Specification F.7/30
From the early 1930s it became apparent that the Royal Air Force’s current fighters were pitifully inadequate barely able to outpace the bombers that were supposed to attack. To address this state of affairs the Air Ministry issued specification F.7/30 for a new day and night fighter. The key performance requirements in the contract included a horizontal speed at 15,000ft of not less than 195 mph, an alighting speed not to exceed 60 mph and service ceiling not less than 28,000 ft. Interestingly it did not mandate that the design should be a monoplane.
Numerous companies responded to the challenge with Supermarine offering its gull-winged monoplane Type 224 which first flew in February 1934.
Although Chief Designer Reginald Mitchell would draw on his experiences in developing the speedy floatplanes for the Schneider trophy, which Britain won outright in 1931, he was to adopt a quite different design essentially only retaining a monoplane configuration for what was to be called Type 224.
However. Mitchell was not happy with the design of the Type 224 whose performance fell short of that of the glamorous winner of the 1931 Schneider Trophy the Supermarine S.6B. So, despite now suffering from bowel cancer, which would later prove terminal, and as a private Supermarine venture, he made some major alterations introducing a closed cockpit and retractable undercarriage while still retaining the Rolls-Royce steam-cooled Goshawk engine.
Further development led to the adoption of the more powerful Roll-Royce PV-12 engine in place of the Goshawk, and removal of the inverted-gull lines for the wings which were also made thinner and elliptical in shape. It was now called Type 300.
Specifications F.37/34 and F.10/35
The design had moved so far away from the original specification F.7/30 that the Air Ministry issued a new specification F.37/14 on 5 January 1935 to encourage the further development of Mitchell’s private venture design. The contest for specification F.7/30 was won by the Gloster Gladiator, which would become the last biplane fighter used by the R.A.F.
Type 300 had been designed to mount four guns – two in each wing – but it was evident that greater firepower was needed to deal with heavily amoured bombers now in existence. So the Air Ministry issued a further Specification F.10/35 for a fighter capable of a speed of at least 310 mph and mounted with at least six and preferably 8 guns. Around the same time the Hawker factory was developing the Hurricane against Specification F.36/34 which was to carrry eight guns.
Having become aware of the progress being made at the Supermarine works, the Air Ministry realised that they need not progress further with the tendering process and on 4 June they agreed to ask Supermarine to bring Type 300 into line with specification F.10/35 and to suspend further tendering. As regards the original specification F.7/30, this was met by the Gloster Gladiator biplane which entered service in February 1937 with No. 72 Squadron, based at Tangmere with wartime service spanning to 1942.
The greatly improved all-metal monoplane, now fitted with the Rolls-Royce Merlin engine, first flew on 5 March 1936 piloted by Captain Joseph ‘Mutt’ Summers. On landing Mutt said ‘I don’t want anything touched‘ by which he meant that there were no snags which required correction or adjustment before he flew the plane again.
Handling trials of K5054 were undertaken in September 1936 by the Aeroplane and Armament Experimental Establishment, Martlesham Heath. Despite earlier concerns about the poor forward vision at take-off and landing due to the Spitfire’s long nose, this was not mentioned in the report and it was deemed simple and easy to fly without any vices and could be flown by the average fully trained service fighter pilot.
Mark I
Following successful trials the Air Ministry placed an order on 3 June 1936 for 310 Spitfires for delivery by March 1939 at a unit cost of £4,500. The first public display took place on 18 June 1936 at Eastleigh aerodrome. Reginald Mitchell was not to see a Spitfire come off the production line as he died on 11 June 1937 aged 42. He was succeeded in charge of the Spitfire project by chief draughtsman Joseph Smith. No. 19 Squadron became the first squadron in the RAF to operate the Supermarine Spitfire Mk.I at RAF Duxford.
Some 1,550 Mk.Is were produced up to January 1942. The vast majority of Spitfires that fought in the Battle of Britain were Mk.Is. The Spitfire was not an easy aircraft to manufacture, each Spitfire Mk.I, for example, required 330,000 man hours to produce.
Spitfire Mk.II
The first Spitfire Mk II left the Lord Nuffield shadow factory at Castle Bromwich as early as August 1940. Shadow factories were established to mirror the operations of their parent factory and thereby increase production – thus Rolls Royce had one at Crewe to help build the Merlin. It was now fitted with a 3-blade propeller as standard and received additional armour for the pilot.
Spitfire Mark IIa, P7290/AF-V
The model company Revell produced a 1:72 scale model of a particular Mk.IIa – serial P7290.
According to records, Spitfire Mk.IIa P7290 was manufactured at the Castle Bromwich Aircraft Factory around July 1940 within contracts B981687/39 and entered service with 611 Squadron August the same year. It was fitted with a Merlin XII engine having 0.477:1 reduction gear and rated at 1,150 hp (860 kW) at 3,000 rpm at 14,000 feet (4,300 m) installed in some Spitfire II’s with three-bladed Rotol constant-speed propeller.
If the records are interpreted correctly, it was damaged in service by a Dornier Do 215 on 21 August 1940 when the Spitfire was piloted by Flying Officer Watkins, but landed safely. At the end of 1940 it was used for air service training at No. 421 (Reconnaissance) Flight, a specialist RAF fighter flight created on 21 September 1940 to patrol the Channel and provide early warning of the types of incoming Luftwaffe raids from occupied France. On 15 January 1941 it made a forced landing near Arlington, East Sussex, after escorting a Wellington across the Channel.
