Eddie Cunningham asked:
Why do wind turbines only have three blades?
Not all wind turbines do have three blades. I’ve seen some in Spain which have four and some older ones only have two. Some old-fashioned windmills have up to six or eight. Three seems to be the optimum for wind turbines. There’s a few reasons behind that. One of them is that if you have too many blades on a wind turbine each blade as it moves through the air leaves a vortex behind it. It’s very like if you look at a plane taking off you can see swirling air behind to two wigs of the plane. Wind turbine blades are very much like a plane’s wing. You get a swirl left behind a moving blade. If those interfere with one another that can cause big problems.
Eddie Cunningham asked the Naked Scientists: Why do wind turbines only have three blades? What do you think? Eddie Cunningham, Sat, 21st Jun 2008
I imagine the number of blades is the result of an optimisation process. Parameters to optimise will include:
Aerodynamics is one of the least intuitive of all bits of Engineering. Just 'looking at' a design can easily give you the wrong impression about its suitability. There are so many parameters involved and 'they' have certainly looked at all possibilities. lyner, Sun, 22nd Jun 2008
Not an expert on wind turbines but generally, fewer blades are cheaper to make and maintain. On a wind turbine however, to sink the same power, reducing the count to two blades would require each of the blades to be quite a lot longer, and to generate the same power, each blade attachment would have to be proportionally stronger, pushing the costs back up. In addition, because the blades would be longer, the tower would have to be higher, further increasing the costs. On the other hand, reducing the blade count from four to three wouldn't incur the same degree of penalty, and as the turbine has to be at a minimum height anyway, three blades seem to be optimimum, all things considered.
Do the all have only three blades? Windmills one sees on farms have more. Alan McDougall, Sun, 29th Jun 2008
Maybe the farmers did not understand aerodynamics. Atomic-S, Mon, 14th Jul 2008
Small turbines (like you get on boats) have multiple blades. (Damned noisy, tho) lyner, Tue, 15th Jul 2008
If you're thinking of the wind-pump windmills one sometimes sees on farms or in western movies, where they are more like a fan than a propeller, I think it's largely due to the blades not being aerodynamically shaped - they're just planks of wood with no aerofoil cross-section, so you need to use lots of them to make up for the inefficiency. Small turbines that you'd be likely to find on small boats can operate at much higher rotation speeds before they run the risk of shedding blades and so don't need a pitch-change mechanism, much reducing the cost per blade - when the wind gets too strong you just collapse or lower it - so you can afford to use lots of blades and reduce the diameter at the same time. LeeE, Tue, 15th Jul 2008
Why do wind turbines have three narrow blades, but ceiling fans have five wide blades?
How about using screw formed devices?
this isn't a question or an answer, but an opinion, adult to adult, ok? this is a question many people will search for and they will find this page, so i think it's important they should read an opinion that shares their skepticism, and offers an alternative view to the witewash presented here. wind farm blades are exactly like a PROPELLOR, not a turbine. it's the elephant in the room and that's why this page is here. these are covert weather weapons / defences. they are so bad at catching the wind, the operators have been caught feeding them power FROM the national grid, just so they go round and people think they're doing something. it's also tragic that clean energy projects have been compromised in the process. lazyhorse, Tue, 28th Sep 2010
Maybe in the olden days the farmers had three blades and now it's a tradition so thy don't want to change it? Ash, Thu, 22nd Sep 2011
lazyhorse is onto something. It's been discovered that the plan is to implement enough of these and run them off the electrical grid. The government would then be able to speed them up and slow them down at will. This would actually speed up and slow down the rotation of the earth and they could get rid of the daylight savings time which causes such a problem twice a year when everyone needs to convert. Not too many people know about this plan. dr_wind, Sun, 9th Oct 2011
If Three vanes reduce the length of the vane required to do the same job as two would , then why not six vanes or more ? Allowance being made for the speed for lift etc. in an aeroplane ? Would smaller, but more vanes, not be better and cause less vane fatigue, and engine percepted noise, and also reduce lift-off speed in mono mini 'planes ? Aidan Colgan, Mon, 30th Apr 2012
The section on prop walk attributes this phenomenon to the difference in water density between the top and bottom of the prop. I doubt, this, though. Water is a very incompressible fluid, so the difference in density between the top and bottom of even a very large prop would be negligible. I would say that other factors have to account for this phenomenon. Tom Einertson, Mon, 4th Jun 2012