Making Aluminum Blades

[SparWeb]

Matt,
Did you ever try to "close" the section?  Meaning, use two pieces of sheet and put the seams together?  Still not sure it would last, but if you did, could it have lasted longer than the open section?  What aluminum were you using?  Different grades/tempers take bending differently.

Smithson,
I still have a half dozen earth-anchors that turned out to be far to small to hold guy wires for a 40-foot tower.  We all seem to start with some of the wrong stuff before zeroing in on the right stuff.  What's next on your shopping list?


Everything made of wood should be treated with care. 
Many things can cause a wind turbine to fail, don't get focused in on just one. 
This example of a Jake that failed in Alaska, proving that spruce isn't a good material for blades, is kind of like saying aluminum shouldn't be used for aircraft because a Boeing 747 would break apart in an Immelman maneuver, or steel shouldn't be used for truck frames because a full load of gravel will bend a F-150 long-box.  No, it's using a machine beyond the range that it was designed for that's to blame in the case of the Jacobs in Alaska.  Under the conditions that I think he's talking about, neither spruce, fir, nor cedar would have held up, not without tuning or re-designing parts of the machine for the conditions.

[ChrisOlson]

This example of a Jake that failed in Alaska, proving that spruce isn't a good material for blades

Actually, it was not one Jake - it was three of them all standing in row, and all three lost their blades in two months.  Admittedly the conditions in Alaska are severe with 70 mph sustained winds off the Bering Sea in the winter.  But supposedly the turbines were designed to handle that.  They retrofit them with fiberglass blades and they have been running with those blades for over 30 years without a failure.  That kind of suggests the blade material being the problem.  And I've seen plenty of other Jake spruce blade failures that also suggest it was not the best material for those turbines.

Marcellus Jacobs used to go to a mill in British Columbia and personally select the spruce planks they made their blades from in Minneapolis.  But he was dealing with a machine that they had limited experience with, compared to the old Model 25's that were so successful.  These new grid-tie machines had different requirements that sitka spruce could not meet.  The turbines ended up getting a bad name for reliability, and eventually the company was sold to Control Data Corp and the switch to fiberglass was made in 1981 on the production line.  There has never been a recorded failure of the glass blades.  WTIC (Wind Turbine Industries Corp) eventually bought the rights to the Jacobs turbines and they are manufactured today at Prior Lake, Minnesota.  The standard upgrade for a 23-10 with the old 1.5" governor and spruce blades is to replace the governor with a 2" unit from a 31-20 and put cut-down glass blades on it that are only manufactured today for the 31-20.

Just to relay the development of a machine where wood did not work for a blade material, and the Jacobs eventually lost market share to the Bergey Excel with its carbon fiber PowerFlex blades.  And I maintain that any turbine that runs at high tip speeds should receive careful consideration on blade material used.  Cedar met my requirements for my severe-duty turbines and fir/pine did not because it is virtually impossible to find fir or pine that does not have defects in it that causes failures.  For the slow-turning turbines in the homebrew books that typically run in a stalled condition, fir or pine might work fine.  But my turbine doesn't do that so I selected a different material based on failures and problems I've seen, just like Marcellus Jacobs did with his turbines.

[SparWeb]

Wind turbine blades are a structure - composed of a material.  Or multiple materials, in the case of fiberglass.  I've seen enough broken FRP blades to know they come in all kinds, good and bad.  I guess I'm about to start another diatribe about the way a thing is designed, how that has a lot to do with the strength of the final product, of which the materials is just one choice out of many.  Maybe I won't.  Maybe I'm splitting hairs here.

Chris, sounds like you've read that long interview with M. Jacobs that was posted on the internet a while ago.

[MattM]

SparWeb-

I did try a triple hem in front and a second piece from the attachment point to the front hem.  Added more weight and still broke at the same point.  Simplicity in design was an advantage in the original.

