If the war had not broken out in 1939 Scotland's central airport might well have been in Grangemouth, in fact KLM operated one flight I believe. This site is about 28 miles from both Edinburgh and Glasgow and would have served the central belt very well. The present M9 is now very close, the Kincardine bridge was already built so connections to Fife were nearby, also railways and in fact the Forth too! How about a river connection to Edinburgh thrown in for good measure. Too late --- it is now a
refinery!! After the war both Edinburgh and Glasgow felt the need to have their own airports at Turnhouse and Renfrew. Grangemouth was history.