The design of any successful aircraft does not just happen by chance, but is the culmination of years of ‘trial and error’, experimentation and progressive development, The family trees of any well-known ‘world-beating’ design is punctuated with numerous disappointments and failures. The old adage ‘onwards and upwards’ was never more appropriate than when applied to developments in aviation.
The Supermarine Aviation Company had more than their share of design difficulties, dating back to its earliest years when founder Noel Pemberton-Billing established his factory in Woolston, Southampton, to build ‘yachts that could fly’. Of several different aircraft designs conceived by NP-B (numbered as PB1 – PB35 using only odd numbering) only two designs ever took to the air and neither of these progressed to production quantities.
However, it is not always the products that make a name for the business – sometimes it is the decisions made and the personnel appointed to carry out the chosen tasks. NP-B’s decision to use a circular cross-section, designed by Linton Hope, for his first flying boat fuselage was radical in its day but had long-lasting endurance in aircraft manufacturing. And his decision to appoint 19-year-old Hubert Scott-Paine as General Manager of his aircraft manufacturing business set the company on a firm footing. HS-P’s subsequent purchase of the entire business, followed by the appointment of the young RJ Mitchell as his assistant, would prove to have lasting positive consequences, despite difficult times ahead.
Mitchell was appointed as Supermarine’s Chief Designer in 1919. One of his first tasks was to improve the tiny Supermarine N1-B ‘Baby’, designed for the Royal Navy by his predecessor FJ Hargreaves and which had been unsuccessful in its bid for selection as a single-seat flying-boat fighter powered by a 200hp Hispano-Suiza pusher engine, giving a top speed of 116mph. The engine was mounted centrally behind the pilot’s position and between the biplane wings. The water-cooling radiator was situated ahead of the engine to catch maximum benefit from airflow created by the forward motion of the aircraft. The First World War had ended before any decision to accept the type into service had been made – the role having been adequately filled since 1916 by Sopwith’s ‘Baby’ seaplane. Mitchell redesigned and rebuilt the N1-B with new wings and tail empennage, achieving improvements in speed at sea level and, with a change of engine (to Sunbeam Arab) it was also capable of reaching over 111mph at 10,000 feet – no mean achievement in those early days of pioneering aviation and engineering design.
Under Mitchell’s guidance the N1-B was improved and reconfigured, including using a 450hp Napier Lion engine and retaining the pusher engine biplane configuration, with fabric covered wings. It was entered in the 1919 Schneider Trophy contest at Bournemouth as the Sea Lion. The Sea Lion had a top seed of over 140mph, but came to grief following a touch-and-go incident during the first lap of the contest when it struck a solid object in the water, and subsequently sank.
Supermarine’s next participation in the annual Schneider Trophy Contests was in 1922 with the Sea Lion II – a much-improved version of the original Sea Lion and its successors the Sea Kings I & II. Again, the pusher-engined biplane construction was used, but new wings and tail were fitted to the modified hull. Napier loaned an improved Lion II engine, enabling the aircraft to exceed 160mph in trials, and in excess of 200mph (indicated) during the 1922 Contest at Naples, resulting in a win for the British design which was also subsequently awarded the first World Records for a marine aircraft for duration (almost 1hr 35 minutes), Longest distance – 230 miles; fastest time over 100kms (28 mins 41.4 secs at 130 mph average speed) and fastest time over 200kms (57 mins 37.4mph at 129.4 mph average speed.
Development on the design continued and the next iteration of the Sea Lion design appeared in 1923 in time for that year’s Schneider Trophy Contest as the Sea Lion III, once again with improvements to the wings and hull, as well as a new 525hp Napier Lion III engine. This would be Supermarine’s last pusher-engined biplane design for the Schneider Trophy Contests.
Having won the 1922 Contest Britain would be the host for the next challenge which started from Cowes on the Isle of Wight and followed an elongated course over the eastern Solent. Despite achieving a creditable speed of 157mph the Sea Lion III was beaten to third place by two Curtiss CR3 slim-line sea planes. RJ Mitchell noted the benefit of the new design and, abandoning his new Sea Urchin flying boat design for the 1924 contest, instead turned his attention to the streamlined advantages of high-speed seaplanes.
The result of this change in design appeared in 1925 as the Supermarine S4 (the ‘S’ in this designation standing for ‘Sealion’). Powered by a 680hp 12-cylinder Napier Lion VII ‘W’ engine and with its all-wood construction featuring a narrow tapering fuselage of oval cross-section with a mid-mounted cantilever one-piece monoplane wing under which were mounted a pair of ‘Lamblin’ radiators for engine cooling. Steel ‘A’-frames and fittings were used for strengthening and all of which was supported by two pairs of slender legs connected to a pair of long narrow wooden floats. The wooden four-bladed propellers that had driven the earlier Sea Lion series was replaced with a twin-blade all-metal unit from Fairey.
The S4 soon proved itself to be a strong contender for the next Contest – setting a new World Air Speed Record of 226.75mph in trials over Calshot. Sadly, this aircraft was destroyed in a crash following a structural failure at 800ft during pre-race practice in Baltimore, USA the day before the 1925 Schneider Trophy Contest; the pilot survived but was badly injured. But Mitchell & Supermarine weren’t beaten and work commenced on designing and building the S4’s successor.
The Supermarine S5 was ready in time for the 1927 Contest, to be held in Venice, Italy and incorporated a number of design changes. The wooden fuselage of the S4 was replaced by a lighter monocoque duralumin construction. The low-mounted monoplane wings remained of wood and were fitted with bracing wires of aerofoil cross-section, which were also attached to the metal floats, offering greater structural rigidity and reduced the net weight. The metal propeller was retained and improved.
