Friday, 14 September 2012

R. J. Mitchell’s Stuka – Type 224.

Supermarine Type 224  (from painting by author)

When the Air Ministry issued specification F.7/30 9 (see my Blogpost:'The Spitfire and its Predecessors - Spec. F.7/30') leading aircraft firms became engaged in a contract race to produce the required interceptor fighter but none found it easy to meet the stringent requirements. Of course, Mitchell’s qualifications for creating  high-speed aircraft were outstanding, thanks to the Schneider trophy contests, but this experience was, in respect of the first Supermarine fighter design, Type 224, more significant for prompting the company to enter the bidding rather than for actually providing a successful formula for winning a contract. As we shall see, much of its under-performance was not of Mitchell’s making but it, at least, gives the lie to the still popular assumption that the legendary Spitfire emerged directly from the Schneider Trophy machines or rose as some single conceptual leap after its designer returned to work at the end of 1933.
As it was, in 1930, Mitchell had had to turn his mind to a military aspect of aviation that he had only briefly been engaged upon with the Sea King II of 1921 – and that aircraft was a flying boat. Also, armament on his flying boats was provided via gunners in cockpits not via guns buried in the wings. Alan Clifton recalled that Mitchell was uneasy about ‘his first venture into military aircraft’, recognising that he was ‘no expert in the field’.and thus, with typical pragmatism, he asked ‘Mutt’ Summers, Vickers chief test pilot, to arrange a visit to the Martlesham test centre to find out what the RAF pilots considered to be most important in a fighting machine.
   Type 224
Mitchell’s resultant design was an all-metal structure with a thick inverted cantilever gull-wing and a short fixed undercarriage with large fairings (hence the comparison with the Junkers Stuka, especially with the prototype which had somewhat similar undercarriage fairings –  see below).
   As Supermarine had been devising undercarriages which could be raised ever since the Commercial Amphibian of 1920, it might have been expected that a fully retractable, drag-reducing, arrangement would have been incorporated in their Type 224 entry in the early 1930s. However, the cranked wing meant that the fixed undercarriage would be reasonably short and light and would be advantageous for certain other new requirements. One of these was that, to satisfy F.7/30, a gun could be housed in each fairing without problems associated with burying the armaments in the wings and conflicting engine cooling considerations.
   Despite some reports to the contrary, the Air Ministry’s only requirement of the type of power plant for the new fighter projects was that it should be an ‘approved British engine’; Supermarine, along with four other manufacturers, chose the Rolls-Royce Goshawk which was predicted to produce an impressive 660 hp (compared with the 450 hp of the Bristol Jupiter VII radial engine of the current fighter, the Bristol Bulldog). As the Goshawk was an in-line engine, its low drag profile might also be expected to contribute to an impressive performance, and it was designed to work with a new water cooling system, the so-called evaporative method, which was expected to bring with it significant reductions in drag – by not requiring conventional radiators to keep the engine coolant below boiling point. With Mitchell's new design, the undercarriage fairings, in conjunction with the cranked wing configuration, produced two very convenient low points where the coolant could collect.
   In view of the Ministry’s slow landing speed requirement, Mitchell’s Schneider experience was not much help as the wing loading of the S.6B had been 42 lbs. per sq. ft. with a resultant landing speed of well over 100 mph. A large air brake was now employed and this could be lowered from the underside of the fuselage but the Air Ministry was, nevertheless, concerned that the projected wing-loading of only 15 lbs. per sq. ft. was too high. Thus the wing span was eventually fixed at a generous 45ft.10in. and, in combination with a fuselage about the same length as the 30 ft span S.6, Type 224 looked somewhat out of proportion.
   But the real difficulty was with the cooling system as a whole. It had been arranged for the water, which had condensed in the leading edges of the wings and had collected in the tanks in the undercarriage fairings, to be pumped up to a header tank behind the engine; however, at the low pressure side of the pump, the water would often turn into steam again, the pump would cease to operate, and plumes of steam would be seen escaping from wing tip vents. Jeffrey Quill, who had joined the company, has recorded how he did not exactly please the Chief Designer when he commented on these problems:
The evaporative cooling system was a real pain in the backside, with the red [warning] lights flashing on all the time. I once made a jocular remark to Mitchell about the system. I said that with the red lights flashing on all over the place, one had to be a plumber to understand what was going on. He didn't say anything, he just looked very sour. He was rather sensitive about the aeroplane and obviously I had trodden on his toes.
As the cooling difficulties occurred particularly during rapid climbs, this meant having to level off until the system was working normally again; one can appreciate why Mitchell was not happy with his prototype, designed for the fastest possible climb to 15,000 feet in order to intercept enemy bombers.

It was during the last days of the Type 224 development that Mitchell’s cancer was diagnosed and when he returned after his operation at the end of 1933, new ideas for Type 224 were submitted in July, 1934. These included a retractable undercarriage and elimination of the cranked wing, which were expected to improve the maximum speed of Type 224 by 30 mph but the official response was lukewarm.
Nevertheless, Mitchell had already made some alterations to improve the performance of Type 224; in particular (and in respect of the future Spitfire): wider span, slimmer chord ailerons were fitted and various sorts of wing root fillets tried out. [None of the Schneider racing machines had had any fairings to smooth the airflow where the wings joined the fuselage even though such an improvement had been evident as early as 1930 with the Northrop Alpha. ]
None of the major modifications proposed were implemented but, at least, test flying continued, for the purposes of producing information for a potential successor, and Vickers even named the aircraft ‘Spitfire’

Meanwhile, Junkers did manufacture, in large numbers, the Stuka which had a somewhat similar configuration, although by the time of its appearance in 1934, Germany had decided upon a two seat formula – hence the very different cockpit canopy. A rival design, the Blohm und Voss Ha 137, which was not proceeded with, also had a cranked wing and a ‘trousered’ undercarriage and, in the versions which had an inline engine, was even more like Mitchell’s Type 224:

It might also be mentioned that this similarity can also be seen in the Kawasaki Ki-5.

Supermarine Type 224
The Ki-5, like the Type 224, first flew in February 1934, thus showing that there was nothing derivative or eccentric about Mitchell’s basic approach even if other factors militated against it – in particular, the wingspan required by the Ministry. The Japanese fighter had a span about 11 feet less and, with over 200 hp more, could reach 240 mph – about 10 mph better than the Supermarine prototype.
Thus, despite Mitchell’s disappointment with his fighter, its top speed of 228 mph speed was quite creditable for its hp, given the imposed penalty of the large wingspan. Thus, whilst Mitchell’s chosen configuration of a cranked wing and a short fixed undercarriage was not an eccentric choice, the landing speed restriction must have been one of the main reasons why the Vickers chairman instructed Mitchell to design a private venture aircraft, which became the future Spitfire.

* See my blog of 25/6/12.

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 Type 224 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.

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