Early evidence of Mitchell’s design work is a contribution to the Nighthawk of 1916 (see my blog of 15.5.11). 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 for conversions for passenger and photographic survey work. However, as we shall see, 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 (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, the requirements for a successful entry 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, pass as high as possible between marker boats 600 yards from the start buoy, and land at the other experimental station at Martlesham Heath. Taxiing on water had to include figures of eight, taking off and landing in rough weather, and mooring out for at least twenty-four hours in moderate weather. Additionally, the hull or floats should still retain positive buoyancy if perforated.
|Commercial Amphibian at Martlesham|
|Channel II Commercial Amphibian|
As regards the Channel precedent, the Commercial Amphibian had a biplane layout in which similar dimensions of wing span, 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 when taxying 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 which Supermarine had adopted.
On the other hand, Mitchell did not just follow the previous precedent of the Channel but also incorporated features of a much smaller aircraft, the Sea Lion I: the fin and rudder outlines were similar although a proportional increase in surface area above the tailplane allowed the 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 mainly to protect the engine and propeller from spray on take-off or landing, but it was the present machine which was most extensively fitted with them. In this respect, it did not look particularly like an advanced design and, 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, whilst the conservatism of the Commercial Amphibian design must by now be clear, 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 significantly remodelled the nose with a prominent boat-like entry to counter spray which 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. 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.
His design was also sturdy – Flight reported:
It is worthy to note that during the firm’s trials at Southampton this aircraft landed in a ploughed field, with furrows of about 15 in., which not only pulled the amphibian up in a very short distance, but allowing for the heaviness of the ground, threw the boat forward on its nose, leaving the section of the hull on the earth for a distance of about 15 yards. The boat was dropped on its tail, which also speaks of the rigidity of tail construction, to put up with such treatment, and was flown off and landed at Eastleigh Aerodrome. No damage of any kind, with the exception of scratched varnish, was experienced, and later during the trials the aircraft was often tipped in this manner by the pilot for turning purposes.
|Commercial Amphibian 'arrival'.|
|Mitchell standing by Commercial Amphibian; Scott-Paine in passenger cabin.|
|Commercial Amphibian taking off; clearing balloons.|
Unfortunately, the low power of the engine of the Commercial Amphibian produced a loss of competition points, resulting in its coming second to the Vickers Viking. However, the final report on October 11th 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 an increase in the second prize money as ‘the proportion of the monetary awards does not adequately represent the relative merits of the first two machines’.
|Commercial Amphibian on Woolston slipway; Mitchell standing nearest bow.|
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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.
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, 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.