New rotor blades

[bigrockcandymountain]

I have carved a new set of blades.  They are lower tsr and stiffer.  I am still deciding whether i need to shim the generator at an angle of just rely on the added stiffness to keep them from hitting. 

The left is an old blade and the right is a new blade. 

I also decided i will add a manual furling lever at the base.  The complicated part is the swivel.  It will all take some time to build so it likely won't fly for awhile. 

[hiker]

Nice blades,,just go ahead and shim,,cheap safe insurance,,,

[kitestrings]

And the new ones are going to spin in the opposite direction, or are the photos from opposite ends of the blades?

[bigrockcandymountain]

You are correct ks.  I had my jig all set up when i noticed they will spin the other way.   I am having trouble balancing this set.  One blade is way lighter wood so its going to take a chunk of 1" thick steel about 6" by 3"  to balance it out. 

[midwoud1]

Hi BigrockCM.

What we do with planks of different weight.
Cut them in strokes of 1,5 inch , make a composition  With the strokes so you have 3 planks ,
Glue them . Start carving.
Balance the blades  in the same center of gravity .
Rgds. F.

[kitestrings]

If the weight difference is small, I usually find it easier to correct at the hub.  On larger differences, we used to drill radially (?), in from the tip, toward the hub at the fattest part of the chord and fill with lead.

[SparWeb]

Filling the tips with lead works, and if it's not enough you can finish with the plate on the hub.
You can get an idea of how much you need by weighing and balancing each blade individually.  The two heavy ones will have a higher weight and a CG further in or out to the tip than the lighter blade.  The difference in weight is what you need to add, partly at the tip, partly at the root.  Before drilling and heating up lead, you can just stack things on the lighter blade's tip and root (I used washers) until you have it approximately matched to the others.  You can also match the CG where it balances at the same time.  There's only so much lead you can get into the tip.  It's never perfect and you can make some final adjustments when the blades are reassembled again.

I still think you should be tilting it "nose-up", too.

[hiker]

Careful not to much weight at the tip,,,it's the weakest part of the blade,,had the same prob a few years back,,,added weight to the tip of one lighter blade,,worked fine,,for balance,,then a big gust hit ,,bang,,rattle rattle,,,their she Blows. ,,,,😜

[bigrockcandymountain]

Ok based on the comments here i think i will just balance at the hub with a big lump of steel. 

I have been working on the manual furling.
  I did cut some shims so my clearance to the tower should gain about 3" at the tips. 

I hope to have it up next week but that is likely not realistic. 

[ontfarmer]

I put apiece of flat steel on the end of the tip  secured with two wood screws.  Drilled pilot holes in from the end towards the hub. Maybe I have been lucky, it has been up there for years!!

[DanB]

On the topic of balancing... (don't wanna start an argument)

Sparweb said 'Filling the tips with lead works, and if it's not enough you can finish with the plate on the hub.'  Maybe that's fine practice but it bothers me.

I would never add weight at the tips.  A heavy tip will do weird things on a fast yaw.  I'd much rather add weight (lots more weight of course) to, or near the hub.

Lately for balancing I drill/tap a steel blade up at exact center, so that I can hang the rotor (from exact center) and add weights until it's perfectly level.  This seems to work well for static balance.

[kitestrings]

Lately for balancing I drill/tap a steel blade up at exact center

Hi Dan, Can you explain this a bit more - its not too clear.  I assume maybe you are hanging a fully mounted set horizontally from the center point of the hub plate, and doing a static balance of the assembly?

I had pretty good results with calculating a moment for each blade.  So, they are weighed individually, but you also measure the distance to COG balanced on a knife's edge.  The weight times the distance give you a moment for each, and you balance from there to bring them within an acceptable percentage - ~2%, I don't recall, but someone has suggested here.  This works well in the rotor is not of a practical size to assemble inside the shop.

Goujon Brothers (not sure of the spelling, pronounced Goo-shon) made blades for Enertech years ago for up to 60kW turbines, and I believe this is the method they used.

