The Junkers G 38 has four propeller blades on the inboard but only two on the outboard engines.
What's the reason for this special arrangement and is there any benefit?
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1$\begingroup$ Your illustration is a drawing by Bruce McCall. His books are very, very funny. I don't recognize this one, but he did create some other WWII aircraft. One had locomotive wheels because of the rubber shortage, Another had a droopy nose because its engineering drawings slipped in the blueprint copier. My favorite was a fighter trainer with lots of student seats, in a line, and one instructor seat. He also wrote an article on golf carts of the Third Reich. $\endgroup$– stretchDec 25, 2017 at 12:39
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$\begingroup$ @stretch Someone else edited the question and added the picture; I've always thought that it was a photograph. But looking at it more closely now I can imagine it to be a drawing. Beautiful artwork. We do know that it depicts a real aircraft from the 1920s well into the 1930s. Thanks for pointing this out. $\endgroup$– user7241Dec 25, 2017 at 13:46
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4$\begingroup$ Of course it is a photograph... It is cropped from a larger one that can be found in the internet, and signed... 1.bp.blogspot.com/-SMGqavIFKx0/Vs9VJR-wWUI/AAAAAAAEJ5M/… $\endgroup$– xxavierDec 26, 2017 at 8:39
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1$\begingroup$ @xxavier Cool, thanks. $\endgroup$– user7241Dec 26, 2017 at 8:54
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2 Answers
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The inboard engines were of a different type, and more powerful than the outboard engines. Hence, the props had to absorb different power, so they were four-bladed for the inboard and two-bladed for the outboard engines.
Yo can see the specs of the airplane here https://en.wikipedia.org/wiki/Junkers_G.38#Specifications_(G.38_1929)
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1$\begingroup$ Wow, thanks for the info. I had never heard of a multi with different engines. You learn something on this list every day :) $\endgroup$ Dec 27, 2017 at 0:04
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What's interesting is the why. In essence this plane is powered by 6 identical engines, which helps with the parts commonality and maintenance.
Owing to the financial difficulties of the interwar period, the inboard bigger engines (L55) are a doubled version of the outboard ones (L8a): an inline-six configuration turned into V12. Which helps save weight and structural reinforcing, and delivers the right amount of power.
Speaking of doubling an engine, you can double a V12.
Junkers had the L2 engine in 1925, it was enlarged to become the L5 (with license from BMW). The L8 is a higher power L5. And the L55 is a double L5. It's a good engineering solution to the lack of engine diversity.
The aircraft was later upgraded to have 4 identical Jumo 204 diesel engines, the original configuration (L55 and L8) was petrol.