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
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.
What wood offers the best combination of strength, light weight,
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?
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
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
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,
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.
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,
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
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.
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
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.
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
Wichita, KS USA
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
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?
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.
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.
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
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
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.
Reply to jclarke at ae tee tee global dot net
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.
Wichita, KS USA
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?)
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