As I've mentioned elsewhere around here, I want an air compressor windmill to run a pneumatic motor, which will run a pulley driven piston water pump. I already have the water pump, and I'm watching Ebay and the dump for a compressor pump and a pneumatic motor. I can fiddle with the blade/compressor speed ratio later by changing pulleys. For the moment, I'm just carving a set of blades that I hope will work. I'm useing Hugh Piggott's Windpower Workshop book as a guide, and while it gives appropriate tip speed ratios for generators and water pumps, it doesn't cover compressors.
Today I went ahead and started carving my blades. I settled on a 3 meter fan. It seems like a decent size to start with, and I had two 10' 2x6 in stock. The plan is for four blades and a 5:1 tip speed ratio. I've been looking at NACA 4415 profiles as I've been playing with the design, and shot for that, although I'm probably a bit off. After running the numbers I found I needed a 10 cm chord on the tip, expanding as I move toward the hub. But 40 cm in, the chord should be about 5.5", so after that I go with whatever I can get out of the 2x6, minding the appropriate angle of attack. Four blades mean that I carve a blade at each end of a single plank, and can check for some level of static balance. I'll put half lap joints in the middles and reinforce when I figure out what hub I'm going to put these on.
I laid out my guidelines with a metric tape, then got to carving using a power plane and belt sander. I started by removing excess material from the windward side (bottom of the airfoil), which is fairly flat. Then I turned the blade over and planed down the leeward side, starting by shooting for a 15% thickness at the tip and tapering from there, best guess, to full thickness at a .5 meter radius.
Here's a shot from where I was almost ready to switch from power planer to belt sander.
When I thought I had decent smooth profiles, I balanced the blade on a pencil on one of the saw rip fence guides, and put a 16 pound hammer as far out as I needed to on the heavier side to see how far out of balance.
Then I planed a little more on the heavy end, and sanded it smooth again. So I've got one plank carved mostly. The hammer still sits 2" to one side of the balance point, but I think a tip weight will balance that. I've got the second plank roughed out but haven't touched it with the sander yet. Once done, I'm going to have to Bondo a few knot holes.
