Monday April 24

[DanB]

Today was fun.  We had some snow last night - seemed like winter came back.  Today we had nothing terribly ambitious to do - we assembled some blades, packed a wind turbine to ship to zubbly and did some unscientific testing on a couple wind turbines to see how much force against the blades would make them furl - and then watched to see if that increased, or decreased as it furled more and more.

first we did it with a 10' machine and used a ups scale against the wall.  We set it up so that the front of the alternator would 'press' on that scale as we pushed on the tail so that it furled (pivoted and raised).  The 10' machine started to furl at about 15 pounds and that increased to about 25 pounds when it was fully furled.  

Then we tested the new 20' machine.  Amazingly - (by luck I think) it took about exactly 4x the force  (the blades are sweep exactly 4x the area).  On this one we attached a spring scale offset 11" from the yaw bearing (same as the alternator) and tied it off to the lathe with a rope.  We tried to keep the rope as square to the magnet rotors and aligned with the spindle as much as possible.  It wasn't perfect.. but close.  George pushed on the tail while I looked at the scale.  It starts furling at 60 pounds and at about half way furled (couldnt go all the way due to ceiling and stuff clearance) it was around 70 pounds.

A shot of the spring scale..

It seems to me 1KW happens around 20 pounds from watching past machines.  It would be really interesting to see a chart (I think we should be able to create one easily but maybe I have a misconception here) that related output in watts to force against the blades.  It would also be useful for thinking about tower loads.  

We did other stuff but not too many fun pictures...

I made this page today about assembling completed blades.

We also made this page about building a 7 foot diameter wind turbine like Rich's and DanF's - although I've not included details about the blades yet.  I'll add that later.

So that's about it for this monday... nothing terribly exciting but lots of fun.

[John II]

Hey... a very interesting and useful experiment ! And yes.... this data would be very useful to calculate tower wind loading as well.

Don't look like the engineers or dogs got to partake of any pizza this time around.... maybe next time ?

: )

John II

[Shadow]

I plucked this table from someones files a long time ago, I think it may have been ifreds to give credit where credit is due. I'm not sure how accurate it is but I use it for estimateing tower strength.Its amazing how the force almost doubles from 25 to 35 miles per hour.

[nanotech]

HOLY BEJEEBURS, BATMAN!!




That 20 ft machine looks like it's going to be putting a full TON of force on the tower in a good wind!!

[Gary D]

Dan B., that chart truely shows why a furling system is needed. Not only for protecting the machine, but the tower. Looks like it was a fun and interesting day up there. Might have been interesting to hang a 5 lb. sack of sugar on the tail (similar to going to 1/2 inch plywood) to see how much extra force would be needed for furling? However I think you'll get quite a bit more power out with the larger swept area without a heavier tail... Nice post! The lilac's are starting to bloom here in Pa. Don't want winter to return!  Gary D.

[jmk]

 I been wondering, with the force of the wind pushing on the blades, how can the little pieces of wood hold all that wieght? It seems to me they would break at about 50 to 100 lbs. Yet we are looking at charts that say 1000's of pounds. Is there some kind of force that helps the blades as they are turning to be able to withstand more pressure?  

[DanB]

I'd really like to get a grip on this stuff.  I question that chart...  I've seen it before and it seems (intuitively looking at machines Ive seen and towers they're perched on) like the numbers are way too high.  I also see this with furling systems.  The 10' machines we make furl pretty early... but if the chart above was true then they'd be furling in winds well below 10mph.

I've found a few pages that contradict it.  The chart talks about .0026 x A x v^2 which is a common formula.  But other places I see they're using mph (miles per hour) - not feet per second.

Here is one place: http://www.arraysolutions.com/Products/windloads.htm

Other places they talk about a flat surface, 1 square foot in area having about 1 pound of force against it in a 20 mph wind.  That would be consistant with the formula if we use mph instead of feet per second.  Another factor that the chart above doesn't address...  the blades are not a solid disk, the slow the wind down and at best they slow it down about 2/3.  My guess is that reduces the force against the blades a great deal - I expect it's less than half what it would be if the blades were a solid disk.

[DanB]

Yes - I'm sure that chart is wrong, the numbers are way too high.  He talks about using feet per second for V and then converting it to mph.  mph should be used for velocity when figuring this.  That would put a solid 10' diameter disk having pressure of about 80 pounds against it in a 20 mph wind.  The fact that blades are not disk  surely reduces that a great deal - how much would depend on the efficiency of the blades.  

[Infinity Steel]

Has anyone ever come up with  testing methods to measure forces on towers, in actual working conditions?

