prime Minister and his government. By the time that political points had been scored and the necessary money allocated, there was less than a year left for all the work required in time for the competition in the coming September. Because of this time constraint and the finite funds available, the British hopes would have to be concentrated upon uprating the current Rolls-Royce engine and upon modifying the S.6 design to handle an expected increased in power.

     Derby had once more to put up with the noise of the engine testing (see my Blogpost:" The Flying Radiators: Pt.1") and the Mayor of Derby had to appeal to the patriotism of its citizens as the tests ran from April 1 to August 12 before the uprated engine could run for an hour at full power– by which time the 1900 hp of the 1929 engine had been increased to 2350 hp.

S.6B

In view of this increase, Supermarine estimated that ‘something like 40,000 B.T. units’ had to be dissipated every minute – the equivalent of over 300 modern fan heaters running at full power. Mitchell had to provide additional radiator surfaces on the floats right down to their chines so that almost half of the 948 sq. feet of the aircraft's available surface area was now to be used for cooling: it is understandable why Mitchell, in a radio broadcast after the competition, described his new aircraft as a ‘flying radiator’.

   Enlarging the cooling area was assisted by an increase in the size of the floats as the anticipated increase in the fuel consumption of the new engine required their capacity to be enlarged, as did a modification to the competition rules: the aircraft were now required to take-off and land immediately prior to the start of the race proper instead of the navigability and seaworthiness tests being carried out with minimal fuel the day before. Wind tunnel and tank testing led to a narrower float design, although of increased length and the side plating was extended by ¾ in. below the chines and as far as the step to improve running and to inhibit spray. And the elevators no longer had a vee cut-out close to the fuselage but were now made to operate with minimum clearance in order to reduce turbulence at this point.

     The revised design was accordingly designated "S.6B" and it was possible to afford two, with the serials S1595 and S1596; additionally, the two 1929 machines, N247 and N248, were uprated and, as such, they were redesignated "S.6A". The only noticeable difference between the two pairs of machines was that the latter had its original floats, which were two feet shorter than those of 1931.

The first of the S.6Bs at Calshot.

As there were to be four Supermarine racers, Flying Officer L. S. Snaith was added to the High Speed Flight which had now consisted of Flight Lieutenants J. N. Boothman, E. J. L. Hope, F. W. Long, and G. H. Stainforth. Flying with the contest machines began when N247 arrived on May the 20th but an alarming oscillation of the rudder developed during an early high speed run, causing the buckling of rear fuselage plates, stress cracks around some of the rivet holes, and stretched control wires. In the short time available, Mitchell had streamlined weights on forward projecting brackets fitted to both sides of the rudder and the ailerons – in order to bring the centre of gravity of these surfaces to coincide with their hinge lines and so to dampen any future oscillations which might occur. The last bay of the fuselage was also strengthened.

     Weights were also needed in response to some instability on take-off and during turns. Mitchell decided that the problem was due to the centre of gravity being too far back and so he had about 25 lb. of lead placed in the nose of each float and reduced the amount of oil (which was, again, carried all the way back to a tank in the fin). However, Orlebar had also reported nose-heaviness during level flight and Mitchell (before the widespread use of trim tabs) ‘produced a splendid gadget to cure the trouble’ – metal strips were fitted to the trailing edge of the elevators and bent downwards by about one degree, thus using the slipstream at high speed to deflect the elevator upwards slightly and to counteract any load on the stick.

    Additionally, the engines were prone to cutting out because of choked fuel filters. This was found to be the result of the fuel mixture causing the excess compound used to seal the joints in the fuel system to come adrift. Mitchell’s response was both practical and blunt: ‘You’ll just have to bloody-well fly them until all this stuff comes out’.

    There was another problem, also experienced in 1927 – the first new S.6B, which had arrived on July 21st, could not be made to fly as it gyrated in ‘a very good imitation of a kitten chasing its tail’. In the course of these rotations, S1595 hit a barge and had to be returned from Calshot for repair.  This year, a different caise was suspected and the smaller diameter propeller of the S.6B was fitted to N247, which then behaved like its younger sister. This suggested that the slipstream from this diameter propeller was producing a side pressure that the rudder was unable to counteract. When S1595 was returned on July 29th a larger diameter unit was fitted and the new machine then took off with no more difficulty than was usual with these machines.

    In view of the peculiar take-off requirements, combined with the increased speed and weight of the new aircraft, it is not surprising that the British team now began to have accidents. Linton Hope virtually wrote off one of the S.6A machines: a piece of the cowling from N248 worked loose in flight and, whilst managing an emergency landing, he encountered the wash from a passing ship which caused the sensitive machine to cartwheel and sink. The pilot survived but was withdrawn from the team because of a punctured eardrum.

