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Discussion Starter · #1 ·
Will someone help me understand how or why 5 bladed fans have their blades clocked at non symmetrical degrees? A recent posting by Mikey, where he is selling an AC 5 blade fan, shows it with what I have guesstimated are the clocking degrees of the blades to be; #1 is at 12:00 or 360 degrees, #2 is at about 2:30 or 070 degrees, #3 is at about 4:30 or another 140 degrees, #4 is at about 7:00 or another 230 degrees and #5 is at 9:00 for 270 degrees. That leave the #5 to #1 blade with another 90 degrees or back to 360 degrees. My thought is the fan assembly looks to be unbalanced as the blades, which appear to be all the same size, are not equally distributed. 4 blade fans have the blades equally distributed, what am I missing?
Thanks
Al
 

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I have noticed that on 5 blade fans but have never questioned, why. Would be nice to know, but that may be a question for the manufacturer’s tech support. I have a 6 blade fan and it to is equally spaced.
 

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I have a five blade fan on the mighty frankensix that mikey hooked me up with at the 2019 nationals. The truth of the matter is that the extra blade, and the more agressive pitch of the blades made a real difference. At idle, you can feel the air being pulled through the radiator, enough to hold a piece of paper on the radiator. I could literally feel the circumference of the fan on the outside of the radiator, and the heat eminating from the corners of the radiator where the fan was not reaching. That was a first for me as I have always run the stock 4 blade fan which I would now say is inadequate for todays drivng.

As for balance, I can attest that I ran my motor up to 6000 rpm going down the track at the nationals this year with that fan installed and not one speck of vibration at any rpm. I had absolutly no cooling issues this year whatsoever, which was not the case previously. In fact, at every single nationls I attended, and any time I would sit in traffic at home or at a show, the temperature would rise to over 230. It was a big pain. This year, steady 180-190. In fact, Drag Six Jr (Patrick) and I were making alternate round robin passes on the dragstirp and no over heating isues.

But then again, every time Mikey advises me to do something (and with his experience I always follow his advice), he is always right.

So while I dont understand how they get the asymetrical blades to balance, those types of fans do work.

Now comes the more courious question which is what other GM applications used a five or six blade fan in the pre clutch fan era that may be swapped into a trifive. Caddy comes to mind but I have so little eexperience with early caddys that I dont know whether they used them or not, although I would think they did as I belive either caddy or packard or both had AC pretty early on.
 

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One possible reason: Generally when spacings aren't chosen 'evenly', it's an attempt by the designer to minimize the amplitude of the resonant frequency associated with the device. By having unequal spacing, there will be multiple resonant frequencies of the fan blade, but the amplitudes will be smaller (which will also correspond to lower noise levels from the fan at those frequencies).
 

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This is a photo of my 5 blade fan from my 71 SAAB. The engine is a German Ford V-4. I wonder if the vibration caused the radial separation on the blade. I'm using a 4 blade fan now, but looking for a 5 bladed SAAB fan.
 

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On those fans with unequal blade spacing, is there any more weight added to the 2 blades on the 'light' side to keep it rotationally balanced?
 

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That's why I'm asking. Are the odd 2 blades heavier than the 3 to make it all balance out, or is it handled with unequal weight in the center hub? I'll admit oddly spaced fans are new to me. As far as I can recall, all my junk has all had evenly spaced fans, or at least balanced across like some 4 blades that have a tight X. The rotating balance has to work out somehow or it would tear itself up.
 

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I have the AC optioned 5 blade fan, the engine temperature can still creep up if you are restricting the air flow through the radiator.
 

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Harmonics and noise


2008-04-14
Development of Low-Noise Cooling Fan Using Uneven Fan Blade Spacing 2008-01-0569
When unifying the functions of widely used two-fan, engine cooling system into a single fan unit, the noise and power issues must be addressed. The noise problem due to the increased fan radius is a serious matter especially as the cabin noise becomes quieter for sedans. Of the fan noise components, discrete noise at BPF's (Blade Passing Frequency) seriously degrades cabin sound quality. Unevenly spaced fan is developed to reduce the tones. The fan blades are spaced such that the center of mass is placed exactly on the fan axis to minimize fan vibration. The resulting fan noise is 11 dBA quieter in discrete noise level than the even bladed fan system.




Abstract:
Methods and systems are provided for a fan. In one example, a fan may include a plurality of blades separated by spacings. The spacing is one of a small blade spacing, a medium blade spacing, and a large blade spacing. The size as well as the arrangement of the small, medium, and the large blade spacings are determined by minimizing an imbalance force and pressure generated by the fan at a plurality of frequencies
 

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Discussion Starter · #13 ·
Well now we know! I asked and I received, what a great thing this site is. Many thanks to all who weighed-in... Al
 

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got to go look at my fan, got me curious. good read here.
 
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