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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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)
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.