Seeking Advice on Choice of Wood for Turbine Blades

I am a mechanical engineer. I am designing a wind turbine (aka windmill) have blades of an atypical design, specifically, having a large area instead of "airplane-like."
I hadn't given a thought to the supports for the blades. When getting down to it, it turns out that steel is pretty heavy and weak stuff. Really surprised me! It seems like wood might be a better choice.
Questions
What wood offers the best combination of strength, light weight, and cheapness?
Where can I find data on wood, specifically, Specific Weight and elastic strength in tension and compression?
From an old text book I found data on Red Oak and Douglas Fir. Is either of these a good choice?
Alan Smith
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So a drag, rather than a lift-driven windmill? You'll have poor to no low wind performance, and your torque curve will roll off rapidly as wind speed goes up.

Yes, wood is the material of choice for home turbine blades.

Douglass Fir is hard to beat for strength, weight, and dimensional stability.

Well, if you use an existing tested design these are moot, but for the sake of argument, I would think a forestry products lab would have information of that type. I know there's a rather complete one in Madison, Wisconsin.

I'd rather use Doug Fir, but red oak smells better when working it. A suggestion - google for "Hugh Piggott" - he has been doing home-made wind turbines for rather a long time, and has plans online as well as several well written and heavily illustrated books on the subject. He goes into all the details you'll need for blade theory, and why a lift rather than a drag system is the way to go in most wind situations.
If you're pumping water, the many-vaned drag systems you're used to seeing on farms makes sense, for generating power the 3-blade lift blades have the efficiency edge. The instructions for machining the turbine blades are very do-able in a reasonably well stocked shop (most exotic tool is a bandsaw).
Hope this helps, Dave Hinz
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wrote:

windmill)
instead
Depending upon how the word "rapidly" is defined in this case, I would agree that the torque will decrease with increasing wind speed if the blade speed increases concommitantly. However, if the blade speed remains slow, the torque, of course, will increase.
As to low wind performance, I think it will be superior to conventional "airplane blade" wind turbines. Can you cite a reference? I would be very interested in seeing it.

Really
It appears it would be superior for small commercial units, too.

I'm lucky that it appears to be readily available.

I eventually found extensive data in my Machinery's Handbook, 24th edition, p. 219

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If it behaved that way, yes.

Hugh Piggott's books have details, and I believe you can find most of them on his site. Scoraig Wind something, in Scotland. Google will find it.

Right, not much distinction really. The Piggott design I'm thinking of is in the 500W range.

Any _good_ lumberyard will have it, it's frequently used in quality (read: not truss) construction for floor joists and rafters. Lucky for us, these are in dimensions which are well suited to turbine blades.

There you go. Yours is newer than mine, but I'm guessing none of the older stuff has changed and I'm just missing newer stuff.

If you can't find his info, let me know and I'll dig a bit.
Dave
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Well, traditionally between strength, weight and cost you get to choose only two, but Spruce was/is used in a lot of applications where strength to weight ratio was important (sailboat masts, airplane parts, etc.). "Understanding Wood" by the Taunton press has the properties you are looking for.
However, I am more than a little confused by your comment that steel is heavy and weak.... It has higher tensile strength & modulus of elasticity (by 3 orders of magnitude or more) than wood.

instead
Really
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I'm fairly sure Mr. Smith was referring to the strength/weight ratio - especially for members subjected to bending stresses - which is why you rarely see steel used in aircraft structures where the part is primarily loaded in bending.
Tom Veatch Wichita, KS USA
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It does seem a little hard to believe, but Grog and the guys back in 2,000,000 BC had, in a way, materials as good as we have today.
My reference is my old materials text book Mechanics of Materials, 2nd Edition, Higdon, Ohlsen, Stiles, and Weese (c) 1967 How quickly we become a geezer.
Elastic Strength Specific Weight Material Tension (ksi) (lb/in3)
Steel 0.4% C hot-rolled 53 0.284 Douglas Fir, air dry 8.1 0.020
So, indeed, steel is almost 7 times stronger that wood, but it weighs 14 times more than wood per unit volume.
Do you have different/better data?

windmill)
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try here
http://www.otherpower.com/otherpowerfront.shtml

instead
Really
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Yup, good site. Also, Al, check out alt.energy.homepower. There are a couple of noisemaking trolls there who will immediately make themselves obvious, but they're easy to spot and the rest of the group is quite helpful and knowledgeable.
Dave Hinz
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instead
Really
You didn't reveal the shape of the blade, but given your stated desires I wouldn't use wood at all. If your an ME, you could design a light weight tube steel frame and cover it with aircraft fabric, this stuff is light and tough as hell. I was forced into covering a 1/2 scale Stearman biplane without any prior experience and the job came out fine, actually fun to do. Having made up some test panels with the fabric and doing some rigorous testing, this stuff is amazing and it has a very good lifetime in exterior apps. If your frame is designed properly you can even get some really nice compound curves. Email me off the NG if you need any more info.
Ed Angell
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windmill)
and
do.
nice
Aircraft fabric??
How expensive is it?
How do you attach it? How do you unattach it?
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Al Smith wrote:

AR400 steel has a yield strength of 140,000 PSI. I don't think there are any woods that go as high as 10,000, and the strongest woods are also quite dense.

Sitka spruce is the most commonly used wood in situations where a high strength-to-weight ratio is desired.
A couple of sources you might find useful <http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm for design of wooden structures (note--their server appears to be down at the moment for some reason) and <http://www2.fpl.fs.fed.us/TechSheets/techmenu.html for properties of commonly available commercially traded wood species.

--
--John
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Back in my young and foolish days as an Aeronautical Engineer, I was seriously considering design and construction of a "homebuilt" aircraft (Experimental, Amateur Built Category). On further consideration, there developed some serious incompatibilities between that activity and my wedded bliss. Alas, my high performance wooden homebuilt remains but a dream.
But, to the point, my investigations led me to some literature from the "Experimental Aircraft Association" (EAA) relative to the materials and techniques for construction of wooden aircraft. According to that literature, the preferred wood for aircraft structural members was one of the varieties of spruce. The precise variety is lost from my memory, but I imagine the requirements for a flight vehicle would not be unlike those of your application.
You might try a Google search for EAA or "Experimental Aircraft Association". If you can contact that organization, I'm quite sure that you can get the information, or sources for all the information you might need.
Tom Veatch Wichita, KS USA
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I'm not an engineer but what about birch? Many aircraft parts are made from birch including props and airframes. That tells me that birch is light and strong. Even the largest propeller driven aircraft ever built, Howard Hugh's Spruce Goose is really made with birch and contains no spruce. (That would make it a Birch Bitch, woulnd't it?)
Wayne
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NoOne N Particular wrote:

Generally speaking birch plywood is used for laminated parts, spruce for parts made of solid wood. Strength-to-weight in bending for Sitka spruce is about 17% higher than for birch.

--
--John
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wrote:

spruce.
us forest service

they're a bit heavy. might work though.
also look into the foam and fiberglas tech used to make surfboards.

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