Carving Cedar Wind Turbine Blades

October 2019

Diameter 10 feet (3.0 meters)

Root Chord 10 inch (40 degree incidence)

Tip Chord 5.0 inch (3 degree incidence)

Airfoil NACA 6415 (root to tip)

Western Red Cedar (13 planks each)

The Spirit of Zubbly has been flying for 10 years with the same 8-foot diameter blades.  Due to age and weathering, the old blades were in absolutely tragic condition. Amazing that they would run at all.  

Like the new blades, the old blades were carved from cedar, stained and varnished. The varnish was touched up several years ago (maybe 2014). About the ONLY thing still intact was the protective tape on the leading edges. Not even a corner peeled up.

The impact damage and chipped trailing edges all happened during one terrible hail storm in 2017. They went through many hail storms before that but none did this much damage. After the 2017 hailstorm I knew I had to replace them, but I was surprised to feel no change in vibrations or funny noises in the tower. There was a marked drop in power output and speed, but they would run if the wind was strong enough. Accepting that running slow is a “fail-safe” mode, I chose to leave them up until the new ones were ready.

For those who want to do woodworking with cedar, it’s becoming a bit of an elite sport.  I hope you can read the prices in the picture below, for CLEAR western red cedar planks.  It’s Canadian dollars but any way you slice it, that’s too expensive for a project that needs nearly 50 planks.

Here is the load of cedar planking that will make the new blades. 1″ x 6″ planks that will need some planing before lamination. The previous blades were 8 feet diameter. After doing some data logging and checking the results, I can tell that it’s running below the target TSR. The new blades will be 3m (10 feet) diameter instead. The furling works very well, so even if they turn out to be too big, it won’t be in much danger.

I had to settle for cedar “fenceboards” from the regular hardware store. They cost 1/3 as much, so I could justify the time to sort through their stock and still throw away about 25% of the boards I bought.

I then sorted the planks, arranged them by their grain patterns for lamination, and glued them all together.

I chose to use a System 3 glue (again). This time it’s T88, which has a higher cure temperature than “Cold-Cure”.  I needed a 1 gallon kit of the 2-part epoxy, and I used almost all of it.  There was very little left over after this job.

I worked out the geometry in CAD, but I did not stick to the plan. The final blades are not actually the same.  Look at the blade roots and compare with the way I trimmed them, and you will see.

I used this occasion to use Onshape, which is an online 3D CAD service that I’m enjoying for many projects. I find it easier to use than Inventor or Solidworks, though not as rich in modeling features and still quite lacking in drawing features. In all domains, Onshape is catching up fast, so having a project or two every year to do in Onshape helps me keep up with its capabilities. (Yes, I know I sound like I’m on their payroll.)

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Large chunks are removed by simply sawing and chipping away with a chisel.  Then it’s cut to an approximate profile with a short  or a long draw-knife.  The short one does the details, the long one for big strokes.  A plane is also helpful but it can only remove so much before it jams.

As I trim and file the blade blank smooth, there is a transition near the root where it goes from the airfoil shape to the shape that holds the blade to the hub.  This transition is a deep corner that requires a lot of work.

 

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One of the problems I ran into was when some of the wood grain became prone to splitting while I was cutting it.

It’s a product of laminating the boards cross-grained, which is great for mutual reinforcement of the structure, not so great for those spots where the grain really plunges away from the axis of the board.

The only solution is to have reeeeeeeeealllllly sharp blades and take shallow cuts. This also slows the work down.

The bottom of that gouge in the photo turned out to be just barely out of the final profile I needed. Whew!

Getting the profile right is difficult with the tools I have, because the airfoil (NACA 6415) has a slight concave curvature on its bottom face. I have a few small spoke-shaves that help with this.

The templates were drawn in CAD, printed on paper at scale 1:1, then glued to sheets of cardboard so that they stand up.

In the photo, the profile cards rest on the surface very close, with slight gaps here and there.  This puts me about 1/4 of the way through the carving process for this blade.  Then comes the other face, carved the same way, then cutting the root ends to mount in the hub.  Finishing the surfaces, mounting and balancing come last.

I got the front sides down to the rough profile quickly. That is the “flat” bottom of the airfoil, which isn’t actually flat in this case.

The paper templates fit together, so putting the top and bottom templates together I can tell how close I am to the final profile. I got all of the profiles “really close” before going back over them all again and doing a pass to “very close”.

I have been keeping track of costs. Since this is a “one-off” set, it’s pretty steep.
One thing I decided to do before even starting was to invest in tools rather than spend extra on wood. Meaning I ducked the cost of clear grain cedar, which would have run at least 900 dollars. I bought regular cedar planking instead. I had to sort carefully at the lumber yard, and after drying them still had to discard a bunch. And go to another store for more after I cleaned the good stuff from the first place.

