20 Foot Page 4 - Blades
The blades for this machine will be 20' in diameter, so each blade needs to be 10' long. As machines get larger the blades turn more slowly (rpm) but the speed at the tips remains about the same. Larger machines need wider/thicker blades. I had our friend/neighbor Scott make these blades for us (he makes most of our blades now). He's got a nice wood shop and gets nice blades done quickly. The plan I gave him for these call for the blades to be 8" wide at the tips, and 18" wide at the widest point (near the root). At the hub they are 10" wide - this fits the steel hubs that will hold them together/attach them to the alternator. I gave Scott numbers for only 3 stations - the root, half the radius, and the tip. The pitch of these blades at the tip will be 3 deg. At the half way point (Radius = 5') the pitch is 6 deg and near the root we went as steep as the board would let us. The airfoil overall is about 1/8 as thick as it is wide (a bit thicker than we normally do). The thickest part of the blades (at the root where they attach) is 3.25" (the width of a planed 2x4)
Scott normally builds our blades from laminated Western Red Cedar 2x4 material. It's fairly strong, light weight and inexpensive. Finding 3 10' long boards this thick and wide without knots and suitable grain would be quite expensive and tricky.
The first step was to laminate up a 10" wide blank. Scott uses polyurethane glue for this.
Next he cuts the 120 deg angle at the root so they fit nicely together. Best to do this now before things get any heavier!
Pictured above he's laminating more wood to the trailing edge to get the full width we need.
The tape shows the actual width we'll need and demonstrates how he didn't waste any material.
Once all the laminating is done he runs the blanks through the planer to get them all uniform thickness and he can taper the thickness this way. The planer removes a lot of material especially at the tips.
Pictured above he's using the bandsaw to cut get the width of the blade right. The picture is blurry and doesn't show it well - but it's a really cool bandsaw that Scott made from scratch years ago. Much of it's made from wood.
Next he cleans up his bandsaw cut and squares up the edges with a jointer.
Now we have 3 blade blanks. The thickness and the width is properly tapered, all that's left now is lots of 'carving'. All of that needs to be done with hand power tools. (none power hand tools would work fine too but it'd take longer - a lot of wood needs to be removed here)
A 'sawzall' with a long blade does a nice job of removing quite a bit of material near the root quickly.
Another shot of the root showing where the sawzall removed material.
Lots and lots of work with a power planer.
The picture above shows the leading edge of the airfoil taking shape as he works the back side with a power planer.
The belt sander makes quick work of cleaning up marks left by the power planer. We normally make the front of the blade (the side that faces the wind) flat. Not a very high tech airfoil but it seems to work fine. So while working the front of the blade a straight edge is often used.
In the picture above he's finishing the back side with a long pneumatic sander.
Same thing on the front side - this does a nice job of keeping things flat/straight.
There they are finished up laying out on the floor of our shop. There are two steel hubs we'll use hold them together, one is laying on top of them. Beautiful Job - Scott does lots of nice wood work. Check out the Canyon Spirit Gallery to see more of his woodworking, and the beautiful/useful pottery that his wife Bonnie makes.
So now we have a 400 pound alternator (including the tail), and 3 huge blades - it's almost finished up. I'm a bit worried about the weight of these blades but I think they'll be fine. Each blade weighs about exactly 30 pounds for a total weight of 90 pounds. The steel blade hubs (30" diameter made from 1/4" steel) each weigh 30 pounds as well -so the total weight of the assembled blade will be 150 pounds.
We started this project in Dec, now its early June and we need a tower to put this on.