Blades

This page shows the construction/shaping of my 8-1/2' long blades.

Here is an image that shows the basic dimensions used for glueing up and cutting the basic blanks for my blades.

Here is what the profiles of the blades should look like:

I have to give credit to the guys at the OtherPower.com website blade carving page for the basic image's used in my airfoil profile diagram above. I have modified it as I found it confusing the way that they originally pictured it, I rearranged the images so that they show (as should be) that the leading edges of the profiles are aligned in a straight line, like they would appear on the actual blades. I had to recalculate the offsets from the blade face to suit the width of my blades, 3 degrees at the tip, 5 degrees at the mid-station and 6 degrees at the root.

The next point that I make turned out to be a major revelation for me - If you were actually looking at the blade from the tip to the root, in the three profiles shown above - the top surface and the right-most edge (shown in RED) are *aligned*.

I have re-researched on how to cut the blade profiles on the OtherPower.com website. I have to say that after a couple of hours I got real confused, until all of the sudden what they were talking about and the diagrams that they were showing very much started to make sense. The principal operation that I was missing is that the first thing that you do is to set the basic blade thickness.

For the 17' diameter blades they defined 3 stations: root, mid-way and tips. The blade thickness for root is 2-1/4", mid-way is 1-1/2" and at the tips it is 3/4".

The first operation will create the basic shape of the blade. To reduce the thickness of the glued up sections, it turns out that what you do is to measure down from the face of the blade at each of the three stations the desired thickness (2-1/4", 1-1/2", 3/4") and draw a line on the existing trailing edge of the blade. On a band-saw you cut off the material (delineated by the line you just drew) from the back side of the blade, with the leading edge flush on the surface of the band saw. If a large enough band saw is not available you have to remove this material by hand. (In Hugh Piggott's book has you performing this operation last and I agree with the OtherPower.com people that it is far easier to do this first while you have nice square edges on the glued up blade blank.)

The next operation is to set the lines that determine the trailing edges the dimensions I am using are mentioned in the above airfoil profile diagram. At the tip measure down from the face of the blade to set your tip angle (5/16" + 1/8"), and mark this spot. Mark another spot at the mid-way station (7/8" + 1/8"), and from the bottom edge of the blade at the root. Now draw a line and connect the dots, draw another line 1/8" away from the previous line (towards the face of the blade). These two lines determine the upper and lower portion of the trailing edge of the blade. These lines may not be perfectly straight lines, transition them so that they are as smooth as possible.

Next you set the blade pitch, on the face of the blade you can now remove all of the wood between the top corner of the leading edge and the top line of the trailing edge. The top corner of the leading edge should not be removed, but used as a reference point. Near the root of the blade you need to make a nice smooth transition between the top hub surface and the area cut out to form the blade pitch.

Turn the blade over and along the back side of the blade draw a line that is 1/3 the width of the blade width away from the leading edge. And remove the material between this mark and the lower mark for the trailing edge. Do this so that it is basically flat between the two points, use a straight edge if necessary.

Contour the remaining 1/3 portion of the back of the blade (bottom portion of the leading edge) that was not removed so that it is forms a nice rounded profile shown in the upper airfoil profile diagram.

Turn the bade over and taper the top of the leading edge side so that it makes a nice radius.

OK - at least that is what I *think* that you do... When I profile my blades I will try to follow this procedure and correct as I go.

I got out a bunch of sassafras out of my stockpile that were of the proper lengths and got it all planed down. The resulting boards ended up being about 1" thick give or take. It doesn't matter what the actual thickness is since I will rip the boards into 2-1/2" wide strips, stack them and glue them up. Once glued up I can run them through the table saw to make them all the same width.

Due to the large amount of glue joints I will not be using a urethane glue (preferred) but instead I got a gallon of TightBond III wood workers glue. I used this glue when building my Vortex chassis and it is a very good glue.

Besides it would just be too expensive in my case using the urethane, in addition once I finish shaping the blades I intend to cover them with a layer of fiberglass cloth and paint them with an epoxy enamel. So the blades will not have their wood exposed to the elements/sun.

Gluing up the blades with all the strips that I previously made. I made up the 'root' section of the blades at about 9" width so that I could run them through my planer, then later on added about a 5" section, again so that I could plane each portion.

My planer (a Dewalt) can only handle at most a 12" width so I did each blade into 2 sections. Each time I glued up a section I let it sit at least 2hrs in the clamps, then put in another section for gluing. After about a day and a half, I have the completed blanks for each of the three blades ready for the final diagonal cut on the trailing edge.

