Anyone know the reason behind the cooling fan on an MX-5, or at least my NB facelift, having unevenly spaced blades on the cooling fan ?
The things you notice when doing a bit of extreme detailing…! 
Some of them broken off?
Or someones protractor was dodgy in Hiroshima?
Just to take an idea from other feilds I’ve come across. Symmetrical blades fans are well balanced but because blades are equally spaced they create the same frequency wave which can increase noise and stress, Asymmetrical blades distrub this noise vibration frequency not allowing it to magnify as it’s always reinventing itself. So quieter and longer life can be had. Saying that is it really required on a plastic fan driven by a small motor ? I stand to be corrected
I’ve just read another explanation; its safety related as a flickering light, even a timing light, can cause a strobe effect and make the fan look like its not moving, so if it has blades spaced at uneven centres that won’t happen (not even sure that’s correct is it ?) . Also spotted references to PC cooling fans being made in a similar way to reduce noise… But does seem extreme measure for something that only runs occiasionally.
I would tend to agree with Martin on this, my explanation would normally concern heavier parts at 3000rpm upwards, How much noise would a little plastic fan make afteral ?
The new Macbook pro retina laptop was touted as having asymmetric fanblades inside the casing, which was for noise purposes.
We have Lenovo desktop computers at work (many of them). A lenovo engineer told me the fans were modelled on an owl’s wings.
They are indeed remarkably quiet. I suppose owls do have a requirement to swoop silently - sadly for little mice, “Death from Above”
Having seen Engine Noise Cell work being carried out, a small change to a fan, ie asymmetric blades or a slight change in design/mat’l thickness to a bracket or cyl block etc [or the funny aditional pipe on the Air Cleaner inlet pipe of a Mk 2.5] can make a large difference not only to the noise recorded in db, but also in the case of the inlet, is it the kind of noise you want to hear!!.
Asymetrical blades produce fewer beat frequencies and are therefore quieter. Check out the tread blocks on your tyres, all slightly different lengths, same thing.
thanks all, I’m plumping for them being quieter then with the wonky blades… altought why that’s an issue in the first place is a mystery, perhaps in other parts of the world the fans run a bit more than they do in our summers…!!!
It’s not just on radiator fans, Eurocopter clicked onto this about 15 years ago…
Well - if we are talking age now - my 1980 series III landrover has a fan blade with assymetric blades - as do all of the 4 cylinder models (haven’t checked the 6 or 8 cylinder ones) so it has been around for longer than 15 years! (and on a landrover diesel engine I don’t think noise was necessarily an issue!)