this exchange 263 squadron became operational, but Petter always regretted that the Whirlwind was not available for the Battle of Britain and blamed Eric Mensforth for the delay in production. Spitfire development By 1942 Westland was building mostly Spitfires under contract. One of the problems with the early marks of Spitfire was variability of longitudinal stability, leading to aircraft getting dangerously out of control and contributing to the risk of structural failure . Petter made a significant contribution to improving the longitudinal stability of the Spitfire because he was the first to appreciate that aerodynamic modification to the elevator could provide additional stability. On his own initiative he had Penrose collect flight test stick force data and trim curves on a Spitfire at various centre of gravity loadings, then produced a prototype elevator with a bulged aerodynamic section, which produced a 'remarkable' improvement in stability, later being known as the 'Westland Elevator'. Welkin In 1940, the Air Ministry was motivated by the threat of high altitude bombers such as the Ju86P, to issue a specification for a high altitude interceptor, F4/40, followed by a revision F7/41 in 1941. Petter submitted two designs. His first was an innovative low-drag aircraft (P13), which featured a pair of staggered Merlins in the fuselage, one behind and slightly above the other, driving a pair of contra-rotating propellers. His second submission was a conventional design (P14), describing it as 'a logical development of the successful Whirlwind...". This was selected and became the Welkin. Specification F7/41 required a minimum speed to 415mph at 33,000ft with a maximum ceiling of 42,000ft. The Air Ministry also wanted low altitude manoeuvrability and a +9G ultimate load factor. The speed was equivalent to a Mach number of 0.62 while the loading condition caused Petter to select a thick wing section which would later be demonstrated to have a critical Mach number of 0.6. The significance of the thick wing section may not have been understood by Petterbecause compressibility effects had only started to be encountered by aircraft designers. During test flying the effect of compressibility was experienced by Penrose who wrote, "In speed runs at the ceiling the wings and fuselage sometimes shook as though the machine was bumping over cobblestones." Petter was reluctant to believe Penrose or accept that the wing would not be acceptable for high speed at altitude. While the cabin pressurisation was innovative and worked well,