From Nighthawk to Spitfire
Page 17
But, at least, personnel were changing in the Air Ministry. The Chief of the Air Staff, Sir Hugh Trenchard, who had favoured concentration on the bomber as a deterrent, had retired and it was fortunate for Mitchell that the Air Ministry and its departments were headed by RAF officers who had by now come to the conclusion that it was fighter development that had to be significantly stepped up.
And so, even before trials of the other, largely unsuccessful, F.7/30 prototypes had been concluded, specification F.5/34 was issued, which now stated that a retracting undercarriage was required as well as eight machine guns to provide ‘the maximum hitting power’. It also specified that ‘the maximum speed at an altitude of 15,000ft shall not be less than 275mph and at 5,000ft not less than 250mph’, and that ‘the time taken to reach 20,000ft is not to exceed 7½ minutes’. As Mitchell’s Type 224 modifications had only promised a top speed of 265mph and a climb to 15,000ft in over eight minutes, it was clear that Supermarine had to do some serious rethinking.
At this time, a new engine was being developed by Rolls-Royce, who had decided that the next generation of fighters would need a new engine. Something between their 21-litre Kestrel and Goshawk engines and their 37-litre Schneider Trophy ‘R’ engine had been proposed. It was designated PV12, where the initials stood for ‘private venture’ and clearly indicated another independent appreciation of the need for Britain to develop better aircraft.
This new engine passed its 100 hour test in the July of 1934, and the board of Vickers (Aviation) Ltd decided on 6 November to finance the design of a machine powered by this new engine. Sir Robert McLean, the chairman of the Vickers board, later described how he had decided that Mitchell and his design team should design a ‘real killer fighter’ in advance of any Air Ministry specification, and that ‘in no circumstances would any technical member of the Air Ministry be consulted or allowed to interfere with the designer’ – no doubt with the unhappy history of Type 224 in mind.
It has been recently revealed that the chairman had not, in fact, been unwavering before his support for the Supermarine designer finally won out. The alternative was the parent company’s Venom, a development of the promising but ill-fated F.7/30 entry, the Type 151 Jockey, which had also succumbed to a flat spin. Like the Spitfire, the Venom had a stressed-skin cantilever wing, retractable undercarriage and a metal monocoque fuselage. In fact, when it did fly, three months after the Spitfire prototype, it attained a top speed only 37mph lower than the Supermarine prototype – and with a less powerful, radial engine.
Clearly, this machine could also have developed into a very serious challenge to the Supermarine project and, indeed, Beverley Shenstone, Mitchell’s aerodynamicist, later reported that in his opinion the Spitfire would not have been born ‘if Mitchell had not been willing to stand up to McLean, particularly in the era when McLean clearly preferred the Venom concept to the Spitfire concept because it was cheaper and lighter’.
Once Mitchell’s proposal had overcome this hurdle, the combination of a Vickers/Supermarine/Rolls-Royce/Mitchell design must have stirred up the new blood within the Air Ministry, for events then moved very quickly. On 1 December £10,000 was allocated for Supermarine to build a prototype and, when a full design conference was called at the Air Ministry on the 5th of the same month, it was headed by Air Marshall Hugh Dowding.
The modifications to Type 224 proposed by Supermarine had been deemed too late and too extensive to qualify for re-entry into the F.7/30 exercise, but fortunately the Air Ministry, in their concern to improve the fighter breed, agreed to Supermarine proceeding independently with their new design and a special specification F.37/34 was drawn up and formally signed on 3 January 1935. It should be noted that this new specification was headed ‘Experimental High Speed Single-Seat Fighter (Supermarine Aviation Works)’ and it stated that, basically, ‘the aircraft shall conform to all the requirements stated in specification F.7/30’ – that is, Mitchell was to design a four-gun aircraft but without other firms being invited to tender in the usual way.
