Blades - lessons learned

[kitestrings]

Spar,

I believe you fashioned new blades for your "Spirit of Zubbly".  I guess new is relative, as I just realized this was done nearly five years ago now:

https://www.fieldlines.com/index.php/topic,149797.msg1048344.html#msg1048344

And, they were quite a bit longer than the set they replaced.  I'm curious how they are working out... if you have data before - and after, or any other observations.  How well is the finish holding up.

BigRock,

Similar questions.  I believe you also carved new blades late 2018/early 2019.  Curious what changes you made and whether they are performing the same, or better? 

https://www.fieldlines.com/index.php/topic,149712.msg1046882.html#msg1046882

Thanks for sharing what you may.  ~ks

[bigrockcandymountain]

I have carved 4 sets so far.  The latest iteration are by far the best, but still not perfect.  The link you posted is my second set of blades.  They are still intact and hanging on the shop wall.  They were quite successful.  I took them down because they were quite unbalanced because one was a way less dense piece of wood than the rest.  The tower always shook when they were running. 

The newest ones are running now, and at 13' diameter, they peak about 3500w.  (If i was selling them on ebay, that number would be 15kw or so ) In normal every day winds they make a steady 500w or so.  Lots of days they make 10kwh.  20kwh is about the highest I see in a 24 hour period.  Any windier than that and it is shut down.

In my case, I have learned a lower tsr works better for me.  I also like a thick airfoil.  I think these are over 20%.  I think the thick airfoil makes them less efficient at higher speeds, which i consider a good thing.  It also helps a lot with stiffness.

I also wouldn't recommend taking any pointers on blades from me.  I always look at yours and spar's posts to learn how to make perfect blades. 

We don't have any real rot issues here.  Bare spruce fence boards that are untreated last 30 or 40 years.  I know that is more of an issue where you're at.

Are you thinking of whittling out a new set?

[kitestrings]

Hmm, I was (maybe still) confused.  I remember you had the rather catastrophic failure during a high wind event.  Then you had to carve a set in pretty-short order as I recall.  Are these you latest set:

https://www.fieldlines.com/index.php/topic,150690.msg1062013.html#msg1062013

So yes, I'm thinking probably next summer to do another set.  Ours have worked remarkably well - more by luck I suspect - but the moisture intrusion is not going to disappear.  Oddly, the balance now is the best we've ever had.

Just starting to think about it really...likely they'll be carved, but possibly modeled, or routed, or CNC.  We live in ski/snow board country (Burton, Rossignol), so it seems like there are lessons to learn from their efforts/successes.

I was also thinking some of a design where there would be no fastener penetrations; just a compression fit.

Those are impressive numbers.  We have higher peaks, but rarely do more than 8 kWh in a day, though I routinely shut down at night.

Perhaps a different post, but I'd be interested to hear how your furling actuator is working.  I'm convinced that automated furling is among the best things we ever did.  It doesn't get much acknowledgement here, as it wasn't proven by complex regression analysis, or tested in a wind tunnel; but it is passive, simple, and it works.
 

[bigrockcandymountain]

Yes, that is the post about my current blades.  I'm really bad at being organised about pictures of a project.  They are good enough that when this set goes to pieces, i will likely just copy them and try to do a better job of finishing and leading edge protection. 

The furling actuator is a wonderful upgrade.  I absolutely wouldn't be without it now.  I used it last night.  I still haven't automated it.  It is just a manual switch, but in the basement instead of climbing the hill.  You have yours set up on aux 1 input voltage high or something like that right?

 I have a voltage monitoring relay that i might use to trigger it.  Then if the classic fails, or the rectifier, it will still shut down.

What wood did you use for your blades? 

I would love to see compression fit blades instead of through bolts.  That would be pretty unique and interesting to see how it would affect the rot resistance. 

One other thing i might try is wood stain instead of paint.  It allows moisture to escape, so anything that gets wet can dry again.  That is what we used on our house cedar siding.  It isn't as shiny or slippery as paint though, so the finish won't be as nice.

Would you do anything different size, airfoil, twist etc on the next set?

[kitestrings]

Yes, we use Aux1 for furling.  You may know this, but for those who don't, Aux1 can be configured as a relay or logic output.  It is non-PWM (unlike Aux2), but can output 12V+, 0V, or continuity when used as a dry-type relay.  Ours is the latter, both jumpers (JP6 & JP8) are set to the right, so it is essentially an isolated switch up to 1A IIRC.  We use "PV V on Hi", so it will shut down if the voltage goes above the input V set-point, and of course you can manually turn it on or off.

Our blades are locally grown balsam fir.  I actually cut the trees these came out of.  These are single piece, without lamination.  Balsam is pretty strong relative to weight, straight grained, with small knots, especially nearest the trunk.  It is not rot-resistant like cedar.  The only local cedar we have is white-cedar, which is rot-resistant - it was used for rail fences for decades - but it is otherwise undesirable: twisting grain, lots of knots and not very strong in moment.

I don't think we will change size.  I'm pretty happy with performance.  Possibly a different airfoil, most likely with twist.