A lot of clamps were used to hold each section while glued up (about 12) and in addition I went through darn close to the entire gallon of TightBond III glue that I purchased (good call on the glue quantity).

In all after planeing, I easily ended up with at least 3 garbage can's full of chips. The nice thing about sassafras is at the mentholated smell coming from the wood is a pure pleasure to work with, I have heard that sassafras is the poor-man's walnut. I have worked with both and I have to say that my favorite all-around-wood is sassafras.

Finally after about a day and a half here are the three blade blanks that I will begin to shape into their final contours. Their length is 8-1/2' and width at the hub is about 9", total width is just under 14" wide just before the root, and after planeing they are about 2-1/4" thick. Their length is made from continuous pieces of wood, there is no splicing to make up the combined length, this is important as the OtherPower.com guys made their 17' diameter blades and spliced the wood - all three blades self-destructed at the joints!

I am trying as much as possible to learn from the mistakes from others in my endeavor, here is a picture of the blade blanks so far:

.I had previously ordered some new blades for my band saw to cut the thickness of the blades, but it did not come in before the weekend so I changed my plans a bit.

Instead of taking down the blade thickness as the first step I decided to cut out the pitch angle on the blade faces. This job progressed pretty fast and quickly I learned the value of my power planer. It was the absolute best tool to help cut out all of the excess wood on the face of the blade. I was able to fairly quickly bring down the surface very close to the line drawn for the upper trailing edge. Using the power belt sander to bring the final surface and to carve out the tight area in the root of the blade.

Between these two tools I was able to carve the pitch angle on all three blades within about 4-5 hours. I used a couple of the 'wedges' that were cut off the blade blanks to help raise the blades to a more comfortable work height on my bench and it helped a lot.

Here are a couple of pictures showing the blade progress so far:

I have to say that in hindsight that I am glad that I did not take the blade thickness down first as setting the blade pitch on the blades made me put a lot of pressure on the blade faces and the extra material helped keep them from flexing too much. I made sure that I did not disturb the leading edge so that once I get in the new blade for my band saw I should be able to easily remove the material from the back side of the blades and set the basic thickness fairly easily.

After removing excess material from the back-sides of the blades I broke out the power planer again so that I could shape the back-sides and leading edges of the blades. This is the perfect tool for working the blade profiles and makes relatively easy work of it. After a couple of hours I finished up the blades with the belt sander - done.

Are there differences between each of the blades - yes. I noticed that by the time that I got the 3rd blade done that I tended to leave a larger radius on the leading edge, in the end I believe that the 3rd blade came out the best (of course...)

Here is a picture showing the blade tips:

And here is one with the blades side-to-side showing the trailing edges:

With all the work done again I weighed them - 19 lbs! A good bit less than the 45 lbs glued up blanks!

I picked up some Dynel fabric from my good friend Paul Schreiner (PS Composites). The plan that I had was to use this fabric on the leading edges of the blades to keep wear down to a minimum.

Paul had suggested that I glass up the blades in two phases, wrap the trailing edge up the body of the blade, and then wrap the leading edge of the blade putting the dynel fabric *underneath* the glass cloth. The reason for putting it underneath is that dynel does not sand very well.

So I cut long sections of 6oz fiberglass cloth shaped to fit, ended up using about 20oz of resin per blade. In addition I used some white pigment as I knew that I was going to paint the blades using the white epoxy paint and it would make it easier for coverage.

Mixed up some epoxy paint and got the blades all done. When I flipped them over to paint the back side the paint stuck on a couple of spots to the paper drop cloth, no big problem will remove any paper after the paint has cured totally.

Here is a picture showing the completed blades, I stood them up outside one of my garage doors to add some perspective:

While the paint was drying I moved onto the 30" diameter plates. I made a wood template the same size as the hub portion of my blades and traced out the pattern onto the 1/4" steel plates. I also drew a circle which represents the 20" rotors, took out my plasma cutter and removed the excess steel from between where the blades will sit.

I made both hubs identical and drilled out the pattern for the center studs that connect to the hub. Later on I will drill additional bolt holes to hold the plates to the blades. Removing this excess material cut the weight of each hub from 30 lbs down to 25 lbs each.:

I will also need to make up a couple of spacers, one spacer that goes between the rotors, and another to go between the front rotor and the back hub. The reason for the second spacer is to allow me to use additional bolts to hold the blades to the hub plates and allow clearance for the bolt heads between these plates.

