blade balancing act

[Ballyk]

hello all,
Some weeks ago posted about a rocking Exmork turbine. I have since discovered that there is a difference of 160 and 150 gramms on two of the blades. the heaviest one being a control. I have brought all the blades to within 2 grammes of each other with stick on wheel balancing weights. I need to know if I should try to exactly balance the centre hub or will that make much diference to the overall position. I have read somewhere that the balance of the blades is far more important. Is this the case or am I just being hopefull.
regards
ballyk

[zvizdic]

Balancing is most important out of balance prop is dangers and can loosen all the screws and nuts and ultimate destroy itself .           

[imsmooth]

Read this for blade balancing

http://www.mindchallenger.com/wind/axial13.html

[Ballyk]

Thanks for the advice all ;

[SparWeb]

Ballyk,

I didn't see the original post, but is sounds like your WT is on the big side.  Have you investigated the cause of the imbalance?  If you have the balance down to 2 grams on the hub, then you are in a much better situation than 150 gm.  The blades may have a foam core with water soaked inside, for example...  in that case they must be dried and inspected for de-lamination before going back in the air.

Sometimes static balance is not enough, which is all you get with simple balance weights on the hub.  If you continue to see rocking or hear any cyclical noises, then you might need to seek a "dynamic" balance.  It's not as hard as some people make it sound.  The physics of it may be based in calculus, but when you understand the principle, the math doesn't have to be hard.

Let me know if you have any concerns about the fiberglass, or about dynamic balance and I'll give you some pointers.

[Ballyk]

hello all,
I still am not getting the result I need. I hung the complete set up on a woir the other day . This after getting all the blades exactly the same weight. The set up tilted right over and I had to put two 17mm spanners on the enda of the two opposite blades to get it into any sort of level position I am really at a loss here as to what to do. My problem is that the two part hud is marked 1,2,3 to alogn the two plates of the hub. There are A,B,C markes on the hub which obviously relate to the blades. the markings are gone off the blades so I dont know which one is A,B, or C. I assumed that by getting them all the same weight I would counteract the ABS marks but I was wrong. I am at a loss as to what to do now.
Regards

[TomW]

I would just point out 2 items:

#1 A pair of blades can be within a gram of one another and not carry the weight in the same areas. This makes balancing the assembly as a whole a must. A gram at the hub is nothing compared to a gram out on the tip of a long blade.

#2 Sometimes it is easier to start from scratch than to recover a muck up.

I have balanced a handful of blade sets but certainly claim no expertise.

I avoid Chinese goods as much as possible because I feel their standards are pretty low.

Tom

[Flux]

How are you checking the weight? In reality weight alone is pretty meaningless. What you need to achieve is the same distribution of weight at a given radius.

You could have identical weight blades but the weight difference could be in different places. You may get some indication by supporting the root on a knife edge and weighing the tips, the tips are the critical region.

It sounds as though you have very different tip weights even though the overall weights are similar.

Normally if blades are physically identical and made of something uniform then the things balance well but if constructed of hollow skins the thickness of the skin ,may be far from uniform.

The other critical factor is that you balance about the exact same point as the blades rotate in real life. If you have a free moving hub then it is best to use that, otherwise you have to be very careful indeed that you use the exact same point. An error of 1/8" in this positioning would make the whole process nonsense.

You have to assemble the blades on your hub and use the complete assembly (hub & blades) to do the balance.

I don't have much faith in hanging complex assemblies on bits of string, it works better for simple things.

Ideally you need to make up a dummy shaft with two very free bearings and mount the hub on that and balance it, trying to avoid any couple from weights all at the back or front. When the hub is right you fit the blades and do a final balance on them. If you can do it easily it is better to try to add weights to the tips, if not possible you will have to do it at the root but you will need large weights and there is again far more chance of introducing a couple.


From your description you have a serious issue with the actual construction of the blades or you have a hub problem or your balance point is way removed from the true shaft axis that it normally runs on.

Without seeing the set up it is tricky to even guess what is wrong. First step may be to try measuring the tip weights with the root supported on a bit of bar or angle. if there is any significant difference then you must completely forget total blade weight.

