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tecon
Starting Member
USA
36 Posts |
 Posted - 04/04/2008 : 01:42:06
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In some ways guys I have just completely confused myself, but the learning is priceless.
For the last 2 years of so of playing with my little wind gen's, I have always used 3 blades, for no other reason then "that's what everybody else uses".
I made a 4 blade version this last Sunday and immediately noticed the rotational start was at a much lower wind speed of course than my 3 blade version, not saying that it reached 12v battery charging status immediately by anymeans, just a sooner rotational start. Running only 2 test blades I would see strong voltages, but those winds required are the exception rather than the rule.
This morning I took the last of my freebie pvc pipe and cut out 6 blades, they are 18" long, coupled with the hub they are roughly 40" tip to tip. Rotational start is somewhere between 3 and 4 mph. No 12v charging at such low speeds, but juice enough for 1.2v charging for sure (I have an array of 10x1.2v batteries on the on charger now, 1 to 5 mph winds).
I have seen 13+ volts at aprox 8 to 10 mph as well this afternoon when the wind was a little stronger. This is about 2+ to 3+ mph earlier than with my 3 blades.
I guess at this point I am thinking small (at least mine) backyard gens are going to need more than 3 blades to be more effective, 4 at bare minimum, but 6 or 8 might be better?
I guess I will know more this Saturday as the winds are forecasted to be gusty. As long as the 6 blade version doesn't peak too soon, then my mind will be made up.
Tim
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MattM
New Member
USA
84 Posts |
Posted - 04/04/2008 : 04:19:17
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The general rule of thumb is more blades does start in slower wind speeds but drag will limit you at the top end.
Has anyone done any research on few long blades coupled with many short blades?
Go Huskers! |
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hillbilly
Starting Member
United Kingdom
22 Posts |
Posted - 04/04/2008 : 08:31:16
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Is it such a problem if drag limits top end performance on multiple blades, since the average wind is generally low/medium speed, surely we should aim to be able to use these winds efficiently and potentially generate more power overall than concentrate on high speed performance. Also would it not be safer if the mill was self limiting due to its poor performance at high wind speeds ?
Regards
Hillbilly |
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Victor
Junior Member
United Kingdom
357 Posts |
Posted - 04/04/2008 : 09:59:58
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Ona dynohub i have tested 2 3 and 4 blade designs.
4 blades work excellent for low start up speeds. I have to say that I noticed that if you have a low wind speed go for 4 blades + and if you have good gusts a 3 blader is good enough.
Remember more blades mean more force on your tower etc. I bent a thin aluminium pole with 4 blades on a dynohub. hope that helps you out. |
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MattM
New Member
USA
84 Posts |
Posted - 04/04/2008 : 13:50:06
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hillbilly-
I think the extra weight of the blades, coupled with the spin and the force of the wind on the blades, actually makes furling more essential in my book. The many blades gets in lockstep with the wind more readily which is probably more dangerous in the heavy stuff.
Go Huskers! |
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Tim L
Starting Member
United Kingdom
43 Posts |
Posted - 04/04/2008 : 14:49:40
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If I'm reading this right, is it the case that even if the blades are designed to self-stall and limit revs to, say, 500 or 600 RPM independent of windspeed, the force on the tower itself is not connected to RPM but instead directly related to the actual windspeed times the frontal area of the prop? It would seem to make initial sense, at first glance.
Presumably therefore, although one could sit smugly back and be reassured that the prop can't overspeed and wreck the genny bearings no matter how high the wind, the whole tower could still get pushed over in a gale, and perhaps that's the primary purpose of furling if you have self-limiting blades. |
Edited by - Tim L on 04/04/2008 15:02:06 |
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tecon
Starting Member
USA
36 Posts |
Posted - 04/04/2008 : 15:02:06
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Victor,
I have immediately noticed there is more thrust on the tower using 6 blades even in lower wind speeds, upgrade will soon follow for sure.
