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Forum Index : Windmills : Furling design
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| maplepink Newbie  Joined: 06/02/2012 Location: PakistanPosts: 4 |
I have following parameters for my windmill (3-blades) Rotor Diameter 5.6 m Rotor Radius 2.8 m Rotor Area 24.63 m2 Offset (7 % of D) 0.392 m Angle of tail bracket from vertical 20 deg. Angle of tail bracket from the plane of the blades 55 deg. Tail Beam length 3.192 m Wind speed where furling begins 20 m/s Area of Vane (7.3 % of rotor area) 1.798 m2 Turbine Thrust 4615 N Turbine Moment 1809.08 Nm I found the tail mass of 206 kg from this formula: Tail Mass=Turbine Moment/Length/sin(20)/sin(55) I read on the forum that I should use the half of the value calculated for mass, and hence the weight will become 103 kg. which I feel is too much. I also tried to find the tail mass as Hugh suggested that first find the power available by the wind from: 1/2*rho*pi*R^2*V^3 and then thrust force by : 1/3(power available in the wind)/V and finally the turbine offset: Turbine Thrust * Offset And then I calculated the mass from: Tail Mass=Turbine Moment/Length/sin(20)/sin(55) Which gives me the tail mass of around 90 kg. Could anyone please guide me on this, and have a look at the numbers that I found for the Tail Mass and Tail arm mass? Is it ok to use 90 kg of tail mass? And which one seems for appropriate: the 103 kg or 90 kg. ? Are they ok for my 5.6 dia rotor at furling wind speed of 20 m/s to start with? Thanks |
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| SnowGhost Newbie Joined: 09/09/2010 Location: AustraliaPosts: 16 |
20 m/s is nearly 45 mph. That seems a little high to start furling. I'm not sure how much power that would generate, but the chart at http://www.otherpower.com/bottom_line.shtml goes to 26 mph and at 18 feet diameter is about 8kw of power. So at nearly twice the speed your talking nearly 8 times the power. Can you cope with 60kw of power? Do you have a dump load that can cope with that. I'd think you want to start furling at about 12 m/s (26 mph) which will get you about 3 - 3.5 kw or power. |
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| maplepink Newbie Joined: 06/02/2012 Location: PakistanPosts: 4 |
Thank you Snowghost. I believe I am not able to cope with the 60 kW of power. You are right in saying that I should start furling at around 12 m/s. Based on this speed, I have now the following parameters: Rotor Diameter 5.6 m Rotor Radius 2.8 m Rotor Area 24.63 m2 Offset (7 % of D) 0.3 m Angle of tail bracket from vertical 20 deg. Angle of tail bracket from the plane of the blades 55 deg. Tail Beam length 3.192 m Wind speed where furling begins 12 m/s Area of Vane (7.3 % of rotor area) 1.798 m2 Turbine Thrust 1652 N Turbine Moment 495.6 Nm I found the tail mass of 56.5 kg from this formula: Tail Mass=Turbine Moment/Length/sin(20)/sin(55) I read on the forum that I should use the half of the value calculated for mass, and hence the weight will become 28.2 kg. So Tail mass=28.2 kg Could you please have a look at all the numbers including the Tail Mass, and let me know what you think of these numbers based on your experience? Are the figures appropriate now for my 5.6 dia rotor at a furling wind speed of 12m/s? Thanks again. |
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| maplepink Newbie Joined: 06/02/2012 Location: PakistanPosts: 4 |
ooops sorry I want to make correction in my earlier post..., my tail beam length is 2.8 m (equal to the radius of rotor) Offset is 0.28 m (5% of D) and area of vane is 1.724 m^2 (7% of Area of rotor) I am confused about the mass of tail arm and tail vane. I want to use mild steel for tail arm, and for 2.8m length the mass of tail arm comes out to be 25 kg (i hope it is not much).Is this ok? Can anyone also help me what should be the mass of the Tail Vane? The material i want to use for the Vane is Plywood. Turbine Moment is 462.56 Nm and I found the Total tail mass of 60 kg I wanna use half of that, that is, 30 kg for Total Tail mass. Total tail mass= Tail arm mass + Tail vane mass My tail arm mass is 25 kg, so can I use 5 kg of Vane. I am so much confused. I know good furling design requires experiments, and I am just on the initial stage and want to have some good numbers to start with..Based on your experiences, what do you suggest me about the Tail arm mass and the Vane mass? Is the length of tail boom (2.8 m) also ok? Thanks, |
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yahoo2 Guru  Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
this may help you, have a look at the bottom of the page on Hugh Piggots FAQ page there is a brief explanation of some of the factors involved. Then have a look at this 17 foot otherpower build and take note of the the tail boom length and more importantly the shape of the tail in the pictures on page 5. the distance that the rotor is set forward from the tower and the vertical angle of the blades (so they don't hit the tower when they flex) will also change things. With a rotor of this size even 12 m/s is pushing the boundaries for safety. Consider how the big wind farms do it. They find the windiest spot they can, with the least turbulence then measure the wind speed and from that calculate the POWER available on that site OVER A GIVEN YEAR for every 0.5 m/s of wind speed. Then produce a graph that has a bulge at the wind speed that AVERAGES the best POWER output, it is usually around 8.3 m/s. They then design a turbine that hits its peak rpm and power output at this wind speed and control the turbine so that it does not go any faster than this in a stronger wind. 
