 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Windmills : Furling - Getting It Right
| Author | Message | ||||
fillm Guru  Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi All, Over the couple of weeks I have been sorting out my furling on the dual as I get closer to geting the AXFX in the air so as both mills furl at a point where the grid inverter will not be overloaded and also to protect the mills from being destroyed in a severe blow as what happened to the " Quad Stator ". Anyway I first consulted the section on " Furling Explained " and with the caculation on force on a 3.15m dia rotor which I wanted to start furling in 10m/s. The caculation given for turbine thrust is [d x d] x [w/s x w/s] / 24 = Turbine thrust , then x offset of .1m came to 4.134kg force acting on the piviot point . Because I already have built my tails and dont really understand all the sin, cos , tan , stuff I decided to use a spring scale and alter the angle of the tail pivot untill I reached the 4 - 5 Kg , and as I can change the blade offset from 0 to 200mm it would be the 1/2 way mark if I wanted to increase or decrease the furling moment . The pics below show how I set all this up using a piece of string to act as the resistance on the tail tip ( the wind) and the spring scale pulling at the point of the rotor centre . 
I ended up with a piviot angle of 5deg to the side and 4deg back , so I put the dual back together and up and waited for the wind which I havn't had a lot of late. A couple of puffs came along and then it started furling in 12klm - 3m/s and when the wind hit the tail from the piviot side the tail would furl and then bring the blades arround , I decreased the offset to 50mm which then increased the wind speed required to furl but the tail weight acting on the blades was to light . So down it came and out with the grinder and welder and I changed the side and back angle to 10 & 8 deg which gave 12kg furl force @ 100mm offset , so its back up in the air again and hopefully right ,now but there has not been enough wind to test if it is starting to furl at 30klm or how smooth the furling action will be. I suppose the question I would like to put up is - How does this caculation I used get the force acting on a turbine so wrong ? Even the example given of a 2m dia turbine in 20m/s ( 72klm) was worked out at 66.6kg thrust , that is almost saying you could quite easily hold a 2m dia rotor in a 72klm wind , to me I think it would have a lot more force than that ? PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
||||
niall1 Senior Member  Joined: 20/11/2008 Location: IrelandPosts: 331 |
hi Phill M the figures are way above me ..  .....trying to set furling is never boring i,ll give it that...
my own mill when wired in star (early stall) had a reasonably predictable furl ...after it was changed to delta (lot faster and kind of out of stall )the furl point was later ...sometimes alarmingly later ....  niall |
||||
SparWeb Senior Member Joined: 17/04/2008 Location: CanadaPosts: 196 |
Phill, Testing things empirically like you are, you are bound to discover that reality is stranger than theory. I want to thank you first, for going to so much trouble to test and measure these quantities. To address your question directly: thrust is related not just to the size and wind speed, but also to how much power is being taken from the wind. If you think of a prop with variable pitch, there are some types that can be set to an angle where the thrust=0. Obviously they also collect virtually no power at this setting, but you will see the point that the angle of attack is playing its part. Now take it a step farther. If thrust is related to angle of attack, then it may also be relative to torque, because torque is also related to angle of attack. You may not often think of it in these terms, but angle of attack is just the inverse-tan of TSR. Blades develop lift, and the lift force is pointed mostly backward on the wind turbine blade. The force points slightly in the direction of rotation - that component creates useful torque. The other component along the axis of rotation is thrust. Changing angle of attack, you exchange one for the other. For example, the power collected by two props could be 1kW for both in the same wind, but if one is at a high TSR, then it turns faster and doesn't need as much torque to make that kilowatt of power. The other turns slower, but it's producing more torque. In that scenario, I would bet that the faster prop has a higher thrust load. So my guess is that high-TSR props have more thrust than slower ones (for the same size and power range). I hope you have overcome the misconception that just because you made a set of blades with a certain TSR in mind, that they do not actually turn at that speed... only rarely... Steven T. Fahey |
