Wednesday, June 13, 2018
Supermarine Spitfire Mk VI
At the time that the Mk V was placed in production there were growing fears that the Luftwaffe were about to start mass-producing very high flying bombers such as the Junkers Ju 86, which could fly above the reach of most fighters of the time. It was decided that a new Spitfire variant would be required with improved high altitude performance. During a meeting held at the RAE at Farnborough on 17 February 1941 the Air Ministry asked "that a Spitfire should be provided with a pressure cabin capable of maintaining a pressure differential of 1 lb per square inch at 40,000 feet." A Marshall-manufactured compressor was to be used, and it was agreed that the sliding canopy could be replaced by one which could not be slid open, as long as it could be jettisoned by the pilot.
The pressurised cabin was used to counter the physiological problems encountered by pilots at high altitudes. The cabin was not like the fully pressurised cabin of a modern airliner; the pressure differential provided by the modified cockpit of the VI was only two pounds per square inch (which was double the Air Ministry requirement). To achieve this, the forward and rear cockpit bulkheads were completely enclosed, with all control and electrical cables exiting though special rubber sealing grommets. In addition, the side cockpit door was replaced with alloy skin and the canopy was no longer a sliding unit: externally there were no slide rails. Once the pilot was in, the canopy was locked in place with four toggles and sealed with an inflatable rubber tube. It could be jettisoned by the pilot in an emergency. The windscreen of production Mk VIs was the same as the type fitted to the Mark III and some Mk Vs although it was fitted with an inward opening clear-view panel on the port quarter pane. The effect was to make 37,000 ft (11,000 m) seem like 28,000 ft (8,500 m) to the pilot, who would still have to wear an oxygen mask. Pressurisation was achieved by a Marshall-manufactured compressor located on the starboard side of the engine, fed by a long intake below the starboard exhaust stubs. Mk VIs were built with the Coffman cartridge starter, with a small teardrop fairing just ahead of the compressor intake.
The engine was a Rolls-Royce Merlin 47 driving a four-bladed Rotol propeller of 10 ft 9 in (3.27 m) diameter; the new propeller provided increased power at high altitudes, where the atmosphere is much thinner. To help smooth out airflow around the wingtips the standard rounded types were replaced by extended, pointed versions extending the wingspan to 40 ft 2 in (12.2 m). Otherwise the wings were Type B.
The maximum speed of the Mk VI was 356 mph (573 km/h) at 21,800 ft (6,600 m). However, because of the limitations of the single stage supercharger, at 38,000 ft (12,000 m) the maximum speed had fallen away to 264 mph (425 km/h). The service ceiling was 39,200 ft (11,900 m).
The threat of a sustained high altitude campaign by the Luftwaffe did not materialise and only 100 of the Mk VIs were built by Supermarine. Only two units, 124 Squadron and 616 Squadron, were fully equipped with this version, although several other units used them in small numbers as a stop-gap. More often than not, the Spitfire VIs were used at lower altitudes where it was outperformed by conventional Spitfires. At high altitudes it was discovered that modified Spitfire Vs could perform almost as well as the Mk VI. At low levels, especially, pilots were often forced to fly with the canopy removed because the cockpit would get uncomfortably hot and they were not confident it would be possible to jettison the canopy in case of an emergency.
In 1943 five Spitfire VIs (BS106, BS124, BS133, BS134 and BS149) were converted to improvised PR Mk VIs by 680 Squadron in Egypt. These aircraft had been "tropicalised" using the same bulky Vokes filter and other equipment used by Spitfire VB Trops, as well as being painted in a "desert" camouflage scheme.
By the time these aircraft arrived they were no longer required to intercept high-flying Junkers Ju 86P reconnaissance aircraft although there was a need for a pressurised RAF photo reconnaissance aircraft to carry out missions over Crete and the rest of Greece. 103 MU at Aboukir carried out the modifications by removing the armament and installing vertical F8 cameras in the rear fuselage. These Spitfires were used a few times in April and May 1943 but were withdrawn from service by August. They were the first pressurised PR Spitfires.