Location and average yearly wind power should be what determines your furl point or the limitations of the generator and blades if the location is in an area with poor average wind. A larger diameter thin walled tube with a brace is the strongest and lightest boom with the weight and leverage supplied by a good sized tail. I'm confused, no wait... maybe I'm not... |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I should have posted this graph as well, it is a good illustration of how small the percentages of energy that is extracted from higher wind speeds are, you can see if the watts of potential energy are multiplied by the 24 meter rotor area, it becomes a huge number, even a tiny amount of lift in a very high wind will destroy a large turbine. 
The figures in the graphs are not accurate and are just general examples but I think it makes the point of why this is important for larger turbines. yahoo I'm confused, no wait... maybe I'm not... |
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ChrisOlson Regular Member  Joined: 19/01/2010 Location: United StatesPosts: 60 |
I have seen this same question posted on several internet forums in the last week for the same machine. With different variations in each one it seems. This is the first variation I have seen at 20 m/s.. As I have stated in a couple other forums on this same machine, and sent in a PM to this same poster, there is no way to precisely arrive at the combination that will be required with a new, unproven design except by trial and error. Calculating rotor thrust is dependent on too many variables, including power output and efficiency of the generator and air density, to arrive at any concrete figures. The only way in the end is to build it and fly it. There has been advice given on this on several forums by several people, and every time the operating parameters of the machine change with a brand set of calculations. At this point I will have to wait to see some photos of the machine to know it is actually built and has a generator in it so those design parameters are locked in. Once folks know what type of blades are being used on it, what sort of generator is used on it (and it's real-world capability), then some of the more experienced builders will be able to supply a good starting point for the furling system that is realistic. On every different forum I've read this on, I've seen different values for head offset, tail boom length, etc.. Once I see some photos of the machine so I know this stuff is welded together and it's not going to change, then I could (maybe) help. -- Chris off-grid in Northern Wisconsin, USA |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Why a person would post the same question on multipul forums with different specs and dimensions is definatly questionable . Maybe this person just wants the answer to their furling and tail design , I would also think that once up into the size he is quoting on this private build if it is at all a private , at 5.8m blade dia, this is no small turbine and other means of controll should also be needed as a precaution from a runaway or overspeed . IMO furling is a black art and there are no caculations that will give you the answer to the universe , it might point you in the right direction but the list of variables are to numerious to make it right the first time. This makes me wonder if you have done any homework PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah I agree, furling is like black magic, only trial and error will get it right. The maths will give you a ball park figure, but you will have to build it and test it out in the real world to see how it furls. A slight change in the tail angle, an extra kg here or there, the turbine dynamics at different wind speeds, etc, can all affect furling. The only way to get it right will involve spending time looking up at the windmill in different wind conditions and making little changes until your happy with the result. Also, this is a big machine, over 18 foot diameter. It might warrant some sort of active furling system or turbine blade pitch control, as a more predictable form of controlling the power. Glenn Phill you just beat me to it. Great minds and all that  The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
The thing I don't like about giving any sort of furling advice is that if this a 5.6 meter 48 volt battery charging machine the homebrew axial generators don't scale up well enough to handle the continuous output possible with that size rotor. If it is a high voltage grid-tie, or high voltage battery charging, machine then it gets less scary. But the fact remains that there is a practical limit to turbine size on low voltage battery charging. And trying to control a large turbine under those circumstances is even more of a "black art". I have already seen calculations on this proposed turbine on this forum and two others that I know beyond a shadow of a doubt, even with an inefficient, poorly designed turbine, would turn it into a flaming wreck on a 48 volt system in any real wind. Details, details. I asked for them on another forum and all that comes back is yet another set of calculations that are not even close assuming a battery charging unit. And I assume it has to be a battery charging unit because a vague "predicted output" was 5 kW at furling speed on another forum, which figures out to less than 55% gen power efficiency (which will turn into nothing a but smoke show in sustained wind). If it was a high-voltage machine it should be doing at least 50% more than that @ 80% efficiency at the gen. Details. Please supply them. Then (maybe) we can help. Otherwise there's a lot of folks that have tried to help that are wasting their time. -- Chris off-grid in Northern Wisconsin, USA |
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shawn Senior Member  Joined: 30/03/2010 Location: New ZealandPosts: 210 |
I am hopless at figures and in the end you will never get furling right the first time it flys!! this helped me (or just gave me more to play with) Its an adjustable offset, the tail can also be adjusted just have to take a picture of that for you to see will do that tomorro if you wish. 
