|Wood Wind Turbine Blades|
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blades have several distinct advantages--they're cheap, light,
efficient, and it's easy to find wood to build them from. However, they
are also pretty time consuming to make with hand tools. In my
experience, they are probbably your best bet if you are not buying
pre-made blades. |
The way I built these blades is probably a fair bit different than how others have done it. My goal was to make it as simple as possible while still using only hand tools. Each blade is cut out from a 2"X8" board that can easily be purchased at any hardware store. I used 8ft 2X8s that I would cut in half so I could use each board to make two blades. Try to find wood that is relatively free of knots and has good straight grain--this will make your life much easier. I used fir for the wood, which was what they had at the hardware store.
This design for wooden blades is not particularly optimized in terms of efficiency. It's actually designed in such a way to do the best with the constraints of working with a 2X8. This makes the design and construction a little interesting. There is a lot of thought that can go into blade construction, but I kept it simple by sticking to two rules: the thickness of the blade should be no less than 1/8 of the width of the blade, and the thickest part of the blade should be 1/3 of the distance from the leading edge to the trailing edge of the blade. This diagram shows the dimensions of the final blade. Keep this in mind while constructing each blade. The steps can get confusing, and having the big picture helps a lot!
The first step is to mark a 4ft 2X8 piece with the basic cuts you need to make.
I first like make the largest cut so that the wood is tapered.
This part gets tricky. To give the blade an angle of attack, you need to make a cut along the entire length of the blade that is at the angle of attack you want the blade to be at. I used five degrees for the angle, and a circular saw for the cut. Almost every circular saw has an angle adjustment to make cuts at angles, which is incredibly useful. If you don't have a circular saw, a normal saw could also work, but it would be a lot of work to keep the angle constant. You also need to decide what direction you want the blades to ultimately spin. The direction shouldn't really matter, as long as all of the blades are made to spin the same way. I designed mine to spin counter-clockwise, which meant that as I cut from the base of the blade to the tip, the bottom of the blade of the saw was farther to the left than the top of the blade, and I kept the right half of the board. This is probably confusing, so I hope the diagram below will help.
I made a jig using some scrap wood so that my cut would be straight. I highly reccomend a setup like this. It wasn't perfect, but it made each blade consistent, which is incredibly important.
This is what the block of wood looked like after the cut. I made the cut from the right side of the picture to the left side.
This is what the end of the block looked like. You can see a slight angle in the wood. Five degrees doesn't look like much, but it does the job. I threw away the right side of the wood (from this angle) and used the left side to make the blade. It would be nice to somehow use both sides, but the left side is too thin, and the resulting blade would spin clockwise, not counter-clockwise.
After making the cut down the blade, I made the other top-down cuts to the wood.
My circular saw did not have a deep enough blade to cut all the way through the wood at some places, so I had to use a hand saw to cut the remainder of the blade in half.This is a lot of work!
This is the blade almost completely cut in half. If the face (wind-side) of the blade is not smooth, it's important to level it out. If there are large chunks of uncut wood, a draw knife works great to remove them. Otherwise, some agressive sandpaper works great. After smoothing out the face of the blade, that side of the blade is done.
At this point, the blade has its basic shape. You just need to taper and shape the blade so it becomes lighter and more areodynamic. I like to start out by marking the thickness of the blade (1/4 of the width, remember!).
Use a planer to shave wood off of the back of the blade until it reaches the needed thickness. A power planer would work great here, but a manual planer works well too. If you don't have a planer, a draw knife will also work, but it requires a lot more care.
This is what the blade should look like after tapering with a planer. Note that the wind-side of the blade is facing down in this picture.
Next, start rounding the back of the blade to create a rough airfoil. I like to mark out in pencil where the tickest part of the blade should be (1/3 of the way back from the leading edge). Then, it's relatively easy to carve the leading edge with a draw knife, and the trailing edge with either a plane or a draw knife.
Towards the base of the blade, I gradually stop carving the blade, creating a nice smooth lead-in to the base of the blade.
The tip of the blade at this point is flat, which is unaerodynamic and not very aesthetic.
I like to round out the end, starting at about 1cm from the tip.
With the back side of the blade finished, sand down the whole blade to get rid of any bumps.
A finished blade
I connected each blade to the hub using two bolts. This worked fine, but I probably should have used larger bolts. Make sure you have washers on both sides of the hub!
I needed to make room for a large washer on the hub, so I chiseled out space. This worked surpisingly well.
The finished product.
It is unlikely that the blades are built perfectly identical, which means that they will be unbalanced while spinning. To combat this, I glued (using epoxy) washers on the lighter blades. It works very well to bolt the blades to the hub and let it rotate freely, either connected to the motor or any other bearing. That way, you can check balance by seeing if the blade assembly will naturally rotate in one direction. Tape washers on the blades, experimenting with weight and location on the blades, until all of the blades are perfectly in balance. Then, glue the washers to the blades.
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