Sunday, 27 May 2012

The Spitfire and its Predecessors




The Air Ministry Specification F. 7/30, which eventually led to the Spitfire, is well enough known (though there are inaccuracies in many accounts). In summary, the Specification called for a four-gun, single-seat day and night fighter with manoeuvrability and a "fighting view";  a speed of not less than 195 mph at 15,000 feet, a landing speed not more than 60 mph, and a climb to 15,000 feet in 8.5 minutes, were also specified. However, the night fighter consideration and current modest grass aerodromes of the RAF required a low landing speed that inhibited the design of a high speed machine.
Gloster Gladiator (painting by the author)

Gloster Gamecock
(painting by the author)
Nevertheless, the F. 7/30 Specification is something of milestone in Ministry thinking as we can see from a brief review of the performances of preceding RAF fighters. The Gloster Gamecock which entered squadron service in 1926 had a top speed of 155 mph and the Bristol Bulldog of 1929 was 20 mph faster – an average increase of about 7 mph per year.
Thus, in 1930, the Ministry might not have expected more than, say, 185 mph if manufacturers responded to the F.7 with the usual biplane configuration, air-cooled radial engine, and fixed undercarriage – especially as double the number of guns was now required.
Bristol Bulldog
(painting by the author)

In order to appreciate the achievement of the 360 mph Spitfire in 1937, it is  necessary to look at previous increases in engine power. This rose from 425 hp (Gamecock) to 830 hp with the Gladiator, an increase of 95% for an improvement in performance of only 66%. Matters were worse later – the Gladiator showed a 12% top speed increase over the Gloster Gauntlet of 1936 for an increase of 30% in hp. Clearly, only an aircraft with a very different configuration would significantly improve matters.  The Spitfire's cantilever flying surfaces,  the additional  improvement of a retracting undercarriage and with the in-line Merlin engine, was the well-known answer and this is shown starkly by a consideration of the present figures and calculations:

Gloster Gauntlet
(painting by the author)










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, 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.

(1) in 1937, the Gladiator could reach 257 mph whereas the Spitfire entered squadron service the following year with a top speed of 362 mph.  Thus, instead of replicating the usual speed increase of less that 10 mph per year, Mitchell’s MkI fighter achieved ten times that figure;
(2) the hp figures are also very relevant – the Spitfire’s 1030 hp,  compared with the Gladiator’s 830 hp,  represents an increase of 24% in power but produced an increase in top speed of over 40%.







































For reference sources see my "Blog Subjects, Sources and References".



