Re: Why are radiator fan blades non-symmetrical?

Radiator fan blades in cars are sometimes asymmetrical, that is very true,
but it has nothing to do with enhancing the performance of the fan itself.
Quite simply, anything less than a maximum length blade is going to impede
the performance of the fan.

However, as anyone who has driven a car with a snapped fan belt can tell 
you,
once the car is moving, the radiator will remain reasonably cool for some
time without the fan due to the passage of air through the engine and grill
created by the car's motion. We can therefore see that the purpose of the 
fan
is chiefly to keep the radiator cool while the car is stationary or moving
slowly - the car with the snapped fan-belt turns it's engine off when
stationary or it overheats in a few moments. Usually the fan points 
forwards
and draws air through the radiator to simulate the air currents of the car 
in
motion. There is an added benefit of circulating air around the engine
cavity.

So, why do we see one blade out of five cut short, offset, or with a hole 
in
it? Moderations to the fan blades that will alter the fan's performance by,
say 5 to 10 percent, are of little consequence. Turn the blade with your 
hand
or attempt to service the engine and you will find that you may require a
screwdriver or some other tool to access a point that would normally be
obscured. Alternatively, the shape of the fan may be determined by its
manufacture. Die-cast plastic items are notorius for modifications for easy
mould extraction, simple die design, and the like. 

That, I'm afraid, is the answer. There is nothing magical about the hole or
the short/offset blade. The choice of electric or belt drives rests with 
the
designer and hinges upon the size of the engine cavity, the dimensions and
orientation of the engine, and economies in production of the car.

In response to further queries from the original questioner, Justin adds:

You are referring to fans used for cooling the radiator in automobiles, yes?
Your question addressed as to why for example, in a five-bladed fan, the
blades were not each set radially at 72 degrees from the next as opposed to
the symmetry of the rake angle of the blades.

Firstly let me re-iterate that many fans (supplied new and unbroken!) have
either: one blade cut short, or a blade with a hole in to facilitate tool
access to the engine or easy manufacture. Both these features will give an
asymmetrical fan. However, since receiving the memo below I have researched
further and can give you something that I think will better meet your
expectations.

Where the number of blades = n, the intonation between the blades should be
360/n degrees for symmetry to exist. However, two blades can be separated by
a lesser angle, OR one blade made a shorter length to set up disharmony in
the aircurrents drawn through the radiator, the effect of this is appreciated
at high rpm (and therefore this is usually seen in high performance engines)
where harmonic noise would be produced leading to vibration in the engine and
auditory discomfort to the driver and those nearby. Electric fans (normally
encountered in economy cars and smaller engines) do not run at sufficient
speed nor draw air in sufficient quantities and high enough flow rates to
produce this phenomenon.

I should imagine that that is more along the lines of what you were looking
for.

Best Regards,
Justin Roux BEng (Hon) AMIMechE