On one hub plate I welded some 1/4" thick retaining plates on each side of the blade 'tabs'. These plates will extend at least 1/4" past the blade thickness so that it gets 'locked in' by the other hub plate, and absolutely keep the blades from ever moving once in position.

I got the blades mounted onto the hubs, ran all 15 bolts through and torqued them down. I also made up a lifting bracket that I bolted on temporarily:

On the day that the tower was erected we used the big crane to lift the alternator so that we could mount the blade and hub assembly onto it so that we could balance the blades. Since the blades were not going to be mounted permanently yet, we simply duct-taped some steel weights onto the blades to determine where and how much weight that was needed to balance it out. Once done we removed the blades and the alternator/yaw bearing was lifted up to the top of the tower and installed along with the thrust bearing.

A couple of days later I removed the temporary weights that I used to balance the blades with and weighed them. They both were with 1/2 oz of each other. When I made up the blades the 3rd one that I cut I knowingly left more material along both the back side and the leading edge. (Eventually Paul Schreiner from PS Composites will take the blade and make a mold from it).

Those of you that have seen my Vortex project know that I used this method to fiberglass on mounting plates onto the chassis. The principal is simple you take a steel plate fill it with holes, next you mount it with glass matt between the plate and the surface. Then you put one (or more) layers of matt over the surface, pushing the matt down where the holes are and then finally put a cover of 6oz glass cloth. The pushed down matt in all of the holes bond to the matt that is below the plate and give you a super strong hold!

The total weight needed to be added was 1 lb 3 oz to the remaining two blades. So I took a couple of pieces of 1/8" thick steel plate and approximated the size and cut it out in the basic shape of the blade, slightly tapered on one edge. I did a pretty good job of approximating it as it came out to be 1.5 lbs, right on target! I filled both of them with a series of 1/2" holes and tapered all of the outside edges.

Once all the holes were in I put a slight bend in along the length just after the first set of holes as they were positioned to start up to the leading edge of the blade. Here is a picture of the final plates, they came in at a hair under 1lb:

The reason that I made them slightly lighter is that I was going to add between 3-4oz of resin and I wanted the total weight added to be my target weight. The entire surface of the blade needed to be sanded to remove the epoxy paint where I was adding the patch, you have to get down to the subsurface layer of glass cloth for best adhesion.

(Post Note: Do not do this - read below as to why)

Here is what the plate looks like where I will be mounting it:

I added some white pigment to the resin so that I did not have to paint when done, in addition I added some milled fibers to help thicken the resin so that it would not run off and create voids.

Here is a picture with the layers of matt and cloth added and waiting for the resin to go off:

Before the resin took a final set I used a sharp knife and trimmed off the bulk of the excess along the leading and trailing edges. Once the resin hardened I lightly sanded around the perimeter to get a smooth finish. Here is what the blade weight looks like when finished:

Now the blades and hub are ready for mounting onto the turbine.

I have had some conversation with a couple of guys on the FieldLines.com discussion board about the placement of my blade weights. According to them this is exactly the *wrong* position for them, they should be at the hub end as they can cause drag at the tips which causes flutter and eventually blade failure due to vibration.

Since I have about a week before the blades get mounted onto the turbine I have decided to rework the blade(s). I say blades as I have the option to take the heavy blade, rework it so that it is inline with the other two and reglass it. Or I can add fiberglass matt/cloth onto the other two blades along the length to bring them in line with the other blade. It may be a toss up as to which method will be easier or harder to do.

I do know that with the heavier blade all of the extra weight was in towards the leading edge of the back side of the blade, so I may opt to rework it instead. I will take out some calipers and do some measuring/comparison of the blades and then make my decision.

Blade balancing act - part II.

During the week I went out and purchased a digital scale at our local WalMart, it is a digital bathroom scale actually. While looking at the various digital bathroom scale I noticed that most of them had a resolution of .2 lbs, there was one manufactured and sold by Weight Watchers however that had a resolution of .1 lb, and at about $33 grabbed it up.

Once I got the scale back I checked it out and it took a little over 11lbs to even trigger the scale, anything below that and it did not register, luck for me my blades were in the 20lb range.

