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Wonder why they didn't try the P47 paddle prop on the Corsair and Hellcat? Or did they?
I read about that P38, it was quite a performer. The reason I asked the original question is that the late model Spitfires had 5 bladed props and I figured if they could use them why not the big radial engined US fighters. They late war Thunderbolt and Corsair both had an abundant amount of hp and I just figured they probably weren't effectively using all of it.
Going by what they "knew" at the time 3 blade props were considered more efficient. But a larger 3 blade prop was needed than a 4 blade.
Leaving aside what FlyboyJ has said, which is correct as far as I know and he has much more experience than me, it does come down to the designers experience, gut feelings and some calculations. The ground clearance thing is a big factor.
Please remember that around 1940 there were only 12-14 wind tunnels in the US of varing sizes and speed capability. By the end of WWII there around 40.
What they "KNEW" or thought they knew in 1940-42 vs what they KNEW in 1945-46 are not always the same thing.