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I just learned that until shortly after world war 2, flying boats had a longer range than land based aircraft

In the 1930s, a flying boat route was the only practical means of transatlantic air travel, as land-based aircraft lacked sufficient range for the crossing. (link)

Why was that? I'm not aware of any reason why land based aircraft should have a shorter range than seaplanes.

JanKanis
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    related: Why were flying boats phased out? – JanKanis Jan 18 '21 at 21:43
  • Seaplanes have less air resistance (mostly due to the lack of landing gears that land planes have). That is why most speed records of older times were held by sea planes. That also means more range with same amount of fuel. – David Balažic Jan 19 '21 at 10:55
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    @DavidBalažic Seaplanes have a worse aerodynamic shape than land planes because they also need to be boat-like. I'm pretty sure the air resistance of a land plane with retractable landing gear will be less than that of a seaplane, but I'm not sure how seaplanes compare to fixed landing gear aircraft. Do you have sources? – JanKanis Jan 19 '21 at 11:00
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    Yes. The speed records. Of course the comparison is to non retractable gears. That is why after they started using retractable gears, the tables turned to favor land planes. – David Balažic Jan 19 '21 at 11:04
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    You're right on the speed record thing, but according to Why were the speed records set by seaplanes in the early thirties? this was due to the runway length needed with the fixed-pitch propellers of the day and not so much aerodynamics. – JanKanis Jan 19 '21 at 11:54
  • The postwar claim of your question is an erroneous misreading of your link; in actuality that article itself indicates 4000 mile landplane flight between Berlin and NY in 1938. – Chris Stratton Jan 21 '21 at 03:38
  • @ChrisStratton And it also indicates a 5,000-mile NYC-Istanbul seaplane flight in 1931. It further indicates that the first trans-Atlantic passenger flight on an airplane (as opposed to an airship) wasn't until 1939... and it was on a Boeing 314 Flying Boat. – reirab Jan 21 '21 at 14:31
  • @JanKanis By the late 30s (still before WWII,) constant-speed (i.e. variable-pitch) props were around. The Boeing 314 had them, for example. Also, I don't think it's quite correct to say that it wasn't until shortly after WWII that flying boats had longer range. The B-29 had a longer range than the Boeing 314, for example, and it was definitely around during the war. – reirab Jan 21 '21 at 14:42
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    @reirab while the wikipedia picture of a CH-300 is of one on floats, every indication I can find is that the NY-Instanbul flight was a land configuration; news articles clearly state that they rolled down the runway at Floyd Bennett, and the picture https://ark.digitalcommonwealth.org/ark:/50959/6682zd329 clearly shows the poise of a taildragger. So no, that's 5000 mile landplane flight in 1931, not a seaplane one. – Chris Stratton Jan 21 '21 at 19:27
  • @ChrisStratton Ah, fair enough. I didn't look further into it and was just going by the picture there. Certainly seems reasonable enough. After all, Lindbergh flew to NYC-Paris in '27 in a landplane. These were, of course, significantly smaller aircraft than passenger airliners of the day like the Boeing 314 or the DC-3, though. – reirab Jan 21 '21 at 21:12

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Since fuel volume in aircraft is a large factor in range, and the volume of an aircraft increases faster than the linear size, larger aircraft have a big advantage for range. In the 1930s, the biggest hurdle for large land-based aircraft was infrastructure. Most airports in the 1930s were not very large, and many parts of the world did not have any at all. The common airliners at the time carried around 20 passengers.

Designing an aircraft that could land on the water allowed aircraft to get larger and have longer ranges without the need for large runways to be built to support them. Aircraft like the Sikorsky S-42 carried almost 40 passengers. It wasn't until at least the late 1930s that land based aircraft were able to reach this size, and after World War II the devolopment of infrastructure and aircraft technology made long-range land-based aircraft much more feasible.

fooot
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    I would also expect it is easier to beef up a fuselage for direct water-landing (as opposed to floats) compared to the weight needed for retractable landing gear. That is, the weight added to make direct water landing reasonable is much less than that needed for retractable landing gear (even if direct water landing requires higher wing mounting or other modification). – SoronelHaetir Jan 19 '21 at 07:18
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    @SoronelHaetir I don't have detailed knowledge of aircraft design, but intuitively I don't share that expectation. A flying boat needs to have a strong bottom along the entire airplane to absorb the forces of hitting the water and waves. In a regular plane the skin only needs to withstand aerodynamic forces, and only the relatively small landing gear need to be strong enough to handle the weight of the plane. My intuition thinks the landing gear will be lighter. – JanKanis Jan 19 '21 at 08:53
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    @JanKanis If the fuselage can't take the weight of the entire plane plus all its cargo, the plane will collapse. So it already has that strength. The skin itself doesn't have to be thick either, because the force of the water is distributed over a wide area. Wooden sailing dinghies are constructed of plywood as thin as 3mm or 4mm. – Graham Jan 19 '21 at 09:29
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    @Graham I wasn't talking about weight, but about the impact forces of hitting the water and the waves. A landing flying boat is a lot faster than a sailing dinghy. The weight can be carried by a few structural beams, but all of the underside of the hull must be able to withstand hitting a wave at high speed. – JanKanis Jan 19 '21 at 09:38
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    This is why the "Spruce Goose" got built, isn't it - the last attempt at scaling up seaplane technology before the jet age. – pjc50 Jan 19 '21 at 14:54
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    Good statement. I had the privilege of growing up in Berlin, which had Hitler's "ORIGINAL" Airport (the first real airport, after the airfields used before) and - it was SMALL. The planes went into Hangars where the terminals where and you walked to the plane IN the hangar. Amazing (at that time only used for mid range turbo prop machines), but it gives a good idea how small the largest aircraft where pre the development of the modern airport and the increase in passenger size and the jet age.Pre this airport basically people used airfields and they were VERY limited. – TomTom Jan 19 '21 at 15:41
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    @JanKanis For the underside of the hull, you're planing - skimming over the water instead of pushing through it. The force from glancing off wave-crests is very different from planting perpendicularly into the water. For sure it needs to be strong, but only on a similar scale to a boat. If you look at Donald Campbell's various speedboats, they were significantly faster than any flying boat, but they weren't that heavily built. Since seaplanes clearly did work, for all the reasons fooot said, I suggest your intuition may not be a totally accurate guide. ;) – Graham Jan 19 '21 at 15:42
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    @Graham A flying boat requires a large area to be a little stronger, a land based plane requires relatively small landing gear which need to carry the full weight of the plane. I don't know which will be heavier, I guess some numbers would be needed at this point, but I don't have them. Do you have numbers or relevant experience? – JanKanis Jan 19 '21 at 16:33
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    I dont think this is worth adding as an answer in its own right, but it does answer some of the questions above - more fuel means more range, which means more weight (as in your answer). Most airfields pre-WW2 were just that, fields. Long, muddy fields. Horrible for heavy aircraft. WW2 happened and all of a sudden there were an excess of long paved runways pretty much everywhere in the world - these long runways allowed lighter built land based aircraft to carry more weight, meaning more fuel. Seaplanes were also much more maintenance heavy (sea water being corrosive and all...) – Moo Jan 19 '21 at 20:40
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    See the progression of London Heathrow from a small unpaved airfield in the 1930s to long paved runways in 1944, with the intention for it to be a hub for long range military transports. It was the paved runways which made the difference - transports and bombers could carry greater bomb loads further than they could taking off of a grass field. – Moo Jan 19 '21 at 20:44