[SparWeb]

Hi,
Well, a question like that one is pure catnip to me!
Here's the story of my blades, with notes about the WT's they were on, but none of the technical details.  I don't think you need dimensions and specs so I'll stick to the experience selecting, carving, protecting, and durability.
I will also ignore many experimental carvings where I was just learning technique and trying new tools.  All blade sets in this list actually flew on a WT.


Blade Set #1, 2007
Designed per Hugh Piggott spreadsheet, however I made adjustments of my own. Actually had excessive twist.  Started up great, limited by stall (but I didn't know that at the time).
Fabricated with laminated cedar.  The laminations went flat-wise, so the tip was was just one layer of cedar.    The root was 6 layers thick.
Durability was limited because I treated them with linseed oil rather than a hard varnish.  I also didn't have the leading-edge tape on for the first 4 months.  The tips were all chewed up so the tape was too little too late.
The generator in that WT started out as an axial flux.  It burned out in 4 months.  I replaced it with my first motor-conversion.
This set of blades was destroyed in an accident lowering the tower.  One survived well enough to be a template for the next set (except... read on...).  I tried to study their performance by building an electronic datalogger, but it didn't work and I couldn't learn anything.


Blade Set #2, 2009
Designed per Hugh Piggott spreadsheet. This time I knew more of what I was doing, and I had written some analysis of my own to learn more.  This is where I found my twist error.  My work gave me a way to input different airfoil profile parameters, and I learned a lot about what matters and what doesn't.  For everyone's sake, sticking with Hugh's spreadsheet almost can't be beat.  My adjustments addressed the tips better and I was delighted how quiet they were.  My analysis also accounts for the power curve of the generator.  If the genny lets the blades run fast (too small) or loads them down too much (too big) I was able to see that.  Approximately.
Fabricated with laminated cedar.  The laminations went flat-wise, so the tip was was just one layer of cedar.  The root was 6 layers thick.  So far these were pretty much the same so that I could re-use the same hub.  This time, while I was carving, I noticed the different hardness of the epoxy I used to bond the laminations and the cedar itself.  I had noticed a "ridge" pattern on th eprevious blades due to this.  Unfortunately I was committed at that point. From this lesson I didn't want to laminate flat-wise again, they should be laminated edge-wise.
Durability was much better because I protected them with a hard varnish and added the leading-edge tape.  Seriously pitted by a hailstorm in 2016 but they were intact enough to keep running.  The next year splits in the cedar started.  The splits followed the wood grain along the trailing edge.
I built another motor-conversion because I was so impressed by the first one.  This one was well de-cogged, ran quietly, and didn't self-limit.  Later, I built a datalogger that did work, and I was able to study performance in detail.  This blade+generator combination had great performance in strong winds but didn't start up in low winds, missing out on a lot of energy.  It wasn't just because they were worn out; I knew I needed bigger blades.  These blades lasted almost 10 years before I replaced them.


Blade Set #3, 2019
Designed with my own analysis, but cross-checked with Hugh's spreadsheet to be sure.  Spent extra time using the data I had logged to make them match the generator's power curve.  Risk was that the bigger blades would run too fast (more power collected) causing more noise, more wear, and the tail to fold excessively often.
Fabricated with laminated cedar (it's an addiction now).  The laminations went edge-wise, so the tip has several pieces of cedar.  The root was 12 pieces thick.
Durability improved again due to their size and arrangement of laminations that should reduce tendency to split.  Same varnish.  Leading-edge tape applied liberally - not just on the leading edge but across the chord.  These blades were also struck by two severe hailstorms (not as bad as 2016 though) and emerged completely unscathed.  What did "scathe" them was a bad job I did with Loctite in the hub, which allowed them to "unscrew" and come flying off in 2021.  They departed as a unit, skidded down the guy-wire, and cartwheeled across the field, knocking down an electric fence.  Despite the drama, it didn't take much work to repair them.  Even the tips were OK.  Some days I do well because I'm smart, but more often I'm just lucky.  These are still going today, and they are still very quiet.  I replaced the generator in 2022, which unfortunately is noisier because I didn't de-cog it properly.


Since that account was focused on building and using the blades, I've skipped a lot of other things, such as sizing them properly.
A couple of months ago, another member asked about durability of various WT blade materials, and if you can find the reply I gave, that might also be helpful.

[SparWeb]

KS,
You're leading the way in many degrees, particularly in your method of securing the WT by shutting it down.
I'm not in any hurry to re-design or re-engineer my WT, it's working well now, but if at some point I do, there are many lessons I can learn from your installation.

I don't have good advice to give on material choices - I use cedar because it's light and easy to carve, and the red cedar I get here is straight-grained, even picking fence boards at Home Depot is good enough.  My weather reports don't have lots of moisture (except in solid form).  I know you don't have these options, so the fir you used before is probably the best bet.  (You cut it yourself, awesome, man.  DIY to the next level)  If you're willing to saw another tree into boards, plane them, and laminate them, then the dimensions of the "blank" are almost unlimited.  You can arrange the root of each blade to form a Grip in a variety of shapes.  That might help you come up with bolt-less grip shapes.  Are you thinking of a dove-tail?