Steel blades fail at low rpm's.  Once fatigue sets in the blade will fold back onto itself and bring the rig to a stop in all but high winds.  It doesn't throw blades, the crack forms perpendicular in the blade - like tearing a paper - and loses rigidity along the attachment point.  I've done up to 24 blades on a single rotor.  It will cease to spin a three blade rig.  It will wobble like hell in even counts with lesser blade counts.  But the 24-blade rotor barely wobbled when one failed.  3/4-inch plywood and wood screws with capped heads proved to be sufficient to survive multiple winters and summers as long as the plywood was kept water tight.

Metal is fun to play with and adding things like winglets to the blade tips is easy.

[kitestrings]

We used balsam fir (not to be confused Douglas fir).  For a local choice, it is pretty strong for it's weight.  If you compare strength properties it is similar to the cedar.  It is a bit poorer in compression perpendicular to the grain, which basically means it can 'dent' more easily.  We tried to compensate for this with as hard a surface finish as we could get.  The other problem is it has nowhere near the natural weather protection of cedar, so keeping the blades well finished will be key I suspect.  Eastern cedar is also readily available here, but it is not at all strong enough.  On a positive side, balsam is readily available, straight grained and doesn't have loose knots like our spruce.  We cut the logs and had blanks locally sawn, then air-dried under cover.  The price is right, but labor input is high.  I do prefer a local solution if one has this option.

I tend to agree with Chris' comments about single piece vs. laminated stock.  Laminated is more stable, but I'm not convinced it is stronger except in very well controlled set-ups.  A buddy of mine laid up a maple butcher-block at a high-end local furniture plant.  It's flawless, but their set-up, glues, and environmental controls are way beyond most home/shops; certainly mine.

My experience has been that the leading edge takes tremendous abuse.  With MPPT I suspect this will only be worse.  Many folks have used blade tape or metal flashing to compensate.  I have a love-hate relationship with the tape - it's good in the early years, but can be more of a nuisance after that.  The copper/bronze flashing seems good if you can get it to stay put.  Any preferences?

~kitestrings

[ChrisOlson]

My experience has been that the leading edge takes tremendous abuse.  With MPPT I suspect this will only be worse.

It is much worse with MPPT, and especially on the blade tips.  I have been using polyurethane enamel (DuPont Imron or PPG Delstar) to seal and protect my blades.

DuPont invented this coating originally for industrial use and it quickly found its way into the classic car restoration business when people found out you can paint a body panel with it, cave the body panel in with a maul and it still won't chip or crack the paint.  The downside is that it is toxic, every expensive, you have to wear breathing apparatus with external air supply when spraying it, and use activated charcoal filters on your paint room exhaust fans to prevent anybody else from breathing in the overspray dust.  I just happen to be set up for it because I use it on semi trailers and tractors when doing a refinish job.  Most people are not set up for it, so would have to take your blades to a body shop to have them painted with it.

The durability of it, though, is excellent.  Far tougher than leading edge tape.  A quart of Imron or Delstar, with the activator and hardener is about $600.

[SparWeb]

Matt,
That's some interesting experimentation you've done.  Too bad you haven't found a long-term design yet.  What size of rotor are we talking about?

Kitestrings,
I'm very happy with the leading-edge tape I use.  It's a 3M polyurethane wear-resistant tape, with self-adhesive backing, designed for composite aircraft propeller blades.  You can order from Aircraft Spruce online catalog.
http://www.aircraftspruce.ca/catalog/cspages/8674_7.php?clickkey=4972

Is this the same kind that you've tried?

I've been using a roll of 2" tape.  I got about 100 feet of it many years ago, and I've only used up half.  Replace the tape every year but it usually doesn't want to come off.  Except once that I left it for 2 years, and then it was peeling a bit.  So what's 20 dollars of blade tape per year, compared to the damage of eroded leading edges?  Same cedar blades I've been talking about all along.  I posted a thread about it at the time...  May be possible to find it.  The erosion on the original blades was pretty bad.

[MattM]

SparWeb-

My hope one day is to rework an old farm windmill with my blades.  I've mainly experimented with 3-4 blade lengths in 16-20 gauge and two foot lengths in 22-24 gauge.  The painted steel didn't work as well but it's a higher carbon content and more prone to fatigue.  Galvanized worked best for strength although weathered badly on the front edge and rusted where cut or worn from air friction.  Rust doesn't really make a difference and the weathering is purely cosmetic.  Painted steel looks much nicer after long term weather exposure.

