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Forum Index : Windmills : Newbie - why 3 blades?
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| emdx Newbie  Joined: 30/11/2009 Location: AustraliaPosts: 3 |
Hi, I'm just starting out on this DIY windmill idea, so please be gentle. I've tried to google the answers but can't find them yet. I've built some little test model mills with stepper motors, with PVC and PET bottle blades, some with 2 blades, some with 3, some with 4, and some with 6, and mounted them on poles in the garden. They get plenty of wind, and it interesting to see which ones spin. What confuses me is that most modern wind-turbines I see are 3 blades, yet in my experiments, the more blades the faster it spins, and starts spinning at lower wind speed. When I drive around the country-side, all the old-time windmills for waterpumps, are 12 blades or more, and they seem to spin even in low winds. The other part is about modern 3 blade turbines all being wide at the hub and narrow at the tip, while old-time blades are narrower at the hub and wider at the tips. In my experiments, the old-style wider-at-the-tip models seems to spin the best. Can someone help me with answers to these questions? I appreciate any reply. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5123 |
More blades actually means a lower RPM. Its true a windmill with more blades looks like its spinning faster, but thats an optical illusion. My old Southern Cross with its many blades looks like its spinning much faster than the 3 blade windmill next to it, but in fact its spinning at less than half the speed. Less blades means more RPM, and more RPM means more power and a more efficient alternator. 1 or 2 blades do work, but have serious vibration problems. 3 blades has a natural balance and minimal vibration. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Perry Senior Member  Joined: 19/11/2009 Location: Posts: 190 |
Hello and welcome to the forum. It can be fun making small models and testing their performance. Just about all of us start out that way. Just as Glenn pointed out, more blades usually mean a lower speed and greater torque. That's why the 'olden' style wind mills have so many blades. They spin slowly but create a great amount of torque. Perfect for their purpose, pumping water. The many blades create a lot of drag which slows them. When you want to make electricity you need to have a fairly fast shaft speed to keep the alternator design simple. So the number of blades is dropped. Less blades means less drag so they can spin faster. There are turbines with two and even one blades but they create stability problems when yawing. Three blades, as Glenn pointed out have a neutral dynamic balance to them. You may have noticed that your tests with more blades spin faster and they may in fact do so. But I think once you make a good set of 3 blades you will see the difference. If the efficiency of the blades you have is low than more blades can perform better. Perry |
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| emdx Newbie Joined: 30/11/2009 Location: AustraliaPosts: 3 |
Thanks Glenn, so fewer blades are faster, once is starts spinning. Is it also true that with more blades, with the same swept area, that it spins in less wind? Any ideas about the narrower-to-wider (is there a proper term?) type blades? Is it that there is no difference, it's just easier and more stable to mount wider-at-the-base blades? And my other related question is, why do most boat/marine wind-turbines have 6 blades, yet land-based turbines almost always have just 3? |
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montyLalor Newbie Joined: 17/12/2008 Location: AustraliaPosts: 21 |
Commercial turbines have narrow tips with a reduced angle of attack because of the peripheral speed of said tips. Peripheral speed in layman’s terms is the speed that the very tip of the turbine’s blade is travelling at during rotation. An example just quickly: diameter = 20 metres; rotational speed = 50 rpm, therefore peripheral speed = 3141.6 metres/minute = 188.5 km/h! To have a blade with a tip of larger cord length than the root (“narrower-to-wider”) operating within the parameters of the previous example would destroy itself quite spectacularly. The wind would push the blades into the tower (if it indeed the blade’s root mountings lasted long enough to reach the inevitable over-speed condition). Longevity of a turbine’s blade requires a large majority of the wind capturing capacity to be concentrated towards the hub. This is where the strength is. Another reason for narrow tips is noise reduction. Large sums of coin are thrown towards wind tunnel development of blades to design almost drag-less aerofoils towards the tips to reduce the air disruption that generates noise. Conversely, water pumping windmills are all about torque. The bore pump that is driven by the windmill takes a lot of energy to operate. A blade design that captures the most from each breeze in a way that turns slowly enough to not over-rev the ‘mill’s gearbox and to operate the bore pump efficiently results in blades that have the “narrower-to-wider” shape. As the wind picks up, the closeness of the blades will cause a wash-out of wind capturing capacity that generally governs its own speed, keeping the whole mill relatively safe. Luke "So are you doin' this project to make us money or cost us money?" she asks again... |
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| frepdx Newbie Joined: 16/11/2009 Location: Posts: 7 |
The perfect wind turbine has an infinite number of infinitely thin blades. The more blades, the higher the efficiency you can achieve. On the other hand, fewer blades spin faster, if correctly designed, so your alternator can be smaller. But they also harness less of the available wind power. One blade with a counterweight works and is fun for testing. Two blades work fine on smaller machines, but for bigger machines a gyroscopic effect wrecks havoc and you need a Teetering hub. Three blades cancels some of those gyroscopic effects and is a good compromise for speed, efficiency and stability. But for a small machine two blades work fine too. |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
This old interview by Mother Earth News of Marcellus Jacobs (*the* small windturbine pioneer) answers all your questions, and more: http://www.motherearthnews.com/Renewable-Energy/1973-11-01/T he-Plowboy-Interview.aspx It's both enjoyable and educative reading to hear him explain what didn't work, and what did, and how he arrived at the solutions to problems. Peter. |
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| Perry Senior Member Joined: 19/11/2009 Location: Posts: 190 |
Thanks for posting that Link Dinges. What a great piece of wind history. Perry |
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oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Emdx: [quote]Thanks Glenn, so fewer blades are faster, once is starts spinning. Is it also true that with more blades, with the same swept area, that it spins in less wind? Any ideas about the narrower-to-wider (is there a proper term?) type blades? Is it that there is no difference, it's just easier and more stable to mount wider-at-the-base blades? And my other related question is, why do most boat/marine wind-turbines have 6 blades, yet land-based turbines almost always have just 3? [/quote] Firstly, we need to distinguish between what is generally thought as true.... and why it isn't really, but appears to be so 
