Supermarine Spitfire Mk VI AB200, 1942
The emergence of high-flying German bombers and reconnaissance planes over Britain in 1941 necessitated the rapid development of Spitfire variants capable of intercepting these threats at altitudes beyond the reach of existing models. At that time, Rolls-Royce was advancing the production of Merlin engines with superchargers that could be fine-tuned for specific applications, such as the low-altitude versions for the Spitfire LF.V.
This customization of power outputs was fully utilized with the introduction of the Merlin 60 and 70 engines, featuring two-speed/two-stage superchargers for later Spitfire models, combined with higher levels of boost and high-octane fuels.
The Spitfire VI, also known as HF.VI (the ‘H’ standing for ‘high’), was based on the Mk.VB wing and airframe and featured the Merlin 47 engine, optimized to deliver maximum power at 14,000 feet (4,267 meters). It included several enhancements for high-altitude performance, such as a four-bladed propeller (the first for a production Spitfire), extended wingtips for improved control at height, and a lightly pressurized cabin (2.0 psi) resulting from research by the Royal Aircraft Establishment and Supermarine during 1940-41.
The pressurized cabin was sealed between bulkheads at the front and rear, and the cockpit canopy, which could not be slid open, was bolted down and sealed after the pilot entered but could be jettisoned in an emergency. An additional accessory drive on the Merlin 47 powered a Marshall blower, which forced air into the cockpit, doubling as a canopy demister. All controls, cables, and pipes passing through the pressure cabin were equipped with airtight seals.
The prototype, converted from a Mk.V (originally a Mk.I), first flew in July 1941, and the Mk.VI entered service in April 1942 with 616 Squadron. Its first engagement was against a Dornier Do 217 in the following month. However, production was limited to 100 aircraft, with the last delivered in November 1942. Five RAF squadrons operated the Mk.VI during 1942-43 with limited success due to its fair performance and inability to effectively engage targets above 35,000 feet (10,670 meters), making it less effective against its primary adversary, the high-altitude Junkers Ju 86P and R bombers.
The sealed canopy was unpopular with pilots, who preferred to taxi, take off, and land with the canopy open. While efficient heating allowed flight in shirtsleeves at high altitudes, this was impractical during combat, causing pilots to overheat in standard flight gear. A bypass system was later implemented to address this issue.
The Spitfire VI was considered an interim solution for high-altitude interception until the more capable Mk.VII with a two-speed/two-stage Merlin engine became available.