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Forum Index : Windmills : Blades 3 or 4 PVC or ?
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| Chris220220 Newbie  Joined: 17/02/2010 Location: AustraliaPosts: 29 |
I have my mill up and running again.(F&P) With 3 40inch blades made from PVC pipe. I have a couple of questions I hope you can all help with. Will 4 blades have more torque and start easier than 3? Does a 4 blade set up need to be better balanced ? As this is not going to be a permanent setup would wooden blades be better ? At this stage it is getting closer to being an easily portable setup. (in the boot of the camper trailer) I have been playing with a differential wiring setup.8 and 6 at present. Was 6 and 2x4 but that didn't get up to 12v very well on the VAWT config. Will go back to that or can you suggest the best 12volt wiring solution for most conditions? Sorry for so many questions. Regards Chris |
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| VK4AYQ Guru  Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Chris I have my mill up and running again.(F&P) With 3 40inch blades made from PVC pipe. I have a couple of questions I hope you can all help with. Will 4 blades have more torque and start easier than 3? Does a 4 blade set up need to be better balanced ? A: More blades will give better low speed ,whether it will start easier depends to some degree on the blade design, as you mention PVC blades it is likely they have a reduced efficiency in relation to a proper airfoil section so wont achieve a high TSR where the actual power is generated. As this is not going to be a permanent setup would wooden blades be better ? A: Wooden blades made to a proper airfoil section will perform better than a compromise airfoil set such as cut out of PVC tube, The higher TSR of a properly carved blade will generate more power across the range of RPM. At this stage it is getting closer to being an easily portable setup. (in the boot of the camper trailer) Sounds Interesting I look forward to some photos in the future. I have been playing with a differential wiring setup.8 and 6 at present. Was 6 and 2x4 but that didn't get up to 12v very well on the VAWT configeration. Will go back to that or can you suggest the best 12volt wiring solution for most conditions? A: Gordon or Qz would guide you here, I personally am going for a switch-mode regulator and leaving the windings in Star on my little one. a 60 F&P. A solar 20 amp switch-mode MPPT regulator I have tried works a treat to 12 and 24 volts, it reads the voltage itself, its a no brand one I got off ebay with a Chinese instruction sheet so slim on other details, it is rated at 15 to 100 volts DC input, it needs some high value 125 volt electrolytic caps on the input after the rectifier to work properly. Sorry for so many questions. Questions are good as we all learn from forum comments often from different points of view. All the best Bob Foolin Around |
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| Chris220220 Newbie Joined: 17/02/2010 Location: AustraliaPosts: 29 |
Thanks for the reply Bob. I have an extra pvc blade I'll try add and see how low wind power output varies. I think I'll try my hand at some wooden blades. Guess I have nothing to loose. I'm also trying to keep the cost down and would prefer not to lash out to much. A switch mode regulator sounds interesting. I'll check the threads and see what I can do. I seem to recall PC's run switch mode power supplies, maybe I can modify one. The more we play the more we learn  Regards Chris |
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MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Chris220220 I'm a bit of a mustang when it comes to blades, but one thing I can tell you is this: Two blades or four blades will yaw similarly. Windmills don't like turning about their vertical tower positions (called "yaw") with two blades. Four blades presents a similar situation and that is, the angular momentum created by spinning things, tends to like to stay in one spot. You can demonstrate this to yourself by taking the front wheel off a bicycle and holding it by the axle (using both hands), touch it to your shoulder and drag it down sharply, imparting spin. Now try to move it left and right. Feel that little "I-don't-wanna-go-there-tug"? That's angular momentum at its finest. When you have two blades and each blade is vertical when the wind changes direction and the tail spins it around back into the flow, no big deal. On the other hand, when the blades are horizontal, the angular momentum is at its steepest and it doesn't want to turn. It will turn, but it will fight it and "bump". This can be seen visually as a large wiggle or vibration and it will shake your tower. If the thing is spinning super fast, it can damage either the tower, the mount (bearing) or cause the blades to bend tempting a tower strike. Still, a two-blade mill is fast and has its place, you just want to limit the length of the blades to what your bearings can handle. If you goof it up, it'll let you know right away. As I stated, a four-bladed prop presents two of its four blades horizontally when the mill yaws nearly every time and thus puts extra strain on things. Using three blades solves 99% of these problems. There's still angular momentum, but it's off-set by there being an imbalance of forces each side of vertical and the "bump" or "wiggle" doesn't happen. Hope this helps you with your decision. If it were I, I'd go with three blades. Then again, I must admit I'm building a two-blade contraption today in my shop to run a PM generator I found in someone's trash. Go figure! Edit: Added links. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| Chris220220 Newbie Joined: 17/02/2010 Location: AustraliaPosts: 29 |
I have noticed how it yaws and how it effects the spin speed. I don't think I'll temp 4 blades in that case. Regards Chris |
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