Murfs New Blade design

[Boss]

We are getting readier for a new set of blades. Kevin Murphy is the designer, I hoped he would post this so you all could ask him questions, alas, it's me translating.
Concerns:
Load.
New battery load of more than twice the AH rating plus sealed cell over flooded cell has our Other-power book built 10 footer, 2" disk magnet, 24 volt 50 foot tower (on top of hill) massively under-performing.
Blade length.
Not realizing the critical correlation between blade length and turbine/tower offset I decided to make the third set of CNC blades 6 inches longer, now the turbine isn't facing the wind properly.

Deep thought and then Action (hopefully, right?).
Kevin said we needed RPM and wind speed data from the mill, but the more he looked, the more it looked like TSR is effectively a "given" and what we need is to set the turbine up with varying loads, and see how it performs at various RPMs.
Then as he worked on the problem of how to get the blades to be more effective aerodynamically something interesting resurfaced, the blade design in the books are pretty much as they say, "tweaked about as good as hand carving needs to get them."  
I am no engineer, never claimed to be, really wish Kevin was explaining this stuff, because frankly, I am more of a "tear it out and change something hopefully radical," type of guy. I kept asking dumb questions like," if you did have RPM data and wind speed data how would this affect the model?
"Not as much as better wind would I would hear as an answer."
Anyway, undeterred by my ignorance, Kevin is forging forth,


I am working more days and hours that ever, my wife is home recovering from surgery on her thumb,
(trapeziectomy with ligament reconstruction and tendon interposition) yeah.

 The Day before yesterday I brought home a 10 inch Craftsman table saw

So I am happy

Kevin is still modifying the design, this morning I see this model in my inbox

I see that the blade, not being constrained with "hand carving techniques" has double edges, I know he said he was working on a multiple twist profile. Stated in the only terms I know, so pardon the crudeness,  this is also a double curve style blade; convex on top, concave on bottom. Maybe if we're lucky, people will ask enticing questions of Kevin, and he will reply here.

Like I said, I am the rip it up, and redo it guy. I bought a new saw, on one visit to Santa Fe to take my wife to the hospital there, another time I went to Home Depot, found and scored 9 rough cut Western Red Cedar full dimensional 2"X4"x8's.
With the saw, and the rough cut cedar in my shop it is my intention to do the lamination here this time, then take the three blanks over to my buddy's shop and let Kevin do his magic with  the CNC router.

So yeah, our new blades will be Cedar, we're going back to five foot, to rid this system on the errors associated with randomly changing the TSR and the off-set
The good news is some of the technology I was beginning to feel was way over my head, isn't, in fact it's all relative.
The trick, is figuring just how to get our foot in the door of understanding

[SparWeb]

There's no way to tell - is there any progression of pitch along the span of that blade drawing?  What TSR are you aiming for, then?

Going backward to the first problem you mentioned:  Larger diameter blades not tracking true to the wind. 

Have you tried the "ribbons" test?  Tie a ribbon to the tail and the tower, then see what the difference is between plane of rotation and the wind direction?  Maybe you have: I seem to recall someone asking about that months ago.

If you have confirmed that the turbine is always yawed (say more than 20 degrees) have you tried changing the position of the tail?  Adjusting it to swing a little farther outward (I'm talking about the normal down position) then it will steer the mill a bit better into the wind.

[Boss]

is there any progression of pitch along the span of that blade drawing?  What TSR are you aiming for, then?

We are looking at that now. The pitch at the tip is negligible; a degree or so. At the base (12 inches wide) the pitch is 13 degrees. From the tip the twist stays pretty much the same for four inches, then there is a gradual twist to 6 degrees to the point marked as plane 2. The twist continues to increase all the way to the base.

We are looking now at what effect wood-use economy might have on the air-foil. For instance if we were to keep the blade slender enough near the tip to only need two pieces of laminated 2X2s down there.

I think we decided that not knowing our turbine (alternator) specifics we wound use what the book says the TSR and calculate it as 7.

What it seems like we are doing in this experiment is sliding values in either direction and looking at the results.

Have you tried the "ribbons" test?  Tie a ribbon to the tail and the tower, then see what the difference is between plane of rotation and the wind direction?  Maybe you have: I seem to recall someone asking about that months ago.

Good idea, I can only attest to the sense that the tail is not perpendicular to the blades in normal wind, I'd guess 5 to 15 mph. Kevin did build the anemometer, but never got to calibrate it. Hopefully we'll get that done when we bring the turbine down to install the new Murfofoil design blade set.
At that time we might look at extending the tail stop so the tail can't go past perpendicular.
I feel guilty, not having tried adding a resistive  load to the system to try and get the turbine matched to the load.
One thing I can see, is these blades are not stalling like the last set, perhaps this is the extra length changing the TSR, I don't know, again I feel bad having not logged more information.

thanks for the input, and I hope this answered some questions
 

[fabricator]

If you make the rotor disc bigger and use the same size tail the disc will over come the tail pressure and turn slightly off the wind to the offset side, you can counter that by making the tail bigger or lengthening the boom or offset the tail closer to the rotor opposite the hub offset.

[97fishmt]

Hi Boss,

Those blades look nice.  I made a set a couple months ago, hand carved,
close to those.  Not with the concave face, and am very happy with their
performance.

