surfboard-like windmill blades

[willib]

i just found some rigid foam insulation made by Dow Chemical .

it is called polyisocyanurate insulation..

it is pretty stiff and cuts like butter..

if i coat the outside with fiberglass for strength will this work as a set of blades?i am thinking of using the whole width ( four feet ) for the radius of a three blade set.

it comes in sheets about four feet wide x eight feet long by an inch thick . i think this will make an excellent formable blade.

i am thinking of laminating it , to make it thicker ?

i found a whole sheet of this stuff so i have a lot to play with

[sh123469]

This type of thing has been done for years in the model aircraft world.  Foam makes great, quick wings that are quite strong for their weight.  Be careful when glassing.  Some types of foam are dissolved by the resin.  

I have no idea how well this would work for blades for a mill.  They take lots and lots of stress.

If you decide to do something like that, be sure to use a good, strong spar in the foam for additional strength.  And, stress test for the highest wind speed you expect to see.

[willib]

Yes it is very , very light and strong for its weight.i found it on the dow site

http://www.dow.com/styrofoam/na/iso/s_tuff_c.htm

yes i will test it , and i will also need  wood for the root? of the blade .

any recomendations for a spar?

[Tropical hydro]

I have some experience with foam sandwich (which is what you are proposing)

Yes when fibre glassed it is very strong! It is very light for it's strength. Most sailing boats are made of it now. It can be shaped into wonderfully complex shapes with ease, making the most aerodynamicaly effecient blades. BUT....as sugested above you will need a spar of some sort to add rigidity otherwise it will flex. This is bad. Flexing will wear away the foam where it connects to the glass as well as weaken the glass. The root and hub can be made entirely of glass but I suggest you add some carbon fibre. CF is not that expensive! If you have a surfboard made of CF it will cost you a fortune because it's a one off....not because CF is costly.

In Ravenshoe QLD Aust' we have a 22Mwatt windfarm and the blades are made of composites as you propose.

Carbon Fibre doesn't stretch but it will bend. It makes a great spar if you laminate it together to make a rectangle (veiwed from the end) rod layed lengthways attatched to the hub. The wide sides of the rod face the leading and trailing edge.

Does that make sense?

CF is good for the leading edge too as it resists abraision.

Good luck with it....This is how I was planning to build mine. Glass and resin is not cheap though.....

[willib]

thanks for the interesting reply , CF was also on my mind ,i just did some ebaying for CF , its not cheep.any idea what they mean by decal material? they apparently use it to make your car look "cool" but i'm not sure if that stuff is what i'm looking for? as it has sticky stuff on one side?

i plan on making the root of ths blades out of wood, maybe , with glass over it.and the rest of the blades will be foam and glass.i'm not quite sure what you mean by your description of a rectangular rod though , what shape blades are you meaning?

[paradigmdesign]

Yes you can use foam for blade inserts, but it the foam that is usually used is a two part polyurethane foam, either a 2lb or 8lb/cu ft density.(usually  They sell it at fiberglastwarehouse.com, it is preaty cheap.  

This is the exact design that I am using for the blades that I am building.  What you need to do is create a mold for the foam insert, then poar 3 cores, and glass them.  The molds for the foam need the spar buildup's figured into them.  I would recomend cutting out foam sections for the blades (the exterior shape) in several inch sections.  You can use a blade calculation program to tell you how to cut the sections.  Then create a top and bottom "negative" fiberglass molds from the foam blade you made from the cut out foam sections.  In thoes molds you can lay the fiberglass that you want to use as the spar cap reinforcments.  So when you poar the foam into the molds, they will have the spar cap section already figured in.  Then you can just take thoes foam cores, and lay the spar cap reinforcements, and then the blade skin.

Ususlly the spar caps are made out of a combination of unidirecional fiberglass, and or carbon composite.  The skin should be either a fiberglass mat, or a bidirectional (+/- 45 degree) glass.

This design can be used for either a hand lay-up, vaccume bagging, or RTM (resin transfer molding) process.

[willib]

have you got any pics of a blade ,in its finished form..? great info , as usual :-)

http://www.otherpower.com/images/scimages/2965/0b_1_b.jpg

there seems to be two styles of blades out there, these and hugh piggots style.although its hard to tell from the pic just how thick they are..

i am thinking of making a combination of this style,but thicker with foam inside..

also those blade programs are a little confusing , as to the shape of blade that you will end up with??

