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3 Meter variable pitch wind turbine project
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Mike&Penny
Ancient Yew
Joined: 11 Sep 2006
Posts: 2969
Location: Berkshire Mtns (Massachusetts USA)
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Posted: Mon Aug 24, 2009 2:07 pm Post subject:
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| justme wrote: |
Is there any mileage in spring loading the blades so they twist with the increase in load applied?
Or even having a blade material that will shape alter under load? |
The latter has been done but generally only in applications where a much smaller change in pitch is required. An airfoil structure where the center of mechanical support is at 25% chord or even closer to the leading edge will flex to higher pitch under load. Not uncommon in the design of "Wakefield" class props and very common in "indoor rubber" (to compensate for the initial power burst of the fully wound "rubber band" -- in these cases, a driven prop, the center of mechanical support is moved rearward to increase pitch under load)
As far as centrifugal controls with springs go, one possibility is the blades themselves are the weights, able to move slightly in and out but in a spiral socket so moving outward the pitch increases. But I'd go with electric controls as at any reasonably high tip speed/wind speed ratio a centrifugal control prop might not self start. BTW -- no need to measure wind speed for pitch control; measure device speed. Up to some predetermined RPM constant pitch and then above this the pitch increases rapidly (so won't ever turn much faster than that)
BTW: I would like an explanation about how the permanent magnet generators are matched to the right behavior over a significant range of power (to have output proportional to the cube of speed).
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rayman
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Joined: 06 Apr 2007
Posts: 264
Location: Rothley, Leicester, England
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Posted: Mon Aug 24, 2009 8:46 pm Post subject:
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| Mike&Penny wrote: |
| An airfoil structure where the center of mechanical support is at 25% chord or even closer to the leading edge will flex to higher pitch under load....the center of mechanical support is moved rearward to increase pitch under load) |
Where on the airfoil should I attach the shaft to support the blade so as to require the smallest force to rotate it? Is it just 50%?
| Mike&Penny wrote: |
| BTW -- no need to measure wind speed for pitch control; measure device speed. Up to some predetermined RPM constant pitch and then above this the pitch increases rapidly (so won't ever turn much faster than that) |
Thanks, that's confirmed what I thought.
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Solar array 191 Watts @ £0.88 per watt!
Making a 3 meter wind turbine.
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rayman
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Location: Rothley, Leicester, England
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Posted: Mon Aug 24, 2009 10:23 pm Post subject:
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[deleted - double post by accident]
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Solar array 191 Watts @ £0.88 per watt!
Making a 3 meter wind turbine.
Last edited by rayman on Mon Aug 24, 2009 10:24 pm; edited 1 time in total
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rayman
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Location: Rothley, Leicester, England
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Posted: Mon Aug 24, 2009 10:23 pm Post subject:
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Oh another thing.... I've got a bit further with the hub today. I've cut out two 300mm diameter discs from 6mm duralium plate (it might be just aluminium, not sure). They're to mount the bearing blocks for the blades, and all the associated parts to make them rotate. I've also machined the other side of two of the bearing blocks, still lots to do however!
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Solar array 191 Watts @ £0.88 per watt!
Making a 3 meter wind turbine.
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Mike&Penny
Ancient Yew
Joined: 11 Sep 2006
Posts: 2969
Location: Berkshire Mtns (Massachusetts USA)
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Posted: Mon Aug 24, 2009 10:45 pm Post subject:
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| rayman wrote: |
Where on the airfoil should I attach the shaft to support the blade so as to require the smallest force to rotate it? Is it just 50%?
I thought. |
That will depend on your airfoil to a certain extent but in the neighborhood of 30% of the chord behind the leading edge won't be far off. It would be a very strange airfoil that had its center of pressure as far rearward as 50%. If you want to go "by the book" rersearch the "center of lift" for your chosen airfoil.
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cadfael
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Joined: 19 Mar 2007
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Location: Noranside, Angus
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Posted: Tue Aug 25, 2009 9:16 am Post subject:
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If you get the opportunity, go gliding, it'll give you an opportunity to experience aerodynamics at first hand.
Great fun too!!
Mike.
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Mike&Penny
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Joined: 11 Sep 2006
Posts: 2969
Location: Berkshire Mtns (Massachusetts USA)
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Posted: Tue Aug 25, 2009 1:35 pm Post subject: practical
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| Mike&Penny wrote: |
That will depend on your airfoil to a certain extent but in the neighborhood of 30% of the chord behind the leading edge won't be far off. It would be a very strange airfoil that had its center of pressure as far rearward as 50%. |
Because of Cadfael's (good) advice I better add something to avoid confusion. The glider is probably operating with its overall balance point in the neighborhood of 50% of the wing chord back from the leading ledge. But that's taking into account the entire glider, not just the wing. The overall balance point has moved rearward because the horizontal stabilizer is producing some lift too and its leverage arm is long.
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rayman
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Location: Rothley, Leicester, England
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Posted: Tue Aug 25, 2009 4:56 pm Post subject:
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| cadfael wrote: |
If you get the opportunity, go gliding, it'll give you an opportunity to experience aerodynamics at first hand.
Great fun too!!
Mike. |
yes, sounds like a good excuse!
| Mike&Penny wrote: |
| Because of Cadfael's (good) advice I better add something to avoid confusion. The glider is probably operating with its overall balance point in the neighborhood of 50% of the wing chord back from the leading ledge. But that's taking into account the entire glider, not just the wing. The overall balance point has moved rearward because the horizontal stabilizer is producing some lift too and its leverage arm is long. |
I see, that makes sense. I assume by 'overall balance point' you are referring to the gliders center of mass? Please could you suggest an airfoil that would be suitable for me? I was thinking of using NACA4412.
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Solar array 191 Watts @ £0.88 per watt!
Making a 3 meter wind turbine.
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Mike&Penny
Ancient Yew
Joined: 11 Sep 2006
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Location: Berkshire Mtns (Massachusetts USA)
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Posted: Tue Aug 25, 2009 6:11 pm Post subject: Can't help
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Can't help with that (those books went in the fire). All I can give is general advice on "type" and a reminder that structural considerations play a role (thus you might use an airfoil thicker tham optimum just because that can result in a stronger prop). Hence something like that 4412.
Note that in this application the wind speed might change (gusts, reverse puffs, etc.) more rapidly than the speed of rotation will change (high angular momentum). Thus you want an airfoil that isn't overly sensitive to changes in angle of attack as the ratio of tip speed to wind speed varies momentarily. That would rule out some of the special "low drag" airfoils that carry their maximum thickness relatively far back as these tend to be very sensitive to small changes in angle of attack (they only work properly in a very narrow range).
The exact airfoil you choose won't matter nearly as much as your ability to make all blades alike.
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