Early evidence of Mitchell’s design work is his contribution to the P.B.31E Nighthawk of 1916. He was, no doubt, also involved with Chief Designer Hargreaves’ Sea Lion I and, after the latter left in 1919, was involved in various conversions of the Admiralty designed aircraft, A. D. Boats, which Supermarine bought back from the RAF to be converted for passenger and photographic survey work. However, the first overall design which might be attributed to R. J. was the so-called Commercial Amphibian of 1920.
The first aircraft involved in commercial flying after the end of World War I were conversions of military machines – like the Channels - the name given to Supermarine’s conversions of the A. D. Boat. As they were by no means well suited to their new roles, the newly formed Department of Civil Aviation at the Air Ministry announced, in March 1920, two competitions for commercial designs ‘of British Empire origin’ to promote ‘Safety, Comfort and Security’ in air travel. With a view to developing international travel, and bearing in mind that there were few airfields available, one of these competitions was specifically for amphibian seaplanes with a first prize of £10,000 and a second one of £4,000.
It was not surprising that Mitchell was asked by Supermarine to design an entry for the seaplane competition, which was to commence on 1 September of that year. By later standards, entry requirements were extremely modest: in the amphibian category, these included seating accommodation for a minimum of two passengers; a range of 350 nautical miles at 1,000 feet at a speed of not less than 70 kt; and a load of 500 lb to include passengers and life belts but not including crew. There was also a requirement of a flight of three minutes at 5,000 ft. to check if the machine would fly itself at this height and with enough height to recover if necessary. Based at the experimental station at Felixstowe, the amphibian competitors should take off from water and fly to the land-based experimental station at Martlesham Heath.
|Commercial Amphibian at Martlesham|
As the Commercial Amphibian can be claimed to be the first comprehensive design by Mitchell, it is surely very understandable that the end-product would be a conservative one. Also, there were only about twenty weeks separating the announcement date of the competition and that of the trials, leaving little time for innovative thinking. Not surprisingly therefore, Supermarine described the new design as ‘practically a "Channel" type boat, with a wheeled undercarriage hinged on each side’ although the comparable side views show considerable changes in the Mitchell design:
|Channel II Commercial Amphibian|
As regards the Channel precedent, the Commercial Amphibian had a biplane layout in which similar dimensions of height, and length were adopted and the sea rudder was similarly placed to that of the Channel – vertically below the leading edge of the tailplane – but now converted to act also as a skid for taxiing over land. The wing-tip floats were also of the Channel sort and the oval hull and the general arrangement of its built-on planing surfaces continued the Linton Hope/Channel principle of hull construction [see my Blogpost: 'Mitchell's Wooden Hulls'].
On the other hand, Mitchell increased the area of the lower wing; he also incorporated features of a much smaller aircraft, Hargreaves' Sea Lion I: the fin and rudder outlines were similar although a proportional increase in surface area above the tailplane allowed our designer to provide a more symmetrical appearance to the fin. And the Sea Lion's outwardly raked inter-plane struts were repeated in the new, and larger, machine.
Between the Amphibian’s struts there were canvas stabilising screens, full length between the inner pairs and quarter length between the middle ones. These screens were relatively uncommon by the end of the 1914-18 War but survived on several later Supermarine designs as well as on the Channel and Sea Lion, perhaps (in the case of the inner ones, at least) mainly to protect the engine and propeller from spray on take-off or landing. It was the present machine which was most extensively fitted with them and, in this respect, it did not look particularly like an advanced design. As many of the features from the Channel and the Sea Lion I were thereafter abandoned by Mitchell, the present design can be regarded as something of a ‘time capsule’, a summing-up of earlier practices rather than a statement of the way forward.
But, nevertheless, Mitchell showed an early instance of boldness and originality by abandoning the biplane tailplane and twin rudders of the Channels (still evident in the Handley Page H.P.42 of the 1930s) in favour of a single fin and tailplane. It is worth noting that the competition rival Vickers Viking III went through three more variants before the Mark VII, the Vanellus, appeared five years later with a more modern-looking single tailplane and rudder. And Mitchell’s rudder was also a departure from the minimalist approach of previous Admiralty inspired rudders – perhaps his work alongside Hargreaves on the Sea Lion I had had some influence in this respect. Additionally, Mitchell remodelling of the nose with a prominent boat-like entry to counter spray was to prove successful in his future Sea Eagle, Scarab and Seagull designs.
A further feature was Mitchell’s design for a retracting undercarriage, necessary because the Air Ministry competition was for an amphibian aircraft. At this time, an American landplane, the Dayton-Wright R. B. Racer, had a fully retracting landing gear designed specially for the Gordon Bennett race of 1920; on the other hand, the Air Ministry commercial competition was satisfied with devices which merely lifted the wheels out of the water, in order to facilitate take-off and alighting. Supermarine’s concern for ‘boats which fly’ offered no previous experience of retractable undercarriages for Mitchell to call upon and so it is noteworthy that, for his specially designed mechanism, he chose a geometry which displaced the wheels outwards rather than forwards – thus avoiding any change of trim when the wheels were moved up or down.
One other particular feature of the Commercial Amphibian must also be mentioned: the enclosed passenger cabin [see the opened cabin roof top in the photo above]. The competition’s intention of ascertaining ‘the best type of Float Seaplanes or Boat Seaplanes which will be safe, comfortable and economical’ might have seemed to make an enclosure for passengers inevitable but it should be noted that the other two amphibian entries had open cockpits for the passengers, one seated next to the pilot and the other two side-by-side behind. Open cockpits at this time were the norm and they saved weight, but they were far from ideal for operation over water and in northern climates – Cozens, a contemporary observer, described how previous Channel passengers ‘had an uncomfortable ride on many occasions’ and, although ‘wearing flying coats and helmets they looked wet and miserable as they got into a boat that was rowed out to meet them’.
No adjustments or replacements to the Mitchell aircraft were required, despite its one-off design and the short notice of the competition, and the Supermarine entry was the only one which completed all the tests that were stipulated and whose landing gear did not give trouble at any time. The judges also noticed with approval an effective tiller arrangement for steering whilst taxiing on water, the equipment for sea use, and the way in which the shape of the forward part of the hull kept spray off the passengers’ compartment. On the other hand, the novel undercarriage gave rise to criticism for being none too clean, from the mechanical and the maintenance points of view. The lateral control of the Commercial Amphibian was also considered not immediately responsive enough.
Unfortunately, the fitting of a Rolls-Royce Eagle engine in order to lift the considerable more loaded Channel design led to a loss of competition points, resulting in its coming second to the Vickers Viking. However, the Air Ministry report on 11 October stated that ‘the results achieved for amphibians show that considerable advance has been attained … and the competing firms deserve congratulations on their enterprises.’ They also recommended a doubling of the second prize money to £8,000 as ‘the proportion of the monetary awards does not adequately represent the relative merits of the first two machines’.
A modest beginning to be sure but, as we shall see later, its general design and its overall performance gave rise to a call from the Air Ministry for a development of this machine which led to the Sea Eagle and the Seagulls between 1923 and 1926.
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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 (details below) which also provides material for wider reading, grouped according to specific areas of interest.
More information, photographs and a three-view drawing of the Commercial Amphibian, 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.