| T O P I C R E V I E W |
| chris_psmith |
Posted - 05/03/2008 : 20:22:13
Starting to get the bits together for my new VAWT but mulling around ideas for the best way to make the blades.
The blades need to be 2m long (3 off)with the profile shown below 23cm wide.
Thoughts are:
Wood - would take ages, be difficult to get exact and thin trailing edge would be fragile, would be heavy as well.
Aluminium - As in Lenz2 design with shape cut from wood with stringers and 0.6mm ally bent round it - Would be quite fragile, stringers would be difficult again due to thin section.
Polystyrene + Fiberglass - cut shape from polystyrene (thinking about feeding through hot wire of correct shape) and cover in fiberglass - hmm, probably harder than it sounds.
Extrusion,- wouldn't that be perfect, but cost a fortune for 3 blades.
Am I missing the obvious answer, thoughts please.
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| 15 L A T E S T R E P L I E S (Newest First) |
| gholt |
Posted - 31/03/2008 : 19:07:46
Here are some links about foams
http://www.favonius.com/soaring/foams/foams.htm
http://building.dow.com/styrofoam/na/res-us/
http://radiocontrol.wikia.com/wiki/Foam
http://www.clarkfoam.net/prods.asp?kc=ha1Lo
kr2bldr is a KR2 (foam/fiberglass aircraft)builder. His KR2 video -"kr gathering 2005" - can be found here: http://www.youtube.com/watch?v=E-jrdHA1MmE
Chris,
You can deal with the "wow" in the trailing edges by extending your fiberglass top and bottom past the trailing edge, then trimming to fit your cord length. Your blades are not an aircraft wing, and thus should be a tad more forgiving, less critical. MattM offers a very astute observation about the need for wood inserts aka "hard points" in the foam so you'll have something to attach to. Wood splines are often used to act as a backbone of sorts in certain applications - surfboards are a common example.
G.
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| MattM |
Posted - 31/03/2008 : 04:47:42
Is that last post a spam bot? I shiver to think what would happen if I follow a link after a brevis post like that. Perhaps he/she was responding somewhere else and accidentily left it here?
chris_psmith-
Polystyrene is available in a closed foam, too. You've probably seen the pink sheets of foam at the lumber yard before. At least around here it is normally pink. The closed cell form of the polystyrene isn't so prone to sucking up liquids. It is not waterproof, but its not so much like a syphon as that beaded polystyrene. It cuts easier, too, because those beads melt inconsistently whereas the closed cell form is much smoother consistency of material to cut through.
And polyethyl methacrylate (not polymethyl methacrylate) is like an acrylic and is compatible with polystyrene. Probond glue is what model plane builders use. While it won't be as tough as fiberglass you can reinforce the wing with a wood slat or two fastened through the center of it. It is pretty easy to disect the wing into smaller long sections for doing the wood slat reinforcement. The advantage of the wood reinforcement is that you have something rigid to fasten onto whereas epoxy covered fiberglass mesh is not really something to which you want to fasten.
Go Huskers! |
| kr2bldr |
Posted - 31/03/2008 : 01:39:01
yup thats me
Keith and Martha Crawford
Lansing MI
martha.simerson.net |
| gholt |
Posted - 28/03/2008 : 07:27:58
Hello Chris,
The link provided by Hillbilly is a good start and provides the typical method for making airfoil sections in foam. There is another method which is well documented by a KR2 (homebuilt aircraft) builder by the name of Langford. (I believe we have a KR2 builder on the forum.)
http://home.hiwaay.net/~langford/
http://home.hiwaay.net/~langford/swings.html
The first link is the home page to his website, the second link describes in detail his method for building airfoils in foam and there are subsequent sections on his glassing techniques.
At this point, from your photos, you may want to sand your foils to get closer to your template. Also, foam is cheap compared to the labor and the glass you will be using.
Styrofoam (polystyrene) will melt when exposed to polyester resin. Polyurethane (Urethane) foam will not - however be forwarned you must NOT use a hot wire to cut urethanes. The fumes are leathal ... they can KILL you.
Typically one uses a layup epoxy resin (find a boat yard or marine supply) when working with polystyrene foam. Another method is to coat the polystyrene foam with a sealant such as latex paint or carpenter's glue. The problem here is even the slightest pinhole will allow the esters to attack the styrene foam. It does work and I've made many bucks from many varieties of styrene foam including beaded styrene - which doesn't sand all that well, but coating and filling can resolve that fairly well.
When you have your foam form ready to glass, the technique is fairly simple and straight forward.
1. Cut and fit your glass (standard woven glass - not matt) to your parts - dry fit
2. Brush a layer of epoxy onto your foam part
3. Lay the glass onto the part
4. Stipple (up and down motion) the glass with a bristle brush to make the glass conform to the part surfaces
5. You will be able to see where the glass is saturated and where it does not have enough resin - add more resin to these areas with your brush sparingly. The thing to remember here is to use as little resin as possible and to make sure there is enough resin to coat everything. Epoxy is fairly forgiving in that you can add as many layers as you wish, either during the first layup or a subsequent layup session after the first has cured whereas polyester resin has a smaller window of opportunity to do so.
Glass does not like to make bends around corners. Typically the trailing edge would have the upper and lower layers of glass just meet together and extend past the foam. They will join during curing and then you can trim them. Trimming during the "green" period when the epoxy is just starting to firm up is the best opportunity, although one needs to be gentle or you'll do more damage than good. You'll get the hang of trimming "green" with a little practice and you'll work your butt off if you don't learn to trim during this time period.