[bob golding]

hi, i made a set of blades out of MDI foam. the sort used for surfboards. they have stood up pretty well. i made a pair using  windstuffnows plans. i live on the coast in cornwall uk where we get a lot of squally ?sp showers. the wind can gust up to 60 mph very quickly. i have no furling so i didnt expect them to last long. they have broken a few times mainly due to my somewhat slipshop methods. i broke them getting the tower down once and when the mounting failed throwing them into the tower. the nice thing about MDI or TDI is that you can use araldite epoxy to glue the bits back together. they have only failed once due to the wind, breaking at the root. i think this is because they are getting britttle after a couple of months. maybe UV degradation?

 they only took about 1/2 hour each to carve and cost 60 dollars. i might get around to  glassing them at some point. MDI will take  standard polyester car body repair kit type resin. if you use polystyrene foam you have to use epoxy resin  which is a lot more expensive. all in all a good experiment. and they are light enough to not cause damage if they do fly off. i dont recommend taking my  somewhat cavalier approach. i have a 5 acre field  and no nearby residents. nearest are a good 1/2 mile away. only thing in the way are my cat and a few rabbits. cat stays well out the way.

have fun and play safe.

bob golding

[Experimental]

     Hi Will,

    I built blades of foam, many years ago -- some I just used the fibre layers, as the front side is in tension, and the back , in compression !

    That worked OK, but you will have a real problem getting the glass cloth, around the leading edge to keep from lifting,and air bubbles forming under the cloth !!  This can be done, by "vacuum bagging" the blade -- and you also need to use epoxy resin, as polyester, will desolve the foam ...

    I also made some with a spar -- I just used a tube, made of carbon fibre -- these tubes were what they make shafts for archery arrows out of !!

    You can buy carbon fibre tube, from "Aircraft spruce Company", in the LA area-- but it,s rather expensive -- the arrow shafting was pretty cheep , but not very long !!

   I finally gave up on this, as it was just to much work, and have been building wood blades, as in the end -- they are much easier (for me, anyway!!)

   The foam fibre blades, work very well, But, lots of work !!

   Much luck to you, and happy building, Bill H......

[DanG]

Detail of a 35m blade 'Courtesy of' from WINDBLATT The ENERCON Magazine Issue 06/2004

If you go the way of foam study commercial methods if you can find them.

Foam, balsa wood, kevlar fabric & epoxy construction - note the stiffening honeycomb of small balsa & foam blocks w/ epoxy coatings bearing the stress ... The black stripes w/ dots are reference points for assembly quality assurance & the black bar is a stress gauge. I assume there is a kevlar structural tube or beam not visible on the surface.

Enercon also puts winglets to reduce vortex losses and noise, they make low rpm direct drive wind turbines - no gearbox! Google enercon and windblatt for more info.

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[paradigmdesign]

I don't have any pictures of completed blades yet, because they are not completed yet.  But I can give you the airfoil sections that I was using.  As far as using the blade programs, I use them to calculate the chord (width) and twist of the blade.  The blade calc program should give you the blade in 1/10'th sections.  Once you have the twist, and the chord, you can calculate the X variable (how thick it is).  Some designs stick to a strict X to chord ratio, but I belive it is better to narrow the inboard sections chord while increasing the X.  This will save on glass for the inboard sections and give you a stronger blade at the root.

These sections are actual size for a 6' blade.  8 inches between each section, will prob modify the 20% section more

[paradigmdesign]

sorry abt the double pic post.

Also, you should go to sandia.gov they have alot of papers about designing fiberglass blades, the one about resin transfer molding is a good one.

[willib]

actual size? cool!!

thanks , i googled sandia.gov and resin transfer molding and found a cool pic of an ocean wind farm off Denmark..i'll keep looking..

[paradigmdesign]

http://www.sandia.gov/wind/topical.htm

Look through the papers in the Manufacturing section.

After reading a couple of thoes, you realize how little you really do know.  But at the same time, it is good to see someone as big as Sandia National Labs is researching wind turbine tech.

[willib]

sorry i did'nt reply earlier , its been a crazy week.

archery arrows , hmmm i'll look into that..

thanks

[willib]

i'll do that thanks..

[willib]

i've been analyzing(sp) your drawings . not regarding the length, the height to width ratio is .3 of the bare board?

so say for example if i was to start with a piece of foam  1" high the width would be 3.3" ??

[willib]

i just made three blades out of that foam i was mentioning , that stuff is soo cool,i can smooth it with a copper scrubber.

i think they came out nice, the first one is a little thin near the root but all in all they came out nice!

[willib]

they come out to only 27.4 " in dia., but hey , they work !!

[willib]

I took the test blades out for a spin in the car today . they worked really well.

it might have looked a little strange , with me holding the props out the window with my left hand but they had to be tested !!

No rpm/MPH readings , maybe someday..

i have not glassed them yet, i may just save the resin for the larger versions..

[paradigmdesign]

you also have to watch how the foam acts in colder temps, and over time, because thoes both usually lead to brittleness.  Even on smaller blades I belive(know) you are going to need atleast some glass.