Savonius Blade Size Ratio Questions
By PushnFords, Section Newbies Posted on Fri Nov 30, 2007 at 05:57:40 PM MST
Trying to power a small LED, have to have blades done in two days.

I'm part of a college team that was assigned a project wind generator to build.  The generator has to use balsa blades and a DC Hobby Motor and no specialized parts.  Tooling required is supposed to be very basic. Our project started out ok.  We made a PVC base and mounted a kit assembled gearbox to the top with 7:1 gears.  Earlier design decisions mandate a vertical axis.  Our initial design used balsa blades that were soaked and then dried while wrapped around soda cans...about the size of a half section of can.  These were mounted on wood dowels with approximately a 5-6" length.  End result was a 3 blade fan with somewhere around a 8" diameter.  In front of the box fans we had available it spun but would not light an LED.  We then changed the gearbox to a 13:1 gearset with the same blades.  In front of a box fan it would turn slowly but did produce enough power to light an LED. This week (project due next week) our instructor finished the test wind tunnel.  He is using a commercial sized 220 fan blowing into a 4.5' by 7' tall wind tunnel.  At the front of the tunnel he has placed a couple of panes of mesh which appear to smooth out the airflow. When we put our turbine in the wind tunnel it wouldn't turn at all and at this point we only have two days worth of free time to throw something together.  I'm thinking we need to make a savonius type blade setup that is bigger than our soda can example...maybe something about the size of a 2 liter bottle...will probably be limited by the size of balsa sheets available locally.  Does this sound like a good plan? How about the gear ratio?  Some of our gears were stolen by another team so we are now limited to either 6:1 or 13:1.  We are allowed to give it a boost by hand to start the initial turning? Any suggestions or specs on size ratios for the blades?  We lost most of our alloted time because it worked with our fan so we fine tuned it to that.... Thanks, Derek

Savonius Blade Size Ratio Questions | 7 comments (7 topical)

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#1) by ghurd on Fri Nov 30, 2007 at 11:29:54 AM MST

The gears are certainly causing troubles. Might try the 6:1 gears with another motor rated at a higher voltage, hopefully with a similar or lower RPM.  Radio Shack might have an 18V motor in stock, not sure of the RPMs. (a stepper would be a lot easier, if you can convince him it is DC) Make it TALL. The ratios are here... http://www.fieldlines.com/story/2004/4/20/122735/478 Also, use a Red LED and no diodes.  They light pretty easy. The VAWT was a mistake if you needed speed. G- Ghurd.info

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#2) by chris psmith on Sat Dec 01, 2007 at 02:52:40 AM MST

Have a look at the benesh photo i uploaded in 'my files', using 3 discs you can make the blade profile out of some flexible plastic sheet, the one in the photo only took about 30mins to make. What speed do you need to get the required power, the little model will easily do 120rpm and it is about 6" tall, the gearing will realy slow it down so would keep the ratio as little as possible. I have added a 4 to 1 chain gear to the larger model i have and it has a fairly drastic effect on the speed of the rotor. Using the cowl makes a stepped inmprovement in the speed. Look up us patent 4,784,568 Wind Turbine System Using A Verticle Axis Savonius-Type Rotor on Google Patent for plans. Hope it helps and good luck in the project. Regards Chris [ Parent ]

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#3) by thefinis on Sat Dec 01, 2007 at 05:32:38 AM MST

In wind power swept area is king but smaller diameter makes more rpms. For an S rotor type go bigger scoops same diameter. Try 3 liter bottles cut in half mounted on the arms. Heh heh use the balsa to create a wind funnel or shroud if design regs will allow such mods. You are only seeing wind from one direction. Drag is the enemy reduce it. I could give more details but a quick search on vawts will show what I am talking about. Good luck vawts are a funny kind of critter and it takes some real ingenuity to improve on the old designs. There are some new innovations out there but you are in the last hours so use what you can of what you have but go bigger, grab as much wind as possible and try to reduce drag on the upwind side. High gearing is no good if it slows the turbines rpms too much try the lower gears to see if it helps. Finis Texas born and bred

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#4) by PushnFords on Sat Dec 01, 2007 at 07:21:05 PM MST

Thanks for the advice...looks like we are up the preverbial creek without a paddle...maybe we should switch to water power!  lol We only had access to the wind tunnel for two days to go over our design...found out on the first day that what worked in front of the box fans we'd been using wouldn't spin in the wind tunnel.  We pulled a late night and got a simple balsa box made up for a gearbox (no direct drives allowed for the design) and tried out the two remaining sets of gears that we had.  The 6:1 gears let our existing blades spin but not fast enough to get any light.  The higher ratio lights a little if we hold a baffle in front of the blades to channel air...out of time and materials to make a true baffle at this point.  They are using solar cells to measure light output so I think we need to go with standard LEDs and not red. We had bought was was listed as a "Gears and Motors Kit" online but most of the gears were unsuitable quality or different shaft sizes so they didn't fit the motor setup they came with.  Some others were stolen by a group members roommate!  I tried to make a new blade but we had to use balsa for the blade construction and no one in town had a sheet big enough to make the blades.  They were all skinny and long...and the wood grain going the wrong direction to bend them.  Balsa blades were mandated in the assignment. Thanks again, Derek

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#5) by ghurd on Sat Dec 01, 2007 at 08:52:17 PM MST

"using solar cells to measure light output so I think we need to go with standard LEDs and not red". Meaning White?  White is a blue LED, with phosphors to add yellow. Red may do just as good. Plus... RED LED will make LIGHT well before white LEDs.  Check the Vf of each. Good luck. Ghurd.info [ Parent ]

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#6) by chris psmith on Sun Dec 02, 2007 at 02:24:48 AM MST

Why couldn't you mount the 'long and skinny' balsa sections next to each other to form larger blade? Chris [ Parent ]

Re: Savonius Blade Size Ratio Questions (3.00 / 0) (#7) by wooferhound on Mon Dec 03, 2007 at 09:23:16 AM MST

Maybe try Shrouding http://www.fieldlines.com/story/2003/6/17/73839/3388 W o o f -={(

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