The Supermarine S.4, S.5, S.6 and S.6B were a series of Schneider Trophy winning floatplanes that were designed by R.J. Mitchell, and that played a part in the design of the Supermarine Spitfire by giving him experience of designing high speed stressed skin monoplanes.
R.J. Mitchell had already won the Schneider Trophy, with the Supermarine Sea Lion II, a biplane pusher that had won the 1922 contest. However in 1923 the Americans turned up with the Curtiss CR.3 float plane, and the modified Sea Lion III had been beaten into third place.
Mitchell's response was the Supermarine S4. This was a radical new design – a fully cantilevered monoplane, with no external bracing. Supermarine received some support from the British government, which was interested in encouraging research into high speed aircraft.
Mitchell's first design was powered by a 700hp Napier Lion VII engine. Cooling was provided by two Lamblin radiators, each with 226 plates, mounted under the wings. Tests in the wind tunnel at the National Physical Laboratory suggested that these radiators might cause problems
The S.4 was of largely wooden construction, with a wooden fuselage and floats. The fuselage was covered with three-ply wood, which formed a stressed skin monocoque that stretched from (and included the tail) forward to the engine compartment. The front of the aircraft used steel tube construction.
The wings had ailerons and flaps, with the flaps inboard of the ailerons. The two were links, so that the ailerons moved when the flaps did.
The aircraft had a rather spindly appearance, with the floats connected to the fuselage by two long narrow struts on each side. It had mid-mounted wings. The nose was rather 'hunched', with the propeller low down. The pilot sat in an open cockpit that was positioned some way behind the wings.
The Air Ministry approved the design on 18 March 1925. Work on the S.4 began on 25 March 1925 and it made its maiden flight on 25 August 1925.
On 13 September 1925 the S.4 set a new World Speed Record of 226.752mph on a 3km course at Calshot.
This was its last achievement. The S.4 was shipped to the United States to take part in the 1925 Schneider Trophy race, but during the seaworthiness and navigability trials on 23 October wing flutter developed at the first turning point, and the pilot (Biard) was unable to regain control. The aircraft stalled, span, crashed into the water and broke into two halves. Biard was able to escape unharmed, but the aircraft was lost. The contest was won by the United States, with a Curtiss R3C.
After the failure of the S.4 Mitchell immediately moved onto a new design, the S.5. At first there was some idea of competing in 1926, and wind tunnel tests began in March 1926, but there wasn't enough time and the Air Ministry decided not to enter the contest that year.
The decision was made to make an all out effort for 1927. The Royal Aero Club was given permission to enter RAF aircraft, and the wind tunnels at Farnborough and Teddington were made available to the team. A new RAF High Speed Flight was formed at the M.A.E.E. Felixstowe, under the command of Sqn Ldr L. H. Slatter, and six aircraft were ordered – three from Supermarine and three from Gloster. Specification S.6/26 was issued to cover the Supermarine design.
Mitchell produced three designs for tests in the Teddington wind tunnel. The idea of an entirely cantilevered design was abandoned, as the reduction in drag was negated by an increase in weight. The first two designs had low mounted wings. Design No1 used W struts to brace the floats to the low wings. No.2 had wires for the outer bracing and between the floats. No.3 had a high mounted wing. This third design was quickly eliminated and the wing tunnel tests suggested that bracing wires produced less drag than struts.
The S.5 was a low winged monoplane, with a similar fuselage to the S.4. The cockpit was moved further forward. The floats were unusual, in that the right hand float contained all of the fuel, was larger than the left hand float and was 8in further from the centreline. This helped compensate for the engine torque from the powerful Lion engine. The wire bracing allowed Mitchell to reduce the size of the strut braces, and remove the inter-float struts. The frontal area of the S.5 was 35% smaller than on the S.4. The floats also had a smaller frontal area.
The S.5 was powered by the Napier Schneider Lion VIIB, a specially developed engine with a smaller frontal area of 4.25sq ft (compared to 5.5sq ft on the Lion XI). The Lamblin radiators of the S.4 were replaced with surface radiators. This was calculated as having improved speed by 24mph.
Two versions of the S.5 were produced. The S.5/21 used a direct-drive 900hp Lion VII engine. One aircraft, N219, was built to this design.
The S.5/25 used a geared 875hp Lion VIIB engine. Two aircraft, N220 and N221 were built to this design.
N219 was used for high speed trials on 24 July 1927, where it reached an average speed of 284mph. N219 and N220 were then shipped out to Italy, to take part in the Schneider Trophy contest, which was held at Venice.