If the tip weights are similar then I can only suspect the hub is not at the exact rotational centre and if it is a heavy hub this will cause serious trouble.

This should give you some pointers to start from and you can always come back with more specific questions when you can see where it is wrong.

Flux

[Ballyk]

Hello all.
I think I have had a "eureka" moment. See if I am right withi analogy. If I hold a broom handle at the end with the tip out srtetcged and add a 1lb weight at my hand I can hold it easily. If I move the weight to the farthest end of the handle I cant hold it even though both are exactly the same weight. !!! Hence the need to achieve the same balance on each blade to avoid a wobble.

please God let me be right. I once had a sergeant who told me that he thought he was wrong once but was in fact mistaken !!!

Regards

[TomW]

Hello all.
I think I have had a "eureka" moment. See if I am right withi analogy. If I hold a broom handle at the end with the tip out srtetcged and add a 1lb weight at my hand I can hold it easily. If I move the weight to the farthest end of the handle I cant hold it even though both are exactly the same weight. !!! Hence the need to achieve the same balance on each blade to avoid a wobble.

please God let me be right. I once had a sergeant who told me that he thought he was wrong once but was in fact mistaken !!!

Regards

Ballyk;

This is exactly what Flux and I are saying in the  2 comments above your last one!

It is pretty simple but critical gotcha (where the weight is).


Tom

[poco dinero]

Hello Ballyk,

I have an Exmork 5 kw turbine.  The owner's manual says that the blades were attached to the hub, and the whole assembly spun up and balanced at the factory.  The blades and hub positions were all marked with numbers so that they could be reassembled in those same positions.  I checked my hub and blades and yea verily they are marked.  Haven't flown the machine yet so I don't know how good the balance is.

My suggestion is to check the blades and hub to see if they were factory balanced and marked.  If they were, and you ignored those markings when you assembled the blades to the hub, that could be your problem.

Other replies in this post have cast suspicions on the balance and "trueness" of the hub.  I doubt that that is the problem, my hub looks like a fairly high precision assembly.

I also have two 2 kw Exmork turbines but haven't inspected them to see if the blades and hub are marked as though they had been factory balanced.  They're up at my other place; will check them out today.

poco

[Ballyk]

Thanks pocco,
That is exactly what happened to my machine. I was working on the tower and ny son was putting the blades together. I didnt check them and am now paying the price. The instructions I got with mine werent that great. Did you get the whole Kit from exmork, controlller inverter etc? When mine is running, complete with wobbles, I am getting great power from it. So much so that I have had heat build up in the six square coming from the turbine to the controller. Not much but it gets warm. I have a 600ah forklift battery connected and it is constantly full and will run my house for days. I have reduced my electricity bill by about 70%. Cant say I agree with the chineese comments above as yet.
Regards,

[poco dinero]

Hello Ballyk,

I went up to the other property and checked my 2 kw Exmork wind turbines, and I found no evidence that the factory had assembled the rotor, spun it up and balanced it, as they had done for my 5 kw turbine.  But I did get another idea of what might be causing your problem.

I was working on the tower and ny son was putting the blades together.   

My owner's manual isn't all that  great either, but one thing that was emphasized very strongly is that it is extremely important to measure the distances between blade tips and to ensure that these distances are exactly the same.   The slightest amount of slop in the blade-to-hub bolts could, when magnified by the length of the blade, throw those distances seriously out of whack.  And that could have a drastic effect on the rotor balance, by moving the center of gravity of the rotor away from its center of rotation; even if all three blades were of equal weight.  If I were you, the first thing I would do is to check those inter-blade tip distances and make sure they are exactly the same.

Did you get the whole Kit from exmork, controlller inverter etc? When mine is running, complete with wobbles, I am getting great power from it. 

Yes, I got the whole kit and kaboodie.  How could anybody pass up a deal like that--for $165 you get an MPPT controller and a 4 kw heat dump.  That controller is too cool.  When you flip the turbine kill switch, it gradually raises the voltage that the alternator has to work against, until the blades are fully stalled, then it shorts the three phases together.  No sudden jolt like you get when you short the phases in one fell swoop.

