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Forum Index : Windmills : Easy to make vs. theoretical blades

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imsmooth

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Joined: 07/02/2008
Location: United States
Posts: 214

Posted: 12:33am 09 Mar 2009

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I asked this at Fieldlines, but no one seems to have an answer. The question is probably best answered by someone who has made their own fiberglass blades with a true theoretical shape. It is probably best answered because I would guess these people started with the basic carved blades.

I would like to know if anyone knows if a theoretically shaped blade offers better startup and better power than a simply carved blade. The carved blades don't get as wide or deep at the root because this would be difficult and expensive to waste that much wood to get this aspect.

So, does anyone know if there is a big enough difference to make it worthwhile, or is the power gained so close to the hub negligible?

GWatPE

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Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 12:44am 09 Mar 2009

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Hi imsmooth,

generator type and torque requirements will dictate the required blade. Lower torque [AxFx type alternators] can use simpler blade shapes.

Gordon.

become more energy aware

imsmooth

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Joined: 07/02/2008
Location: United States
Posts: 214

Posted: 02:10am 09 Mar 2009

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Thanks Gordon. But does this mean that the theoretical profile offers more
torque and power than simple blade shapes? If so, how much of a
difference are we talking about?

I'm just wondering if it is worth carving some deep pitch blades with some
creative wood laminating.

oztules

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Joined: 26/07/2007
Location: Australia
Posts: 1686

Posted: 10:49am 09 Mar 2009

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Imsmooth, I wouldn't bother... even with the thickness you have now. The inner area is of virtually no use for power gathering, it is only useful for start up if you have a cogging or heavily ironed machine or geared up machine that is a little hard to start.

You have no hope of emulating the theoretical dimensions in close, so big bits of wood not helping as they should will in reality be holding things up with drag.

With an axial..... Remember, these simple blade calculators don't take into account loading..... the result is a finely tuned blade, that runs at some other TSR than it was designed for... probably right through the rpm range. All the time it is off it's theoretical optimum TSR, the big root will likely be even further off song. It may help a badly dragged down blade, but by then the efficiency is so shot to pieces, it is immaterial.

If you design a good MPPT, then it may be of more value, but in the usual stall regulated machine, I can't see it helping at all.... I mean at all. Do the sums, the inner 25% or so helps very little... even if it is perfect.... and your not gunna get that.

Further, I suspect that the straight taper is better suited to battery charging for the same reasons. With curved taper, you end up with very skinny blade tips, which are fine if their TSR is right, small=less drag... better performance... but...

Once you load these things down with an alternator capable of stall control (which most seem to build so they don't have screamers in the back yard), then the skinny chord will actually hurt production, as when you start to overload the blade tips, a little bit more surface area is more useful for torque, than a skinny chord trying to avoid drag that is not there due to slower than desirable rpm.

I think you may do better with Danb's simple blades than with your better designed ones.... just my opinion though.

This opinion is more intended for axial flux machines, as F@P and other current limited machines will be less likely to overpower the blades... but a well built axial calls all the shots, and the blades have to conform as best they can.... and in these cases, compromise blades will perform under compromising conditions probably better than a thoroughbred pulling a plow.... horses for courses I guess is what I'm getting at..

From all that you may think that getting a little bit from the center is better than nothing, but a little bit of help because the outer is not working,is like piddling into the ocean and waiting for the sealevel to rise.

If your going to drive a grid tie inverter straight, then you may have a good case for curved taper, as I assume you can program in the power curve into those gadgets. With the voltage rising with rpm, you will follow the ideal TSR a bit better, and the inner may help a little bit more then.

........oztules

Village idiot...or... just another hack out of his depth

imsmooth

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Joined: 07/02/2008
Location: United States
Posts: 214

Posted: 12:29pm 09 Mar 2009

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Thanks. I am up to epoxy coating my blades now. They seem significantly lighter than the cedar blades. I have to weigh them.

Making the second set went a lot easier than the first. I also made the tips 3.5" wide instead of 3" wide like my first set.

Thanks again, both of you for your answers.

Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 5119

Posted: 12:37pm 09 Mar 2009

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Oztules is right, the inner part of the blade really isn't that important. Its all about swept area.

Example. If you blade was 1m long, and its mounted 0.1m from the turbine center, then you have a turbine diameter of 2.2 meters. The total swept area is 3.799m2, using Pi r squared.

The first 0.1m from the center is taken up by the hub and blade mounting, and has a swept area of 0.0314m2. Thats only 1/120th of the total swept area, so not worth worrying about in terms of lost power.

Now lets look at the first 0.1m of blade beyond the hub. This area is usually called the root section. Total diameter is 0.4m ( including the hub diameter ), gives us a area of 0.1256m. Take away the hub area of 0.314m2 leaves us 0.0942m2 swept by the first 0.1 meter of blade. This is only 1/40th of the total turbine area, again not much to worry about. However this part of the blade has the most complex shape if we want to make it perfect, a lot of fabrication effort needed for a small percentage of the total swept area.

Lets look at the other end, the last 0.1m of the blade from the tip. First we work out the middle bit, 1m from center gives us a swept area of 3.14m2, we take this away from the total swept area of 3.799m2, leaves 0.659m2, or about 1/6th of the total swept area. Now this is more important.

So in this example, the outside 0.1m of our blade near the tip has almost 7 times the swept area of the inside 0.1m of our blade near the turbine center.

So dont spent too much time trying to get that complicated blade root shape correct, its just not going to make any detectable difference to you power output.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
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