The Prototype

Mitchell's project was an all-metal semi-monocoque. This is a self-carrying construction without the use of earlier days tubes and canvas. This construction was more demanding when building the aircraft, but it secured better aerodynamics and thereby higher performance. Hawker chose the traditional methods on their Hurricane.

The fuselage soon looked like the one we know from the production Spitfires, however, the wings were still rather thick and had a straight leading edge. At a later stage it was decided to construct a slimmer wing which had a thickness of 13 percent of the wings chord measured at the root. This caused problems with the stowage of ammunition and room for the retractable main undercarriage. The problems were solved and thus the wings turned out as we know them - the unmistakable elliptic wings. One of the advantages of these wings were the reduction of induced drag, which is the drag which comes especially around the wing tips, due to the fact that air at the wings underside, where the air pressure normally is high, will attempt to get to the upper side of the wing where the air pressure is low. This is generally known as vortex. This low drag gave later Photo Recognisance and High Altitude interceptor versions of the Spitfire very good high altitude performance. The unusual solution with elliptic wings was no coincidence, but was a well-considered construction based on big knowledge about aerodynamics. Actually a Spitfire had higher Vne (velocity never exceed) than the German swept wing Messerschmitt Me262, which was the world's first operational jet fighter. A later LF Mk. IXE's maximum speed when diving was March 0.85 equivalent to 600 mph (960 km/t) at 20,000 feet (6,100 meters). Actually the first aircraft to exceed the high-speed capabilities of the Spitfire was the American F-86 Sabre.

The engine, the PV12, was a new type, but it had its roots in the R25R. This engine was also liquid cooled and that fact caused problems constructing an efficient cooling system. Water has a quite low point of boiling, so it took quite big radiators to keep the engine coolant temperature down. Rolls-Royce solved this problem by using a mixture of ethylene and glycol. It turned out that this liquid has a boiling point of approximately 380F (190 Celsius), and implementing only small modifications to the PV12 engine could use it. Supermarine tested a radiator, which could be lowered from the wing, however, this idea was scrapped. At the end the radiator was fitted into a housing under starboard (right) wing. This housing was aerodynamically shaped so that the air passing through the radiator would have better cooling capabilities. By opening or closing a flap on the backside of the radiator housing the airflow through the radiator could be increased or reduced. All in all the surface of the radiator was decreased by 50 percent. This solution, which was very depending of the air speed of the aircraft, had the drawback that later versions of the Spitfire equipped with larger engines did not like to be on the ground with the engine running idle before start for at prolonged period of time. The coolant temperature could get very high.

After many discussions about armament of the Spitfire (and Hurricane) the Air Ministry finally chose 8 Browning 0.303" machineguns, which were manufactured by Colt in the USA. This weapon used the same ammunition as the standard rifle of the British Army. This decision caused problems with the wings both in Supermarine's and Hawker's projects. Luckily Supermarine succeeded in keeping the optimal chord of the wing without adding humps changing the profile of the wing. Ironically it was a Squadron Leader form Air Staff's Operational Requirements Branch, who indirectly caused Supermarine problems by being the champion of the 8-gun solution, who was so happy about the Spitfire-project and found it so promising that he pushed for an order for Spitfires to be effected even before the Spitfire prototype flew for the first time. Many problems with the new engine, which in the meantime had been re-named Merlin, were constant causing delays. In December 1935 the Merlin got a temporary certificate of airworthiness and was finally fitted to the Spitfire prototype carrying serial number K5054. On the 6th of March 1935 the Spitfire took off for the first time with Joseph "Mutt" Summers at the controls. (According to Jeffrey Quill, who was the test-pilot no. one on the project, writes that the maiden flight was carried out on the 6th and not the 5th as history normally says.) The maiden flight went well and there were no snags.

Later flight tests were disappointing; K5054 could not reach the speed of 350mph (560 km/h) which "R.J." (Mitchell's nickname) had hoped for. The top speed was only 335mph (536 km/t). The problem was the propeller. Different propellers were tested, however, none of them gave a good result. Not until a propeller made by Supermarine was tested and the speed of 347mph (556 km/h) was reached, R.J. thought that it was close enough.

K5054 was assembled by using only flushed rivets, which was expensive and time consuming to do. In order to simulate the use of normal round headed rivets split peas were glued on top of the flushed rivets. The result was a significant reduction of the performance of K5054. In conclusion flushed rivets were used where it was found critical for the airflow and normal rivets were used where it was less critical.

When Supermarine had ended flight tests, K5054 were handed over to the government and Royal Air Force for further tests. After small changes had been done to panels in the wings and some major changes of the ailerons had been carried out by Supermarine, K5054 was painted grey/blue and once more handed over to the authorities for the final tests. Using an updated 1172 hp Merlin "C" engine remarkable results were achieved: 349mph (558 km/h) in 16.800 feet and from 0 to 15.000 feet in 5 minutes and 42 seconds.

More minor changes were done and orders for the first 310 Spitfires were placed. The first operational Spitfire, K9789, arrived at squadron no. 19 at Duxford on the 4th of August 1938. Instead of the flat canopy it had got a domed canopy instead, which offered the pilot improved visibility. An external bulletproof panel was added on the outside of the windscreen

It should be mentioned that what Mitchell meant for the design and birth of the Spitfire, Joseph "Joe" Smith, Mitchells right hand, meant the survival and development of the Spitfire. Joe Smith had been on the project from the start and when Mitchell died he took over. With his knowledge of the project and idea about what was needed he right away started working on the idea of fitting Rolls-Royce's new engine, the Griffon, to the Spitfire.
 

 

 

A constructions drawing of the Spitfire.

Click for larger picture

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Spitfire prototype - K5054 in its gray-blue livery on Martlesham Heath where it was sent for flighttests with Aircraft and Armament Experimental Establishment . Note the 2 blade prop.