The first 77 Mk Is had a two-bladed, fixed-pitch propeller. Subsequent aircraft received three-bladed, two-position airscrews, with fine pitch for take-off and coarse pitch for cruising, and these were subsequently retro-fitted to the earlier aircraft. Taller pilots found the headroom very restricitive and this led to the original flat cockpit canopy being replaced by the bulged version which was to become a feature of all future marks. Other improvements included the provision of an armour-plated windscreen and 6mm armour panels on the rear engine bulkhead and behind the pilot's seat. Heating for the guns was also installed after it was found that they froze at high altitude. The original armament of eight .303 Browning machine guns had been chosen because of the ready availability of this weapon but, in June 1939, two 20mm Hispano cannon were fitted to L1007 for trails. These proved unsuccessful as the Hispano had been designed to be mounted on top of a fighter's engine block which would be solid enough to absorb the recoil. The mountings in the Spitfire's wings were too flexible causing the guns to jam. Nevertheless, the Hispano was ordered into production, pending a satisfactory solution to this mounting problem.
Many pilots found the new aircraft diffucult to adapt to - those used to open cockpits, often found the closed canopy claustrophobic and left it fully open. Additionally, these pilots were unfamiliar with the retractable undercarriage, and numerous early accidents were caused by their forgetting to lower the Spitfire's wheels. The aircraft did have a warning klaxon but, as this tended to sound whenever vibration increased, it was often switched off - with embarrassing consequences! Taxying was a zig-zag process requiring the aircraft's tail to be swung from side to side so that the pilot could see ahead beyond the aeroplane's long nose. Combined with the narrow-track and somewhat fragile undercarriage , this made crosswind landings hazardous. Nevertheless, it was considered that the aircraft could be flown without risk by the average fully-trained fighter pilot. New pilots came to the Spitfire via Magister and Master trainers and a short spell at an Operational Training Unit (OTU). Experienced pilots converted to type directly on the squadrons.
Before the outbreak of war, considerable interest in buying Spitfires or arranging licence production had been shown by many foreign countries, including Japan. In the event, one example was flown to the French before war dictated that all future production would be earmarked for the RAF. Orders placed before September 1939 amounted to 1,160 to be built by Supermarine with a further 1,000 to be produced by the Nuffield organisation.
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