Author Topic: Calculating furling  (Read 5759 times)

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poikkis

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Calculating furling
« on: December 21, 2008, 03:34:33 PM »
Hi,


I have tryed to make some sense on my calculations about tail furling.

I think i allmost got it, but i'm not absolutely sure.

I builded one webpage about the furling calculations where i can make some quick tests.


Could someone how knows better, check this page and comment are there

any sense on it´s calculations,please.

http://www.saunalahti.fi/~jaripoi/furling.php


Oh yes, and then there is this thing, wich i don´t understand at all.

Dosen´t the tail fysicall size mean nothing when furling point is calculated.?

Least i didn´t find any formula for it.

So it dosen't matter if the tail size is 1m2 or 5m2 ??


Thanks

Poikkis

« Last Edit: December 21, 2008, 03:34:33 PM by (unknown) »

scoraigwind

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Re: Calculating furling
« Reply #1 on: December 21, 2008, 03:39:01 PM »
A couple of comments.


You can calculate the theoretical behaviour of the tail, but the reality is a little different.


You need a large enough offset or it will nt furl at all when the wind gets stronger.  The centre of pressure on the blade moves off centre when running in yaw.  so allow a big offset (150 mm for 3 m diameter).


You can figure out the thrust of the wind and the weights and angles, but I recommend you allow a reduction factor afterwards because the force ends up less than you would expect due to the movement of the centre of pressure toward the yaw bearing.  We also say that the blades try to 'seek the wind'.


As for the size of the tail, that length boom and the area of the vane should be as big as possible.  To be honest they are often smaller than they ought to be and the turbine runs a bit further off the wind than it ought to in low winds, because the tail does not have enough grip.  But it also needs to be strong, and also the right moment of  weight so those are separate considerations you need to work around.


The area is what holds it to face low winds.  Choosing the correct weight makes it turn off at the right time in stronger winds.

« Last Edit: December 21, 2008, 03:39:01 PM by scoraigwind »
Hugh Piggott scoraigwind.co.uk

oztules

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Re: Calculating furling
« Reply #2 on: December 21, 2008, 04:12:57 PM »
Thanks for this little gem Hugh:


 "You need a large enough offset or it will nt furl at all when the wind gets stronger.  The centre of pressure on the blade moves off centre when running in yaw."


I seem to have missed this completely up till now. Obvious as the nose on your face... but I missed it.... Now it makes more sense to me.


Thank you


oztules

« Last Edit: December 21, 2008, 04:12:57 PM by oztules »
Flinders Island Australia

SparWeb

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Re: Calculating furling
« Reply #3 on: December 21, 2008, 09:58:02 PM »
Try this and tell me what you think:


http://www.sparweb.ca/Wind_Turbine_Comparison.htm


It is a web-enabled Excel table, so if you don't have Excel, it may not work, but I could enter the numbers into the table myself and print the results for you.


The static format is on this page:


http://www.sparweb.ca/Forum/Wind%20Turbine%20Comparison%20Table.html


Sorry it's not a "cookbook" tail sizing & design spreadsheet, but comparing your windmill size and proportions to others could help.


On my first pass through your spreadsheet, I think that your estimate of rotor thrust is probably based on the total wind power available in the rotor disk, not the portion that is actually being captured by the rotor itself.  Since the rotor can only harness a part of the wind's power (ie. Betz) then only that part will create thrust.


Did you perhaps use some "aircraft" assumptions in your analysis?  I can't reverse-engineer your numbers, unless I use a "85%" prop efficiency factor, which an aerodynamics student would use for aircraft propeller thrust.  You can drop that 85% figure for now.


Like Hugh suggested, use a fraction like 1/3 of the wind's total power, divide by wind speed, collect units, and you'll have a good enough estimate of thrust for your design.


After that, I think your math adds up.  Good work!

« Last Edit: December 21, 2008, 09:58:02 PM by SparWeb »
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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poikkis

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Re: Calculating furling
« Reply #4 on: December 21, 2008, 11:18:57 PM »
Hi,


Thanks for your comments.


And no. I don´t use any of my own calculations.

(I have no idea and my math really sucks, lol)


I have just tryed to put some numbers together to myself, based on this page.

http://www.thebackshed.com/windmill/Docs/Furling.asp to help


I have only check the calculations againts this sample on that page.


So for our windmill to start furling at 72kmh, it needs a tail pivot angle of 20, a tail lenght of 1.378m and weight of 20kg.


It´s seems to add up, least one those numbers, and gives me some basic info

about the forces wich are playing along there.


Yes i have an excel, but the first link dosen't work for me. Either Firefox or IE7

The second one is ok.Very nice, now i have some data where to compare.


regards.

Poikkis

« Last Edit: December 21, 2008, 11:18:57 PM by poikkis »

scoraigwind

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Re: Calculating furling
« Reply #5 on: December 22, 2008, 12:18:05 AM »
The thrust is about 4/9 x density of air x rotor area x windspeed squared. (metric units metres and metres per second.)  I can't deal with km/hour.  The force there will be in Newton, so you need to divide by ten to get kg.


force in kg = (rotor diameter squared) x (windspeed in m/s squared) /24


You can compute the effect of the wind thrust trying to to turn the blades away (torque around the yaw axis) by multiplying the thrust by the offset distance.


Compare this with the maximum restoring force the tail can exert.  This is found by multiplying the moment of weight of the tail by the tangent of the angle of inclination (off vertical).  Moment of weight is the kg (or Newtons) weight times the distance of the centre of gravity from the hinge.  Or you can weigh the tip while the other end is supported by the hinge, and then use this tip weight times the total length of the tail.