 If you were able to put some sort of load sensing device on guy wires-then couple it in real time recording, to a linked anemometer........you'd have  a failsafe way to figure out a bullet proof table, to end all of this speculation.............

 And we'd have a lot less towers going down.

[TomW]

IS;

Well, thank you so much for volunteering to gather this data.

T

[Titantornado]

Yea, it's called centrifugal force.  It's the same thing that keeps helicopter blades from collapsing under the weight of the machine and cargo.

[Flux]

The pressure on a disc is given by 1/2 x rho x A x V^2.

The pressure on a prop is related to power extracted and it is more like half this value.

For a 10 ft prop at 20 mph I make the thrust on a disc about 70lb ( converting back from metric)

A prop is likely to see about 35 lb.

If you measure 20 lb to start furling this is probably about right. There is a wind seeking force keeping the prop into the wind that is a variable depending on many things.

Once you furl the loading on the tower will be in the 25 to 30lb region if you furl at 20 mph.

There will also be the area of the tower added to this so it will be higher.

For machines that don't furl the loading continues to rise with wind speed and things such as the Air probably have a higher wind loading than a furling 10 ft machine in high winds.

[DanB]

Thats about what I came up with Flux using the .00256 x V^2 formula.  A 10' blade  - if it were a solid disk would come in close to 80 pounds of force against it in a 20mph wind.  I'm not sure if my thinking was right - but I figured if we're slowing the wind down by about 2/3 with spinning blades then the blades would behave about like a solid disk in a 13ish mph wind, which gives about 35 pounds of force.  Am I thinking along the right lines here?

  I expect on many of these machines that lean towards stalling as windspeed increases it's less because the blades are not slowing the wind as well as they could.  The 10' machines we make need about 20 pounds of force agaist the alternator to start furling.  I forgot when I posted about this last night that we didn't have the tail on - adding the plywood tail increased things about 5 pounds.  The larger one seems to need about 65 which I think I'll go with for now to keep things safe.

[SparWeb]

Infinity,

Your question may have been rhetorical, but:

"Has anyone ever come up with  testing methods to measure forces on towers, in actual working conditions?"

You mean the American Society of Mechanical Engineers.  They have developed codes and methods for analyzing wind loads on towers, which have evolved in lock-step with regulatory codes that define loads and conditions that towers must withstand.

Flux probably came close with his estimates, but the "extra" things that increase the load are significant, and cannot be ignored.  The 35 pounds looks like a "middle of the road" load, but a more efficient rotor would be more highly loaded than that.  Also, a tower built "just strong enough" would sway too much.

BTW, I've done the math, and the killer is the furling load, due to gyroscopic forces.  Particularly on large rotors without much taper.

[richhagen]

The blade page seemed very easy to follow.  No blank steel rotor on the front of the small machine, I'm guessing that is for simplicity of construction, it seems it would save the hassle and some of the excitement (read as hazard) of a assembling a dual rotor, but I wonder how much performance just a blank rotor with no magnets would add to the same design, adjusting the number of turns to match the change in flux?  Thanks for the well written pages.  

The load chart presented is probably a less than proposition, meaning that if your tower can withstand that loading it will be OK.  As the numbers don't appear to take into account furling or a balance of the loading.  The force acting on a blade is the integral sum of all of the forces acting on the blade surface, front and rear, of the blades in actual conditions, which would seem to me to be a very complex calculation which would have a very long list of variables.  I don't think you can base this on the power output of the unit except as a very rough approximation based upon empirical data.  It would seem to me that the amount of force required to make the machine furl would be larger than that measured in your shop because of the force of the wind on the tail attempting to straighten it out.  The net force between the wind force on the tail and the blades is what you would be measuring in the shop I believe.  Rich

[Dave B]

Dan (or anyone)

  I need your advice on my furling set up. 12' 3 blade but with only 4" offset. My boom and tail combined length is 7' with mount angle etc. same as what you use. Tail is sheet aluminum probably at about minimum area to what is suggested here. Everything works fine but I want to furl earlier (safer) before mounting way up on the tilt tower and possibly adjust later. Extending the offset is most diffacult with tail weight, size, boom length easiest to work with. I am tempted to shorten the boom maybe a foot but with the minimal offset it seems like it needs the length but maybe just lighter ? It gets confusing to me with the angles, leverage weight etc. and I thought with your testing you could offer a suggestion in the right direction. Thank you,  Dave B.

[WXYZCIENCE]

DanB, I read your page on assembly of wood blades. Well you got me working on wood blades. Just one question? In reading several articles, I ran acoss discussions on leading edge tape for protection on wooden blades. Do you add this or is it not required? Joe.  

[Infinity Steel]

So what's the multiplier you would use as a safety Factor Steven? Times 4 perhaps?

