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Fans that look like this:

enter image description here

Is it to somehow help the balance of the engine?

30

It isn't something restricted to old vehicles; my Lumina has fans like this, though not as irregular as the picture shown in the question.

Lumina SS Cooling Fans

As far as I can recall, the chief reason cited for this by the manufacturer is noise reduction. You'll notice the additional weight on some of the blades to ensure that rotational balance is maintained despite the asymmetry.

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    It's true that a symmetrical blade would produce harmonic noise which would be more unpleasant than a more "white" noise produced by asymmetric blades. I believe another added benefit is maintenance/tool access because of the larger gap. – ghoppe Jan 7 '16 at 16:32
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    This has also been done in recent Apple laptops for noise reduction - see here for a picture of the fan in a MacBook Pro. – Scot Jan 7 '16 at 20:30
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    Many tail rotors on helicopters are like this as well... – Moo Jan 7 '16 at 22:07
0

It may be to allow a hand/tool to access a screw or bolt that would otherwise be inaccessible if there wasn't one opening between the fan blades large enough. Although I can't think of how that would apply to electric fans

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