Westland Welkin

The Westland Welkin was a British twin-engine heavy fighter designed by William E.W. Petter and manufactured by Westland Aircraft Company that was designed to fight at extremely high altitudes, in the stratosfere. As a matter of fact, the word "Welkin" means "the vault of heaven" or "the upper atmosfere".
Westland placed their P.14 project, which was basically a high altitude adaptation of the Whirlwind, to meet Air Ministry Specification F.4 of 1940 for a high altitude fighter. Welkin's most predominant feature was its enormous high aspect ratio wings with a span on the production aircraft of 70 ft (21 m). The troublesome engines of the Whirlwind were replaced by the more powerful two-stage Rolls-Royce Merlin Mk.76/77 and it featured a pressurized cockpit made entirely out of heavy-gauge duraluminium bolted directly to the front of the main spar. The cockpit hood used an internal layer of thick perspex the hold the pressure, and an outer thin layer to form a smooth line. It featured a mechanism to blow heated air between the two layers to keep the canopy clear of frost.
The Ministry of Aircraft Production authorised the building of two prototypes in January 1941. The specification F.4/40 was revised into F.7/41 that same year, so the Welkin design was now in competition against the Vickers Type 432 which was powered with Merlin 61 engines.
The pressurisation system was driven by a Rotol supercharger attached to the left-engine,which was the difference between the Merlin 76 and 77 engines, which provided a constant pressure of 3.5 psi over the exterior pressure. This, resulted in an apparent cabin altitude of 24.000 ft (7.300 m) when the aircraft operated at the altitude it was designed for, 45.000 ft (14.000 m). Given that this cabin altitude was too high for the pilot the breathe, he had to wear oxygen mask when flying. A rubber gasket filled with pressurized air sealed the canopy when the system was turned on and a valve ensured the pressure was controlled automatically. The pilot must also wear a high altitude suit as he may had to bail-out at high altitude in case he was shot down.
The Welkin also featured a sophisticated electrical system, which was done to minimise the number of seals and points of entry in the cockpit for the control and instrumentation. An experienced electrician in the electrical features of the Welkin needed 4 hours to undertake a pre-flight check of this system. The wings were so large that the fowler flaps of the Whirlwind weren't needed so they were replaced by a simple split flap. The extra wing area also required more stability and the tail was lengthened to provide a longer moment arm. It was armed with four Hispano 20mm cannons, just like the Whirlwind, but they were carried in a tray in its belly to make their loading easier. In that position muzzle flash was less likely to dazzle the pilot.
Welkin's long aspect-ratio seriously handicapped its compressibility, so it needed to be thick at root for strength reasons. Compressibility caused the flight envelope (flyable speed range) between high-incidence stall and shock-stall became very small at high altitudes (any decrease in air speed caused a "normal" stall, while an increase caused a "shock-stall" due to the aircraft's limited critical match number). This reduction of speed envelope is common to all sub-sonic high altitude designs and also happened with later designs like the Lockheed U-2.
The Welkin was kept as top-secret during World War 2 and any information about its existence was only released after the end of the conflict. It was powered by two Rolls-Royce Merlin 76 or 77 V-12 liquid-cooled piston engines which were rated at 1233 hp each.
A two-seat radar-equipped night fighter version was also developed, known as the Westland Welkin NF.Mk.II, for F.9/43 specification, but eventually just one was produced as the variant wasn't ordered into production.
When the Welkin Mk.I was completed and under manufacture, it was evident that the Luftwaffe was no longer performing high altitude missions due mainly to the successful interceptions made by specially modified Spitfires. Just 77 machines were completed at Westland's factory in Yeovil, Somerset, plus further 26 engine-less frames. Only two Welkins served with the Fighter Interception Unit, based at RAF Wittering from May to November 1944, where they were used to gain experience and formulate tactics for high altitude fighter operations, but, eventually, the Welkin was never used operationally by the Royal Air Force.

Sources:
1. https://en.wikipedia.org/wiki/Westland_Welkin
2. https://www.valka.cz/Westland-Welkin-F-Mk-I-t10702
3. https://www.valka.cz/Westland-Welkin-NF-Mk-II-t35453
4. Salamander Books - The Complete Book of Fighters
5. 4+ Publications - 20 - Westland Welkin