[taylorp035]

I've seen a lot of different hub designs on here and often times you see various plywood designs fail once they rot.  After watching my wood hub fail, I switched to two thick steel plates and applied ~90,000 lbs of clamp load by using (9) M12 grade 12.9 bolts torque to 90 ft*lbs.  Each blade has two holes in the middle if the clamp load for some reason decides to go away.  My math says it should be good to ~60 mph wind speed with the very, very thick and heavy blades I have at the moment.  You could definitely scale it up and/or weld some hard stops so they can't slide out.

https://www.fieldlines.com/index.php/topic,150017.58.html


Now that I have a working system, I hope to see if I can improve the performance with better blades.  I recently took 2 degrees of pitch out of them (~7 deg tip to now a 5 deg tip).  They have no taper, but have negative curvature on the front face in an attempt for a higher lift profile at the expense of high speed performance.  I've been quite happy with their low speed performance.  They are probably too big in diameter for the alternator at the moment, so I'll probably go slightly smaller in diameter next time and thinner for better mid and high wind speed performance.

I don't profess to be an expert on blades.  I've had good luck doing the math for the air speed based on the slope of the chord line.  When I did it on my last mini CNC blades, they worked amazingly well.  I've never seen something spin so fast and they produced a lot of power (at least until it crashed and they weren't the perfect shape any more).

[Mary B]

Leave a band of full thickness wood at the base of each blade that sticks up behind the hub. The hub clamps and centers the blades via steel angles welded to the bottom, the lip of wood prevents blade pull thru if fasteners loosen. Loctite and yearly fastener checks added for safety. Could even safety wire the nuts like aircraft do.

[kitestrings]

Thanks for the great responses all.  We've started into our (maple) sugaring season, so much of this will have to wait except for the scheming and dreaming  .

A friend of mine sawed these logs out on his permanent/portable mill.  I had him saw both full size one-piece stock, but also quite a bit of 2 x 6 dimension planks.  I assumed the first set I carved would need modifications, but that future sets would be laminated.  As I looked for some photos I was reminded of a couple of things: 1) Mary was quite close to delivery of our second son, and 2) times flies.  The little one climbing up here is now in his first year of college.



We've used a few of these, but most are still sitting in my shop.  The easiest thing of course would be to just plane, laminate and carve another set; "rinse & repeat."  By laminating we'd reduce larger checking cracks.  Perhaps a stain or other method of finish, but otherwise straight ahead.  The set we have has been working since about 2013, and could probably go for another year or three.  We don't always do things the easy way (Mary reminds me of this frequently).

We really have had no issues with how they are fastened.  We use four SS thru-bolts and nylocks with blade plates sandwiching the blades.


I've always thought though, that the holes were a weak point for moisture, and that for serviceability a tool-less compression clamp would be pretty slick.  I was envisioning something between what Spar & Mary have suggested.  Some sort of keyway, or locating pins with hard-fitted metal stops on the sides of the root end.  This is very crude, but something like:


Ideally you could set a blade in position, clamp it in position, and then torque it to the desired compression spec for the area and material.  The same center pin could secure a nose-cone.

Regarding other possible materials - I did revisit a list that Spar developed that looked at durability, cost, complexity, etc.  Found that interesting; good reference.  Btw, I hadn't realized you had a set cartwheel down and still survive.

I've also have enjoyed seeing Taylor's progress with CNC.  Very slick.  Perhaps a future set in AL?

Much more to ponder, but again thanks for the input.  Oh, and I will start another topic on power furling to hopefully not derail this line of discussion.  ~ks

[SparWeb]

I've always thought though, that the holes were a weak point for moisture...

I can't disagree - they are.
But if you haven't had problems for years, I'd counter with "if it ain't broke...".
I'm still curious to see what you'd invent - as a spectator I win either way! 

One thing I sometimes don't see addressed in the mounting of WT blades is bending at the bolts.  If there is only one hub joint where the bolt passes from blade to hub, then there is some bending at the bolts.  Think of the thrust on an airplane propeller - there's some of that in a wind-turbine, too.  The thrust is greatest at high TSR.  On my blades I estimated about 170 pounds on the set of 3, with a 10' diameter.  The NREL test report TP500-38550 measured 560 pounds thrust on an 18' Bergey Excel.

To see some examples of double-plate hubs, you can see them on the front and back of Hugh Piggott's old blade design recipies. This allowed Hugh to get away with plywood hubs.  These instructions were followed by Fieldlines member ClockmanFRA and Zubbly. While Zubbly used metal plates, ClockmanFRA prefers to replace the plywood regularly as a way to deal with wood rot.  If you go looking, you can see that my blades have double plates, and so do the blades made by BRCM.

So if there was any design change that I would suggest to make your blade attachments more robust, I think you can add a plate on the front.  This would make the entire rotor stiffer, and maybe less water would get into joints and seams.  That's based on an assumption that your blades flex at the hub joint, and a gust adding thrust in one moment and relieving thrust a moment later would open and close narrow gaps.  Water absolutely loves little gaps and seams.

This advice comes from a guy living in a region that got only 5 inches of rain last year (semi-arid) so it might not be worth much.