As of 23 March 1942 it was based at Duxford with the the Air Fighting Development (AFDU) where comparative trials were performed, and tactics developed to be effective against opposing aircraft types. It is later recorded at Royal Navy Air Service (RNAS) Yeovilton with 759 Squadron from around August 1943 to January 1945.
Four guns to eight guns
Mk.IIa’s armed with eight .303 inch machine guns saw limited action in the latter stages of the Battle of Britian. The adoption of eight guns was necessary as it was calculated that 256 bullets would be required in two seconds to bring down an enemy bomber at the increased speeds of the new aircraft. The calculations needed to prove this were largely attributed to 13 year old Hazel Hill who helped her father calculate how many guns the new generation of Spitfires and Hurricanes would need to bring down enemy aircraft.
Solving the loss of Merlin engine power
The carburettors in early versions of the Merlin engine forced fuel to the top of the carburettor’s float chamber rather than down into the engine, leading to loss of power when the aircraft manoeuvred created a negative G force. The problem was solved by an engineer Beatrice ‘Tilly’ Shilling who designed a restrictor plate which could be fitted into the engine’s carburettor without taking the aircraft out of service. It was just what the pilots needed and the device was nicknamed Miss Shilling’s orifice!
Merlin to Griffon
The long-term future of the Spitfire was in part assured by the adoption of the Rolls-Royce Griffon engine which had 2.3 times the power of the original Merlin. The Griffon arose from the Merlin R engine which had been specifically designed for racing in the Schneider competition. At the outset the engine was not considered suitable for a fighter due to its weight and large size. However, by reconfiguring its dimensions the frontal area was reduced to within 0.4 square feet of the Merlin so that it could now be fitted in Spitfire. The first flight of a Griffon-powered Spitfire was in November 1941.
The adoption of the Rolls-Royce Griffon engine provided the Spitfire with a significant increase in performance, but presented both Supermarine designers and pilots with challenges when converting to Spitfires powered by the new engine. Thus the Griffon engine rotated the propeller in the opposite direction to that of the Merlin, creating torque that c ould take pilots by surprise.
There were three Air Ministry specifications relating to the production of Griffon-powered Spitfires:
- F.4/41 for the Supermarine Spitfire XXI
- F.1/43 for development of a Spitfire with Griffon & laminar flow wing as the Supermarine Spiteful
- N.4/43 for Carrier-based fighter – Seafire with Griffon engine as the Supermarine Seafire XV
The Spiteful was intended as a successor to the Spitfire but was not progressed with due to the coming of jet-powered aircraft.
Altogether, 2,570 Seafires were produced in eight major different marks, ranging from the first Seafire Mark Ib through to the Mark 47, which first flew just after the war and, fully loaded, weighed over 10,000 pounds – more than twice the weight of Mitchell’s prototype.
It nearly went supersonic
Given the high speed achievable by the Spitfire it was used by the Royal Aircraft Establishment (RAE) to research compressibility of airflow at transonic speeds – that is between 250 and 760 mph. The plane chosen was a Merlin-powered photo-reconnaissance PR Spitfire XI stripped of all operational equipment but fitted with boffins’ recording gear. Flown by Eric ‘Winkle’ Brown it was taken to 40,000 feet, flown flat-out and then put into a steep dive until the Machmeter read 0.83.
Following a gentle shaking of the tail the Spitfire started to nose down into a steeper dive without any pushing by the pilot and began shaking badly and rolling side-to-side. At Mach 0.86 the pilot had to pull the equivalent of 60 lb to stop the dive becoming ever steeper.
Later a Mach reading of 0.92 was achieved by Squadron Leader Tony Martindale at which point he was pulling 110 lb on the control column to recover when the over-speeding propeller became detached, together with its reduction gear. He was able to exert such a force as he was of much bigger build than the diminutive ‘Winkle’ Brown. With this lost weight the Spitfire became tail heavy and zoomed vertically upwards causing the pilot to black out under a force of 11g. When he recovered the plane was at around 40,000 ft with its straight wings slightly swept-back (an indication of future wing shapes). Despite all this, Martindale managed to glide it back to Farnborough landing on its wheels with the valuable camera records intact. The speed achieved, 620 mph in that dive was the highest ever recorded by a piston-engined aircraft.
An adaptable aircraft
No other Allied aircraft than the Spitfire was produced in so many variants to deal with the ever-changing threats posed by the enemy. In total there were no fewer than 19 different marks and 52 variants produced across the various factories, as shown below.
By the time production ended, over 20,000 Spitfires had been built, and the aircraft had changed engine (Merlin to Griffon), its loaded weight had doubled and its maximum speed increased by 90 miles per hour.
Sources include:
Air history site attempts to list every Vickers-Supermarine Spitfire and Seafire aircraft built
Dinger’s Aviation Pages – Spitfire
Spitfire Performance Testing
All Spitfire pilots
Chapter 11 ‘Spitfire’ in ‘Great Aircraft’ by Wing Commmander Norman MacMillan. Published by G.Bell and Sons 1960
‘Spitfire – Portrait of a Legend’ by Leo McKinstry. Published by John Murray 2007
http://www.historyofwar.org/articles/weapons_spitfire_mkII.html