Westland Welkin The Westland Welkin was a British twin - engine heavy fighter from the Westland Aircraft Company , designed to fight at extremely high altitudes , in the stratosphere ; the word " welkin " meaning " the vault of heaven " or the upper atmosphere . First conceived in 1940 , it was built from 1942 – 43 in response to the arrival of modified Junkers Ju 86P bombers flying reconnaissance missions that suggested the Luftwaffe might attempt to re - open the bombing of England from high altitude . The threat never materialised ; consequently , Westland produced only a small number of Welkins and few of these flew . Design and development . Westland put forward their P . 14 , essentially an adaptation of Westland ' s Whirlwind fighter layout ( and a more experimental twin , the P . 13 ) to meet Air Ministry Specification F . 4 of 1940 for a high altitude fighter . The most obvious feature was the enormous high aspect ratio wing , with a span on the production aircraft of . The compact but troublesome Rolls - Royce Peregrine engines of the Whirlwind were replaced by the more powerful Rolls Royce Merlin Mk . 76 / 77 . However , the most significant feature was a pressurised cockpit . The last item required the majority of the effort in designing what would become the Welkin . After extensive development a new cockpit was developed that was built out of heavy - gauge duraluminium bolted directly to the front of the main spar . The cockpit hood used an internal layer of thick perspex to hold the pressure , and an outer thin layer to form a smooth line . Heated air was blown between the two to keep the canopy clear of frost . In January 1941 , the Ministry of Aircraft Production authorised the building of two P . 14 prototypes . The F . 4 / 40 specification was revised into F . 7 / 41 that year . The Welkin design was now in competition with the Vickers Type 432 with Merlin 61 engines . The pressurisation system was driven by a Rotol supercharger attached to the left - hand engine ( this was the difference between the Merlin 76 and 77 ) , providing a constant pressure of over the exterior pressure . This resulted in an apparent cabin altitude of when the aircraft was operating at its design altitude of . This cabin altitude was still too high for normal breathing , so the pilot had to wear an oxygen mask during flight . A rubber gasket filled with the pressurized air sealed the canopy when the system was turned on , and a valve ensured the pressure was controlled automatically . Moreover , the pilot also had to wear a high altitude suit as he might have been required to bail out at altitude . The Welkin required a sophisticated electrical system . This was to minimise the number of seals and points of entry in the cockpit for the controls and instrumentation . It took an electrician experienced in the features of the Welkin four hours to undertake a pre - flight check of this system . The wings were so large that the high lift Fowler flaps of the Whirlwind were not needed , and were replaced by a simple split flap . The extra wing area also required more stability , so the tail was lengthened to provide a longer moment arm . The armament − four Hispano 20 mm cannon − was the same as the Whirlwind ' s , but the Welkin carried the guns in a tray in its belly , which facilitated loading . In that position , muzzle flash was also less likely to dazzle the pilot . Operational history . By the time the Welkin Mk . I was complete and rolling off the line , it was apparent that the " Luftwaffe " was no longer conducting high altitude missions , due largely to successful interceptions by specially modified Supermarine Spitfires . In the end , only 77 complete Welkins were produced , plus a further 26 as engine - less airframes . 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 . A two - seat , night fighter version known as the Welkin NF . Mk . II for specification F . 9 / 43 was developed but only one was eventually produced as the variant was not ordered into production . Information on the Welkin was only released at the end of the war . The Welkin was seriously handicapped by compressibility problems exacerbated by its long but thick wings , causing the flight envelope ( flyable speed range ) between high - incidence stall and shock - stall to become very small at high altitudes - any decrease in airspeed causing a " normal " stall , any increase causing a shock - stall due to the aircraft ' s limiting critical Mach number . This reduction of the speed envelope is a problem common to all subsonic , high altitude designs and also occurred with the later Lockheed U - 2 . When W . E . W . Petter came to design his next high altitude aircraft , the English Electric Canberra jet bomber , it was distinguished by noticeably short wings .