While with hindsight it is quite obvious to us that the future in the 1930s was heavier than air aircraft, this was not necessarily obvious at the time. The problem with heavier than air aircraft in the 1930s for long range was the lack of sufficiently powerful and reliable engines to allow an aeroplane to built big enough to have adequate range and still have a useful payload. There has almost never been successful civil aircraft with more than four engines, so that the maximum size was dictated by four of the largest available engines at any time. There have been attempts to use more, but a good example would be the twelve engined Dornier Do X flying boat or the eight engined Tupolev ANT-20 landplane. Both were complete flops
Another problem facing long range aeroplanes because of their necessarily large size was that they had to be flying boats because of the lack of large airfields. Designing an aerodynamically clean hull that also was acceptable on water was not simple, and the problem was not solved until the end of the thirties. Even then, the newer landplane designs were a lot cleaner aerodynamically, as a result of retractable undercarriage and effective wing flaps.. 
The airfield problem was solved as a result of WW2, during which it was found on all sides that the easiest way to extend the range and bomb load of bombers was to make the runway longer. Since the war extended worldwide, this meant that by 1945 most potential aviation destinations were equipped with runways suitable for the new long range aeroplanes such as the Lockheed Constellation, DC-6, Boeing Stratocruiser, as well as the turbojets being designed as soon as the war finished. The failure to realise this meant that a lot of money was spent on designing new flying boats such as the Convair R3Y and the Saro Princess. The last large transport flying boat design to actually enter service was the Martin Mars, first flying in 1942. But only seven were built. Two are still (sort of) in service as firefighting aircraft. (The Sandringham flying boats of the immediate post war period were conversions of military Sunderlands, not new aircraft.) There have been quite a few experimental medium to large flying boats since the war, and some have even flown, if only briefly, like the Hughes 'Spruce Goose'.
John
There have been
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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