I was glad that you are getting great power from your machine, wobble and all.

Cant say I agree with the chineese comments above as yet.

Glad to hear that too, as there is at least one individual on this forum who appears to be heavily prejudiced against Chinese products and likes to knock them every chance he gets.  He claims to have documented evidence, but when you ask him for the evidence, he flipflops and says that his documented facts are just his opinions.

This is exactly what Flux and I are saying in the  2 comments above your last one!

That statement reminds me of the 1988 vice presidential dabates on TV, when Senator Dan Quayle (a lightweight) was trying to associate himself with John Kennedy (considered by many to be a heavyweight).  Lloyd Bentsen popped up and said  "Senator, I knew John Kennedy, John Kennedy was a friend of mine, and, Senator, you are no John Kennedy".  Quayle looked like a whipped puppy, as well he might.  One of the greatest moments in American politics.

Let us know if checking the distances between blade tips solves the problem.

poco

[DamonHD]

Damning all Chinese products seems to be as popular a current US sport as our damning Japanese and then Hong Kong products in the UK, until we found that out local production sucked worse in terms of performance, reliability AND price.  I think we get what we pay for, and I note that the Chinese are now in turn having to launch action to protect their brands against cheaper knock-offs from elsewhere in the world...  Plus ca change, as we say in London.  %-P

Rgds

Damon

[Flux]

If the blades are bolted to the hub in such a way that you can get the tip spacing wrong then that is indeed a significant factor. The tip spacing should ideally be better than 1/4" before you even attempt balancing.

Unlike tracking this affects static balance. Spacing fore and aft , tracking, is also important but will not show on any static balance tests you do. It will however introduce a couple that causes dynamic unbalance and this will also cause a wag.

The serious errors you reported earlier could indeed be caused by incorrect tip spacing. I really can't see why the hub and blades needs to be marked, there is no reason to ship something with an unbalanced hub but if the blades are drilled and individually fitted to the hub you may have to mark them to get the tip spacing right.

Your broom handle analogy perfectly sums up what Tom and I were trying to explain, total blade weight with no reference to position is no criteria for balance. Ideal blades will all have the same weight at the centre of mass and this will be in the same radial distance on all the blades.  The actual weight really doesn't matter that much if you add balance weights but it becomes very messy if there is serious non uniformity on the blades.

From the weight you had to add in your balance test i can only assume something is very wrong, normally a few grams error at the tips shows up, I wouldn't expect you to be able to fly it with the sort of errors you described without it shaking the tower violently.

Flux

[Ballyk]

hello again,
This afternoon after watching the heineken Cup semi final, I again went out to my little troublesome machine.I weighed the blades with the root on an edge and after removing the weights I had added. The tip weights were all within a couple of ounces of each other. I found that the weights I had added at the root really had no effect on the tip weights. There are only 9 different combinations of blade positions, and tomorrow I will try them all if the day is calm enough.
regards

[poco dinero]

Hello Mr Ballyk,

I think I may have some good news for you, especially if your exmork is the 2 kw or the 2.5 kw machine.  In reality they are the same machine, only the 2 kw is rated 2 kw at 22 mph wind, and the 2.5 is rated 2.5 kw at 27 mph wind.  The two machines have exactly the same power curve, and they are in fact the same machine.  The 2.5 kw is no longer advertised on the company's website.

hello all,
I still am not getting the result I need. I hung the complete set up on a woir the other day . This after getting all the blades exactly the same weight. The set up tilted right over and I had to put two 17mm spanners on the enda of the two opposite blades to get it into any sort of level position I am really at a loss here as to what to do. My problem is that the two part hud is marked 1,2,3 to alogn the two plates of the hub. There are A,B,C markes on the hub which obviously relate to the blades. the markings are gone off the blades so I dont know which one is A,B, or C. I assumed that by getting them all the same weight I would counteract the ABS marks but I was wrong. I am at a loss as to what to do now.
Regards

Maybe you weren't wrong.  My apologies!  I completely missed this post of yours first time around.  But here's the good news.  I went back up and further examined my 2 kw exmorks, and they have that same ABC marking on the two halves of the two piece hub.  BUT THEY HAVE NO MARKING ON THE BLADES.  So what that means is, you haven't lost the identification of which blade goes on which hub position; there never was any requirement to put a certain blade in a specific hub position.  The ABC marks are only there for the purpose of aligning the two halves of the two piece hubs.