The word ‘experimental’ might very well have reflected decreasing confidence in Supermarine among Air Ministry officials after the experience of Type 224, or that the three successive Schneider Trophy wins by the Rolls-Royce/Supermarine combination had not been forgotten by the new Air Ministry officials and was used to deflect criticism that the normal method of aircraft procurement was being bypassed.
Whatever support there might have been for a Supermarine project, it was not lost on the company that Hawker had also been encouraged to substantially modify their Super Fury F.7/30 entry with a rival ‘experimental’ offering. The Air Ministry additionally issued F.10/35, three months after the Supermarine requirement, calling for at least six, and preferably eight, guns to ‘produce the maximum hitting power possible in the short time available for one attack’. They also issued specification F.37/35, for a single-seat day and night fighter armed with four cannon.
The Westland response, the Whirlwind, when it first flew in 1938, would have been a formidable aircraft that might have soon replaced a Supermarine fighter. It had an extremely low frontal area, an excellent pilot view, a top speed of 360mph, and its four 20mm cannons promised to make it the most heavily armed fighter aircraft of its era. Also, as the armament was mounted in a nose cluster, there were no convergence problems as with wing-mounted guns.
Thus, the Supermarine fighter being designed would have to offer something special, for F.10/35 called for a maximum speed of ‘not less than 310mph’. In the event, engine problems with the Whirlwind (and with the later Hurricane replacement, the Typhoon) left the field clear for Spitfire development. At the same time, one suspects that Supermarine and Vickers were looking well beyond their designated four-gun model, and towards the greater speed and armament requirements, when their elliptical wing shape was decided upon – the thin wing, which Mitchell had come to believe would give him the speed, would only accommodate any increased weaponry via the broad chord of an elliptical wing, given the need also to allow for a retracted undercarriage.
Clifton later recorded that Mitchell specified a wing that had a thickness/chord ratio of 13 per cent at the root and 6 per cent at the tip, although advised by the National Physical Laboratory that wind tunnel tests showed that there was no advantage in going below a thickness chord ratio of 15 per cent. Fortunately Mitchell’s instincts were proved correct when the Spitfire prototype’s top speed was eventually achieved.
It was also fortunate that, about this time, F.W. Meredith of the Royal Aircraft Establishment, Farnborough, had come up with a ducted underwing radiator that not only impacted less on the streamlines of a machine but actually used the heat exchange of the radiator to produce some thrust at high speed – thereby encouraging confidence in Supermarine’s predicted 350mph for their new fighter.
Nevertheless, when the new prototype first flew early in 1936, Mitchell had to acknowledge that the top speed was ‘a lot slower than I had hoped for’ – and as its test pilot, Jeffrey Quill, said, ‘Unless the Spitfire offered some very substantial speed advantage over the Hurricane, it was unlikely to be put into production. Thus the disappointing speed performance of our prototype at that early stage was something of a crisis and Mitchell was a very worried man.’
The prototype was given a special new paint job and, by 9 May 1936, re-emerged with a very smooth light blue-grey finish, thanks to the application of fillers and automobile paint supplied by Rolls-Royce. Despite the smooth new finish, the speed of the aircraft was still less than hoped for (bearing in mind F.10/35) and the aircraft’s top speed of 335mph was still thought to be too close to that of the Hurricane, which was believed to be around 330mph.
Fortunately, the fitting of a particular propeller (Quill mentioned flight testing ‘some 15 to 20 different designs’) on 15 May produced a very impressive speed increase to 349mph. The following chart, comparing performances with those of preceding aircraft, clearly shows the achievement that this new aircraft represented
and the reason why it was ordered with such alacrity after its first service test at Martlesham:
Aircraft
Power
Top Speed
Power Increase
Speed Increase
Gamecock (1926)
425hp
155mph
Bulldog II (1929)
440hp
178mph
3.5 per cent
15 per cent
Gauntlet (1936)
640hp
230mph
45 per cent
30 per cent
Gladiator (1937)
830hp
253mph
30 per cent
12 per cent
Spitfire Mk I (1938)
1079hp
362mph
24 per cent
41 per cent
It can be seen that the increase in top speeds from 1926 to 1937 was 98mph. Over the eleven years this averaged 9mph per year, and the Gladiator (which had eventually been awarded the F.7/30 contract) registered an increase of only 23mph for the 30 per cent power increase over the Gauntlet of the year before.
