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Forum Index : Windmills : Offset & Tail Angle to Blade Attack Angle
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fillm Guru  Joined: 10/02/2007 Location: AustraliaPosts: 730 |
A few months back I was sitting looking at my mill not turning and thinking it normaly started in this wind , this was before I put the new PVC blades on, and it usually took 10 - 12klm to start , and had been like this for quite a while so I thought back to the last change I had made and remembered I had increased the offset to almost the max I could swing it out to 170mm approx with out changing the tail angle , so I lowered it and attached a piece of light marker tape to the tail end and raised it . The result quickly showed me why it would not start , the angle of attack of the blade face was 30 to 40 deg of the wind and the blades where not turning , basicly in a 1/2 furled position to try and start due to the force on one side of the blades not being counter balanced by the tail angle and tail surface area , I have crudely drawn an example and snaped a couple of pics of what I mean . 
So even when it did start the wind power was already gone due to the angle of attack getting worse as the force increased with the wind speed , basicly furling when you don't want it . I then lowered it and decreased the offset and kept increasing the tail angle untill I got to the point where the blades faced slightly to the opposite side to start to allow then the wind / offset / tail angle / tail surface to have the blades as square to the wind as possible in the light to moderate windspeed as shown below. 
The result was of course a massive increase in output , the other thing I startedto notice is the lack of wind behind the blades when they are getting up in Rpm and also the disturbance with even 100mm offset the tail was having trouble keeping the blades square , this then started my thinking that to have as minimum offset that I could achieve with as light and smooth furling would achieve the highest blade efficiency . It would also keep the blades as close to the wind as possible untill the furl moment starts . I would encorage you all to put a streamer on your tail end and check what is happening when it starts and as the wind increases ,. you also need to know the apparent wind with another streamer attached to a guy wire . Make sure the tail streamer can not foul the blades of course . The results may surprise you . 
PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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SparWeb Senior Member  Joined: 17/04/2008 Location: CanadaPosts: 196 |
Hi again Phill, Last summer I took lots of photos like that with streamers on the tail. The motor conversion was mounted to the tower with lots of offset, an no easy way to reduce it. Instead I set the tail to swing out farther. When running normally, the tail would be about 20 degrees from the axis of the windmill, getting the rotor within 10 degrees of the wind. With so much offset I wasn't risking messing up the furling. In strong winds below the furling speed the tail would often lift off the stop and just float around. Also, when starting in slow winds the windmill would usually look left then right before getting going! Have you observed things like this? Steven T. Fahey |
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| fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi Steve , Yes I have noticed exatcly what you have as well , this is one of the reasons that has motivated me to getting the furling right with as minimal offset as I can ,so there is minimal force trying to yaw the blades off the wind. I have noticed since I have put the PVC blades there seems to be a lot more force on the tail and I am having to increase the the tail balance angle to get the best angle of attack , the other thing would be to increase the tail surface area or lengthen the tail and reduce the furling angles to over come the additional weight to keep the offset to a minium . With so much focus being put on getting generator effiency up sometimes we loose the basic fundementals and that being , if the blades are not at the right angle to the wind the power lost is like a cloudy day on a solar panel. This is partly the reason for caculating the force on a turbine , which seems to be in the to hard basket as it seems the variables seem to be a bit beyond my capabilities in maths , thanks for your input on the furling thread . PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331 |
hi Phill i think the yaw bearing might come into play here too my own uses pipe inside pipe with grease ...simple and effective ...but the grease eventually dries out and the yaw can take on a slightly squeakly motion ...in very light winds the mill can sometimes sit a bit off waiting for an extra little gust to correct it it,s always nice to see a mill spinning up in low wind ..looks like yours will be nicely set up .... 
a bit of topic is a downwind machine , i,ve only ever seen one type (proven) ..the thing just sat there in the wind as if it was locked on .....  niall |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Niall, This downwind is the type of thing I wish to do some testing with. I have come to be fairly non-plussed with the gravity furling. It just seems too "accidental" and wastes more power than I like to admit. There must be a better way. Whiskey seems to feel these are good things, and thats enough for me to consider them carefully. .............oztules Village idiot...or... just another hack out of his depth |
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