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Author Topic: Help needed modifying blades.  (Read 1687 times)

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Simen

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Help needed modifying blades.
« on: March 23, 2009, 08:41:20 AM »
Had some good winds yesterday to test my 7' blades/generator described in this diary:


http://www.fieldlines.com/story/2008/11/29/235516/47


I didn't have any rpm counter, but it seems that my initial tests are close; around 13V at 180-190rpm.


It seems that the gen reached that speed in about 8m/s (ca. 18mph), which seems slow for that wind... (at that speed, i connected a (charged) battery, and got around 4A of charging at 10m/s for a few sec. (gusty winds.))


It seems that there is good torque in the blades, once cutin has been reached, but i wish for cutin in lower winds.


Looking at my blades in my diary story; anything i can do to the profile of the blades? Are they too fat overall?

« Last Edit: March 23, 2009, 08:41:20 AM by (unknown) »
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Flux

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Re: Help needed modifying blades.
« Reply #1 on: March 23, 2009, 03:31:16 AM »
Very difficult to decide from pictures. The blades do look very fat at the tip.


The thickness at the thickest point at the tips of a 7ft dia prop need not be above 1/4" yours look to be at least double that and perhaps more. The thickness can increase as you reach the root but at about 2ft radius there is no need for them to be thicker than 1/2".


from your pictures I am not sure blades are your problem. You look to have a very turbulent site and if your test tower is still at ground level as in the pictures you will have no real wind. Your winds may well be down in the 8mph region and that would bring you in the cut in wind speed region. Everything else you have done seems to be about right and your alternator cut in should be ok.


If the blades really are as thick as they look then thin them down from 1/4" at the tip down to whatever you have at the root. Mainly concentrate on reducing the thickness from 2 ft radius outwards. A bit thick near the root will make no difference.


Otherwise try to get it up on a higher test tower and check that you have it in decent wind.


Flux

« Last Edit: March 23, 2009, 03:31:16 AM by Flux »
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Simen

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Re: Help needed modifying blades.
« Reply #2 on: March 23, 2009, 06:03:08 AM »
Thanks Flux; i had a feeling they was a bit fat... ;)


Close to the hub, they're 1 3/16"; at 2' radius, they're 1", at 3': 11/16, and at the tip: 1/2" I'll take off some more... (inches for the benefit of the majority here... ;D )


As for the testsite at groundlevel... Yes, lots of turbulent winds, and hard to get steady readings and results. The windspeed was measured with the easter-egg anemometer, shown in the last pic in the story...


But every beginner testing his first windgen, are optimistic beyond real life... :D


I'm also thinking of making a new stator, with 70-80 turns of 1,4mm wire, instead of 100 turns of 1mm which i now use.

« Last Edit: March 23, 2009, 06:03:08 AM by Simen »
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Flux

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Re: Help needed modifying blades.
« Reply #3 on: March 23, 2009, 07:44:44 AM »
"I'm also thinking of making a new stator, with 70-80 turns of 1,4mm wire, instead of 100 turns of 1mm which i now use."


With the present information available that looks a backward move you will make cut in issues worse and there is no indication at present that you need to do any changes to the alternator.


If your anemometer is home made and not calibrated I suspect even more that you have a wind problem.


Flux

« Last Edit: March 23, 2009, 07:44:44 AM by Flux »
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Simen

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Re: Help needed modifying blades.
« Reply #4 on: March 23, 2009, 08:14:01 AM »
The anemometer are calibrated as good as it's possible with a homemade one, and does agree with my weatherstations anemometer.

But i do agree with you that turbulent winds are the major factor at the moment. :)


I won't insert a new stator before i see any problems; either electrical or mechanical with the current one... (There are one or two cracks in the epoxy.) I just feel that 1mm are a bit on the thin side.

A new stator with 1,4mm or 1,6mm will probably be applied together with a secondary rotor WITH magnets. :) (But then i would probably need a new set of blades too...)

« Last Edit: March 23, 2009, 08:14:01 AM by Simen »
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Janne

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Re: Help needed modifying blades.
« Reply #5 on: March 23, 2009, 09:49:31 AM »
God kveld Simen.


I'd also agree with Flux about the blades being a tad too thick. The blade's thickness should be roughly 1/8 of the chord width at any chosen spot.


I looked in your picture gallery, and it looks like your test stand doesn't allow the turbine to yaw around. In ground level with gusty wind this might be the nr. 1 cause for slow speed.


 

« Last Edit: March 23, 2009, 09:49:31 AM by Janne »
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Simen

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Re: Help needed modifying blades.
« Reply #6 on: March 23, 2009, 10:24:59 AM »
God kveld Janne. ;)


Heh, so you discovered that litle detail... ;)

The forecast was for 7-10m/s wind from due west, and that flagpole-stand are oriented so that i could just clamp down the alternatoer facing it west for the test... I haven't made any yaw, tail or anything yet; just eager to test... :)


I've heard that placing a windgen at groundlevel would be the equivalent to placing a solarpanel in the shadow... I'm starting to agree more and more. :D (but one can't blame a beginner to try for oneself... ;) )


Just let me ask a bit about some theory...

In my case; by making the blades thinner by taking away wood on the backside, the wind get a shorter route there compared to before, and therefore a bit less lift, but also less resistance and turbulence; making it a bit faster in the same amount of wind? (did this make sense? :) )

« Last Edit: March 23, 2009, 10:24:59 AM by Simen »
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I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)

Flux

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Re: Help needed modifying blades.
« Reply #7 on: March 23, 2009, 11:02:55 AM »
The thing that matters is the ratio of lift to drag.


At these low Reynolds numbers there is not a lot of difference between any of the shapes regarding lift, it depends mainly on the angle of attack. there is a lot of drag from very thick sections.


Flux

« Last Edit: March 23, 2009, 11:02:55 AM by Flux »
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Simen

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Re: Help needed modifying blades.
« Reply #8 on: October 28, 2010, 03:09:56 PM »
Now, i've Finally got around to make my blades thinner, and boy, that did a world of difference! ;D
I've thinned them down to around 6mm at the tip, and 12mm 2' from the root, and then gradually up to full thickness towards the root. I've also made the trailing edge close to razorsharp. (that is, around 0.5-1mm...)

I've again raised the gen 2m above ground ;), but this time at another site with more stable wind, and now with yaw. :) So far, it hasn't been much wind (ofcourse not; i've just put up an windgen! :D ), but now i reach cutin at 2-2.5m/s, and i've seen around 3A++ at 4-4.5m/s; this into an almost full 72Ah battery.

I'm very pleased with the results. Thank you all for help! :)

I'm also mostly done with the metalwork; a couple of welds and som sandblasting and paint remains. I also have to treat the blades with something; either oil, sealer, or epoxy...

Also just finished a simple dump-load controller based on a PicAxe 08M with an irfz48 fet. So far, it only turns on the load @ 14.50V, and off at 12,55V, but i'll be adding a temp sensor later. I'm a little bit in doubt about when to turn on and off the load; anyone got some suggestions? :-/
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I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)
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