Author Topic: Hugh Piggott blades, Fiberglass, testing and failures.  (Read 8816 times)

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clockmanFRA

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Hugh Piggott blades, Fiberglass, testing and failures.
« on: January 04, 2014, 04:18:04 PM »
Fiberglass blades, testing and failures.

I have 3off Hugh Piggott 3.7m (12 footer) diameter Wind turbines. No 1 Turbine has American Cedar solid wood blades, but when I came to make my No 2 turbine I really struggled to get good size pieces of Cedar for the blades in the UK and France. I therefore decided to make a mould from Hugh's blade design and manufacture my own fiberglass blades.

Engineers Without Borders (EWB-UK) & Sibol ng Agham at Teknolohiya, (SIBAT – Philippines) produced a document, 1st may 2008, FIBRE GLASS WIND TURBINE BLADE MANUFACTURING GUIDE This guide used Hugh's 3.6m (12 Ft.) design of blade.

My fist set of fibreglass blades had been up a good 3 years, but had shown me that joining the 2 half's together was not as straight forward as described. Separation splits had occurred after one year at the root of all three blades. This was repaired and re-enforced at the root.
 
This Christmas we had a good blow, real stinker of a storm, not a hurricane but as you can see in the Picture I lost a blade and the tail dropped off. The tail was at the base of the turbine, the blade had managed 200 meters distance!. (In Pic No 3 turbine is down).

Interestingly my No 1 turbine has survived an official French Hurricane 5 years ago; just the top section of the pole got bent.

With No 2 down I noticed that the tail fully furled stop, was not exactly stopping flat/square on the main pole, i.e., it could push its way inside and up the pole, and hence the whole tail could lift off. I have now modified the stop bar with a longer one that still stops the furling at the correct position, but has a safe stop position. There are marks on the tail that could have been where the tail hit the blade?

The other two blades have fractures at the root, but these can be repaired, and I am now making a new replacement blade.

My No 3, has fiberglass blades Mk2, but these are really heavily re-enforced at the root and I use folded over woven matt,  at all the seams when I am putting the blade half's together but leaving clearance for the foam filling process. See pic of a blade before the top goes on.

It seems I am not alone with the root problem as this Irish manufacture of Hugh's design,  also using fiberglass blades,  http://www.hevenergies.ie/index.php/products/hevair-3k/  has extended his root mounting.  see the last Picture.
« Last Edit: January 04, 2014, 04:24:41 PM by clockmanFRA »
Everything is possible, just give me time.

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http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

SparWeb

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #1 on: January 05, 2014, 01:46:40 PM »
One shouldn't be surprised to see wind turbine blades fail in a hurricane!
Kinda voids the warranty.

Still, I was curious enough to read the manufacturing guide you mentioned.

I found a copy of the blade manufacturing manual here:  http://www.engineeringforchange.org/static/content/Energy/S00019/blade_manufacture_guide.pdf

My reading of it is that Hugh had little to do with its writing or design, he simply inspired the designers in creating a copy of his wooden blade shape, with fiberglass material.
I applaud their noble goal, but I see errors in the design as I read through it.  The authors know a lot about making parts with fiberglass, but I also have some criticism there.

Design:
Fiberglass is a layered material.  It forms a shell-like part, therefore many loads applied to the part become stresses in its skin.  Those stresses cannot distribute themselves through the thickness of the part because the core of it is a light foam filler material.  While that foam can support a bit of shear and compression, it cannot resist much tension at all.  Furthermore, the foam can only do so much to maintain the "cross-section" of the part.  Any force on the part that could cause the cross-section to change could cause the part to collapse.
To be more specific, the blade is subjected to bending when the thrust of wind load is applied to it.  The bending pushes the tips backward, putting the wind-ward face in tension, and the back face in compression.  For this to be successful, the tension side must not have the ability to stretch.  But it does - there is an elbow near the root where the airfoil cross-section ends and the root cross section blends in.  This blend is rather sharp, however.  This will be forced to straighten out under the radial tension from rotation and windward face tension from thrust.  The other skin in compression is fairly uniform, but it's not reinforced by any internal structure other than the "stringer".  If the back skin is even slightly wavy, the compression could cause the skin to buckle and then it would abruptly collapse.

Fabrication:
The lay-up procedure is well detailed and relatively complete.  However the lay-up misses an important factor known as "volume fraction" of fiber to resin.  In the lay-up, one should take care to press the layers together with a roller so that excess resin between the layers is squeezed out.  If this is not done, the layers are too far apart, floating here and there in a sea of glue, so they lose their ability to work together.  The final step of pouring all of the left-over resin into the mould is just about the worst thing you could do.