The Spitfire entered squadron service the following year with a power increase of less than 25 per cent and, instead of replicating the usual speed increase of less than 10mph per year, achieved ten times that figure. It is perhaps not always appreciated that it was the initial order for the Spitfire which concentrated minds in Germany and led to the order for the Messerschmitt Bf.109 (see p.152).
Believed to have been taken during Mitchell’s last airborne viewing of his fighter prototype. (Courtesy of P. Jarrett)
When one remembers how two F.7/30 contenders were eliminated when the one, and only, model crashed, it is worth recalling that the equally unique Spitfire prototype nearly came to grief when handed over to Flight Lieutenant (later Air Marshall Sir) Humphrey Edwardes-Jones at the Martlesham Heath test centre. In his account to Price of his first landing, he confessed that owing to a distraction he nearly forgot to lower the still novel undercarriage. One wonders how a crash-landing of the sole untried prototype would have affected its future, or at least its delivery, when being required for the Battle of Britain. (For the German contract exercise, equivalent to the British F.7/30 requirement, four firms had each been authorised to build three prototypes.)
Events in Europe were certainly now creating an even greater urgency to find an adequate replacement for the standard RAF fighters of the day and, after the many doubts along the way, high hopes were now being felt for this latest Rolls-Royce/Supermarine/Mitchell ‘experimental’ machine. Thus it was that Edwardes-Jones’ first flight took place, unusually, as soon as the aircraft had been delivered to Martlesham. Additionally, the pilot was instructed to telephone the Air Ministry as soon as he got down and report on its suitability for squadron pilots.
Eight days later, on the strength of that brief conversation, and just less than three months since the prototype’s first flight on 5 March 1936, the Air Ministry signed a contract for the first 310 Spitfires.
THE SPITFIRE WING
The film about Mitchell, The First of the Few, shows him looking at seagulls, presumably for inspiration when contemplating his future fighter – had he been concerned with the sort of glider development going on in Germany at the time, a case might have been made for his apparent interest in such birds. His keeping racing pigeons during his youth was evidence of an interest in flight, no doubt, but – as Jeffrey Quill observed – Mitchell had better things to do when designing aircraft than ‘looking at bloody seagulls’. One might perhaps note that his first foray into fighter design, Type 224, had a gull wing – but it was inverted, and every effort was made to avoid the current aerodynamic problems of ‘flutter’!
Other assumptions that the legendary Spitfire was a direct development of his Schneider Trophy machines, or that it was conceived in some moment of inspiration following his operation for cancer, are also far from the truth – as reference to the changing proposals around this time will show. Supermarine drawing No. 300000, Sheet 2, shows how the Type 224 wingspan was reduced by nearly 10ft with an almost straight trailing edge and a swept-back, straight leading edge with a rearwards sloping main spar. Sheet 11 also shows a continuance of the main spar positioning, but a distinct movement towards the elliptically shaped wing that was to become, pre-eminently, the distinctive feature of the Spitfire.
The successor to Mitchell’s first fighter design was, at first, still to be designed around the evaporation cooled Rolls-Royce Goshawk engine as, whatever the problems, it offered a considerable reduction in drag. Then, in October 1934, the Air Ministry suggested that the proposed fighter should be fitted with a Napier Dagger engine, expected to be more powerful than the Goshawk. But, Rolls-Royce had, by then, produced their PV12 engine and the board of Vickers decided to finance a design powered by the new engine – which was, of course, to become the famous Merlin. As the new engine from Rolls-Royce was significantly heavier than the one that had been previously allowed for, it would need a less swept-back wing and so the eventual Spitfire wing shape became a modified ellipse, with a much straighter leading edge.
Sheet 2.
