Carving wind turbine blades (2.4 meters)
I can’t think of much I would rather do than carve wood. Particularly when I get to give it all these pretty curves. The sad part is that it’s the quickest step when building the wind turbine! I find that each blade takes only a few hours to carve, including the time it takes to trace out the lines, especially when doing several at once and you can get some “economies of scale”.
Laying out the shape of the blade requires lines simply traced on in pencil. I cut my blades with taper, twist, so there are several cuts to make with the hand-saw before really settling down to carving.
Here is one (below) that has been tapered in two ways. Notice how I stopped cutting at the tip, and left an un-cut block. This block is very useful when working on the other side of the blade. I can clamp it to the table and prevent it from moving as I carve there.
This tapered side was cleaned up with the chisel but not really brought to any special shape. Then it was flipped over and clamped to the table again to start on the “front” face. This side is the “bottom of the wing”, where the airfoil would be flat or even concave.
It is vitally important not to become confused about which face of the blade goes where, otherwise the blade could be carved to turn backwards relative to its mates!
With it clamped here I could start carving out the deep curve, but before I start, I grab the saw! By sawing down into the crook of the curve, I can much more quickly remove this extra material. The photos are pretty self-explanatory.
This process of shaving and filing takes the wood off quickly and in small steps, so it isn’t likely to take too much. As I go, I use the straight edge of a ruler to check that I’m not making it too round or concave. I still don’t have the trailing edge down sharp yet. Actually, I’m cutting down to a “safety line” on this side – traced 1/8″ above the “drop line” to tell me where to stop. To finish carving this side down closer to the safety line, I used a smaller spokeshave and take off much less wood per cut.
I didn’t actually finish the flat side all the way. I flipped the blade over and started working on the other, rounded side. Before carving the curvature, I had to remove the extra block. A few whacks with the chisel. Of course I split it with the grain, and nowhere near where the final contour should be. I did not risk splintering away several inches of tip. Since I hadn’t taken the flat side all the way down to the chord lines, I could still see the pencil line and watch it as I shaved the curvature.
To keep track of my progress on the curvature of the airfoil, I placed shape templates over the blade at pre-determined stations. These shapes were printed out from the computer, and the paper was glued to a stiff card, then the shaped was cut out. In the photos below you can see how the curvature gets closer and closer (light under the template), trimming here and there until the wood matches the card.
The front (flat) face still isn’t finished. I left the final cutting down to the “drop line” for last. This is done where it is the easiest – the flat side. At this point I carved slowly to avoid making mistakes, and after about 20 minutes of cut-file-sand-repeat, I had a sharp trailing edge.
Well, I thought they were perfectly identical at this point. Then I mounted them to the hub and tried to balance them.
Claims of making one windmill blade “stronger” than another are not justified unless they are built in a controlled engineering environment, where rules and procedures prevail. That is not what happens in a hobbyist’s garage. Wooden wind turbine blades have lots of structural safety margin and durability when built correctly, and there isn’t much that you can do so wrong, that it wouldn’t be obvious that there is a mistake.
That said, I’ve seen some folks get very inventive with their blades. Some have tried to make complex blades with ribs, some with fiberglass shells, or other interesting features in the name of “lightness”. What they are usually trading away is “stiffness” and “safety margin”.