Bigger blade sizes require heavier rotors, which I didn't improve upon.  If I kept 16 gauge blades down to around 30" they didn't break so much as the rotors tended to wear out.  IMHO plywood, even marine grade, is a bad rotor material.

[Flux]

My experience with the polyurethane prop tape is that it lasts about a year, if you are happy to change it yearly it should be fine, much beyond a year it starts to become more trouble than help when it blisters, peels and flaps about. I have never tried the stainless steel foil, that should be ok as long as it stays on.

Copper foil works fine if you can keep it on, but the traditional method of keeping it on with staples causes a lot of weakness.

Certainly the tip erosion will be greater if you are to take the full advantage of mppt in high winds.

The thing that surprised me is that with conventional profiles significant leading edge wear seems to have little effect on performance. This is probably not true for the high lift profiles but they are not necessary for mppt.

I have a fast prop on a freelite and a friend stuck neoprene pond liner on the leading edge of that and it has stood up extremely well.

Flux

[MattM]

So laminate the polyurethane prop tape with your copper foil before applying the tape and have the best of both worlds.  Copper should hold up to the elements well and it's corrosion should arrest any biological processes.  Copper kills everything in nature, which is why hypoallergenic ductwork uses it.  They make rubber roof seaming rolls that would bond permanently to a leading edge, too.  If pond liner is durable than rubber should work.

In my first paragraph above, SparWeb, I meant to say 3-4 foot blade lengths.  Note I am talking about blade lengths.  I typically use a twelve inch diameter plywood circle with 2x12 circle cutouts behind it to hold bearings.  My wood jigs are a good match to the bearing diameter.  An arbor on the axle makes a simple stop..  A washer and cotter pin through the axle keep it on.  Purely yard art.

[SparWeb]

Well I can admit I've screwed up the poly tape, but each time riped it off and replaced it before putting the tower back up, when catching my mistakes.  And I have to be careful that the surface of the blade is clean smooth dry etc.  But that,s normal for any adhesive that is exposed to the elements.
But I also assert that I've taken the old tape off several times now and even 2 years old it peels the wood finish off with it.
Perhaps my dry climate is giving me an unfair advantage.

[SparWeb]

The old time aircraft props often had curved leading edges to which you would often see stainless or copper strip form-fit and nailed with miniature 1/32" nails.  Maybe the EAA knows more about this subject than we do.

[joestue]

The old time aircraft props often had curved leading edges to which you would often see stainless or copper strip form-fit and nailed with miniature 1/32" nails.  Maybe the EAA knows more about this subject than we do.

what i read about that was a lot of people got killed when that strip of steel departed from the blade...

i would look into perhaps electroless nickel plating the entire blade.

[SparWeb]

This video explains it.  Rather dry.  Skip the airplane lingo for about 3-4 minutes, and then he gets down to business.

http://www.eaavideo.org/video.aspx?v=2616988676001

He forgets to use gloves (neoprene not powdered latex) to prevent his fingers from touching the tape.
Notice that he gets a kink in it on the first pass but the next few passes with his card work it out.

If it's good enough for 3000 RPM, I don't see a problem at 800 RPM.  No matter what the blade is made of.
Composite aircraft blades also require protection.

Sensenich still sells (beautiful) wood props with brass leading edges.  Good illustrations here:  http://www.sensenich.com/products/item/9

[SparWeb]

Yiannie,
Hope you're still reading this.

I went back to the OP question and realized that you were asking about formed-aluminum shapes, but I had assumed you meant extruded aluminum.  There's a big difference.  I would not recommend formed aluminum tubes for a wind turbine.  Are you considering aluminum extrusions instead, perhaps?

PS there is a way to make "streamline" steel tubing by rolling round tube, but steel is much more forgiving of that process than aluminum can be.  Since aluminum can be extruded in almost any shape you want, there is no need to squeeze aluminum tube.