The "stringer" is a beneficial feature, and this is done well enough given the materials and tools available.  What has been neglected are "ribs".  There are none!  This is a major problem.  The blade changes cross-section a number of times but there are no ribs inside to define what the cross-section should be.  Rather, stiffness of the shells is relied upon to enforce the cross-section, but experience proves that this is not adequate.  The elbows at the roots will "unfold" under the bending stress applied by wind thrust, causing the blade to flex substantially more than a wooden blade would, furthermore putting a sharp flex at the inside corner of that elbow.  I think you mention some cracking in this area, and that's why.  Those ribs are also crucial in preventing the skin from buckling when it is in compression, as I mentioned above.


Test:
The first test looks like a good approach, but not nearly enough information is provided to allow a builder to do it correctly. 
The second test seems so irrelevant that I cannot begin to see the point of it.  10 Newtons?

The thrust load on a wind turbine rotor can be many pounds per square foot.  On a 12-foot rotor, without furling, that may be 200-300 pounds.  With correct and complete furling this can be reduced by a fair fraction, however the rotor is now spinning its wing-like blades edge-on to the wind, so the problems don't actually stop, they just change.  Rather than rely on brute strength, the blades must also be stiff to handle this kind of swinging about, otherwise their flexibility will provoke either fatigue or a tower strike.
None of the tests shown in the builder's manual address this well enough to show that the blades should survive.

So...  It's a good guide, but it doesn't leave you with a part guaranteed to last for long (even if a hurricane doesn't blow through).
The authors should take some of these failures to heart, and produce a revision that introduces ribs, make more gradual transitions in shape at the elbows, and squeeze out the excess resin from the parts.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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fabricator

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #2 on: January 05, 2014, 10:12:23 PM »
Spar is right it's all about the fiber not the resin, you would also do well to look into vacuum bagging, then you have no air bubbles, I can't believe an actual engineer would form a fiberglass blade that way, with fiberglass blades you use flowing curves not sharp bends like the wood blade pattern they are using.
Pour the extra resin into the form? OMG, you are being a little charitable Spar.
I aint skeerd of nuthin.......Holy Crap! What was that!!!!!
11 Miles east of Lake Michigan, Ottawa County, Robinson township, (home of the defacto residential wind ban) Michigan, USA.

Flux

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #3 on: January 06, 2014, 03:49:30 AM »
Can't add much to Spar's excellent comments except to say that it looks to be a direct copy of the wooden blade with that dreadful stress raising notch where the blade joins the root. Wood is one of the few things that will stand that. Even on my wooden blades I blend that area out in a radius rather than the sharp notch. There is virtually nothing produced in that inner radius and for fibreglass it needs to be designed mainly for strength ( and to some extent for low drag).

For fibreglass the flat front face is probably not ideal but with a reasonable rounding of the leading edge it may be fine.

I have never tried fibreglass but the joining of the two skins always seemed to be a point of extreme weakness as you have confirmed.

Having almost survived a huricane the design can't be far from the mark, with Spar's suggestions and a bit of change of shape at the root to reduce the bending stress I am sure you will get a decent working blade.

Flux

clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #4 on: January 06, 2014, 12:29:38 PM »
Thanks Flux for your encouragement, and yes my next set of blades, Mk2 type, have the alterations at the root.

Fabricator, I have seen blades being formed using a vacuum bagging process. But with just small number of blades like mine, vacuum bagging is a heck of a lot of extra work.

SparWeb, Many thanks for your comments and observations on that fiberglass blade making process. I will modify my Mk3 design with a couple of ribs at the root section of the blade, and I will add an extra stiffener I beam along side the other.
I will still allow the foam to pass through and around these new ribs. The foam once mixed starts to expand very quick, about 4 seconds, so I inject the mixed chemicals at the tip with a very large medical syringe. The foam goes every where as you can imagine, and I get the completed blade back in its moulds pronto and bolted up as I have concerns with the foam distorting the blade out of its proper shape.
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

SparWeb

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #5 on: January 06, 2014, 10:28:47 PM »
Clockman,
Glad you are not discouraged.  I'm always afraid of getting people down when I get critical, even when I think it's due.
I failed to follow your last comment.  Are you preparing to fix the broken blades?  With ribs or without, there is no way they can be repaired, and I worry for safety or collateral damage to the tower or generator if you try.

If you plan to make new blades instead, then I would suggest making new molds first.  If you are at a loss about vacuum-bagging, fiber-to-resin ratios, or curing with heat, as is normally done with quality fiberglass parts, then I can help you with that stuff. 
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #6 on: January 07, 2014, 04:14:32 PM »
SparWeb,

oops, sorry, I did not explain correctly.

I am at this moment making a new set, but adding the ribs, the extra stiffener and altering the root design.

