I've got about a two acre market garden. It's on the opposite side of the road from the house and barn where the electric power is. I've got a pond at the bottom end of the garden that I irrigate out of with a gas powered 2" pump. I've been through a couple pump engines, and I don't like handling gasoline so near the water. I'd rather not go through the trouble and expense of building a generator and getting an electric pump that would work with the variable voltage. I can scavenge a lot of things locally, including black plastic water pipe. I'd rather run that than wire. What I want is a mechanical water pumping windmill. I'd pump either directly into improvised drip lines in the garden or into cisterns made from pallets, old cable, worn out carpet, and blue tarpaulin.
My plan so far is to build a moderately large blade set (~15') and put it on a relatively short tower (also about 15'). It would be pointed south-southwest, toward reliable summer sea breezes and away from storm winds. (If there is danger of high winds, the mill could have a break set and spoiler covers put over the blades. (?) ) It would not pivot with the wind direction. The blades would have a sprocket between them and the hub, which would chain drive a larger sprocket for speed reduction. (Possibly there should be another chain and sprocket set to further reduce speed and increase torque.) The large sprocket would have a stud where a rod end bearing would attach, and that rod would drive a piston or diaphragm pump, which would run whenever the wind blew. The mill would be about 300' horizontally and 20' vertically from the water.
Airfoil: A NACA 4415 or a Clark Y looks easy enough to carve. I keep my eye open though. In the book Gaviotas, they use an airfoil for an airplane flap from a NASA catalog. It has a concave lower surface, and I'm trying to figure out why that would be selected. I'm also trying to decide what angle the foil should be at for any particular percentage of the radius. I suppose this depends on tip speed to wind speed ratio. Since I want modern airfoil efficiency but also want good starting torque, I have to consider some tradeoffs here. I may be better with a larger chord size, and designed for a lower tip speed, resulting in a more angled blade set.
Regarding built up blades rather than carved ones: I could carve a leading edge and cut some ribs, glue them to the spar, and cover with thin marine plywood. There's also a middle ground I've been thinking about, where several boards are laminated into a rough blank, then carved. This would make it possible to carve relatively large blades without starting out with 4x12's. In the blade carving PDF I downloaded from somewhere around here, the blades taper quite a bit. I'm trying to decide why this is important, other than making the effective spar thicker where the sheer and bending moments are highest.
Number of blades: I had started by thinking three, but I'm not sure. Four would be easy enough to build with spars jointed as they cross each other, and might work better with a four bolt hub.
The tower will be made of scavenged pipe, braced by two struts to leeward. I'll have to find a way to weight the bottoms of the tower and struts, but I don't see that as a huge problem. I'd like to be able to take the whole thing down in the winter and store it in the barn. I'd probably also need to go up to hub height to grease things once a week or so.
All right then. Just how cracked am I?
Dan