I usually find that i get good results if the tail restoring moment is about 70% of the theoretical value needed to hold the machine to face the wind.  But there are a lot of factors (size of offset, size of overhang into the wind, tip speed ratio, etc..)


Also I use the maximum angle of inclination but you also need to think about how much side angle and how much back angle you have.  If you want to play safe in strong winds, then use a small amount of back angle (like a bergey) and a large forward overhang into the wind.  The only thing is that this tends to be unstable.  And you don't get much power in strong winds, which gets annoying after a while.

« Last Edit: December 22, 2008, 12:18:05 AM by scoraigwind »
Hugh Piggott scoraigwind.co.uk

Flux

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Re: Calculating furling
« Reply #6 on: December 22, 2008, 01:03:05 AM »
I agree that there are too many factors in this to make calculation worth the effort.


Hugh's value of thrust seems to be in agreement from what I have measured on guy wire loading, I found it to be a bit less, but near enough.


On the machine that drove me to distraction when the offset was too small, I did some tests after I got it furling and found that the seeking force was over 1/2 of the thrust so in my case perhaps 50% rather than 70%.


It seems to me that if the offset is on the small side then the seeking force holds it into the wind until the prop is at about 45 deg to the wind and then it looses seeking force and the thrust takes full control. The result is as Hugh described for the Bergey, it is virtually bistable and the output drops rapidly as it furls beyond 45 deg.


If you make the offset large then the power stays much more constant but you then have a much bigger issue with it running at an angle to the wind below furling. ( It does that a lot more than most realise anyway).


From reading comments here for years I also believe the load matching is a big factor and what happens for mine with the things running clear of stall doesn't seem to tie up with experiences here.


My only conclusion is that you need plenty of offset ( the 1/2" per foot looks marginal for this stalled operating) and you need to start with a nice light tail.


Never convince yourself that it is furling until you see the tail round to at least 60deg, otherwise you might get a rude awakening in the first real storm.


Maths has its uses but this case is more of an art than a science as far as I can tell.


Flux

« Last Edit: December 22, 2008, 01:03:05 AM by Flux »

Flux

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Re: Calculating furling
« Reply #7 on: December 22, 2008, 01:13:36 AM »
Just one more thing I forgot to mention.


I have never been sure what effect the overhang of the alternator has but I have now more or less come to the conclusion that Hugh is right in saying that it helps to make the furling safe. Moving the alternator forward gives more clearance from the tower and I am fairly sure it may help some of the machines mentioned here where the power still continues to increase as the thing goes well into furling.


I have never seen a case where the power continues to rise as it goes into hard furling. All mine show the tendency to drop power as they furl. If that effect is great then it does become irritating to see it producing very little in a high wind but if you see some drop in power then you have some reassurance that it is really furling and in high winds the duty cycle becomes much higher so for cooling alone I wouldn't want the power to keep rising.


Flux

« Last Edit: December 22, 2008, 01:13:36 AM by Flux »

SparWeb

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Re: Calculating furling
« Reply #8 on: December 22, 2008, 12:39:41 PM »
Something important that I should add to my earlier posting:


The wind turbines shown in the list above have different furling characteristics.  If you look through the links at the bottom of that webpage, you'll find the discussions that set off the process of making that table.  I'm currently very satisfied with the furling behaviour of my windmill, but it comes after over a year of trying this, trying that, gradually adding more weight to increase the speed.  I started slow and boy that was a good idea!  


Others with whom I compare, like Dan and Janne, are seeing furling that isn't complete until very high wind speeds, and they also suspect that it "comes out" of furling when the wind speed increases even further.

« Last Edit: December 22, 2008, 12:39:41 PM by SparWeb »
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

poikkis

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Re: Calculating furling
« Reply #9 on: December 22, 2008, 01:45:07 PM »
>they also suspect that it "comes out" of furling when the wind speed increases even further.


Sorry i´m not understanding what you mean by this.Could you clarify, please.


Regards

Poikkis

« Last Edit: December 22, 2008, 01:45:07 PM by poikkis »

poikkis

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Re: Calculating furling
« Reply #10 on: December 22, 2008, 01:59:46 PM »
Oh and then there is also this question:


How does the generator loading effect the tail furling. ?

Will the tail also furl without the load if that ever happens. (hope not)


I still don´t quite understand the all the forces wich affect the furling.


btw.

I also can´t figure, how to edit my older posts. Can someone help, please.

« Last Edit: December 22, 2008, 01:59:46 PM by poikkis »

scoraigwind

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Re: Calculating furling
« Reply #11 on: December 23, 2008, 12:59:23 AM »
If a turbine begins to overspeed then it can acquire a very strong wind-seeking tendency and this can overcome the furling force.  Typically this happens in stronger winds and gives the turbine a 'kama kazi' mode that is unexpected.  The best remedies that I know for this are to make sure you have a big enough offset and to increase the forward overhang.  Both of these help as insurance against the shift of the centre of lift toward the windward side of the swept disk.  A heavily loaded rotor in yaw to the wind acts like a wing, and the centre of lift moves toward its leading edge.


Furling is a bit of a black art, but in most cases I can now predict the way the tail will work and get it to furl correctly first time.  I guess this is based on following certain rules about minimum offsets that work.  It's always better to start off a bit cautious and to add more weight later, if you are experimenting.  Either that or use the brake switch in the beginning if you need to.

« Last Edit: December 23, 2008, 12:59:23 AM by scoraigwind »
Hugh Piggott scoraigwind.co.uk