As Fabricator said to get machine to balance between furling and staying
pointed into the wind, it's a tail issue.  It's better to start out with the tail lite,
like it sounds like you have, and add weight to get more power out of it.

Cut in is the RPM you are shooting for. If it's starting to charge in reasonable
winds and just furling to early it's a tail to loading of the prop issue.  Although
it could help to add resistance or change the air gap.  You have a lot of variables
with the axial flux set up.

On my machine I increased the offset ,more to the side and more out front,
made a larger prop and had it furling to soon.  I fixed it to my liking by adding
a larger vane and adding about 10 pounds to the tail boom.

I had it working great with other props before but changing the props changes
the balance with the furling tail.  You'll be happy with new blades but that isn't
the end of tweeking it to have a safe and reliable machine.

Mike

[SparWeb]

...
I think we decided that not knowing our turbine (alternator) specifics we wound use what the book says the TSR and calculate it as 7.

A good start.  My understanding of the rest of your description of the blades is that you are changing the way to carbe the blade for economy of the material.  Did you start out with Dans blades on the first try?  BTW the Dans have thoroughly tested their alternators, especially the one for the 10-ft genny, so looking back through the site you should turn up some reports about what the appropriate RPM speeds should be.  Relating that to the diameter and TSR you target, the formula gives you wind speed at which all these variables coincide.

The basic formula looks like this: 


and you can turn it around to give you V the wind speed with a little algebra.

...
At that time we might look at extending the tail stop so the tail can't go past perpendicular....

Actually I meant the opposite.  By allowing the tail to swing beyond the perpendicular point (when it's down), it will counter-act the tendency of the rotor to turn out of the wind due to its thrust and offset.  This diagram may help picture that.  You're talking about situation "b)".  If you let the tail swing out even more, then the increased force upon it would push the rotor back to face the wind better.  This kind of adjustment has little effect on complete furling.

[Boss]

I think it is a good idea to add a windsock (ribbons) and have another look, because what I am seeing is not the same as what is happening in either image. Our 11 footer (modified from 10 foot) which indeed is right out of the book, and built within a year of Kevin and I taking a week long seminar in Colorado with Dan, Dan, Rich, and George where we built our first turbine also a 10 footer.
Anyway, our machine is pushed all the way against the tail stop most of the time, has been ever since we made the blades 5.5 feet, Makes all the sense in the world now that you all explained what is happening. The turbine axis isn't off-set from the tower by enough and normal wind can not produce the pressure to spin the assembly into the tail. When a high enough velocity wind hits the turbine, we can see it jolted out of the locked position and begin to furl. I thought perhaps I could better the balance point where the tails rests. I don't know though, it seems like a bandage to a problem I created with longer blades.
The bottom line, either build a turbine housing made for the longer blades, or fake it and extend the tail stop. Instead I choose to go back to five foot blades and keep the turbine the same for now. 
I mean having a guy like Kevin who knows his stuff with Solidworks, G-Code, CNC and has history with airplanes,  it is logical to want to experiment with blade design. 
This is why I bought the table saw, now I can do the preliminary blade blanks development here. The last sey of blades took a ridiculously short amount of time to carve. This time we'll let the machine do the whole process, right down to the finish. Maybe it will take 45 minutes per blade including the time to flip the blank over.
At some point we will sell off the prototype blade sets.

For now it's all good.

[SparWeb]

The last sey of blades took a ridiculously short amount of time to carve. This time we'll let the machine do the whole process, right down to the finish. Maybe it will take 45 minutes per blade including the time to flip the blank over.

Gee, isn't it a shame that the funnest things in life take the least amount of time?

... yah that, and carving blades goes quickly, too.

[Boss]

Close up of Cedar as I was gluing

This is not using the proper setting on the camera, just zoomed  to about 10power, when I went all the way to 12X the image went out of focus. There are more settings on this Panasonic DMC-ZS7 showing up every time I find new new cool button with submenu

The first blank for our new Murfoil blade design, yeah man this time I found Western Red Cedar in rough cut 2X4s
I sorted through a hundred boards to get nine with fairly clear grain
I am very happy with the way they are turning out. Btw, the 2X4 Cedar boards were around $6 a piece at Home Depot in Santa Fe NM, USA
Also I shot a bunch of images in the wind yesterday, trying to capture the tail angles

This may not be the best picture, no worries, I got lot's

[97fishmt]

I don't know about your blade design now.
From what I have done and read and seen
the leading edge should be straight from the
hub and the beginning of the airfoil.

If you are tapering on the leading edge side
of the blade that seems a little out of the ordinary.

And I'm not sure you got the tail balance stuff.
You can make your machine work with other
diameter rotors but it is a compromise with getting
out of it what works for your winds. 

So let us know how it works but changing one
thing leads to another.

[SparWeb]

From that perspective, I'd have to say that the tail already swings past the perpendicular point, doesn't it?  So if I was suggesting before to increase that angle past perpendicular it seems you already have that situation.  The next thing to consider is the size of the tail relative to the diameter of the rotor.  I'll take a look at those other pix you posted in a minute, but of course, picutres get misleading when you try to size things up.  I could even be wrong about the tail angle here- it's just one photo from a funny angle and who knows what kind of optical affects occur in one lens or another.