Dissatisfied with my first effort to create a wooden windmill propeller because it took a strong wind to get it turning and was clunky, I decided to do some research and I learned two things. For a windmill, apparently, the shape of the blade isn't all that critical because the main benefit of an efficient airfoil is reduced drag for a given amount of lift (long story). Drag increases with the square of velocity and the propeller won't turn very fast. Second, the pitch of the blades should apparently be quite low. It appears something around 10 degrees is about right.
I needn't have bothered with the research because I could have just walked into the front yard:
This little thing made of plastic and nylon fabric spins like crazy in barely perceptible winds. The blades are very thin and they are affixed at about an 8 degree angle.
At this point I had a brainstorm. The angled slots that whirligig makers use don't seem like a great idea to me, more something that takes advantage of a dado blade and a tablesaw jig. They aren't suitable for an eight-bladed propeller. I dimly recalled a windmill I made as a small child with Tinkertoys. Why not just drill holes in the hub and use a short dowel to attach the blades? I cut out a round hub with a holesaw then drilled eight evenly spaced 3/8" holes around the perimeter. Then I created a flat on a part of some short 3/8" dowels that I would use to affix the blades. Doing it this way allows you to easily attach the blades at any pitch angle you want.
This one minute video shows it turning up with a fan set on low about 6' away. Since my camera records at 30 fps and the blades appear stationary at speed, I think it is turning about 225 rpm. The main thing that appears to hold it back is that the bronze bushings are .252" and the stainless steel shaft is .242" so it starts to shimmy a bit. This is fast enough anyway.
I needn't have bothered with the research because I could have just walked into the front yard:
This little thing made of plastic and nylon fabric spins like crazy in barely perceptible winds. The blades are very thin and they are affixed at about an 8 degree angle.
At this point I had a brainstorm. The angled slots that whirligig makers use don't seem like a great idea to me, more something that takes advantage of a dado blade and a tablesaw jig. They aren't suitable for an eight-bladed propeller. I dimly recalled a windmill I made as a small child with Tinkertoys. Why not just drill holes in the hub and use a short dowel to attach the blades? I cut out a round hub with a holesaw then drilled eight evenly spaced 3/8" holes around the perimeter. Then I created a flat on a part of some short 3/8" dowels that I would use to affix the blades. Doing it this way allows you to easily attach the blades at any pitch angle you want.
I'm satisfied with this so I'm going to move on to the next step. If I were going to experiment further, I would try different pitches to determine experimentally what angle works best. I would also use my spokeshave to shape the blades into a better airfoil, which would produce modest improvement via reduced drag.
I think this method works pretty well with hand tools. It's definitely a challenge to drill the holes accurately and I might try to come up with some sort of fixture if I make more hubs. A hole saw works with a brace or an electric drill.
If you want to optimise the windmill, foils must be warped like an aircraft propeller.
ReplyDeleteThe optimum will always be for a range of combination wind speed/ windmill rotation speed giving the best "angle of attack".
What you write is correct but, from the research I found anyway, not all that critical in my application. What a rectangular foil does is increase drag because a higher angle of attack is required and for other reasons related to the pressure differential between the front and back of it. That is very important for an aircraft propeller at thousands of rpms but not for a windmill operating at 225 rpm. Mine starts readily and has a surprising amount of torque. The main problem I have is that the stainless steel bolt I used as a shaft is .242" so it chatters in the bronze bearing.
DeleteTinkertoys were a blast from the past. Made me think too of my erector set. I like the simplicity of the blade set up. Very clever how you solved that. Are variable pitch blades next?
ReplyDelete