Company decreed that there would be a technology sharing between the UK and US, the UK had solved the problems with the S duct intake for the centre engine and this was part of the technical information taken over to the US and presented to Boeing by HS, when Boeing came over to do the likewise sharing they basically didn't. Thus Boeing was able to take the gifted information and solve the problems with the 727 design and take the mid market.
Size did come into it as well, and some of the BOAC BEA sizes were madness, one was written based on the seating capacity of their bus fleet, so the aircraft was built to that, rather than the cheaper and more efficient way of altering the buses or running two.
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Last edited by NutLoose; 11th Feb 2018 at 12:52.