Most of Mitchell’s seaplanes were designed for war, although naval reconnaissance rather than bombing was their main requirement from the Air Ministry. They were, basically, traditional biplanes and Mitchell was also encouraged to design two other biplane flying-boats, closer to the outbreak of World War II, despite the obsolescence of the configuration.
The first of the new designs, to become known as the Scapa, was ordered as a successor to the long-serving Southampton, to be fitted with a metal superstructure and powered bythe new Rolls-Royce Kestrel engines. Thus the Southampton IV/Scapa has been regarded as, essentially, an ‘improved Southampton’, particularly as its hull planing geometry was closely based on that of the earlier aircraft. This similarity was very much a compliment to the intuitive designers of the Southampton hull of 1925, as the later advantage of tank testing did not suggest any real need to depart from the basic shape of its predecessor – indeed, a wider beam behind the step, to discourage water striking the tail surfaces, was replaced by the older Southampton after-portion when actual take-offs revealed an unpleasant pitching when the rear step made contact with the water.
Nevertheless, the eventual Scapa was, effectively, a new design. The need to ‘stretch’ the Southampton design resulted in a lengthened bow with a deepened forefoot which, with a flatter top-decking, altered the overall appearance of the previous Southampton hull. The Air Yacht tradition of flat plates and rectangular sections was still evident although the upswept tail section and other curvatures restored something of the elegance of the Southampton.
Two fins now replaced the triple-fin arrangement of the Air Yacht or standard Southampton and the engines were now positioned directly under the top wing.
This upward re-siting of the engines was probably influenced by the water ingestion problems experienced by the Seamew and allowed Mitchell to dispense with the engine supports of his previous inter-wing-engined designs and the large Warren bracing of the Southampton superstructure. His reversion with the S.5 and S.6 to wire bracing must also have been a factor here and also led to a single bay structure, even though the new machine’s wingspan was to be as large as 75 feet. Beverley Shenstone, Mitchell’s aerodynamicist, considered the resulting aircraft ‘perhaps the cleanest biplane flying-boat ever built, with minimum struttage and clean nacelles faired into the wing’. He did not mention the very ‘boxy’ radiators which projected on either side to the rear of these nacelles although this positioning least compromised the overall lines of the new design.
No new name had been chosen when ‘Mutt’ Summers took S1648 up for its first flight on 8th July, 1932. After numerous tests, it was delivered on 29th October to MAEE, Felixstowe, for further trials which included a maximum duration flight of 10 hours over the North Sea. In the following May, the prototype was flown to the Kalafrana flying-boat base, Malta, for overseas acceptance trials with No. 202 Squadron and these involved a long-distance flight to Gibraltar and back and a cruise to Port Sudan via Sollum, Aboukir, and Lake Timsah. On its return, the Scapa took part in the 1934 fly-past of ‘the competition’ at the Hendon R.A.F. Display with, as Penrose reported, ‘the clean Supermarine twin-engined Scapa leading, followed by the four-engined Short Singapore, triple engined Blackburn Perth, the distinctive gull-winged Short Knucklduster, Saro R24/31 London and the three Saro Cloud trainers’.
The Hendon event was clearly designed to impress foreign governments of Britain’s military capabilities and, during the 1935 – 1939 period, the Scapa fulfilled its required purpose with anti-submarine patrols to protect neutral shipping during the Spanish Civil War. Some of the aircraft of No. 202 squadron were later transferred to No. 204 Squadron and were sent to Egypt during the Italian-Abyssinian confrontation. There was also a single Scapa attached to No. 228 Squadron whose contribution to the developing war preparations was involvement in early radar trials.
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 (details below) which also provides material for wider reading, grouped according to specific areas of interest.
More information on ….., photographs and three-view drawing… of the ….., 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.