 

There are only 9 different combinations of blade positions, and tomorrow I will try them all if the day is calm enough.
 

You don't need to do this.  The manufacturer never intended for you to put a specific blade into a specific slot in the hub. 

Mr. Ballyk, you probably have never built a DanB turbine, but you need to read and follow what he says about assembling and balancing your rotor even though you bought a commercially availaqble machine.    It will make your problems go away.  DanB (Hugh Piggott also) didn't worry particularly about whether one blade was a little heavier than the others (fix that problem with statically balancing the rotor).  He didn't worry aqbout weight distribution differences between one blade and another (they're a second or third order effect that also go away with static balancing of the rotor).   And he didn't worry about dynamic balance, because these blades are only an inch and a half thick, and if the blades track each other, most anything else that affects dynamic balance is insignificant.  Wood is not nearly as homogeneous as fiberglass, and your rotor balancing problems are a piece of cake compared to what he had to contend with. 

What he did worry about was the following:

1.  When assembling the rotor, you gotta make sure the blade tips follow each other around the plane of rotation of the rotor.  If one tip is 1 inch behind the plane, and another one is an inch ahead of the plane that introduces a couple that will cause dynamic imbalance.  So you really need to address this issue, so that your rotor is not dynamically imbalanced.  Anything else that could possibly affect dynamic balance is insignificant, and was not addressed by DanB.  You needn't worry about it either.  If there is a problem with the blade tips following each other around the plane of rotation, DanB tells you how to fix it (shims).

2.  When assembling the rotor, make sure the blade tips are equidistant from each other.  DanB tells you how to do this.  I've already elaborated in a previous post how drastically the tip spacing can affect your rotor balance.

3.  NOW, and not until you've completed the above steps, statically balance the assembled rotor.  DanB tells you how to do this.  I couldn't possibly improve.

If you carefully follow DanB directions, I'll bet you dollars to doughnuts that your problems will all go away. 

Best Regards and keep us posted,

poco



   

[zvizdic]

OK , factory balance blades and hub together  so  you need to assemble  and balance like that . Forget tracing, tip spacing and tip wight  your problem is finding blade order on a hub end of story .

[Perry S]

OK , factory balance blades and hub together  so  you need to assemble  and balance like that . Forget tracing, tip spacing and tip wight  your problem is finding blade order on a hub end of story .

End of story? I don't agree. Even if the rotor was prebalanced at the factory you need to recreate the assembled geometries that were present at the time of balancing. Proper tracking and especially tip to tip distances need to be properly maintained to have any hope of balancing the rotor. Never seen an Exmork hub but I imagine that if you loosened the blade bolts you could move the tips an inch or two in either direction. Tolerances are like that. Identifying the proper blade for the proper 'slot' is very important but once that's done, you need to check tracking and tip spacing for sure.

Perry

[TomW]

OK , factory balance blades and hub together  so  you need to assemble  and balance like that . Forget tracing, tip spacing and tip wight  your problem is finding blade order on a hub end of story .

End of story? I don't agree. Even if the rotor was prebalanced at the factory you need to recreate the assembled geometries that were present at the time of balancing. Proper tracking and especially tip to tip distances need to be properly maintained to have any hope of balancing the rotor. Never seen an Exmork hub but I imagine that if you loosened the blade bolts you could move the tips an inch or two in either direction. Tolerances are like that. Identifying the proper blade for the proper 'slot' is very important but once that's done, you need to check tracking and tip spacing for sure.

Perry

I have to say that Perry is right here and  zvizdic is giving very bad advice that should NOT be followed or believed!.