Did a quick check and easily saw that my heavy blade in face was the same weight as remaining two, but remember that I added about a pound to the others, so the first job was to remove the glassed on weights, sand them down and get an accurate value for the weight of all three blades.

Once the weights were off I weighed them and marked the weight on each of the blade. The offending blade was a little over a pound from the other two, and one of them was about 5 ounces lighter than the other. So using the middle blade as my target weight I decided on a plan of action. I would add weight to the lightest one by adding a layer of fiberglass, and the heaviest one I would bring the weight down

It took a little while to bring down the weight of the heaviest one, and every step I kept weighing it to see how I was progressing, until I finally got it to about 8oz less than the target weight. I did this as I knew that I had to basically glass the entire back side of the blade, and add in the weight of glass and resin. I knew from previous work that it takes about 8 oz of resin to glass one side of the blades.

The lightest one I added a layer of 4oz glass cloth along with the resin and basically squeegeed off as much of the resin that I could without starving the glass cloth. The heaviest one I again used 6oz cloth and made sure that I used a full cup of resin.

When it was all said and done, I was extremely impressed with the results - according to the digital scale they were all the same weight! Now with a .1 lb resolution I figured that at worst all three blades were within 1 oz of each other. Perfect!

After letting the resin set I trimmed it back and completely sanded both sides of all the blades. Once done I applied a fresh coat of paint on both sides of all the blades.

I had seen one suggestion from one person saying that they use paint as the method of equalizing the weight of blades. On the lightest blade they would apply more paint than the other ones, building up the weight in a very slow and controlled manner until all were the same.

After final sanding one blade registered about 1 oz lighter than the other two, so I used this method to bring up the weight very slightly.

All said an done, initially I did not understand the importance of keeping as clean a blade surface as possible, and the implications of not doing this. I am glad that I had the opportunity to learn about the importance of blade surfaces as well as keeping a balanced system.

If anyone were to build their own blades I hope that they learn from my mistakes and certainly this is one job that having a digital scale is absolutely the tool that makes the outcome the best that you can obtain!

Once I got the blades in the hubs I torqued down all of the bolts to about 90 ft/lbs and used lock washers along with red (permanent) lock-tight as I hope never to have to remove the blades from the hub again.

Next in I had to make up a spacer so that I could use the duplicate trailer hub and mount it directly to the hub plates, I had to turn a piece of pipe on the lathe to be about 5-1/2" OD and 5" wide, I then used this between the trailer hub and my blade assembly. Made up three lengths of threaded rod and connected them to together. Next with the help of my wife we lifted the assembly up and inserted it into the end of my engine hoist, using a couple of pieces of angle and a spacer to help fill the gap clamped it down

Here is what the assembly looked like in the end of the crane:

I was able to lift the blade assembly quite easily using the hoist, it angled it slightly as it rose but in all it worked well. The whole thing is so large that I could not do the balancing inside the garage, and it was a relatively calm day outside. The area in front of my garage is pretty well protected from the wind. Once on the hoist, I put a little bit of blue paint on the tips of the blades so that when they are turning it is easier to see the tips of them.

Here is a picture of the blade assembly on the hoist:

Next thing was to rotate the assembly to see where the heavy side was. I ended up using what the guys on FieldLines.com refereed to as the 'pendulum method' and it works like this: "spin prop and let it settle place the "arm" at the 12 o'clock position bent inward so to be as close to the center plane of rotation as possible. Spin and let settle and add weight [nuts / washers] until it no longer settles in one spot. The arm can be adjusted left or right, also. It works very well. Once you get it dialed in, lock the nut with some shellac or nail polish so it stays on. You may need to experiment on the arm length and weights. The arm is simply a flat strap of steel with appropriate holes drilled in it and the right length."

It worked out fantastic and super easy to do, within about 15 minutes I got the weight dialed and when rotating the blades they stopped at different random points. Here is a picture of it all painted and permanently mounted:

The wind picked up slightly and I decided to let it turn freely and get a couple of pictures, in the amount of time that it took to take about 3 photos the wind gusted and the whole blade assembly was spinning pretty quickly! I was able to stop it turning eventually by using a length of 2x4 wood and applied pressure to the hub assembly from the back side - lets just say that the blades were spinning pretty quickly, probably about 200rpm and were making a nice swooshing sound, interestingly I did not notice vibration in the hoist so the whole unit must be balanced pretty well!

Once stopped I got my wife to take a picture for posterity with me and our two dogs in front of the blades to give it some perspective:

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