Fan balance

I am trying to watch Wimbledon in this hot weather with a fan to cool me, but the noise and vibration from the fan is distracting. Can anybody suggest
a methodical way of balancing the fan, or is the trial and error method of attaching weights to the blades at random my only option?
The fan is a 3-bladed plastic job that does not stop at any particular position when switched off so there is no obviously heavy blade. The motor runs smoothly and without vibration with the fan removed. I suspect that, like car tyres, dynamic balancing is needed rather than static balancing, but I would be glad to hear otherwise.
Cliff Coggin.
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Cliff Coggin wrote:

"COME ON TIM!!"
BugBear
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Blu Tak.
John S.
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If it was a model aeroplane prop or impeller you would use a prop balancer which is a shaft on low-friction bearings, sometimes magnetic, and sand off the heavy blade. You won't find one that will do a fan blade though.
Also, it's possible and maybe likely that the noise and vibration somes from the airflow over the blades themselves. They're not "designed" in any aerodynamic sense but are most likely cobbled up from whatever looked good to the designer. Well probably, anyhow, unless it's a very expensive fan. The poor airflow characteristics and the vortexes that'll be forming all over the place at the tips and the trailing edges of the blades will cause quite a bit of noise and vibration anyway.
On a model aeroplane the prop shape can have a marked effect on the noise of the model - some blade shapes (and blade materials) are much quieter than others, even when heard over the noise of an IC engine, and that's largely for the reasons I've given above.
You can buy special "quiet" fans for electronic equipment, for instance, which have had some effort into making good blade shapes for quieter airflow, but the cost for doing that for a fan would be prohibitive.
Nobby
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First see that the blades are not bent and are tracking in-line properly, both front and back. If you can verify that the blade lengths are equal, when running the heavy blade will be the first to touch a piece of chalk moved slowly toward the outer edges.
Unless the blades are not tracking, static balance is adequate but you need a low friction mount to do it.
Don Young (USA)
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Thanks Don and Nobby.
The blades are all the same length but they do not project forward each to the same extent so that is one problem that is not curable. The other thing I should have mentioned is that it is a cheap Micromark piece of kit so I daresay aesthetics took precedence over aerodynamics during design. Anyway, experimenting with small blobs of BluTak has reduced the vibration to an acceptable level.
Cliff.
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