My reasoning is that when folks like Hugh Piggott, Dan Bartmann (in published books) and Flux (here) say it matters I take it as truth.

To even claim that it does not matter is ludicrous and defies basic physics.

Tracking and tip spacing all matter on every prop regardless of the material or manufacturer.

Take it or leave it but bad advice must be countered with proper advice.

IT MATTERS

Listen to the pros in the field. Anyone can prove this simply and easily just balance a prop perfectly then displace one tip and see how it balances now? That ain't rocket science and anyone can do it.

I know it because I have seen it.

Of course if the factory did all of this properly then you should be able to just assemble it as they did and it will be as balanced as they made it.  Even then I would still check the parameters myself anyway. Regardless of the origin.

The turbine Zubbly [RIP] built for me was pre balanced and had index marks on the parts. It assembled simply and easily with perfect balance after a trip from Canada in a box.  He used a neat trick with a drill bit to set each blade properly before tightening the bolts. Loosely assemble align hole in hub with hole in blade root insert bit and snug it down. So prebalanced works if done properly, also.

If there are no index marks or other method to show where each blade goes you gotta go from scratch which seems to have been the case from the original posters comments.

Tom

[Ballyk]

Thanks to everyone for the advice so far. I think I will do what everyone has said if thatsd possible. I will start from scratch and do a blade balance on the hub on the shaft.( shed isnt big enough so will have to wait here for a calm day. Atlantic coast or Ireland doesnt have too many of those) I will then check tip alignment and then recheck blade alignment and balance again. I will do this after going through all the possible combinations of blade positioning on the hub.
Watch this happen: after all that there will be no wind here for days. Oh the joys of wind turbines!!!!
This site is one great asset though. Its a pity you all dont live down my road!!!

Regards

[DamonHD]

Hey, I put up my new solar grid-tie and banished sunshine from the UK for the bank holiday...  B^>

Rgds

Damon

[poco dinero]

I will do this after going through all the possible combinations of blade positioning on the hub.

You will be wasting your time if you go through all the possible conbinations.  For each combination you would have to check tip tracking, then intertip distances, and then do a static balance.  What you will find is that each of the (9, I believe you stated) combination works.

The reason I am sure of this is because I have a brand new exmork 2 kw and the blades are still in the shipping crate, and THEY HAVE NO INDEX MARKS.  Tells me it doesn't matter which blade goes into which hub position.  The manufacturer did not preassemble your rotor and balance it.

I don't think it matters whether you check and correct the tip tracikng first or the intertip distances first.   I would do the tracking first, the intertip distances second, and then come back and recheck the tracking.  Do the static balance last.

poco

[Ballyk]

Hello poco,
I am beginning to agree with you. I sent an e-mail to exmork and as ususl they were back to me in under an hour. I asked them about blade order/combinations etc and I have copied their answer below:

Hello Eamonn
Sorry, i can't help you because each set blades are different.
 
But you could try. It doesn't have large infulence.
 
Best Regards
 
James Ye

The "it doesnt have a large influence" bit suggests that they can be placed anywhere on the hub. I have taken a very accurate tip to tip measurement and Have found a diference of over 1 inch. I will spend the afternoon setting them up by tip distance and maybe this will settle other problems as you said. The three bolt holes in each blade are very tight with no real movement in them so I am hoping they will line up with moving them aroundand hopefully get back to the correct factory setting that way.
Regards

[Ballyk]

Just a note to add to my last post. I spent the afternoon looking at tip to tip measurements and found that I had about about 18mm of discrepency. I added the three tip distances and then divided by three and came up with 2890mm as the optimal distance. I coaxed my better half to help me as she is getting slightly bored with this project. Using a ratchet strap, I now have exactly 2890mm from tip to tip on all three. Tightned up all the bolts and put it to bed for now before I start to hate the thing !!!!!!

Will have a go at the tip plane during the week and post results

Thanks to all again

regards

[zvizdic]

I did not wont to offend any one. I merely seed, order is mist and he need  to find proper place for each blade and he is good to go.
Tip out 1" no big deal couple of grams or there.Tracing the closer the better  but wont vibrate badly as out of balance blades.I build many different machines and ran into  balancing issues so I know little.

