Lawn Art

[Dan 04617]

I've been collecting parts from the dump for a while now.  A boat trailer hub here, a few used 2x4 there.  Finally I just had to throw something together.  

The tower I'm using is just a piece of 1 1/2" pipe. The whole thing pivots within a pipe that someone years ago placed in the lawn, probably for stepping a flag pole into.  The mast is welded to a stub mast, which is welded to the old spring mounting bracket of the hub. The mast flexes quite a bit in the wind.  I angled the blades upward a little to keep them clear of the mast, which means my tail looks funny, but then I'm probably not keeping the tail anyway.

The blades were made by ripping 2x4 in half on a 20° angle, cutting them to 6' long, sandwiching the ends between squares of plywood, and screwing on the blade faces.  I put 2 1/2" of face sticking out in front of the spar, shooting for having the worst of the spar at about 30% of the chord.  The faces are 12x43, because I had two 48x43" pieces of plywood from the dump.  The downwind surfaces would, of course, be better if I covered them with canvas or plastic or something, to smooth the transition over the spar.  

I drilled four holes through the hub wheel rim and used long carriage bolts to attach the blades.  The tail is a piece of tubing from a trampoline frame, which fits over the stub of the axle fairly well.  I cut slots in the sides so it could flex enough to tighten down with hose clamps.  

My long term need is not electricity, but water onto my garden.  That means a high torque, low speed design.  I've got a pulley driven piston water pump.  What I'm thinking is that I could make a loop of rope, take a couple turns around the wheel that the blades are bolted to, and use it as a windlass.  I could put the pump at the bottom of the tower and have the rope fit into the v-belt pulley.  It would mean that the blades stayed pointing in one direction, probably south, into the reliable afternoon sea breezes.

Anyway, the whole thing is cheap, ugly, and big.  It's not really built to last, but I can swap out blade sets easily enough.  I'll be able to play with it to learn simple ways to transfer power off of it and into a water pump or other machine.

Dan

[ghurd]

Hi Dan,

How did you decide on 20 degrees?

Intersesting, but it looks like the blade placement on the 2x2 could act as a pseudo airfoil.  Is that the intention?

G-

[Dan 04617]

I decided on 20° based on running a number of excel spreadsheets calculating angle and chord for a few different tip speed ratios.  This was a while back and not for this particular mill, but when I went back and looked at the drawings I made based on those spreadsheets, 20° was about where I wanted to be, 20% of the way in from the tip on a 2:1 TSR blade. This is rough, but it seemed like a reasonable approximaton.

Yes, I was trying to approximate an airfoil with the spar placement.  I wonder if it would work to get some 18" flashing, attach 3" to the leading edge windward side, then bend the remaining 15" around the leeward side, fastening it somehow to the trailing edge, enveloping the spar and ending up with the blade better shaped as an airfoil.  I may have to experiment.

Dan

[ghurd]

Possibly think 2-liter soda bottle material for a test?

I believe it would help.  Maybe more if the tips were narrower and thinner than the roots.

G-

[Dan 04617]

This is a 12' diameter blade set.  I don't think soda bottles are going to help, and besides, I never touch the stuff.  I stick to beer in glass bottles, milk, or water.

Dan

[Dan 04617]

Okay, here's what would happen if I took 16" flashing, stapled 2 1/2" of it to the windward side of the leading edge, then bent the rest around and stapeled the opposite edge of the flashing to the leeward side of the trailing edge.  Except that I'm sure something better than staples would be required.  At the very least I'd need some construction adhesive at the leading edge attachement point, and maybe a bevel strip and more construction adhesive on the trailing edge.  I could even make the bevel strip so that it covered the edge of the plywood and came out to a sharp trailing edge.

Dan

[ghurd]

I didn't see it as being so large!

It would be interesting to see how much the flashing helps.  I have read flat blades do quite well, better than most people would expect.

G-