    Hope was replaced by Lieutenant G. L. Brinton who, on 18 August, on his first take-off in N247, was killed. Greig told how, if porpoising developed during take-off, it was imperative to close the throttle and start again; otherwise, the pitching of the aircraft invariably increased until the machine was eventually thrown into the air without flying speed and an accident was inevitable.  It appeared that  Brinton did not heed the advice in time and the aircraft cleared the water for a second and then dropped back. The S-6A bounced 40 feet in the air and then plunged down into the water. It was first assumed that Brinton's body had been lost at sea, but later was found jammed into the rear of the fuselage. It is not recorded how the actual discovery of the body was received by Mitchell but, in view of his well-known concern for the pilots of his machines, an explanation of the need to cut into the damaged machine for its recovery must have required considerable tact.

At about the same time, a French machine was considerably damaged in a landing accident and another was completely destroyed, killing its pilot. Meanwhile, Macchi were developing their M.67 layout into a new machine, which was also to kill one of its pilots.

As a result of such accidents and other setbacks, the French and Italian teams requested a postponement of at least six months but, by the time it was received on 3 September by the Royal Aero Club, it seemed that all the significant problems with the S.6Bs had been solved and Hope’s N248 had been salvaged and was well on the way to being restored to flying condition. Holding the competition on the due date was insisted on and, as a result, the Air Ministry was informed on the 5th of September that neither France nor Italy would be able to compete. 

In the end, the only postponement of the twelfth contest, again at Calshot, was for one day owing to bad weather and the following day was almost perfect with visibility of over ten miles. In view of the fly-over situation, it was decided that S1595 was to complete the course without putting undue strain on the engine – the increase in propeller diameter had produced a higher engine temperature and, again, Mitchell had had to accept a slightly lower airspeed than his design was capable of. If this attempt were to fail, then the repaired S.6A would aim to finish the course and, therefore, to win the Trophy outright. The second S.6B would be available to make trebly sure of a win but it was hoped to retain it to give the crowds the additional thrill of seeing the setting of a new World Speed Record.

   The Commanding Officer, Orlebar, gave the senior pilot, Stainforth, first choice and he opted for the proposed attempt on the speed record; the next most senior man, Boothman, then chose to fly first in the competition itself and, hopefully, to have the honour of winning the trophy. And so, just before 1 p.m. on September 13, Boothman taxied out in S1595, which had never having flown in practice for longer than 27 minutes. Nor had it been considered wise to practice the landing which the 1931 rules required, with the full load of fuel for the 350 kms of the competition course and as well as for the required preliminary maneouvres. Nevertheless, he took off without any apparent difficulty, landed at about 160 mph without mishap, and then flew the required seven laps, all within about four mph of each other, slightly throttled back, taking the turns wide and with a gentle bank and finished with an average speed of 340.08 mph – just over 11 mph faster than Waghorn in 1929. Then, as if to emphasise the superiority of the Rolls-Royce/Supermarine partnership and also to post a more impressive speed, Stainforth took out the other S.6B a little later and proceeded to capture the World Absolute Air Speed Record at 379.05 mph.

    Lady Houston had attended on her steam yacht, Liberty, to watch her machines and, two days later, gave a celebratory lunch on board, attended by Mitchell and his wife and by the High Speed Flight. She had been afforded the rare privilege of mooring on the RAF buoys at Calshot and in the evenings Liberty had a string of electric lights from her bowsprit to the mastheads and down to the stern.

The Air Ministry then set about disbanding the High Speed Flight and restoring the Calshot base to its normal flying-boat duties but Rolls-Royce particularly wanted to have produced the first aero engine to exceed the new magic mark of 400 mph and Mitchell had indicated the same in an interview with the Southampton Daily Echo:‘with a specially tuned up engine, I am very hopeful we may get very near to an average speed of 400 mph, which is our ambition’.  For this special sprint machine, Mitchell had the wing-tip air scoops removed and a specially prepared engine was to be supplied, using a new more potent fuel mixture. After delays caused by bad weather, Stainforth squeezed into the cockpit of S1595 and the required runs were photographically measured. There was some concern that bad light and a low evening sun might prevent confirmation but eventually, at 4 a.m., the results were telephoned through and Mitchell was informed; it was reported that he was ‘too sleepy to be more than mildly enthusiastic’ that the World Absolute Speed Record had just been raised by nearly 30 mph to 407.5 mph.

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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.