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Author
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Topic: Why are fans so weird-looking?
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Fastback68
Gearhead Posts: 4511
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 05-12-2002 10:30 AM
 
I just got a five-blade 68 radiator fan off Ed (bluetech). Thanks mate! Why are radiator fans so out of whack? I think the term is "radially assymetrical".
Very curious ...
Simon
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V8 Thumper
Gearhead Posts: 4777
From: Phoenix, Arizona
Registered: Dec 2001
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posted 05-12-2002 11:18 AM
Maybe for balance?
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bluetech
Gearhead Posts: 254
From: Far and Away
Registered: Feb 2001
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posted 05-12-2002 11:38 AM
quote:
Originally posted by Fastback68:
"radially assymetrical".
You really couldn't resist asking  Let's hope the answer is not as complicated as "radially assymetrical".
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Fastback68
Gearhead Posts: 4511
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 05-12-2002 12:16 PM
 Balance must be the answer, but usually radial or bilateral symmetry does the job. The human body is bilaterally symmetrical. If we had two of everything on one side, and nothing on the other, we'd fall over. A golf ball is radially symmetrical or it wouldn't fly straight. Why does a radiator fan not follow the same rules???
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mellowyellow
Gearhead Posts: 8198
From: So. Fl.
Registered: Aug 2000
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posted 05-12-2002 07:58 PM
If you got the 68 AC fan, you got a good one. They do a good job.
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Fastback68
Gearhead Posts: 4511
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 05-14-2002 01:47 AM
I've got another theory. Maybe it has nothing to do with balance. Maybe the aim is to create turbulent air flow to stop a pocket of hot air forming in a sort of eddy in front of the engine. Asymmetrical fan blades could be the way to do this.
Someone must know the answer, don't they? Alex, you know EVERYTHING
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AKBOSSMAN
Gearhead Posts: 2993
From: WASILLA , AK USA M&M #743 MCA #40857
Registered: Oct 2000
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posted 05-14-2002 02:19 AM
quote:
Originally posted by Fastback68:
I just got a five-blade 68 radiator fan off Ed (bluetech). Thanks mate! Why are radiator fans so out of whack? I think the term is "radially assymetrical".
Very curious ...
Simon
Answer, All fans were made on Monday 1st Shift or .........
I'm wrong 
My Boss fan looks weird too.
------------------
WAYNE CURTIS
70 GRABBER GREEN BOSS 302No fake scoops here!
01 Sangria Red Focus ZX3 Going Solo II !
03 Boss 302 (5.0) (waiting on order form)
92 SHELBY 4X4 (K-9 IDITAROD LEAD DOG)
My old '71Torino GT
"If your under control then your going too slow!" Parnelli Jones
"When it's on it's top and burning,you've gone fast enough." unknown
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richard bohm
Gearhead Posts: 373
From: tucson,az-luray,va
Registered: May 2001
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posted 05-14-2002 04:23 AM
the key here is airflow across the radiator, and keeping the air flow balanced. my falcon has a four bladed fan(it has the 6) where the fan blades are not perpendicular to each other. my mustang had a five bladed fan where the blades were not evenly set. remember that the fan blade is nothing more than a spinning airfoil. it has to contend with votexes and the offset blades compensate for the disruption in airflow they cause to the blade behind them. if you have ever been to an airport, and noticed that light aircraft land well behind a jumbo jet, where as jumbo's land at equal intervals, you begin to understand the flow disruption caused by a vortex for the wing that comes in behind it. i hope this clears the mud a little.
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Fastback68
Gearhead Posts: 4511
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 05-14-2002 05:59 AM
So if that's the theory, why would a regular electric fan be symmetrical? Could that be because of the sheer number of blades? And/or because they're light and flexible?
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Daniel Jones
Gearhead Posts: 972
From: St. Louis, MO
Registered: Aug 99
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posted 05-14-2002 12:00 PM
My guess is it's done for NVH (noise-vibration-harshness) reasons.
Some basic vibration theory. Any physical mechanism or structure
with mass and elasticity is subject to vibration. There are two
basic types of vibration: free and forced. A free vibration is one
that occurs due to the motion and mass distribution inherent in the
mechanism itself. It occurs without any external force acting upon
the mechanism and will occur at the natural frequencies of the
mechanism which are a function of the mechanism's stiffness and mass
distributions. A forced vibration is one that is due to external
forces. If the external force is oscillatory, the mechanism will
vibrate at the excitation frequency. If that frequency is near one
of the natural frequencies, the amplitude (the distance the vibrating
part moves) of the vibration can grow dramatically. Such a mode is
said to be resonant. The stiffer the structure, the higher the natural
frequencies will be. All physical structures are also subject to
damping due to friction (both internal and external). The amount of
damping can greatly affect the amplitude of the response. The greater
the damping, the smaller the amplitude.Okay, so how does this apply to the cooling fan question? Imperfect
mass distributions (both static and dynamic) and a flexible structure
will induce a free vibration. Airflow (both external and induced)
will provide the forcing function to drive the fan into resonance.
Vortex shedding at the tips of the fan blades could also be a forcing
function, though I'd have to calculate the tip speed to see if that's
much of a problem. A symmetric fan will tend to vibrate all the blades
at the same frequencies. By making the fan asymmetric, no one frequency
will excite the entire fan so the peak vibration and noise will be lower.
The trade off is the noise and vibration will occur over a wider range
of frequencies. There are, of course, other ways to address the problem.
If you stiffen the structure sufficiently, you can shift the natural
frequencies to something higher than the frequencies encountered
during operation. You can also balance the device to a high degree
so that the natural osciallation amplitudes are lower. These things
are often done to driveshafts, for example. Dan Jones
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kcode
Gearhead Posts: 3415
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99
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posted 05-14-2002 01:08 PM
Yea, what Dan said......The same principals apply to the Alternator fan blades also. Early to mid 60's all had concentric blades, then ford changed to ugly alt fans with a couple of blades that looked like they were mis-stamped Mike
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Fastback68
Gearhead Posts: 4511
From: Sucat, Paranaque, Philippines
Registered: Jul 99
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posted 05-14-2002 01:16 PM
Fascinating! Thanks a lot!
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AKBOSSMAN
Gearhead Posts: 2993
From: WASILLA , AK USA M&M #743 MCA #40857
Registered: Oct 2000
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posted 05-14-2002 03:10 PM
D@#$ we have some smart DUDES here
 But.........do you know how Grrls work? and why are they asymmetrical ???? ------------------
WAYNE CURTIS
70 GRABBER GREEN BOSS 302No fake scoops here!
01 Sangria Red Focus ZX3 Going Solo II !
03 Boss 302 (5.0) (waiting on order form)
92 SHELBY 4X4 (K-9 IDITAROD LEAD DOG)
My old '71Torino GT
"If your under control then your going too slow!" Parnelli Jones
"When it's on it's top and burning,you've gone fast enough." unknown
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Daniel Jones
Gearhead Posts: 972
From: St. Louis, MO
Registered: Aug 99
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posted 05-14-2002 04:53 PM
> But.........do you know how Grrls work? I'm afraid that is outside the comprehension of mere mortal science! > and why are they asymmetrical ???? If you don't know that by now ... Dan Jones
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