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The image below is of a "Predator 670" engine with the fan shroud removed. These engines are available in the United States at Harbor Freight. It is a simple air cooled twin cylinder design, commonly used for farm implements and go karts.

With the shroud removed, we can see the blades of the engine. If you look closely, you can see that some of the fan blades are closer together compared to the others. They gradually get closer together, then gradually get farther apart again in 180 degrees. So the blade opposite any given blade has identical spacing.

What purpose does this variation in the spacing of the fan blades serve?

enter image description here Image credit: CarsandCameras YouTube video

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    I believe it all has to do with acoustics. With the unevenly spaced blades, you don't get the rhythmic vibrations through the air as you would if evenly spaced. The same sort of thing is done for engine driven fans on automobiles. At least that's my theory and I'm sticking to it :o)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 26, 2017 at 1:32
  • I agree with Paulster2. Also, look closely at a car tire, and you'll see similarities - the tread pattern varies slightly around the wheel.
    – PeteCon
    Sep 26, 2017 at 2:40
  • possible duplicate mechanics.stackexchange.com/q/24327/15036
    – agentp
    Sep 26, 2017 at 17:29

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The comments point to noise which is one reason , the other is that it can spread the working range in terms of delivery or efficiency.

We now have variable geometry turbos for the same reason...

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  • I feel like this answer is correct, but should provide a physical explanation of the phenomena.
    – Eric Urban
    Sep 26, 2017 at 21:44
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    If you want all the maths involved in velocity triangles then the post should probably be on engineering... However if you wish to read up on the theory this link may help you - enjoy the read : mvsengineering.com/files/Subsurface-Book/MVS-SVE_Chapter10.pdf - a velocity diagram is shown on page 7 ....
    – Solar Mike
    Sep 27, 2017 at 4:24

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