[Frank S]

Having made numerous aluminum gas tanks for motorcycles in the past. One thing is for sure when trying to form aluminum you first must anneal it to soften it otherwise it cracks. I used a torch with highly carbonized flame to do this. Aluminum work hardens horribly when stretching or bending. This is pretty much true for most any grade or alloy unless you have a way to hold it at a certain temperature then to return it to its full strength potential it needs to be placed in a kiln, normalized & re heat treated.
 Cold forming a tube such as a section of irrigation tubing could be done I guess by maybe running it through a tapered rolling mill. But when the sharp bend of the trailing edge is formed that area would have to be softened by heat. Or  after the tubing was formed the trailing area could have a section cut out of it then the edges welded together.
 This might make a suitable blade skin as long as there was something placed inside to become the mounting structure.
 I imagine the results would be about the same as the time I helped carve an  air-boat prop out of Louisiana Cyprus (DISMAL)   

[MattM]

Extruded aluminum is one third to one sixth glass material and it's crystalline properties would make it more brittle than purified aluminum alloys.  Heat treatments afterwards may fix the cracking of the material, but it's not going to solve the inherent brittleness of extruded materials.

[kitestrings]

I have been using polyurethane enamel (DuPont Imron or PPG Delstar) to seal and protect my blades.

...

The downside is that it is toxic, every expensive, you have to wear breathing apparatus

Yes, this is what we used as well.  Imron ''5000" I think it was, follow by top coats of single-part poly PPG Concept IIRC.  Mick Sagrillo, formerly with Lake Michigan Wind swore by this primer.  A buddy of mine painted ours and he looked like he was preparing to enter a nuke plant after a spill, but my understanding is (like that) you only are allowed so many parts per million over your life-time before you start growing extra toes.

SparW, I'm not sure of the name of tape we used.  It was made by 3M; the good stuff.  Went on nicely, but after just a couple years it seemed to always get either a bubble, or a 'hang-nail' tear/flap of some sort.  I could always hear the latter.  I guess your method of early replacement avoids this, but I'd like to get to a ~7-year finish cycle if possible.  I confess I've also refinished blades in place without rebalancing.

Some of the Winchargers had copper flashing with the little brads.  I like the material, just not sure the method of attachment solves more problems than it creates.  I can't say I've ever heard of never heard of the electroless nickel plating

Still not a big fan of aluminum for blades.

~ks

[SparWeb]

Matt,
Have you seen glass fiber in Al extrusions?  While I don't think it's impossible, it sounds very improbable.  Any evidence you can give I would greatly appreciate seeing.  A very novel thing!

[SparWeb]

KS,
Can't dispute that the 3M tape probably won't last 7 years.  Will the paint, bearings, grease, etc. last that long?

Electroless nickel plate is a "luxury" proce$$.  Eg. Aircraft landing gear.

[MattM]

Matt,
Have you seen glass fiber in Al extrusions?  While I don't think it's impossible, it sounds very improbable.  Any evidence you can give I would greatly appreciate seeing.  A very novel thing!

Oops, should have read...
one third to one sixth of a percent glass

Glass acts as a lubricant in the alloy and allows for significantly lower extrusion pressures.  I get interrupted a lot when typing and lose track of where I am when posting.  You'll see cheap Chinese extrusions in multiple percentages of silicon/glass.  It's very brittle compared to the typical process using .3-.4 percent.  Any carbon or silicon impurities takes away from strength when it comes to metal alloys.  But when you annually sell millions of pounds of the stuff you get a financial incentive for keeping costs down.

[SparWeb]

Matt,
Any suppliers who would do this would SERIOUSLY undermine my faith in their products.  As far as I know, this is not present in the aluminum grades I deal with on a daily basis (2024/6061/7075 etc.) even in extruded form.  At work I sometimes review the material test data from the mill that accompanies our orders.  I've never seen a silicon composition that would indicate the inclusion of these fibers, but, I've never looked for that, either.  I want to do some research into your claim.  Any starting point you can give me would be helpful.  Specs, supplier names, alloys, etc. 