The base mould takes the aero curved side of the blade, while I can just modify the top mould with that sharp/thin bit and make it curve more gently.

Its winter here in Normandy France so I do have good control of the mixing process and heat & times required.
 
I weigh the 200gm weight woven matt and normally do about 1.5 times more resin, as I have small rollers that ensure good penetration.

When joining the 2 half's, I now use folded over woven matt inside along all the 45 degree joints, this seems to work well on my Mk2 set for my No3 turbine, however any other suggestions would be very welcome.

Pic's shows my old moulds. (as I said I will keep the bottom and modify/new the other side). And a Pic of the moulds bolted together with the blade inside, during the blade half's coming together and also for the foam filling process.
« Last Edit: January 07, 2014, 04:23:01 PM by clockmanFRA »
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

SparWeb

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #7 on: January 08, 2014, 05:00:11 PM »
Quote
...Its winter here in Normandy France so I do have good control of the mixing process and heat & times required...

Haha - if I said something like that they'd never stop teasing me (I live in Canada).

Change only one of the mould halves?  I had difficulty with that approach once.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #8 on: February 12, 2014, 03:20:41 PM »
My No2 3.7m diameter is Up and running.

These new blades are my Mk3 design with a changed root and a stiffer internal rib and a small extra internal rib along side.

I put it up and would you believe it the weather forecaster in the UK says, gusting Hurricane force 12. Gulp! I am about 300 miles away from the storm, but still very strong winds here in Normandy.

First photo shows the 6.3m Gin Pole still attached to the main mast, as I will take the turbine down in the next few days to have a real close look at the new blades.

Second photo shows the turbine coming up to furl fully at about 32amps at 58vdc, but seems happy to run at 25 -28 amps before furling starts.
Just at the tail spigot I have used a very strong bronze flexible wire that now prevents the tail from lifting off the tail spigot.

Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

kitestrings

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #9 on: February 12, 2014, 04:13:12 PM »
Your blades look awesome. Nice work.  Congrats!

~kitestrings

sunbelt57

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #10 on: April 15, 2014, 06:40:06 PM »
Have you ever considered using Kevlar fabric? Its a little more expensive but you can use the same epoxy resin you would use for fiberglass fabric. The other alternative would be carbon fiber, which is stronger and lighter than fiberglass and Kevlar, but Kevlar will give more than fiberglass and carbon fiber and I think its stronger than fiberglass too.

http://www.uscomposites.com/kevlar.html

kitestrings

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #11 on: April 15, 2014, 09:26:14 PM »
clockr,

I'm not experienced at all in this type of blade design process, so I can't add anything meaningful.  I did enjoy the photo-journal of your build and in particular the blade mold and fabrication on Nav*> (that other site ;)).  It's impressive workmanship.

I also know Jim Sencenbaugh had moved to wood-core blades near the end.  He described a process he'd starting using where he dropped the formed wood core (Sitka spruce I believe) into a heat-shrinkable Dacron "sock".  Over that he used a hard fiberglass or epoxy skin.  I may have a better description from him.  Gotta look a bit.

~kitestrings


clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #12 on: April 17, 2014, 04:51:36 PM »
sunbelt57, yes, I also considered Kevlar, but flipping heck the price is nearly 5 times the price  :o of normal 200gm woven fiberglass cloth.

Hiya Kitestrings, thanks for your kind words re. nav....n postings. as..... http://www.navitron.org.uk/forum/index.php/topic,17156.45.html

To be honest I would now prefer to use one piece bits of American Cedar for each blade, but as I said its real difficult finding quarter sawn lumps at the right size here in Europe. I have tried laminating with cedar but one test blade just exploded so hence me going to a complete Fiberglass solution.


I was never particularly happy with the blade joining process where the internal foam was the adhesive and then just a thin cloth on the edge seams. So I was prepared for the blades to start opening/splitting.

No doubt about it, the root has to handle some real stresses and needs to be flexible and yet strong enough to stop the joints from splitting open, then water gets in to the blade, soaks into the plywood root and it expands, blades get out of balance etc etc..
I still want sharpish leading and trailing edges so now spend time ensuring the joins are well coated and then nicely finished. I use the good old jigger bug/orbital sander.

My latest set have a sock arrangement around them at the root and then gently finished. This seems to be working real well, but I will await the next winter storms to see if their is progress with this latest design Mk3 type.

My marine plywood core in the root is to stop the blade from crushing and give the screws something to bite into, (Hugh's concept of disc and triangular clamping the blades), but,  I now have 2 I beams coming away/out of the plywood root block.

Another problem arrives with all these modifications, its weight, agghh.
 Each blade is now about 6kgs and that's double the weight of good American Cedar wood.  So I can not seem to win at the moment.