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| ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
I am a little confused now. Is this the same machine being discussed? That looks like a 9 coil stator in the photo, which can't be for a 5.6m machine? -- Chris off-grid in Northern Wisconsin, USA |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I kinda don't mind maplepink's post, when I first saw it I thought, it's a paper turbine for a university project, a bit of googling soon turned up the other forum's posts. Everyone has got to start somewhere, every journey starts with a single step, etc,etc... Everyone who has posted replies have highlighted areas where the designer needs to improve their understanding of the concepts behind working turbines and aired a few thoughts about larger fixed pitch designs in general. Forum has done its job! Where this topic goes from here is up to the members who would like to be involved, gizmo is not holding a gun to my head to "stay on topic" and directly answer the question asked and I am not going to. I think chris's comment applies to a number of commercial turbines as well. If a potential 5 Kw turbine buyer can get a feel for the battering that a turbine needs to withstand to survive by reading this topic, then it is not a wasted effort.  I'm confused, no wait... maybe I'm not... |
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| ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
I don't mind outlining the calculations I would use as a starting point for a machine this size, as long as I know the details of it. But I have to stress that when I see calculations that will yield a dangerous machine in high winds, along with a new builder that is trying to blindly apply mathematics to something that even computer modeling can't predict, that it enters a "grey area". If it was a 10 footer the smoke trail and subsequent explosion from a runaway is not that big. With an 18+ it turns into a disaster real fast. Furling is not even a reliable method to control power on that size machine in the first place. Even Bergey knows that and they simply unload the turbine and let it go if the furling fails to hold power within reasonable limits. But the Bergey's PowerFlex rotor is designed to handle free-spin up to 119 mph wind speed. Few homebrew turbines can do that. From my standpoint, and burning up several up them myself, this one makes the alarms go off lest other people read the thread and get the mistaken idea that you can calculate furling to a precise thing. You can't. If it's an unproven design it's 2% using some basic calculations to arrive at a starting point and cutting the result in half, and 98% trial and error with your hand on the emergency brake switch the first time the wind blows until you can see what's it's going to do on the tower. -- Chris off-grid in Northern Wisconsin, USA |
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| maplepink Newbie Joined: 06/02/2012 Location: PakistanPosts: 4 |
Hello Everyone, Thank you so much for your replies. I am very sorry if my questions on furling didn't make any sense to you. I really need to study more about the furling behaviour and must do my homework first before I ask any question. Actually I am doing a project on wind turbine and it's just on paper and is not built yet. I just wanted to have inital furling design and wanted to have some values that make some sense. From your replies and from literature and books, I have started to understand better about furling. I don't have any real model,therefore, I can not compare my theoretical results with real values. But your experiences and your ideas/comments about my design would really help me to improve my design and will help me to get some sensible values as a starting point. thanks and kind regards. |
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| ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
OK, this makes a LOT more sense. You will find no correlation from theoretical results with real values. An example is my 3.2 meter machine with MPPT, where I can basically run it at any speed I want to by pushing buttons on the controller and re-programming its power curve. If I run it max 450 rpm it furls beautifully at 12.5 m/s, reaching fully furled around 15 m/s. If I let it run faster, at 500+ rpm, it makes considerably more power, and hence should be making more rotor thrust. But it refuses to furl at all at those speeds, even at 20 m/s wind speed developing grossly more power than its generator can handle continuously. That is why furling, as a method of power control, is described as more of a "black art" than science. It is not fully reliable. It is not a good method to control power to a set level because even a machine in furl can develop very high power spikes well over its acceptable power rating. All furling does is reduce the swept area of the rotor by presenting an oblique rotor disc to the wind blast. In order for it to work the rotor cannot go 90 degrees to the wind direction because it will stop making any power or thrust and simply come out of furl again and go ballistic. It is always a balance between two torque moments at the yaw shaft - one from the rotor, the other from the tail. The one from the rotor is very hard to model or calculate because wind is never a constant, and neither is the swept area once it starts to furl. If you calculate how much kinetic energy is available in the wind at normal furling speeds, then calculate how much is available in the smaller swept area of a furled rotor @ 50 m/s in a thunderstorm - it is totally different and possible to see huge levels of power that will destroy your furled turbine. In the end, I think the only real way to control power precisely is with a variable pitch governor, especially on larger machines. -- Chris off-grid in Northern Wisconsin, USA |
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| shawn Senior Member Joined: 30/03/2010 Location: New ZealandPosts: 210 |
Your comments are corect Chris, my reply I thought would be good in this thread as furling always comes up and science (somtimes) just has to many variables to work with so adding an adjustable mast offset and tail could help. My early mills I had to cut and reweld them a few times to get it right now I can play it safe. |
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| ChrisOlson Regular Member Joined: 19/01/2010 Location: United StatesPosts: 60 |
Ah, OK. When I saw that I liked it and my first reaction was, "Well, somebody knows what they're doing....."
-- Chris off-grid in Northern Wisconsin, USA |
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