[MattM]

I'm not talking about glass fibers, I'm talking about alloys.

5052 is a good example.  Not many consumer grades are free of Silicon.  Not even 2024.

[SparWeb]

Then I don't think you meant "glass" at all.  SI can come from many sources, and some grades tolerate trace impurities better than others. Grades 3000 5000 and 6000 always have some and up to 2% is in the spec for 4000 series welding rods.  SI drastically reduces the ductility of aluminum so there cannot be too much.

[MattM]

Well, in reality it is silica added in, not pure glass.  It's also not purified silicon added, as that's very expensive.  With all chemical processes there are byproducts removed from the final product and the silicon element is left behind in the alloy.

[joestue]

It is silicon dissolved in the aluminum unless there is enough of it to crystallize out, and you may be able to see these crystals. I mixed up a 40 pound pot of aluminum scrap recently and under a microscope I saw grain of sand sized crystals that were softer than silica. i'm pretty sure they were silicon, and the aluminum alloy i ended up with was very brittle, nearly useless.

[MattM]

I believe you have to use Nitrogen to fill the air void as you work Aluminum or it oxidizes the ingredients.  Silica is SiO if my old brain is working right, so those crystals may be a byproduct of your work.  When they smelt aluminum ingots they are very keen to keep impurities out of the process, which ir probably why they use electricity to melt it and not fuels.

[tanner0441]

Hi

When I was welding aluminium it was pure argon used as a shielding gas. Any trace of oxygen resulted in a blackened burned weld. The rods I used were pure aluminium or for a harder weld with better wear resistance 2% silicon but the work piece had to be annealed afterwards or it soon cracked on cooling.

Argon shield (MIG gas 5% O2 ), or CO2, were a definite no, no.  I don't remember trying Helium, I used that on stainless, though it was more difficult to keep the arc going than Argon.

I still feel that at the end of the day, wood is the easiest and cheapest option.

Brian

[dugajimi]

My experience with aluminum blades turns out to be very positive. For three years our turbine run uninterruptedly without any problem. The blades are made ​​of aluminum with a CNC milling machine. Each blade is 600 mm long (the maximum length allowed by our machine). Our turbine is installed at medium intensity windy site but we had no breakage. This is the link to our project so you can see:
http://antedoro.blogspot.it/2012/06/istallazione-turbina-eolica.html

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[SparWeb]

Dugajimi,
Thank you for all the pictures, and especially thank you for signing up to the group in order to share them.  I have not seen many CNC machined blades that have actually been used, though I keep hearing about them.

I looked at the website link too.  My first thought is "many hands make light work".  Have you met Antonio Cecere, from southern Italy? 

[dugajimi]

Dugajimi,
Thank you for all the pictures, and especially thank you for signing up to the group in order to share them.  I have not seen many CNC machined blades that have actually been used, though I keep hearing about them.

This is a school work of my students at italian high school. We teach metalworking technology therefore our first goal is to teach machining.
Perhaps it is better to make the blades wood.

Have you met Antonio Cecere, from southern Italy?

I do not know Antonio Cerece but I knew other teachers who have built a wind turbine at high school in Italy like this: http://www.openventolone.com/

[SparWeb]

Yes, Antonio's "Microeolico" projects are very similar.  He closely follows the Hugh Piggott designs. He contacted me for some translations several years ago, but I've kept track of his projects since then.

[MattM]

I'd argue that plastics would make better mass manufactured blades than wood simply because you could use far less labor, energy, and material.

Solid cnc-machined parts are always cool.  But the coolness of using sheet metal was that the blade flexed and became more efficient as wind pressure against it increased.  Stiff designs cannot do that.  Plus the machining process is bookoo expensive compared to sheet stock.

I think my ogee design would be best done in plastic, where it too could conform to wind pressures.  Being able to reduce your blade pitch as wind pressure picks up increases your rpm's over a fixed design.  I've seen plenty of wood blades take quite a stiff wind to spin up to their optimal speeds.   The lightweight metal blades take little time to go from stationary in dead wind to a blur in a short gust.  Plastic or carbon fiber blades should perform way better.