The race took place on 26 September, and was between the British entries and the Italian Macchi M.52 monoplane. The race was dominated by the British. It was won by the geared N220, piloted by Flt Lieut Sidney Norman Webster. He averaged 281.65mph, and also set a 100km world speed record of 283.66mph. Second was the N219, flown y Flt Lieut Oswald Ewart Worsely, at 272.91mph. The Gloster IVB had been going at 272.53mph but was forced to drop out after five of the seven laps. The Italian aircraft all had to withdraw during the race, after failing to keep up with the S.5 or the Gloster IVB.
Sadly N221 was lost during an attempt on the World Air Speed Record on 12 March 1928, and the pilot, Flt Lieut S.M. Kinkead was killed
In November 1928 S.5 was also used to set a new British record of 319.57mph with Flt Lieut D'Arcy Greig at the controls, although the World Speed Record eluded him.
N219, in a modified form, served as the third aircraft in the 1929 Schneider Trophy team.
The Supermarine S.6 was produced for the 1929 Schneider Trophy. Supermarine decided to use the new 1,900hp Rolls Royce R engine, a rather larger engine than the Napier Lion. As a result the S.6 was a larger and heavier aircraft, but it followed the same basic design, with a low mounted wing and wire bracing. The float struts were also used to support the heavier engine.
The S.6 used surface radiators, which were fitting to the wings and to the top of both floats. The floats had finer lines than on the S.5. Fuel was stored in both floats.
The Air Ministry decided to enter the 1929 contest, and on February 1929 ordered two S.6s, N247 and N248. N247 was delivered in August 1929. The RAF High Speed Flight was reformed for the contest, and was equipped with the S.6 and the Gloster VI.
The original version of the S.6 proved to be rather difficult to get to take off. This was solved by altering the floats, storing more fuel in the starboard float and making the port float longer. N247 made its maiden flight on 10 August and N248 on 25 August.
The contest was held on 6-7 September 1929 on the Solent. Britain entered the two S.6s and one S.5, and Italy entered the Macchi M.52bis. The contest was won by Ft Lieut H.R.D. Waghorn in S.6 N247, with an average speed of 328.63mph. The S.6 was significantly faster than any other design – second place was Wt Off T. Dal Molin, in the Macchi M.52bis, but he only achieved 284.2mph. Third was Flt lt D'Arcy A. Greig in the S.5 N220, who reached 282.11.
The second S.6, N248, piloted by Flg Off R.L.R. Atcherley was disqualified for missing the Hayling Island pylon during the first lap, but reached an average speed of 325.54mph, which would have put him in second place, and also set a world speed record of 332mph for 50km and 331mph for 100km.
In the aftermath of the Schneider Trophy races an attempt was made on the World Speed Record, which required a 3km measured track. On 10 Septemebr 1929 the Glostor VI set a record of 336.31mph, but this only lasted for a few hours. Later o the same day Sqn Ldr A.H. Orlebar reecorded an average speed of 355.8mph with S.6 N247, then two days later broke his own record, reaching 357.7mph.
Early in 1931 N247 and N248 were given larger floats (to increase their fuel capacity) and otherwise upgraded to the S.6B standard. N247 was lost in a fatal crash on 18 August 1931. N248 took part in the 1931 contest.
The Supermarine S.6B was the last of R.J. Mitchell's Schneider Trophy winning float planes. In 1929 the Air Ministry had funded the British effort, and this had produced the successful S.6, but in 1931, with the Great Depression in full swing, the Government decided not to fund the team. The Royal Aero Club launched an appeal to raise £80,000, and received a donation of £100,000 from Lady Houston. This allowed the club to fund the aircraft, while the Government allowed the RAF to provide the pilots once again.
Mitchell was give an even more powerful version of the Rolls-Royce 'R' engine, this time producing 2,300hp. As a result the Gloster VI was no longer able to compete, and the two aircraft were used as trainers by the team. Two Supermarine S.6Bs were ordered (S1595 and S1596).
The S.6B received 24ft long floats, carrying extra fuel. The rules for the 1931 contest required the seaworthiness tests to be carried out just before the race, so extra range was required. Water cooling radiators took up almost the whole of the wing surfaces and the upper part of the floats, and lourves were fitted to the wings to allow cooling air to get to the inner sides of the radiators. Oil cooling ducts lined the fuselage.
The contest was held at Spithead on 12 September 1931. In the event the French and Italian teams didn't arrive and so the contest was a walkover. Even so Flt Lieut J.N. Boothmam did set a new 100km closed circuit record of 342.87mph and finished the race at an average speed of 340.08mph, 15mph up on the 1929 times. Britain won the trophy outright as this was her third successive victory.
On 13 September 1931 Flt Lieut G. H. Stainforth set a world speed record of 379.05mph in S.6B S1596.
On 29 September this was raised to 407.5mph, this time in S1595, S1596 having been lost in an accident soon after setting her record. Stainforth thus became the first person to fly at over 400mph.