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Brian Vogel
Prolific User
Username: guyslp

Post Number: 226
Registered: 6-2009
Posted on Sunday, 03 February, 2013 - 04:18 am:   Edit PostDelete PostView Post/Check IP

My life of late has been somewhat focused on viscous fan clutches and fan blade sets. There are many "symmetric" (for lack of a better term) blade sets where the blades are equidistant from each other around the circumference of the blade hub. Others are asymmetric, like the original fan blade set on the II/2 series cars, which has three sets of blade pairs, two of which appear to be spaced equally, the third of which is spaced a bit more widely, and then a single blade not "connected" with the rest.

Does anyone know, in fairly simple terms, what the reasoning is for the asymmetric designs? These would seem much more difficult to produce as far as having a balanced weight distribution, so there must be some reason (or claimed reason - we all know how some design reasons really aren't) for this choice.

Brian, ever curious
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 951
Registered: 6-2006
Posted on Sunday, 03 February, 2013 - 07:12 am:   Edit PostDelete PostView Post/Check IP

Resonance, and trying to prevent it. They are quieter than equidistant blades.

Well that was what I was taught. :-)
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Chris Browne
Frequent User
Username: chrisb

Post Number: 100
Registered: 2-2010
Posted on Sunday, 03 February, 2013 - 08:19 pm:   Edit PostDelete PostView Post/Check IP

Hello Brian,
I have just been researching the cooling system on the Cloud 1 which I am restoring and came across a very detailed article about fans on a website called KDA132. I am not sure whether this applies to modern plastic fans (I suspect it does) but the humble cooling fan is an incredibly complex assembly. Without going into too much nerdy detail, each blade is set at a different angle to the hub to prevent rhythmic droning noises at particular rpm. The radius of the blade leading edge tip is different to the trailing edge so it is more aerodynamically efficient and the blades themselves are twisted through two different radii along their length for the same reason. I had no idea that there was such complicated engineering in a cooling fan but then again, when did Rolls-Royce take the easy route? Hope I haven't bored you!
Kind regards,
Chris
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Bill Coburn
Moderator
Username: bill_coburn

Post Number: 1477
Registered: 4-2003
Posted on Sunday, 03 February, 2013 - 08:44 pm:   Edit PostDelete PostView Post/Check IP

Chris Thanks for that, I suspected as much. What I did read about yonks ago was the science involved in the design of a propeller for a huge container ship. Best I go consult Dr Google!!
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Brian Vogel
Prolific User
Username: guyslp

Post Number: 228
Registered: 6-2009
Posted on Monday, 04 February, 2013 - 01:09 am:   Edit PostDelete PostView Post/Check IP

Chris,

Thanks for that info, though I think that some of it does not apply to the newer plastic fan.

I'll have to check again, but on the fan from my car the pitch of each of the blades (angle to the hub) is the same. The other things seem to be the same, too, unless these differences are almost imperceptibly small to the naked eye. I know that they are not on some of the older metal fans I've seen in photos or "up close and personal."

These days it appears to be very rare to find an asymmetrical fan design, which is why I asked. Any aftermarket fan blade designs I've seen from all the major manufacturers are all symmetric designs as are all of the electric fan blade sets I've seen so far.

Brian
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David Gore
Moderator
Username: david_gore

Post Number: 1202
Registered: 4-2003
Posted on Monday, 04 February, 2013 - 09:56 am:   Edit PostDelete PostView Post/Check IP

I suspect the switch from asymmetric fans to symmetrical fans occurred when the metal blades were replaced with "plastic" equivalents. Fatigue failure of metal fans due to induced vibration from blade flutter/harmonic vibration led to the asymmetric design to remove the underlying causes of this vibration. The significantly reduced notch sensitivity of the "plastic" materials meant their resistance to fatigue was much greater and the extra manufacturing costs involved in making asymmetric metal fans could be eliminated.

Further to Bill's comments about ship propeller design: not only did they have to worry about fatigue; they also had to worry about erosion of the blades over time due to cavitation changing the conditions of operation for the propeller blades.
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Brian Vogel
Prolific User
Username: guyslp

Post Number: 230
Registered: 6-2009
Posted on Monday, 04 February, 2013 - 10:17 am:   Edit PostDelete PostView Post/Check IP

David,

This question is not meant to be snarky (but I know it might read that way): So how does that explain the virtual non-existence of asymmetrical metal blade sets these days?

I have really been surprised that asymmetrical metal and plastic blade sets were so common for so long and really seem to have vanished from the aftermarket these days. Has something changed about the metals used, or what?

Brian, who sees this thread as yet another demonstration of why one should always solicit information from the cohort regarding arcana
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David Gore
Moderator
Username: david_gore

Post Number: 1203
Registered: 4-2003
Posted on Monday, 04 February, 2013 - 05:37 pm:   Edit PostDelete PostView Post/Check IP

Brian, I doubt it has anything to do with the type of material selected and everything to do with design and construction of the fan assembly where everything from blade numbers and profiles through to actual mass and construction of the assembly can influence harmonic vibration, noise levels and air flow.

Of course, the manufacturing cost for the assembly will also be an important consideration. Fortuitous circumstances, astute observation with a degree of lateral thinking often helps solve otherwise impossible problems over time.

David, who believes in putting difficult problems to one side for a day or more in the expectation a solution will become apparent; more often than not, a solution or a partial fix eventually becomes apparent which solves the problem or allows it to be bequeathed to someone else who then continues the process.
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Bill Coburn
Moderator
Username: bill_coburn

Post Number: 1479
Registered: 4-2003
Posted on Monday, 04 February, 2013 - 06:10 pm:   Edit PostDelete PostView Post/Check IP

David, There are more things twixt Heaven and Earth etc!! On the subject of fan blades, you are not quite old enough to remember the aluminium (?) blades on the Ghosts. There was one piece of juicy apocrypha circulating of a car that the owner was demonstrating the upper revs and a fan blade detached itself which pieced the aluminium upper bonnet like a hot knife through butter. The laundry bill was quite high for all concerned.

I gather the plastic fan on the front on my Spur is symmetric. It looks so discreet there and even at full puff is quite unobtrusive!!
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 955
Registered: 6-2006
Posted on Monday, 04 February, 2013 - 07:22 pm:   Edit PostDelete PostView Post/Check IP

I did a water pump an a '94 car last week. Still metricate there.

A big plus when working on the cars is that if the fan won't slip out one way, you can usually spin it around to a different set of gaps and it will slide out easily.

I suspect that because the newer model engines are noisier, fan noise is drowned out and is much less of a factor?
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 2768
Registered: 4-2003
Posted on Monday, 04 February, 2013 - 07:43 pm:   Edit PostDelete PostView Post/Check IP

Certainly all postwar Crewe cars, at least until those occasional BMW engines from 1998 onwards which may be different, have asymmetrically-arranged main fan blades on the fan attached to the water pump whether riveted (like on my R-Type and later cars) or plastic. That is because they must not resonate throughout the range of RPM they endure. As Bill points out, the electric fans in front of SY2 cars onwards have symmetrically-arranged blades. That is because they run at a constant speed and are tuned suitably not to operate on load at a resonant frequency. The qualification is that when they are passively freewheeling at any odd speed, as they usually do, the fans are not applying effort so do not have resonant phases.

RT.

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