Monday, 21 May 2012

R. J. Mitchell’s Wooden Hulls – structure and finish.



One of the fortuitous circumstances in R. J. Mitchell’s career was joining a firm which had adopted a method of flying-boat hull construction of considerable potential. G. A. Cozens relates how the Pemberton Billing firm that he joined began 'in a part of Mr.Kemp’s boatyard' and how the workforce were largely 'Kemp boatyard men.' Thus it would be no surprise that Pemberton Billing, in his stated concern to produce  'boats that would fly', basically adopted current boat-building techniques.  As Cozens related:
In 1914 the firm built the small flying boat PB I which was a credit to the workforce, and indeed it was judged to be the best example of aircraft construction at the 1914 Olympia Aero Show. The PB I hull was of round construction, built by small boat methods with closely spaced wooden ribs of half inch square section like girl’s hoops, joined by longitudinal stringers and covered by two layers of mahogany or cedarwood planking, laid so that the outside layer was sloping the opposite way to the inner layer, this was known as ‘opposed diagonal planking’. There was a layer of doped fabric between the layers and the whole fastened by brass screws or copper nails and in some cases the nails were turned over and clinched or riveted over a small washer.
With the outbreak of World War I, an Admiralty design team was sent down to Woolston to develop flying-boat construction and the basic structure with which Supermarine afterwards continued was established. F. Cowlin, the Technical Supervisor at the Royal Naval Air Station has recorded how he went down to the then Pemberton-Billing firm and ‘learned a great deal about hull design from Linton Hope'. As Penrose wrote: ‘the Admiralty had found floatplanes too dependent on smooth water; they were interested in the far heavier flying-boat hull which in the Linton Hope approach consisted of a double skin of mahogany planking with fabric in between, with rock elm strips forming almost circular ribs, longitudinally stiffened by closely spaced stringers’. 
The standard large flying-boat of World War I was the Felixstowe, but it had structural and hydrodynamic deficiencies and the Air Ministry were concerned to see if the Linton-Hope type of hull could be adopted on aircraft of the Felixstowe size. However, these types could not be contracted out immediately and it was to Supermarine and Mitchell’s good fortune was that the building of two prototype machines - known as A.D. Boats - had meantime been contracted out to the then Pemberton Billing firm. Although Hope did not entirely get his own way, as he indicated in a Flight article, the less than perfect machine embodied the alternative and much more promising approach to flying-boat hull design:
these boats were very difficult to get off the water . . . and with later experience it was obvious that the  main step was too far aft and the rear step much too far forward. . .  In spite of these faults in design, the A.D. boats showed the great strength of the flexible construction, and some bending and crushing tests carried out by the R.A.E. works at Farnborough show what they were able to resist.
The essential rightness of the Linton Hope approach was its flexibility, whereby a flying-boat could absorb the shocks of sea landings and take-offs when there could be no recourse to the normal springing of an aircraft undercarriage. Mitchell, as a young Chief Designer with a background in locomotive engineering and from the land-locked Potteries, was thus fortunate to inherit considerable sophisticated marine know-how and also a much more advantageous approach than that of the slab-sided Felixstowe flying boats. 
   He seized upon the basic principle of mounting necessarily rigid flying surfaces and engine mountings upon the Linton Hope flexible and robust hull.  [A small example of attention to the interface between rigidity and flexibility is mentioned in a Flight article about the Sea King:  ‘[the pilot’s] controls are mounted on the triangular tubular frame so well known in all Supermarine boats, and whose function it is to allow the circular hull to flex and "give" in a seaway, without interfering with the smooth working of the controls.’]
    As mentioned earlier, the Linton Hope hull was double planked although the positioning of the outer layer at about 90 degrees to the inner, diagonal, one was usually superceded by the former being laid longitudinally. What is less certain is whether the final finish was wood, varnished, or a doped-on fabric, also varnished. The fine reconstruction of the Southampton hull in the RAF Museum at Hendon [well worth a visit] shows the culmination of the Linton Hope approach which a Southampton apprentice of the time described as having the final layer of planking sanded down and varnished until it had a finish "akin to the best kept dining room table".  There is no final layer of fabric. And Cozens wrote of Supermarine aircraft having an "air of sturdy solidarity . . . due to the beautiful diagonal mahogany or Red Cedar planking of the hulls, covered by four coats of Copal varnish".
    However, Supermarine give a variant description of the construction with reference to their Sea King II: ‘the hull is of circular construction with built-on steps. . . the top side being of single-skin planking, covered with fabric treated with a tropical doping scheme’. Also a Flight description of the machine says that ‘the boat hull is of the typical Supermarine type, boat-built and through fastened, with copper or brass fixings throughout. The mahogany single-skin planking is riveted to rock elm timbers and frames, and covered externally with fabric suitably treated with pigmented dope.'
    Specific centre of gravity considerations or design requirements might very well have led to Mitchell requiring variations in planking and finishing in other of the firm’s aircraft at about this time – for example a Supermarine patent allows for planking to be omitted where external steps are to be added:
If it is desired to reduce the weight of the hull to the greatest possible extent, the skin-planking on the hull proper may be omitted where side wings or other projections cover that portion of the hull. Where this planking is omitted it is preferred to use a fabric covering for the hull proper so that it is maintained watertight, even although the wing or other projection may be perforated. The close spacing of the bent timbers and stringers provide sufficient support for the fabric to be a satisfactory watertight skin in cases of emergency.

Thus, when we consider Mitchell’s first complete designs, the Commercial Amphibian, Sea Eagle, Seagull II/III, Seal II, and Scarab/Sheldrake, which can be regarded as coming from a common stable, some or all may also have canvas exteriors – indeed, Flight describes the Seal II as ‘boat-built of planking over a light skeleton of timbers and stringers, and covered in fabric on the outside." The machines in this group are, however, all about 11 feet longer than the previously mentioned machines and would probably require the stiffening of double planking – with or without fabric doped on. There is an intriguing report in Flight of the visit of HRH the Prince of Wales to Supermarine where it is said that ‘the building of the Seagull flying boat hulls was greatly admired by His Royal Highness’: perhaps the company saw an advantage in producing hulls that were, like the Southampton, fine examples of the boat-builder’s art, bearing in mind their British, Australian and Spanish naval customers. But it could be that the prince merely saw well finished planking awaiting a protective layer of fabric to be doped on.

