Sunday, 26 March 2017

Precursors of R.J. Mitchell's Walrus.

Seagull III, Papua (from painting by author).

The first success of Mitchell as Chief Designer was the Commercial Amphibian and this had brought an order from the Air Ministry for a  military development of the type as part of its policy to assist the struggling aviation companies to stay in business. This was obviously not a philanthropic gesture but a recognition that British air power needed the support of a healthy aviation industry especially as, by 1920, the RAF had been in action again with the new military tactic of ‘control without occupation’ – a very economical and swift-acting aerial alternative to the employment of large army land forces in the policing of colonial and League of Nations mandated territories – against tribesmen in Mesopotamia, Transjordania, the Sudan, and Somaliland, against the Bolsheviks in Russia, and against the Afghans on the Indian frontier.

In order to fulfill the Air Ministry requirement, Mitchell’s next design was to be a three-seat amphibian for use as a fleet spotter; it would be required to be extremely seaworthy and to have the lowest possible landing speed with good control – in order to land on to aircraft carriers. Mitchell’s response was known as the Seal II, presumably with the Commercial Amphibian being regarded as the Mark I predecessor.
     The Seal II had the outwardly-retracting landing wheel geometry first introduced on the Commercial Amphibian but the detail of the Seal’s retracting system shows that something had been learned from the criticisms of the earlier plane’s mechanism: the earlier machine had an undercarriage of two steel tubes, hinged below the lower centre-section join with the lower main planes and the wheels were raised or lowered by sideways pressure from a tube in the hull to the wheel axles; the new undercarriage now had a single strut, suspended from the lower wing and braced by two tubes hinged to points on the hull; it was moved inwards for retraction by means of a worm and bevel gear under the wing. This revised method was utilized on all future Supermarine amphibians up to and including the Sheldrake of 1927.
   Of the flying surfaces, only the tailplane followed the previous Hargreaves Sea Lion outlines and, although Mitchell placed the stabiliser lower on the fin, he retained the inverted aerofoil principle. This feature was also continued in aircraft up to the Sheldrake, being necessitated by the high thrust line of the engine which caused increasing nose-down forces as power increased. With the tailplane being ‘of the depressing kind’, as it was described in the publicity for Hargreaves’ earlier Sea King, the need for constant back pressures on the stick was therefore obviated at cruising speeds.
   On the other hand, the wing shape was new and this planform was retained by Mitchell for all his subsequent single-seat naval aircraft, up to and including the Sheldrake. But the rearward-folding wing requirement for a shipboard aircraft had not been tackled by the Supermarine company since the Baby of World War I and Mitchell adopted a similar approach – and one which he, again, continued with in military aircraft until the Sheldrake: the forward wing strut at the joint between the wing centre section and the main plane was doubled so that one of these members carried the weight of the leading section of the wing when folded back. The need for wing-folding also required large cut-outs to be made in the trailing edges of the wings, so that they could fold close to the plane’s centre-line to keep storage space to a minimum, and the wings were placed further forward than in the Commercial Amphibian so as not to project behind the trailing edges of the tail assembly when folded.
   The wing-tip floats, with their decreased side-area, were less clumsy than before and could be carried on struts to the waterline. The pilot was placed well forward and supplied with a machine gun which could be retracted and shielded during take-off and landing; the wireless operator was just aft of the wings and the rear gunner behind him but with the fuel tanks separating off the pilot from these other two crew members. 
    Because two of the crew members were placed behind the wings, a tractor layout had to be chosen for the engine to prevent the centre of gravity moving too far back. In Supermarine’s publicity for the Seal, attention is drawn to this placement, no doubt because of its novelty (at least in single-engined machines):
the engine is the Napier of 450 h.p. The engine mounting is unusual in that it is of the tractor type. This has been rendered possible by the fact that in this case the greater part of the useful load carried is aft of the wings in the tandem cockpits, and the success of the tractor mounting will allow this type of boat to be arranged either as a tractor in such a case as this or as a pusher in cases where the greatest useful load is concentrated forward. Very great attention has been paid in designing this engine installation to securing accessibility for inspection and adjustment of the engine and its accessories.
One notices the offer of the more conventional pusher layout – presumably in the hope of a civilian version which would not, one might reasonably assume, need a gunner behind the wings. The company publicity also draws attention to the new designer’s typical concern with the practicality of his machines – in this case, the ease of access to the engine.

As N146, the Seal II first flew in May, 1921 and, in the following year, one machine was sold to Japan who were keen to be kept abreast of Western technological developments. Despite this general lack of orders, the Seal is important in our story as it is the one early Mitchell design which most clearly looks forward to one of his three main aircraft types – the Seagull II to Seagull V/Walrus series of medium sized amphibians.

Meanwhile, the company had to come to terms with the post-war Anti-Waste League and the resultant Geddes Committee Report which led to a drastic reduction in all Government expenditure. The new Secretary of State for Air, Sir Samuel Hoare, reported that, in 1923, only 371 front line aircraft remained, either in the British Isles or abroad, and assessed the current situation thus: ‘Orders for military planes had almost come to an end and a demand for civil planes did not yet exist … Only two thousand five hundred men and women were left in the industry and the few firms engaged on machines and engines were on the verge of closing down.’
   On the other hand, the actions mentioned earlier of ‘control without occupation’ had to be backed up by support for the ailing aircraft industry. Thus it had been decided that, over the next five years, thirty-four new squadrons would be formed, bringing the air defence of Great Britain up to fifty-two squadrons by 1928. [In the event, the total home squadron numbers only rose to thirty-four by the date proposed.]

