Editing Westland Welkin (section)

== Design and development ==
Westland put forward their '''P.14''', essentially an adaptation of Westland's [[Westland Whirlwind (fighter)|Whirlwind]] fighter layout (and a more experimental twin, the P.13) to meet [[Air Ministry]] [[List of Air Ministry specifications|Specification F.4 of 1940]] for a high altitude fighter.<ref>{{Harvnb|Buttler|2004|p=47.}}</ref> The most obvious feature was the enormous [[aspect ratio|high aspect ratio]] wing, with a span on the production aircraft of {{convert|70|ft|m}}.{{efn|For comparison, the [[Avro Lancaster]] four-engined heavy bomber had a span of {{convert|102|ft|m}} and the [[de Havilland Mosquito]] twin-engined light bomber and fighter had a {{convert|54|ft|m|adj=on}} span.{{cn|date=April 2021}}}} The compact but troublesome [[Rolls-Royce Peregrine]] engines of the Whirlwind were replaced by the more powerful two-stage [[Rolls-Royce Merlin]] Mk.76/77. The most significant feature was a pressurised cockpit, which took the majority of effort in the design. 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 DG558 & DG562. 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 {{convert|3.5|psi|kPa|abbr=on}} over the exterior pressure. This resulted in an apparent [[cabin pressurization#Cabin altitude|cabin altitude]] of {{convert|24000|ft|m}} when the aircraft was operating at its design altitude of {{convert|45000|ft|m|abbr=on}}. 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 [[flap (aircraft)|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 (physics)|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.

The Welkin was seriously handicapped by compressibility problems caused by its long, high aspect-ratio wing which needed to be thick at the root ([[thickness-to-chord ratio]] of about 19%) for strength reasons.{{sfn|Boot|1990 |p=12}} Compressibility caused the [[flight envelope]] (flyable speed range) between high-incidence [[Stall (flight)|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 aircraft|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, the required wing area was distinguished by noticeably short wings, with thickness-to-chord ratio (t/c) at the root of 12%,{{sfn|Gunston|1973|p=16}} a t/c ratio which delays compressibility effects to an aircraft speed of about Mach 0.85.{{sfn|Gunston|1962|p=26}}

Information on the Welkin was only released at the end of the war.<ref>{{citation |url=http://www.flightglobal.com/pdfarchive/view/1945/1945%20-%201216.html |work=Flight |date=21 June 1945 |title=New Aircraft Types |page=664 }}</ref>

A two-seat radar-equipped [[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.<ref>{{Harvnb|Buttler|2004|p=48.}}</ref>{{sfn|Velek|Ovčáčík|Susa|2005|pp=13–14}}