Ps, in the nav....n forum link above you will see a link to a vacuum process using moulds with fiberglass process. The process is good but very, very time consuming. http://www.youtube.com/watch?v=XVj_YyvgMbE

A set of Mk3's getting their finishing coats.
 



 
 
« Last Edit: April 17, 2014, 04:57:03 PM by clockmanFRA »
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

SparWeb

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #13 on: April 17, 2014, 08:27:41 PM »
Quote
...I have tried laminating with cedar but one test blade just exploded...

That's troubling to hear.  Do you mind if I ask about what happened, in detail?
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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kitestrings

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #14 on: April 17, 2014, 09:08:57 PM »
Quote
Another problem arrives with all these modifications, its weight, agghh.
 Each blade is now about 6kgs and that's double the weight of good American Cedar wood.  So I can not seem to win at the moment./quote]

Ours (wood, balsam fir) are clocking in at about 23# ea (10.4 kg), so that actually sounds good to me.  I suspect the combined rotor/stator weight is the larger factor in most builds, except where the alternator offset (e.g. Chris O's, Ont Farmer's).

~ks

clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #15 on: April 18, 2014, 03:55:35 AM »
Hi SparWeb, I have one that survived but has a join opening, will post some pics later, I used this laminated blade as the master for my moulds.

Hi kitestrings, might have to amend those weight figures, just realised that weight I quoted was the woven cloth weights and i need to add total. I have a set of fiberglass blades awaiting use so will actually weigh each.
Mrs CM is here so best go to work!!!
 
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #16 on: April 18, 2014, 02:24:26 PM »
A new set of 3.7m diameter blades MK3 type fiberglass awaiting fitting.


American Cedar laminated blade, with the centre joint starting to open.

Fiberglass blade MK3, finished, weight. About 6.5kgs.


Cedar blade weight is 2.6kgs.


« Last Edit: April 18, 2014, 02:34:28 PM by clockmanFRA »
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

SparWeb

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #17 on: April 19, 2014, 12:19:50 AM »
The split seam is very interesting to see.  I once had to discard a hundred dollars of cedar that I had not glued properly when laminating.  Perhaps a case similar to yours, either the surfaces were not flat enough, or I did not use enough epoxy in some of the joints, I made both kinds of mistakes..  On the other hand, the cedar blades that I laminated and carved 6 years ago are still going strong.

One think that I think I can see in your cracked blade (correct me if this is wrong) is that the separate pieces that were bonded together started out quite thick.  There appears to be only 2 or 3 pieces that were bonded together.

This was a factor in the laminating process that also failed for me.  I believe that I was bonding pieces together that were too thick to "squish" together properly.  I had planed a number of "2x4" boards, which are about 4cm by 7cm if you are not familiar with american lumber measurements.  I thought I had done an excellent job of planing them super smooth, but after the glue was dry I had many joints that were several millimeter thick, and many gaps where the glue had even receded into the joint.  Who knows how many "void" areas were hiding inside.  I had so much doubt about the quality that I didn't even try to make blades from them.

My successful blades were made from boards only about 2 cm thick, so that small imperfections in their flatness after I planed and sanded them were closed easily by the clamping.  They squeezed tightly enough for the epoxy to thoroughly spread through the space in all the joints, and I have had no split joints.  Interestingly, there is some cracking in the blades, but these are drying cracks, and they do not threaten the integrity of the blades.  I did put filler into some of the cracks and sealed them - allowing water to penetrate inside would lead to problems in the future.

Still, it's clear to me now, what trouble you had, but thankfully you stuck with the project.  The fiberglass blades really do look nice, and I hope you get many many years of trouble-free service from them.

The density of fiberglass is about 4x greater than wood, so your arrival at a fiberglass blade weight 2x greater than the wood sounds just about right to me.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca

clockmanFRA

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #18 on: April 21, 2014, 06:21:27 AM »
Hi SparWeb, Yes I concur with your too thick synopsis.

I also noticed that the grain was also a possible contributing factor to the joint opening, as each piece of cedar was different in density and probably had different movement characteristics, just a thought.

I think if they were thinner, then as you say, I could clamp them up and squish the pieces real good.

Regards the present fiberglass blades, I will keep on flying them and see what happens.
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

SparWeb

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Re: Hugh Piggott blades, Fiberglass, testing and failures.
« Reply #19 on: April 21, 2014, 10:14:36 PM »
Quote
...I also noticed that the grain was also a possible contributing factor to the joint opening, as each piece of cedar was different in density and probably had different movement characteristics, just a thought...

Yes that's plausible, but a strong epoxy joint should resist this.

Quote
...Regards the present fiberglass blades, I will keep on flying them and see what happens.

Of course.  They're too beautiful not to fly!  :)
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
www.sparweb.ca