||||
| SparWeb Senior Member Joined: 17/04/2008 Location: CanadaPosts: 196 |
By the way- Why did you use such shallow angles? Steven T. Fahey |
||||
| fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi Steve , Thanks for your input on this subject , I can see what you are saying about thrust being all important when designing furling , but then why have a caculation for thrust ? There must be a way of caculating to a reasonably close number to how much force a certain dia set of blades exert in a specified wind , it might be just the divide by number needs to be worked out first. If it was 8 instead of 24 then it would have worked out as 124kg force @ 10m/s giving 12.4 kg at 100mm offset which would have been closer to the mark , so where do we get 24 from ? The reason I did the shallow angles is thats what gave 4.5Kg of force to counter balance the furl force moment from the equasion . I am trying to get softest furl I can . By having minimal offset and the lightest tail weight I can achieve with my tail design , my tail is down to 4.5 Kg and I am trying to keep the offset under 100mm because to keep these down I think will give a soft furl with minimal gyro forces on the blades/head/ tower, also I have been watching the effect larger offsets have on blade attack angle in light breeze and will start a new thread on that subject soon as it causes massive power loss in light to medium wind. PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
||||
| Janne Senior Member Joined: 20/06/2008 Location: FinlandPosts: 121 |
fillm, There are many other factors included in the calculation of thrust. Seeking force is one factor, and the way how the prop is loaded also plays a big role in the thrust the rotor is making.. These factors add to the conclusion that it's almost impossible to solve the furling with siple equations. If at first you don't succeed, try again. My projects |
||||
Bryan1 Guru Joined: 22/02/2006 Location: AustraliaPosts: 1463 |
G'day Phill, I did use the furling formula's off Glenn's website for my 2 hp motor conversion. I set the tail to furl at 20/ms and I have seen 16 amps out and off the hertz the genny was producing the genny was doing about 300 rpm, the tail didn't furl. All of a sudden the amp gauge topped ( 20 amp) and I did see the tail furl, the hertz told me around 500rpm and yea I guess the wind was near 100 K and the tail was fully furled as I hoped. Anyway mate I have the Machinery Handbook 26th edition in pdf so if you want a copy let me know as it does deal with thrust in very technical terms. Cheers Bryan  |
||||
| SparWeb Senior Member Joined: 17/04/2008 Location: CanadaPosts: 196 |
Hi Phill, Sure you can calculate something, but it takes several pages of algebra and an integration table to get the final number. I worked on this problem all spring, in fact, and I'm still not sure I'm in the ballpark. What I do have is really fancy, and it spits out lots of nice info at the bottom. After a lot of work, I still need to do validation tests (like what you are doing) to prove that the math is correct. GIGO, you know... Last night I did take a stab at making a rough "plug-and-chug" formula, but again, all I have to check it against is a bunch of other un-checked notes. By trying to skip the steps of finding angle of attack and Reynolds Number at multiple points along the span of the blade, the formula is simpler, but you might be able to fool it by putting in data out of the expected range. 
The graphic is a screen shot from MathCAD. The result jives with my more elaborate version for the data I entered. I tried playing with the diameter and wind speed, still good comparisons. I used sea level air density, 3 meter diameter blades, 40 kph wind, and a 10% rotor solidity (the blades cover 10% of the surface area of the whole circle). I also needed to consider tip speed ratio, because the rotating motion of the blades make a lot of the thrust. The thrust coefficient takes into account that the tips make more thrust than the roots. Ratio of the wind speed going in and going out is a bit of a nuance - are you familiar with this fact? To use it, you have to know this: Cp = 0, then gamma=1 (no power captured) Cp = 0.3, gamma = approx 0.5 (typical power) Cp = 0.59, gamma = 0.333 (at Betz limit) In high winds, some windmills have high TSR before furling, in others it slows or can even stall-regulate. I assumed TSR=8 as a reasonable rough number. I've seen 10 or 12 on mine. I just noticed that I went off the metric system at the end. Kilogram isn't a force, must have been a default for units of force. Hope that helps. Heck, I hope it makes sense! Steven T. Fahey |
||||
 Print this page |
 |
The Back Shed's forum code is written, and hosted, in Australia. | © JAQ Software 2025 |