[SparWeb]

Dan's caution is warranted.  Attaching weight to the tip "just any way you want" is probably what makes people uneasy with it.  If I'm going to suggest doing that, then I should also be suggesting how to make it secure.  IMO, a balance weight in the tip can only be used if it can be retained inside the blade well enough to resist the tremendous centrifugal force of being spun at ~800 RPM on a 6-foot radius.  Work out the force per ounce (or gram, since BigRock is Canadian, eh) and it's pretty big, like 600x the force of gravity.

Since a 4" lag bolt weighs about 1/2 ounce (15 grams) but can easily hold against 60 pounds (30 kg) of pull-out, a factor of 2000x, there is reason to believe that a weight can be safely held in the tip of the blade.  Make the hole 8" deep and you can get some lead in there before putting in the lag bolt.

It goes without saying that every step must be done with care.  And there's a point where the effort needed to put all this together is more than the effort to just make another blade...  or if it takes more than a few ounces to get the blade balanced.

A heavy tip will do weird things on a fast yaw.

That's also a good point, and an issue if the machine is going to run fast.  Since BigRock is using a motor-conversion, which will have a "soft" top speed if it's anything like mine, it's less likely to do that.  I would counter that the light blade tip can do weird things, too, just in the opposite direction.

One of the reasons I'm not too concerned about weights at the blade tips is that many helicopters have them.  They don't need much weight, due to strict process control at the factory making the blades come out very close to the target weight.  Usually it's nothing more complicated than a self-locking screw with some washers.

[bigrockcandymountain]

So these blades are up and running now.  I haven't had too much time to observe them but here is what i see so far.


They start turning in lighter winds than the old ones but stay stalled until almost the same wind speed.  When they pop out of stall they immediately produce 300w or so.  I think the large iron loss in this generator is to blame. 

I have the furling set extremely low so at 1000w it is half closed up.  1600w is max so far and i am happy with that. 

They are wildly different airfoil and pitch than the last ones and yet perform fairly similar.  It seems tsr is more defined by the generator curve than the blades.  They are also 2" longer so 4" more diameter than the old ones.  Old ones were 12' 6" these are 12'10" so i call them 13'

They meet my design goal of 500w in normal winds and 2000w max pretty much perfectly. 

[SparWeb]

Awesome!

It seems tsr is more defined by the generator curve than the blades.

See, I get so absorbed in the details that I can't say it as clearly, but you are so right.

Did you decide on "perfect" or "close enough" on the balancing?

[kitestrings]

Sounds like they are working out pretty well, and the turbine overall doing what you'd hoped.  Congrat's.  Furling early is better than late IMO. And, you can always tweak things to suit.

IMO, a balance weight in the tip can only be used if it can be retained inside the blade well enough to resist the tremendous centrifugal force of being spun at ~800 RPM on a 6-foot radius.

I carved a 4', two-blade rotor many years ago on our picnic table and put it up on a post on the barn.  It was just for fun; interest.  I've never made a 6'-ter, but 800 rpm seems a bit on the extreme side of things and the amount of weight should be minimal with reasonable material selections and carving consistency.  I can't say as I've ever run into what Dan is suggesting as a problem.

[bigrockcandymountain]

The balancing ended up being perfect as far as i could measure.  I have an odd vibration issue but i am not sure it is the blades.  The guy wires and tower vibrate  right after cut in fairly noticeably.  Oddly, it goes away at higher speeds.  I kind of think the natural frequency of the tower is interacting with the rotor.  I might try slacking off the guy wires a bit and see if that helps.

Otherwise i am still happy.  The classic logged a peak of 2500w but mostly it runs 1200w or so in moderate winds.  We had some 100km/h forecast here the other day.  I used the manual furling in sone pretty rough weather. It works way better than expected.  Pull the lever and the rotor is stopped in about 30 seconds and it stays stopped no matter what.  And that is with no shorting or load whatsoever. 

I think the efficiency is lower than the higher tsr blades but the slow and steady makes me happy anyway. 

I might play with 1y connections in the spring.  It would cut in at 80 rpm as opposed to 160 rpm.  For the time being it will stay doing its thing.  We have had a week of windy cloudy weather, so it has been great to have. 

[SparWeb]

My motor-conversion has it too.
Harmonics at cut-in, due to the clipping of the sine wave peaks.
Nothing to worry about.

[bigrockcandymountain]

Hey now that you say that, i remember a discussion on harmonics from sine wave clipping.  That makes me feel better.