The pilot, whose cockpit had been mounted behind the wings in the S4, was now positioned further forward, but his visibility was little better, with a very poor forward view from his low position. Engine oil was cooled by running the supply pipes from the rear-mounted oil tank along the outside of the fuselage to and from the engine; water cooling was achieved by covering most of the internal wing surface area with slim copper radiator panels and owing to the narrow fuselage section, the fuel tank was now located in the starboard float. Re-profiling of the nose and forward contours of the floats helped to increase performance and the extra weight in the starboard float helped compensate for the increased torque of the new improved Napier Lion VIIa engine which now offered 900hp and the VIIb (875hp) in the second S5. The winning aircraft returned a new record-breaking and winning speed of 319.57mph with the VIIb engine but handling difficulties during the take-off run, and a fatal crash during an attempt to raise the World Air Speed Record in March 1928 convinced RJ Mitchell that more improvements were required – pilot safety being of paramount importance, particularly in this new unknown territory of high-speed flight.
The Napier Lion series of engines had by now reached the end of its development potential and with that company not wishing to pursue high-performance, short life power units, a new engine supplier was needed. Eventually Henry Royce at Rolls Royce (who up to this time had also pursued a policy of avoiding racing engines in favour of building long-lived reliable engines) was persuaded to develop the company’s 12-cylinder 800hp ‘Vee’ Buzzard engine. The uprated supercharged engine, designated the Rolls Royce ‘R’ was produced within a few months and developed 1900hp. Extensive factory bench-running revealed a number of component problems which were identified and redesigned, thus curing the difficulties – but the engine, which required a new special fuel ‘cocktail’ mix, still had to be flown.
The new racing seaplane from Supermarine would require a major re-design of the fuselage, to incorporate not only the new engine which was considerably longer than the Napier, but narrower in cross-section. Also, space had to be found for the additional fuel and cooling systems that would be required to ensure sustained reliable performance at high speed for a duration of potentially in excess of 1 hour. The new aircraft was designated S6 and was of all-metal construction. Once again the wings doubled as water-cooling radiators and oil-cooling pipes again extended along the fuselage sides to the tail and fin, which now housed the oil tank and pump. The aerofoil cross-section bracing wires of the S5 were continued in the new airframe design. The pilot had even less forward visibility than in the S5 – the forward view being totally obstructed by a bulkhead in the cockpit. Navigation was by line of sight only and the pilot’s view was through the side window panels, between the nose of the aircraft and the wing leading edges.
Exhaust emissions from the engine had caused severe problems to the Italian opposition, resulting in the loss of two of their top pilots. Mitchel contoured the fuselage to direct the slipstream below the open cockpit, and added a small periscope-like tube to direct fresh air from outside, onto the pilot’s face.
Following early test flights handling difficulties were found due to the high engine torque, so Mitchell decided to lengthen the starboard float and increase its fuel capacity to 90 gallons; the port float carried 25 gallons. The extra weight made it necessary to move the centre-of-gravity forward slightly. Air scoops were provided on the nose to supply cold air directly onto the engine & manifold, and more scoops were fitted beneath the wing-tips to draw cold air in, to cool the radiator water inside the wings. This air exited the wings via small ports situated above the wings, at the roots, thereby providing extra lift with airflow over that area of wing at no weight cost but additional cooling was required, resulting in surface tanks being fitted to the aircraft floats – the cooling water being pumped to the engine via pipes attached to the forward edges of the legs.
The new Supermarine S6 proved itself by winning the 1929 Schneider Trophy Contest at an average speed of 328.6mph over The Solent and later set a new straight-line World Air Speed record of 357.7mph from the RAF Station at Calshot.
Italian competition was still strong and interest again grew from France and America – both previous winners of the coveted prize. More work was needed on the Supermarine & Rolls Royce designs if the Schneider Trophy was to be retained for a third consecutive time by Britain, which would mean its permanent retention.
Rolls Royce commenced work to improve the ‘R’ engine, believing they could increase its performance by 25mph. This resulted in an increase to the overall size and weight of the unit, which in turned demanded modifications to the S6 airframe. Mitchell wanted to make as few alterations as possible, but it was necessary to extend the nose by four inches to accommodate the improved engine. A new propeller was also developed by Fairey, who trialled a number of different profiles to determine the most efficient unit. Following flight trials it was found that the tail began to flutter dangerously, so a trim tab was added to counter this effect.
It was decided that two new S6s would be built – designated S6B. The original (1929) aircraft would be modified to take the new engines and were redesignated S6A. During trials in the summer of 1931 one of the S6As crashed, killing the pilot. The aircraft had stalled on take-off; the Board of Enquiry found that the pilot had ignored the instruction to maintain full power until an altitude of 200ft had been reached, and had throttled back early (as in more conventional aircraft) causing the S6A to fall backwards into the sea.
In the event the British met with no opposition due to the early withdrawal of France and America and late technical problems experienced by the Italian team, causing their withdrawal. The S6B achieved another Contest speed record of 340mph and two weeks later established a new World Air speed Record of 407.5mph, becoming the fastest manned object on the planet.
In just twelve years Supermarine-designed aircraft had increased the top speed of these racing flying boats and sea planes from 140mph to over 400mph, setting a
number of new air speed records and had won the coveted Schneider Trophy four times. Rolls Royce Managing Director Arthur Sidgreave said at the time
“It is not too much to say that research for the Schneider Trophy Contest
over the past two years is what our aero-engine department would other-
wise have taken six to ten years to learn.”
In the coming years the benefits of these developments would be seen by the whole world.