Note that this isn’t as great a hardship as it sounds – a lot of material ends up on the floor. The original laminates weighed almost 40 pounds each, the final parts only 10 pounds. To deal with more defects in the cedar I had to place the laminates carefully – strategically locating knots and cracks where they would be lost in the chips anyway. In the end, that strategy worked… almost… perfectly… There are a few knots where I didn’t expect them to end up, but none larger than a pencil, and overall I’ve been able to deal with every trick they threw at me.

All dollars in Canadian currency:

Wood purchase 1: 250
Wood purchase 2: 86
Power planer: 439
Heavy-duty bar clamps: 130
Adhesive: 242
Stain & Finish: 50
Electric Sander: 80
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Total So Far: 1,277

The sander was not a planned purchase – my old one quit while sanding these.

This project was an occasion for my friend Richard, more interested in woodworking than me, to bring over some of his tools for me to try. The red-handled drawknife became my favourite over my smaller one. For starters my small one was too small, so I was going to need a bigger one anyway. Second, his keeps a sharp edge really well, while my old drawknife was probably sharpened on a grinding wheel (not by me) because it can’t keep an edge now (annealed by the heat).

My spokeshaves allowed just enough curve to get the concave shape of the front face. I was expecting to buy some hand-carving tools but in the end I got the whole thing done with what I had, plus Richard’s drawknife.

I still have a lot of sanding to do, and then the staining process begins.

Here are the parts of the hub.

The matching flat plates were laser cut.

The hub collar was made on a lathe by a new machine shop that just opened near my workplace. A simple test of their capabilities and business methods before sending some real work their way.

I carefully GMAW welded the plate onto the collar. 

They were then powder-coated by another local shop.

Notice that I made 4 blades. Then I could pick the 3 best.  While carving, I had not yet made a mistake causing me to discard a “bad” blade.  They were all of roughly equal quality so far.  It’s 25% more work and 100% risk-reduction. Some mistakes I made along the way came close to scrapping one, but so far recovered each time.

Notice that I made 4 blades. Then I could pick the 3 best.  While carving, I had not yet made a mistake causing me to discard a “bad” blade.  They were all of roughly equal quality so far.  It’s 25% more work and 100% risk-reduction. Some mistakes I made along the way came close to scrapping one, but so far recovered each time.

I trimmed down the grips at the root. The “block” laminates were 10″ by 5.5″, and now the grip ends are trimmed to 7″ by 3″. Still lots of meat to bolt together, and saves 2 pounds per blade.

Assembling the blades can also be called a balancing act.

haha

(Sorry)

Anyway… in the photo you can see how I discovered a way to use my laser level as a guide to get the blades aligned.
It was also useful for setting the cone angle, although I admit to making a mistake in the process.  And that, my friends, is why I made 4 blades, because I was bound to make a mistake eventually.

Balancing has been a B****H.  I made 4 and ended up with 2 lighter than the other 2 by a whole pound, somehow.  Like this, after final profiling:

1  8.8 lb
2  9.1 lb
3  9.9 lb
4  9.6 lb

So I saved myself slightly more trouble by eliminating blade 3.  But not much.
I tried some selecting shaving to make blade 4 thinner but I couldn’t go too far without changing the aerodynamics.
Stuck with about 1/2 pound of steel in blade 1 and a bit more in blade 2.

I’m too ashamed to post pictures of this.

Decided to go back to the fun part of the project, and post videos from the carving process (Youtube links).

1: Rough block removal with hammer and chisel 
2: Rough shaping with the drawknife 
3: Close shaving with spokeshaves, and checking with template cards 

Sorry the timelapse just rushes past too quickly. You can adjust playback speed, but I don’t know how to change it after it’s been uploaded already.

It’s finally balanced!

 

The finish also turned out much nicer than I expected.

 

You can’t really see it, but there is a protective tape on the leading edge.  It is a common “propeller” tape used on the leading edge of aircraft propellers, and it completely eliminates erosion of the leading edge as it passes through rain and airborne dust.

There are several varieties, but I’m still using the roll of 3M protective tape 8672  that I bought many years ago.  I haven’t used up the 30 meter roll yet!

 

Blades are mounted!

 

Generator and tail have been checked out, and ready to go, too.

It’s UP!

Still enjoying the thrill of seeing it go.  

After 3 weeks of data logging, the results are in.

I tried really hard to get it right, I hope you agree, but I really didn’t expect to hit the mark so well. CP=50% between 15-25 kph (10-20 mph).  At higher speeds, the furling system of the tail begins turning the blades away from the wind, leveling off the power output.