      Thanks to all     

[SparWeb]

Ballyk:  Will have a go at the tip plane during the week and post results

Good luck, and yes I'm keen to learn how it improves, with all the extra attention you're giving it. 

The wind is howling out here, and because of your thread I put my hand on my tower to feel the vibrations when I last went out to check on things.
The tower used to really shake with my first genny and its blades.  After making new blades last year, and balancing them much better than last time, it is gratifying to feel just a coarse hum in the tower, now.  I know, it seems like a black art, but then you have balanced blades and it purrs no matter what speed the wind blows.

[poco dinero]

Will have a go at the tip plane during the week and post results

Best way to do this is to clamp the alternator so that it can't move, assemble the rotor onto the alternator shaft and slowly turn the rotor so that each blade tip passes a stationary object placed near the blade tips.  Each tip should clear the object by the same amount, say, a millimeter or so.

Most likely they will be a little out of whack.  I don't know how much is too much, but my gut feeling is that if they are all within a millimeter of each other, that's about as good as you can get them.  This adjustment is important, but it's not nearly as important as the blade tip spacing. 

I seem to remember a post on this forum that tells how to fix them if they're out of whack.  It involved fitting various thickness steel shims at various places between the hub surface and the blade surface where it makes contact with the hub.  There should be one blade that you use for a reference, so you will be shimming, at most, only two blades.  It's possible that you might be shimming the front side of one blade and the backside of another one.  Sort of a hit or miss, trial and error proposition.  When you get it  right, I would glue those shims on and also mark the blades  A, B, and C (the hub is already marked) so that if you ever disassemble the rotor again, you put each blade back where it came from.  I would also mark the alternator shaft and the hub so that each time you assemble the hub onto the alternator shaft, it goes back in the same posotion as it was when you placed the shims.  That will protect you against the possibility that the alternator shaft is slightly bent.

Be sure you go back and recheck your tip spacing to make sure it's still 2890 mm after you adjust the tip tracking, because you're going to have to loosen those bolts to insert the shims.

Best Regards and good luck.

poco

[Ballyk]

hello all again,
cant wait for the weekend now to put all my new knowledge into practice !! I will post a few pictures of my own design tower hinge system. I think its cool and works very easily. I can now raise/lower it on my own.
Will spend weekend at the blades and post how I get on.
Regards

[Ballyk]

Hello All.
I am back again with another instalment of this saga. Today I checked the plane or rotation of the blades. They are in about 1/4 inch of each other so I assume thats O.K.. I also started the dynamic balance of the blades using stick in car wheel weights located within 4 inches of the tips. Results as follows: Balde B heaviest and always at the botton at the start. Blade A 100 gramms added and blade C needed 80 grams added. Again all weights within 4 inches of the tip. After this exercise I saw no apparent "Dominant" Blade. I would say that It could be even more exact but I only used 10 and 5 gramm  weights.

Question 1: Does the needed weight seem excessive or normal.
Question 2: I placed half the weight on the front and rear of the blade. Is this correct ?
Question 3: I will replace the car weights with lead strips. How are they normally fixed to fiberglass blades?

I feel I may be getting to the end of this process. Please tell me I am !!!

Regards,
BallyK

[zvizdic]

OK blades off the hub , steel rod trough the hub and spin it should always stop with heavy side on the bottom that were the lightest blade is going rest should be easy. I assume hub is ware they install balancing wight even if you cant see them .   

[SparWeb]

Hello Eamonn,

You sound a bit daunted, and I agree this is delicate.  Requiring 100 grams (nearly 1/4 pound) near the tip of two blades is a very large correction.  I've been confronted with much less imbalance in my past experience, and found that objectionable.  The corresponding weight near the hub, of course, would be enormous - 100g * radius of tip / radius of hub = A REALLY BIG MASS.  In your case, I would seriously investigate returning or exchanging the blades you have for a more closely matched set from Exmork. 