Your foam foils look great for a start! Bravo. Most people find cutting a long foil is difficult because the hot wire cuts faster at the templates than it does in the middle of the foil which causes some distortion. The remedy is to have a friend help during the cutting and to go slowly to allow the middle of the hot wire to keep up with the ends. You've done a great job - keep up the good work!.
Garland
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| MattM |
Posted - 26/03/2008 : 23:03:46
You're using a beaded styrofoam, one of the more sensitive materials you'll run across. Although it looks like it should have closed cells it is rather porous and fragile. Be careful using any liquids, caustic or not, due to the material's porous characteristic that will suck in the liquids. (Moisture in the air winds up getting soaked into that stuff.) Even a latex paint could get messy, so use it sparingly. Definitely do not use spray paint or paint thinner mixed with your paint on this stuff, it will dissolve it instantly.
But in all fairness to the material this is the very same material they use in modern construction to give large flat wall surfaces a rock appearance. They cover it in mesh and apply a blow on cement sand/latex paint to get whatever rock appearance is desired. Look into EIFS (pronounced 'ee-fiss') for more details.
Here's a post on fieldlines that deals with the issues: http://www.fieldlines.com/story/2007/5/22/152719/000
Go Huskers! |
| chris_psmith |
Posted - 26/03/2008 : 22:07:09
Well had some blade cutting fun, I guess all the web sites that talk about using a battery charger aren't trying to cut 2m long blades, stone cold. In desperation opted for 240v mains and a dimmer switch....ooooooh, don't reccomend that, the singing wire cut one brilliant cut, went orange and snapped, yee haaah.
So with sensible head on, borrowed a comercial DC transformer from work, took some practice, and got better every go, but results below, look ok to me.
Any suggestions from anyone on how easiest to coat them for strength, fiberglass mat would be messy, must be some sort of paint on stuff?
Chris
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| ghurd |
Posted - 07/03/2008 : 23:09:56
I am not going to look it up, but some 'styrofaom' melts with some types of epoxy and/or resin.
Make sure what you have before spending a lot of time and money.
G- |
| chris_psmith |
Posted - 07/03/2008 : 20:44:04
Del, I didn't see the program but sounds like the description of how they built the lenz2, my version will have such small sections for the aerofoils that i reckon it will be best suited to polystyrene.
Regards
Chris |
| chris_psmith |
Posted - 07/03/2008 : 20:42:06
Thanks all, looks like hot wire on polystyrene will be the way to go. Got a 2m length of drain pipe (50mm id) to fit over the scaffold pole (48mm od) as a secondary axel today (will stand the pmg on the end of the scaffold mounted down the middle of the tube - wanted the secondary axel to avoid putting too much side load on the bearings), ordered 5m of nicrome wire for the cutter, so should start to come together soon.
Fungus, was going with the Lenz2, but the more i looked at it the less i liked it (on a personal point of view of course), it just seemed like a course approximation to an aerofoil shape. I have given up on any atempt to gear up a turbine, just found it frustrating and sapped loads of energy, got the Benesh up to a succesful level by going small dia and tall (350 to 400rpm) but couldn't cope with gearing. So will be making the new one to about 56cm diameter so want it built for speed, the proper aerofoil should give me the best chance of this. If I struggle with self starting i will probably mount a small benesh on the top to get it going.
Thanks for all your thoughts, nobody tell the wife i am hatching another turbine in the garage!
Regards
Chris |
| Del |
Posted - 07/03/2008 : 17:51:45
Hi Chris,
I think I know what you're after. Did you see the series on TV called "Down on the Farm"? The chap on there built a darrius vawt, a big one. He cut a number of aerofoil profiles from plywood and linked them together with some more ply and then wrapped the lot in sheet aluminium (from an old caravan roof).
If you didn't see the series it's worth a peek - not sure if it's downloadable from anywhere?
Cheers
Del. |
| fungus |
Posted - 07/03/2008 : 12:46:22
Just be aware that what you're proposing ( a closed airfoil design) is a darrius vawt and acts a lot differently than other vawts, it depends solely on lift and often has problems self starting, I'd go for a lenz2. |
| hillbilly |
Posted - 07/03/2008 : 08:35:32
Chris
I suggest you have a look around the Model Aircraft sites. It is a well tried method of creating "Wings" using foam and cut with a hot wire to shape and section. You cannot use Fiberglass resin with polystyrene because it Melts. Lots of experience and tips are available on these sites. The one advantage that you have is that weight is not an issue so you can strengthen your "Wings" quite easily.
Try this site for a start ( http://webpages.charter.net/rcfu/ConstGuide/FoamWing.html )
Regards
Hillbilly |
| Hefhoover |
Posted - 07/03/2008 : 06:07:27
How about a cutter made in a "cookie cutter" style, i.e cut a lot of sections the same profile and then fibreglass the lot together, perhaps with a dowel or threaded rod through them all for extra strength. Maybe even make a template of that section, lay it on top of your foam board and run a hot wire around it to get identical sections, then laminate them as above. Hope it's of help, Hef  |
| MattM |
Posted - 05/03/2008 : 23:23:09
Polystyrene/closed cell foams covered in epoxy is easier than fiberglass imo.
Go Huskers! |
| donniedingle |
Posted - 05/03/2008 : 23:17:28
hi chris.
i think the polystyrene covered in fibreglas might be easier than you think, i like the idea of the heated wire shaped to the profile and pushed through the wire, and besides you can get the denser foam, (kingspan used in insulation) and its not expensive, you could sure have some fun with an 8ft x 4ft x 2" sheet of foam chris. and then coat it with fibreglass matting and resin, ring round the builders merchants you may get some broken sheets chris, i got some from travis perkins to lay under some concrete, the called them damaged / seconds.
regards donnie.
just keep her spinning.. |
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