Friday, 10 March 2017

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:
(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%.

Gloster Gauntlet
(painting by the author)

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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. 

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

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