Vertical Windmill Mk2
by miiiike, published
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Here's my 'Mk2' Vertical Axis Wind Turbine (VAWT). I've always liked the lines of these, but they really caught my attention a few weeks ago when I realised that I couldn't explain how they work.
I'm now a lot less confused about how a wing works (really good explanation here if you're interested: http://www.av8n.com/how/htm/airfoils.html), and I've learnt a few new tricks with the printer.
This is my second version and it can generate a much bigger turbine regardless of the printer that's used; it's a lot more efficient with filament too.
At the moment I don't have this driving anything, but the plan is to attach a small water pump (printed of course) to lift water into a gravity feed system.
update: I've added a couple of different blades sizes to suit different build volumes. If you're after something different, have a play with the SCAD file or if that's not your thing, let me know and I can generate some.
update 2: Well it's been up for a while now and I've got a better idea of how it behaves in different winds. It takes a fair bit to get it going and unless it's really blowing it doesn't spin too fast. I think the idea of driving something with it was probably a bit hopeful, but as a sculpture, I still quite like it. The next version will probably have a few more blades and a bigger diameter. All this takes is a few different settings int he scad file, but I haven't gotten around to it yet.
Changes from Mk1
You can find a copy of my first go here http://www.thingiverse.com/thing:940911.
- The blade profile has changed to a NACA0024 which is apparently better suited to a VAWT than my 'that looks about right' effort;
- The blades are hollow now. This reduces filament use (obviously) and reduces the weight in the blade which in turn reduces the 'centrifugal' force. This is a big deal as the core of the blade adds very little to its strength, but does increase the force acting on it. The other nice thing about this is that there were lots of tiny delaminations along the tail in the Mk1 which are gone now.
- The blade is now made up of 'n' sections (3 in this print) which slot together. By splitting it up like this, the blade can really be as long as you like regardless of build area.
- The first version used pins to align the two halves (didn't work all that well). This version prints a socket on one side of the join that slots into the hollow section of the other piece.
- The blade is now connected to the central shaft via 'n' struts (again, 3 in this case). One of the big problems with the first one was that the joint (weakest point) was in the middle of the blade where the force is greatest and each half was supported from it's end. This arrangement should be much stronger.
- By getting rid of the base, the diameter of the windmill can be as long the struts are. Between this and the segmented blades, you can probably make quite a big version if you're keen.
Ideas for Mk3
- A transverse web across the widest part of the blade; this will improve the strength without adding much weight.
- Apparently a 'solidity' ratio of 88% (i.e. 88% of the space is taken up by blade) is the optimum for a VAWT. This would mean blade about double the size of this one.
- I've got an idea for how to make the struts printable with a very strong connection to the blade.
There are a few push-fit's in here, so the design may take a bit of tweaking from printer to printer. I'd suggest printing two of the 'JointTest' pieces first and checking the fit before printing much more. If they're not a good fit, the .SCAD isn't too bad to modify, but if anyone would like a hand, let me know.
I found that the print quality was much better when I printed three of the segments at once. I'm assuming that a single segment doesn't get enough time for the first layer to cool before adding the second. With the large overhang this seems to have a big effect.
For the gluing I used Loctite super glue (plastic adhesive) which made a really good bond, with the plastic and the carbon rods.
Make it smooth
One of the things I learnt was how much of an effect a rough finish has on the efficiency. It's worth sanding the blade to a smooth finish; the more time you can spend on this the better.
I've made this originally to go on an 8mm straight rod, but couldn't get my hands on one so ended up with threaded rod instead.
For the Straight Rod it should be a push fit through the mount. To keep it in place I planned to use the cross-hole as a guide to drill a 3mm hole through the rod. Next insert a 30mm M3 screw and with a matching nut into the recess on the other side, alternatively drill a smaller hole and tap the M3 thread directly into the rod.
To use the threaded rod, insert an M8 nut into the recess and wind it down the rod. To keep it in place use another nut and lock the two against each other.
The best way I found to assemble it is to get the spacing of the mounts correct before trying to attach the blades. The spacings for the 3 versions I've published are:
375mm tall = 117.5mm
440mm tall = 150mm
625mm tall = 222.5mm
My carbon rods were cut from an old golf umbrella; they are 150mm long and have a 3.6mm diameter. These need to be a pretty snug fit with the sockets, so I'd suggest you print the 'JointTest' piece to make sure it's a good fit before printing the real thing. If not, you can change the parameter in the scad or just drill it out.
Please be careful with these, carbon splinters are not something you want.
It's important that there's very little difference in the lengths of the struts as it can throw off the balance of the whole windmill. Given that it's only supported from one end, I'm fairly sure it will fail in high wind if it's not balanced well.
When you insert them, do it with the axle already in and push them in as far as they go (same for the blade). This should make sure the blade has the same off-set at each strut.
I printed 2 x Bearing Mounts with 2 x roller-blade bearings in each. I used 2xM8 nuts locked against each other above the top mount and below the bottom one to keep them in place.
The holes are sized to suit a 10g screw with a 6mm washer.
Don't be afraid of tinkering with this to get the blade shape / size you want. Have a go at changing these settings first:
plug_os: this is the amount to shirk the plug in from the edge of the blade.
strut_id: inner diameter of the strut hole.
blade_height: overall height
num_segs: sets the number of segments (per blade) to print.
chord_len: length of the blade in mm.
end_os: distance from the blade tip for the top & bottom strut sockets.
num_sockets: total number of strut sockets.
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Vertical Windmill Mk2 by miiiike is licensed under the Creative Commons - Attribution - Share Alike license.
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