Minggu, 05 Oktober 2008

Wind Blades

Building my own blades.

Part of the allure of building a windmill is to carve the blades. The craftsman in me drools over the prospect. I love carving wood. But I didn't know where to start, so I found a web site with some books on the subject and ordered a few books. Two in particular were quite valuable. These were both by Hugh Pigott; Windpower Workshop and Brakedrum Windmill Plans. The web site I bought these from was picoturbine.com. They are also available from otherpower.com. Hugh Piggot has a very helpful web site at Scoraigh Wind Electric which includes alternator information also.

Blade materials

I used basswood (linden) for the blades. I don't believe it is the best wood to use, that would be quartersawn sitka spruce. Sitka spruce is hard to come by here in Colorado and I'm not up to any trips to the coast for a while. I'm certainly not going to buy expensive wood sight unseen. Basswood has numerous good qualities however. It is soft (easy to carve), has little difference between flatsawn and quartersawn sections, and is more available than sitka sprice. It takes finishes well and is very light in weight. It is not brittle like redwood or western red cedar and so shouldn't shatter quite as easily. I purchased three 8' planks of rough 2x6 basswood lumber with the intent of making at least 10' diameter blades. I discovered that I could stagger the blades such that I could build a smaller 7' diameter set first, just like the ones in Hugh's Brakedrum windmill book. So I chose the less desirable ends of the three planks and cut the outlines of the blades.

Templates for the Blades

I've created a .pdf file which should print out to full size templates for the blades I made using Hugh's book. These are available here. There is some chatty info I've included as well. Below is a photo of the templates in use on a blade in the carving process.

Carving the Blades

I carved the blades by hand after cutting the shape roughly on a band saw. I made a template for the overall outline, shown to the right of the photo below. The blades were first cut to this outline and then the shape and angle of the twist was drawn on each one.

Two sides of the blades shown in different stages of cutting. Note the airfoil profile on the near ends.

Three stages of carving shown from the leading edge (back side) of the blade. Note the burn marks from bandsawing the basswood. The blade needs to be sharp and with few hook teeth per inch.

The same three stages from the trailing edge of the blade, again with the back side up.

Attaching the blades together

I pretty much went by Hugh Piggott's Windpower Workshop book (pg 57) for attaching the blades to the hub. See that for a diagram. Most of the screws were applied from the back side of the hub. The weights were applied from the front side (see balancing the blades, below).

Finishing the blades

I decided to use 3 coats of oil based primer followed by 4 or more coats of oil based epoxy resin high gloss enamal. I wanted the superior adhesion of the oil based paint (over latex) and I wanted the extra hardness of the enamal. I brushed each coat heavily taking care not to get many runs. I sanded with 220 grit open coat silicon carbide sandpaper between coats. There were still brush marks on the final coats, so I may spray the last few coats to achieve the desired smoothness and mirror finish.

Balancing the Blades

While all aspects of building a windmill are important, this one is crucial. Without proper balancing, a wind machine will exhibit signs of instability, affecting performance and safety. An unbalanced blade can wear out bearings quickly, run noisily, stall out prematurely and even shatter.

There are several aspects to balancing wind generator blades. I prefer to have the rotating axis of the blades in the exact center of the circle described by the blades. This is relatively easily done by making each blade the same exact length and aligning them on the hub so that the tips of the blades are all the same distance from the center of the hub. It is necessary to measure the distance of the tips from each other, to make sure they are equidistant from the other blades as well. See Hugh Piggott's Brakedrum windmill book for a good description of this. For a hub, I followed Hugh Piggot's suggestion and sandwitch the blades between two (exterior grade) plywood circles, ~11" in diameter.

Balancing means adding weights somewhere. Initial testing showed that one of the blades was considerably heavier than the other two. This appeared to be the actual density of the wood, so I was stuck. I had to add a lot of weight to the other side(s). I decided that I didn't want to put holes in the blades themselves and so I purchased about 2 lbs of ¼" round lead fishing weight. This is like a ¼" string of lead spagetti. I drilled ¼" holes in the plywood circles and epoxied lead spagetti into the holes. In the photo below, the heavy blade is on the right. Note the numerous dots (the holes filled with lead) on the opposite side of the plywood circle. I used the setup shown below to finish the balancing. By spinning the blade and noting where it stopped, I was able to discover the heavy areas and compensate with lead on the other side.

Blades mounted in the makeshift bearing shaft (really long nail) for dynamic balancing. Note the longer basswood planks against the wall for the next set of blades. (10' diameter planned)

View of the front of the blades up, showing the flat front side, curved back side and twist.

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