Putting weights near the tips of blades is troublesome.  If you lose one the consequences can be disastrous.  When spinning, the centrifugal force on the weight will be tremendous.  Adding the weight to the exterior of the blade also introduces bumps that wreck the aerodynamics, also a problem for various reasons.  Attempting to make it smooth can help, but without some means built into the tip of the blade, you're cutting and drilling the blade yourself.  If Exmork didn't provide any balance weight attachments near the tip, you need to be very careful if you're going to add one.  If as you say, you can put one on either side, that is okay from a dynamic stability point of view, but I'd rather consider all other options first.

I e-mailed you a spreadsheet about shaving down selective areas of a wooden rotor blade to make it balance with the others, but that won't get you very far with your fiberglass blades.  Sorry to get that mixed up.  However, it gets me thinking in a certain direction and I hope this doesn't sound too crazy.

Instead of adding weight to the lighter blades, how can you remove weight from the heavy one?  What I think you could do now is find the center of gravity of each blade by resting it on a tube or piece of angle iron until it's balanced.  The point where the blade touches is its center of gravity.  Do this for all three and each will have different c.g.'s, I expect.  Next weigh each blade as carefully as you can - to the nearest 0.1 pound (0.05 kg).  Post the weights here, and feet them into pages 2 and 3 of the spreadsheet I sent to you.

The spreadsheet isn't "automatic", but by adjusting the numbers, you can find the weight changes that will fix the problem.  Sheet 2's fix is to add weight on a hub bolt.  Sheet 3's fix helps you find the amount of weight that should be removed from the heavy blades.  In my case, one blade was obviously lighter, one was heavy with excess weight near the tip, and the third was heavy, but its excess weight was near the root.  From this interesting result, I was able to shave from the correct places and balance them up.

So maybe you have a chance, if you do this check, to find that you could REMOVE weight from the root of your heaviest blade (maybe).  If it's possible and you get a better balance as a result, then you've got at least a partial solution.  Of course, I haven't seen the Exmork blade so maybe cutting out material from the root would be stupid or impossible.  It worked on my wood blade with no problem so that's the only reason I suggest it.

Well, I've had a really long day and I'm getting bleary-eyed.  Third baby horse born this morning, before dawn, of course.  At least this experienced mare didn't need much help. 
I hope this made some sense, but don't be afraid to get me to repeat or tell more because I feel like I'm cutting the explanation short.  Going to bed now.  'Til later.

[TomW]

I have to agree with SparWeb here about the weight on the tips.

It is a poor option due to all the reasons he stated.

Zubbly (R.I.P) showed me a fairly simple and foolproof method that has worked well on the 3 props I used it to balance.

I am in the middle of balancing my 12 footer but it is a icky day overcast and windy. I had to stop at dark last night and today wind is too strong to balance it properly. It makes it easy to explain as the prop is on the alternator and on the test stand so I got a picture:





See the blue steel strap about 04:00. The blade across from it was the heavy one. It always ended up on the bottom at 06:00 even if i stopped it at the top. to get some idea of how much weight I needed I used a small C clamp to attach that blue bar to the hub plate directly across from the heavy blade and straight out from the hub.

I then gave the prop a spin so it rotated a couple full turns. The clamp and bar were still obviously too light so I added another 2" clamp out at the end of the bar. Too heavy. I then split the difference from it to the hub end, did the spin test again and repeat sliding the clamp in or out until the prop does not settle in any one spot. I do about 10 or 15 spins and if it is random where it settles I call this balanced.

Now you could leave the clamps as is but I doubt it would last. I bolt the blue bar to the hub and then I can add weight as needed in the form of nuts, bolts / washers to attain my balance. This one will need a lot of weight so I will use decoy weights on the inner angle of those 2 blades behind the bar Screwed right to the blade roots probably will need 3 or 4 and then whatever I need to add to the bar to fine tune it.

Does that make sense?

Sometimes you can just use a blade mounting bolt to add the arm. I chose to bolt it to the steel hub plate as there was a space for it right where I needed it.

This has worked well for me in the past so it is my preferred method. It keeps the weight in tight to the hub where it is less stress on everything.

Good Luck with it.

Tom