Lets start with just the blades on a stick with frictionless bearings.... and no alternator. If we build a blade set with three blades TSR=X and put it in the air, it will spin... unloaded at about TSRx2 If we now use 6 of the same blades, it will spin at about half the speed... but with twice the torque. The extra blades are slowing things up, but show twice the surface area... more torque and slower, the blades disturb the air with twice the realestate, and only move half the distance in the same time to replace the air between blades.... ie we run into the wake of the blade in front of us twice as soon, or more practically, we go half as fast. If we make them skinnier, they take up less realestate, and can get back up to the same TSR as before. Play with this : http://www.alton-moore.net/wind_calculations.html and change the blade number but keep the TSR the same. If we replace these blades with 6 blades of a different profile that had the same TSR=X as the three blades had, they will spin at the same speed as the three blades did.... but the profile will be very much different. skinnier and thinner. It is the profile that dictates the TSR, not the blade number.... but you can see that if we just keep adding more of the same blades, we get what people generally talk about .... more means slower.......but only if we don't change the profile. There is a problem, as you add more blades, to keep the TSR the same they start getting very thin, very quickly. This makes them practically impossible to build in the real world. For a 4meter set of 3 blades and TSR of 7, the tip would be about 115mm wide and 15mm thick Half way in, it would be 220mm wide and 32mm thick.... no problems to build... for a 12 blade design of the same TSR, and same diameter, the tips would be 29mm wide and 3mm thick, and even at the halfway mark, only 50mm wide and 7mm thick. This is unbuildable..... thats why they don't build them this way... and if we could...a waste of blades for the same outcome. So the generalisation is more is slower, whilst not technically necessarily true, is usually practically so. So your second point is no. It just looks that way from your experiments. It is all to do with start up torque,..from cogging, bearing loss and iron loss. Here the extra torque generated by more of the same type of blade at rest, found it easier to supply the start up torque required to get going (extra surface area) but to keep going... it has to match the load. With little blades and iron cored gennies, it is more likely to occur with lots of torque (relatively).... so more blades. If you had a little axial flux genny, with the grease taken out of the bearings, and the seals too, you would do just as well as multiple blades...provided you wound the coils to match the prop/s. Your built in losses never allowed the low torque 3 blader to get out of stall or maybe even start. Ok, the water pumper blades. Design criteria is slow rpm, high torque.... so drag blades are best for this. Power is power, so slow speed high torque or high speed low torque... same power, different speed. The majority of power in a blade set comes from the outer 1/3, not the inner. In fact for water pumpers, they don't even bother to fill in the inner 1/3..... it's empty, as there is so little power to catch there. These are Drag style blades. Their TSR is always less than 1. They behave the same way as a spinnaker on a yacht when it is running down wind. In this case, the yacht cannot exceed the speed of the wind. If we pull in the spinnaker, and hoist the mainsail, we can run side on to the wind, and "reach", where particularly with land yachts, we can get up to very high speeds.... much faster than the wind. That is using lift from the mainsail, not drag as we were with the spinnaker. Now, if all the real power is concentrated out nearer the tips, than the inner, and we are using drag like a spinnaker.... where bigger is better, guess where you need the most "sail".... out on the ends. The boat marine thing is related to your experiments. Space necessitates a small rotor. You have found that starting small rotors is tough, because you have drag overheads that are not dissimilar to bigger machines, but with less surface area to get it going. Remember, when it is stopped, a lift airfoil has precious little torque. It is not until it is running closer to it's design speed TSR, that the power becomes appreciable. So, my guess is, as a manufacturer, you will use iron core to build it, with nice water proof bearing housings..... and this will have iron drag and bearing drag. Very tough for small high TSR blades to start. So lots of blades with a wide inner to get the thing past all the drag imposed at startup, and skinny at the tips to get a high TSR. Thats why some scream like banshees too... the high tip speed with the skinny flexing tips when they do get going. A three blader would need a much larger wind to get it going up to where it could do it's job properly. It is the static overheads that stop little blades from starting up. Bigger blade sets have roughly the same sort of overheads, but a much larger surface area to achieve startup. Once going, the three blades come into their own..... providing they are sensibly matched to the load. Does that make any sense. .............oztules Village idiot...or... just another hack out of his depth |
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Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi emdx Welcome to the forum. There is very little to add to the comments from Oztules and the others above, but here goes. One of the considerations in wind turbine design is the solidity ratio. This is simply the amount of the area swept by the turbine that is actually filled with blade. High solidity ratio equals many blades, high torque, generally low speed, low tip speed ratio (the ratio of the speed of the blade tips relative to the wind speed), and lower efficiency in terms of extracting the maximum amount of energy from the wind. Like virtually everything in life, you have to grapple with trade-offs. Low solidity equals few blades, low starting torque, higher speed, and potentially higher efficency in extracting wind power. Few blades are consequently better for power generation, while many blades are best for running reciprocating water pumps, air compressors, etc. Anything less that three blades has its problems with gyroscopic effects when it yaws with changes in wind direction. Allan Lucas, a prolific yachting author, once commented on the matter of choosing the best material for building a yacht, that it is a really a question of choosing your favourite problem. I guess that, in the matter of turbine design, the same principle applies. Regards Don B |
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| emdx Newbie Joined: 30/11/2009 Location: AustraliaPosts: 3 |
Fantastic replies from everyone, thanks so much for taking the time! It's appreciated. |
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