The first positive result of the new situation was seen when Supermarine received a letter from the Air Ministry which suggested that it ‘might be inexpedient’ to close down the works entirely as Supply and Research were considering an order, ‘the exact amount of which cannot yet be stated, but which might approach 18 machines, spread over the period ending March 31st, 1924’.
   In response to this Air Ministry lifeline, Mitchell modified the Seal II machine. A more powerful Napier Lion II engine was fitted – again in a tractor layout – and the fin consequently increased in area and the wingspan reduced. Thereafter, two new aircraft, N158 and N159, were completed by March, 1922, by which time, the wing-tip floats had been redesigned, the wings given a slight sweep back, the ailerons redesigned, and the fin area further enlarged. The number of modifications resulted in these first production Seagulls being designated Mark II.
   There was one particular modification of the Seal type which ought to be mentioned: the fuel tanks had now been moved from the fuselage to positions under the top wing centre-section, thus supplying petrol to the engine by gravity feed. [A Supermarine comment on the previous Sea King II design had been that ‘the petrol supply is by pressure, and every effort has been made to reduce the length of piping and eliminate as much as possible the carrying of piping into the hull’.] Perhaps because of the weight distribution of the basic Supermarine flying boat configuration, there appears to have been no undue problems created by the new fuel arrangements and there was also a particular bonus: Supermarine was now able to announce that ‘inter-communication between crew has been considered fully, and a through passage is arranged for this purpose’. [In passing, it should be noted that the constructional methods of the Linton Hope hull that Supermarine had adopted conferred another advantage as there were no internal bracings or structural bulkheads to be weakened by making a passageway between the pilot and the other crew members – see my Blogpost: ‘Mitchell’s Wooden Hulls, Structure and Finish’.]

A competitive test on HMS Argus between the Seagull and the Mark VII version of the Vickers Viking had found in favour of Mitchell’s machine and an  order for five Seagulls, N9562 to N9566, was then received in February 1923. This was followed by a requirement for another thirteen (N9642 to N9654), also in 1922. These aircraft equipped No. 440 (RAF) Fleet Reconnaissance Flight and some were placed aboard the aircraft carrier HMS Eagle. An additional machine was again sold to Japan.
    Further orders for the Seagull came in when the Australian government decided that their Air Force should assist in the hydrographic survey of the Great Barrier Reef. 101 (Fleet Cooperation) Flight was formed on 1 July 1925 and six Supermarine Seagull III amphibians were ordered (A9-1 to A9-6). These machines were essentially Mk IIs with larger radiators and the first of these was ready by February, 1926 – by which time six of the RAF aircraft had served a tour of duty with HMS Eagle but, thereafter, the type was pronounced as having ‘no potential naval use’.
  In sharp contrast, the Australian Seagulls were used more thoroughly, as their survey work extended into 1927 and continued on northwards to include some 10,000 square miles of Papua and one staged flight of 13,000 miles. Interestingly, three RAF Seagulls, engineless, had been acquired at the scrap price of one hundred pounds each and were intended to be used for spares. However, they were found to be in such excellent condition that they were re-erected and quickly put into service.
   This traditional Supermarine ruggedness was also evident after the survey work was completed, as the Seagulls were assigned to the newly constructed seaplane tender HMAS Albatross, commissioned in 1929, and continued in carrier use until 1933 when the vessel was placed in reserve.

The Seagulls had, by this time, lost their appeal for the RAF but, as a result, the Seagull design continued to occupy the minds of Mitchell’s design team even until 1928: fitting hydro-vanes was considered and, because of its porpoising tendencies, various permutations of the hull step position were tried out on N9565 and on N9606. And one aircraft, N9605, was fitted with Handley Page slots and a new tail unit with twin fins and rudders. This aircraft, designated Mark IV, was converted to take five passengers in 1929, when the Supermarine company was looking forward to the old Sea Eagle Southampton–Channel Islands routes being resumed [see my Blogpost: 'R.J. Mitchell's Sea Eagle Fleet'] – with a small fleet of Seagull IVs. A pilot service was begun in July by the prototype five-passenger conversion (G-AAIZ) but most of August was void owing to serious damage to the hull caused by its hitting a barely submerged rock. Then, on 2 September, the short-lived business ceased when the aircraft ran into engine trouble.
   Two other Seagulls, N9653 and N9654, were converted for civilian use: registered as G-EBXH and G-EBXI respectively they began a coastal service at Shoreham but this also failed, owing to inadequate public response.
   However, two other modifications of the Seagull were of great significance to Mitchell’s team. One was concerned with equipping a Seagull to initiate the testing of catapults for launching aircraft and the second was the changing of the usual water-cooled Napier engine to an air-cooled radial engine in a pusher configuration. As we shall see later, when the Seagull V/Walrus appeared, it was an aircraft engined in this particular way and stressed for catapult launching.

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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  which also provides material for wider reading, grouped according to specific areas of interest. 

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