Sheet 11.
The Elliptical Wing
By this time, the elliptical wing was coming to be regarded as the most efficient shape for the sorts of speeds and altitudes that were now being contemplated. As Shenstone said, ‘Aerodynamically it was the best for our purposes because the induced drag – that caused in producing lift – was lowest when this shape was used.’ The Heinkel He.70 transport, which first flew in 1932, has sometimes been cited as influencing the Spitfire wing design, but such a transport aircraft was an unlikely model for the new breed of fighter, where climb, speed and manoeuvrability were paramount considerations. On the other hand, it did illustrate the appreciation that the elliptical wing was also a very efficient way of accommodating stress loads and hence of allowing a lighter structure.
The enormous Kalinin K-7 was being built at the same time and embodied, perhaps, the ultimate symmetrical elliptical wing to support its 174ft span and seven engines. But it should be remembered that its generic elliptical wing shape had been considered by Mitchell in connection with his 185ft Giant – projected before the Russian aircraft first flew. It is also worth mentioning that there was an earlier precedent for the elliptical wing approach – the two-seat light aircraft, the Bäumer Sausewind, notable for its all-cantilever structure as early as 1925. It was designed by the Günter brothers before they joined Heinkel and produced the above mentioned He70. Again, one finds that in that same year, Mitchell had also produced something approaching an elliptical wing with his S4 – which also featured all-cantilever flying surfaces despite being required to withstand far greater loads than those of the German light aircraft.
The Kalinin K-7.
The question arises: why did his S5/6 series of racers, which were the immediate forerunners of the Spitfire, not follow this precedent? It may very well be that, with the short development times available before the 1927–1931 competitions, the introduction of wing-surface radiators with the S5 was sufficient for Supermarine, without the added complexity of following curving leading and trailing edges; and the move to metal wings with the S6s was, again, perhaps enough to be going on with in the limited time available before the competition. Additionally, the wire bracing reintroduced with these later aircraft made the strength/weight advantages of a more complex wing shape less obvious. It is, however, interesting to note that their uncomplicated and unbraced cantilever tailplanes were perfectly elliptical in shape.
The overseas aircraft mentioned above featured symmetrical ellipses, particularly with reference to structural considerations. Other, somewhat elliptical, wing shapes closer to home, where speed was a foremost consideration, were familiar to the Supermarine design team. Both the Short Crusader of 1927 and the Gloster VI of 1929 employed shapes which approached the elliptical, but for the narrower chord close to the fuselage – wh
ere control surfaces were not involved.
However, in the present context, the most intriguing shape was that of the Italian Piaggio P.7. While it never progressed beyond its taxiing stage, the general arrangement of this rival Italian Schneider design would surely have been known to Mitchell, and the elliptical wing shape modified by a straighter leading edge is very similar to that which was developed for the Spitfire (see drawings pp.152–3).
Also, a return to Heinkel aircraft is especially called for – in respect of the He.112 which was, in effect, a scaled-down version of the He.70 mentioned earlier and a contender for the contract which produced the Messerschmitt Bf.109. This latter was chosen for the Luftwaffe in October 1936 as it was smaller, lighter, and therefore offered a better performance, at the particular point in time when a decision had to be made – when it became known that the Spitfire had been ordered into production. The much improved He.112B appeared in July of the following year but did not go into service with the Luftwaffe; as its wing shape is very similar to that of the Spitfire, one wonders how formidable a fighter it might have been developed into.
The Short Crusader.
The Gloster VI.
The present concern, however, is to note that while some German designers were coming to similar conclusions as Mitchell, his design predated that of this rival aircraft, having been arrived at as a solution to various requirements – for both the engine weight reasons and for the aerodynamic reasons mentioned above. And it equally suited the decision to now employ a retractable undercarriage. The necessary arrangements for its housing meant that, if the machine guns were to be sited in the wing, they would have to be placed well outboard. In this respect, an elliptical form of wing was also attractive as it tapers towards the tip very slowly at first.