Matt, I was glancing at TLG stats for his mini might(?) 5 blade design (closest in size to my 6 blade rig). According to his info on that particular rig, peak rpm was reached at 70 mph (wow!) and that was 2750 rpm. On his 10 blade rig, peak rpm was aprox. 2000 rpm at 70mph.
I am wondering if too much emphasis is being placed on drag in real world winds and servo motor generators?
Best,
Tim |
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Mertz
Starting Member
USA
42 Posts |
Posted - 04/04/2008 : 20:59:50
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I am running both a 3 blade and 2 blade generators. They are both 30" long blades. The 2 blade gennie will turn at 1 to 2 mph winds. The 3 blades needs at least 5 to start spinning. The 3 blade is on the Indiana which takes more power to start. The 2 blade is on the Ametek which spins easily. The 2 blade can spin very fast in high winds. The 3 blade never spins as fast as the 2 blade. Blade profiles are almost identical using 6" pvc cut in thirds.
I will be replacing the 2 blade with a wood 3 blade 4' dia this weekend. I will report on the difference.
Mertz |
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tecon
Starting Member
USA
36 Posts |
Posted - 05/04/2008 : 01:15:45
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Mertz,
Would love to see pics of your blades that start rotation in 1-2 mph winds.
I think you have us all there, I am not even sure my daughters little plastic pinwheels on a straw turn at such low speeds.
Best,
Tim |
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MattM
New Member
USA
84 Posts |
Posted - 05/04/2008 : 02:09:29
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No kidding, tecon/Tim. He must have something special going on with those blades.
After watching Tim's two rigs this afternoon it is clear the more blades makes it start sooner. It just seems the fewer blades seems to spin faster overall. I wonder if the higher peak speed of the 10 blades has anything to do with stalling of the blades. It would make sense that as power falls off from the stall the extra blades would continue to seed enough power to continue on past the peak speed of the less blades. (Which means drag at the top end is not the issue, but rather it would be stalling.) According to his chart does the fewer blades keep ahead in overall efficiency at any speed ranges before stall/peak rotation?
Tim L-
The climbing force of the wind on the turbine is a valid reason to furl.
Go Huskers! |
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tecon
Starting Member
USA
36 Posts |
Posted - 06/04/2008 : 00:20:50
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Matt, it did indeed survive today! That is a plus 
My testing today using my Ametek 30v, both the 6 blade setup, and my 3 blade setup.
Overall the 6 blade setup exceeded 12v much more frequently, perhaps ten fold by the several numbers I jotted down than the 3 blade; the 3 blade setup would of course reach higher rpms in stronger wind than the 6 blades.
I think one would need to meter 2 exact motors set in very close proximity, one running 6 blades, one running 3 blades to guage the difference, the end result may be exactly the same.
With that said, my personal opinion, 0-15mph winds (summer?) - the 6 blades will easily out perform 3 blades, visa versa in winter months here (our best winds).
A couple other things, much concrete and sink your pole deep if using a non-furling design for the 6 blades. I had a high anxiety moment yesterday in 20 mph winds with the 6 blades on my 90v as my basketball hoop pole (weighted down with at least 500 lbs!) really seemed like it wanted to tip!
3 blades emit very little noise compared to 6 blades, perhaps noise isn't the correct word, an audible "whoosh" may be more appropriate. It doesn't exceed the noise of the wind itself, but if your are close by to the gen, you will hear it and it will make you look up.
All for now,
Best,
Tim |
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MyCattMaxx
Starting Member
USA
9 Posts |
Posted - 06/04/2008 : 01:22:54
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| You sure that noise wasn't swisssh...lol |
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tecon
Starting Member
USA
36 Posts |
Posted - 06/04/2008 : 02:17:30
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Maxx,
All I could hear in my head was Tim Allen saying "uh-oh"! 
More power! |
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MattM
New Member
USA
84 Posts |
Posted - 06/04/2008 : 03:48:35
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Tim, those metal blades should run pretty quiet. They have such a low profile they don't swoosh much.
Did you break 100W at any time today/tonight?
Go Huskers! |
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3-4 mph rotational start