[Infinity Steel]

And yes,more drag with  more powerful magnets equal more wind resistance...........The blades shed the force slower..hmmmmm

[DanB]

Hi Steven -

I maybe wrong but if I understand it correctly - the 35 pound figure would be a nearly perfect blade extracting as much power as possible from the wind... not a 'middle of the road' figure.  I expect most homebrew machines are less.

of course vibration and oscillations in the tower would definitely call for some serious safety factor here - but as far as just the load presented to the tower by the wind I don't think its that much.

the killer furling load you talk about is also just as bad (maybe worse) in yawing - thats when blades come apart in my experience is in a fast yaw.  One reason not to have the tail too big or too close to the yaw bearing I suppose.

[Infinity Steel]

Give it time- I have some upcoming installations to do in some rather windy conditions.

(Refference my post on mounting solar panels in mechanical) I'm afraid I'll have no choice.

  http://www.fieldlines.com/story/2006/4/25/16114/0421

 And I'll be the first to admit any mistakes I make- I'm far from expert at this-yet.

 But that's why I'm here-I can theorize-you guys DO-same as me, later.

 I have no doubt that you'll be seeing me post pictures of a few crashes next winter   As well as some that keep flying.

[jmk]

 Ah okay, I see now. That explanes why those flying crain helecopter blades don't break. You sure can see them bend though!

[DanB]

Hi Joe - leading edge tape is probably a very good idea.  Ive not had problems here with the way we do things.  (A very thick coat of linseed oil which gets refreshed at least once a year)  In some more humid - or dusty places, I expect a different finish with leading edge tape would be important.  Especially if the finish is hard and brittle I think (linseed oil builds up to be soft and rubbery).  A couple machines up here we've glued/stapled copper flashing on for leading edge tape - it looks neat and should help, but again - I've seen no problems with erosion here except on 1 machine that was not well finished before it was raised and left unmaintained for a year - there was minor erosion.

[Flux]

Dave

I am not sure that a 12ft machine will furl with a 4" offset. You will be extremely lucky to get away with it.

You will certainly get to the point where it looks to be furling, with the tail at an angle but I suspect it will never escape from the prop seeking force.

5" is the absolute safe minimum. I hope it works but if it does I think it will virtually stop when it does pull away from the seeking force.

Flux

[jmk]

 Hugh puts an eight inch offset on his 12' machine. He has a differant style of a tail though.

[DanB]

Hi Flux -

what would be yourn comments about the bergey xl1.  They seem to work about the same way, but the offset on those seems very small for a 8' diameter machine.  Im thinking maybe an inch or two...

[Flux]

Yes Dan, all the Bergey machines have a small offset compared with what everyone else seems to need.

I suspect it has something to do with the pulltruded blades that they use.

I have a feeling that if you put a set of your blades on a Bergey it will never furl.

From the look of the Bergey power curve it looks to be close to the limit with their blades and when it does furl it goes almost into shut down mode with a marked drop in output power. Probably safer for the machine and tower if you can make it work reliably. That is the beauty of commercial production, once the prototype works you just keep making it. We have to get out prototype to work first time with a one off machine.

If Dave can get his blades to behave like Bergey he should be ok with the small offset but I can promise you that 2" offset didn't work on 8ft blades when I tried it and I tried a lot of tricks to make it do so before finally admitting defeat.

If you can get it past 45 deg it will work and I am convinced it has something to do with the way the blades behave at an angle to the wind and how near they are to stall at the point you try to furl it.

Flux

[SparWeb]

While it is tempting to go on about how to calculate the loads and stresses on a tower supporting a wind turbine, I have to step back and admit that I shouldn't do so publicly - there's a ring on my finger that reminds me not to give too much advice outside my field of expertise.  Rich Hagen pointed out above that there are a lot of variables, which is very true.  Any attempt to simplify the analysis to a single line of algebra will be an estimate only.  There are estimates, and then there are estimates.  Some over-estimate the number, others under-estimate it.

I want the estimate that over-estimates the loads.

I did say I've done the math, before, and it falls into the "estimate" category, and the same goes for what Dan and Flux did.  We, in fact, do get the same numbers, but via different means, and by adjusting variables that Dan and Flux didn't use, I can show that a windmill's drag could be 50% higher.

I'll never be able to design a windmill that captures all available energy within the Betz criterion through its disk, but I will design a tower that can handle it, plus the safety factor (4 sounds right, but I'd look that up, too).

On to furling...

Has anyone tried changing the angle of incidence of the tail?  The plate on the tail, whatever its shape, is generating a force to one side or the other, depending on its angle of attack.  You could change that angle of attack by, say, shimming the plate away from the boom.