Incidentally, it is worth considering that wherever there was double planking, with the usual layer of canvas in between, fabric might also have been applied externally to protect the woodwork from splitting in the sun as well as to prevent the soaking up of water – one thinks particularly of the ‘tropicalised’ Seagull IIIs for Australia (see painting above). And the use of "pigmented dope", as quoted earlier, might account for the commonly held view that Supermarine hulls were a mahogany colour – either because of a varnished wood finish or because the doped-on fabric would allow the colour of the timber to show through – or, the ‘pigmented dope’ chosen might be mahogany in hue and none too opaque, given that Supermarine might very well have been strongly attached to reminders of their boating heritage.



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, 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.






Thursday, 10 May 2012

Naming the Spitfire



When did R. J. Mitchell say that "Spitfire" was "a bloody silly name" for his fighter ? Dr. Alfred Price (The Spitfire Story) noted, via the logbook of test pilot George Pickering, that the earlier Type 224 was known as the Spitfire some time before July, 1935 and so Mitchell’s remark might have been made about this time or earlier – when this fighter was so designated in a brief announcement by Supermarine in 1934: "The ‘Spitfire’ is a single-seat day and night fighter monoplane built to the Air Ministry specification."
Unfortunately Type 224 was not a success, mainly because of engine cooling problems and Air Ministry requirements that militated against an efficient machine.

Thereafter,  its proposed successor, Type 300 – which eventually evolved into the Spitfire – was usually referred to at the works as "the fighter", Gordon Mitchell's book copies a Supermarine document of 29 February, 1936, which refers to the soon-to-fly aircraft as the 'Modified Single-seater Fighter', and his father on occasions erroneously referred to it in his diary as F.37/35.*  When this new fighter was named "Spitfire", at the end of April, 1936, it is probable that this was the time when Mitchell made the well known comment – perhaps he didn’t want reminding of the disappointment of the first Spitfire and/or, after two years, the brief appearance of the name in 1934 had faded from his mind, which was looking towards the success of the new Type 300  and the 8-gun specification of F.10/35.*

 Vickers’ suggestion of a name for the new fighter was accepted by the Air Ministry, in all probability inspired by Ann McLean, the chairman’s daughter, who had habitually been referred to as ‘a right Spitfire’. [As well as the comment ’It’s the sort of bloody silly name they would give it", Mitchell was also reported as saying that it could be called ‘Spit-Blood’ for all he cared. ‘Shrew’ and ‘Shrike’ had also been considered and it is a matter of speculation as to whether our Chief Designer would have preferred either of these.]
    By this time, the Aircraft Nomenclature Committee was no more and names were now selected, in discussion with the manufacturer, by the Air Member for Supply. For fighters, especially, words indicating speed and aggression were now being chosen (Fury, Gladiator, Gauntlet, Whirlwind, Hurricane etc.) and 'Spitfire' more or less fell into this general category. However, it also had a ‘British’ pedigree as well: it had been applied in previous times to cannons emitting fire, to angry cats, and to anyone displaying irascibility or a hot temper, especially women – as evidenced in 1762 when Lord Amhurst is quoted as saying to his mistress: ‘Not so fast, I beg of you, my dear little spitfire’; and Shakespeare echoed the general sentiment when King Lear defies the elements: ‘Rumble thy bellyful! Spit fire! spout rain!’. In 1778, a Royal Navy galley was named ‘Spitfire’ – a euphemistic rewording of ‘Cacafuego’, a Spanish treasure galleon captured by Sir Francis Drake; thereafter the Navy used the name eight other times until 1912. It was also used in the titles of six pre-war films and thus at that time was not just a forgotten part of the English vocabulary – as it probably would be today without the wartime actions involving Mitchell’s fighter.

* * * * *


* A confusion between Spec F.37/34 (actually drawn up for the 'Experimental High Speed Single Seat Fighter', the 4 gun aircraft that was gradually taking the shape we all know as the Spitfire)  and the slightly later F.10/35 requirement for an 8 gun machine which, it is likely, Mitchell was also intent on satisfying [see my Blogpost" 'R.J. Mitchell's Spitfire - a Close Run Thing'].




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, 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.