Author Topic: New blade contruction  (Read 3042 times)

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force9BOAT

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New blade contruction
« on: May 04, 2006, 06:50:02 AM »
Those of you who have seen my earlier posts about my wind turbine no that I need a new and better set of blades.  This past week I started that project with the purchase of 12 eight foot cedar 2x4 boards.  Since this is my first attempt at blade carving I decided the best approach would be to do one blade completely from start to finish before working on the second and third.  That way if I get it wrong then I've ruined one blade.  The following is the sequence of work done so far.


The new blades will be just under five feet long for a machine slightly under 10' diameter.  I used Alton's Blade Calculator for a TSR of 6.  Pictured below is the stock wood laminated with West Marine epoxy.  


Pictured below is the stock with an extra plank laminated on.  It will give me some extra cord width at the root.


Pictured below is the stock with trailing edge taper cut away.


Starting to cut away everything that does not look like a blade.








Here finishing sanding with a belt sander in progress...


Here the front side shaping is complete.  The brown "wet" area is epoxy mixed with sawdust that I had to use to fill in an area where the chisel cut too deeply.  The extra epoxy will be sanded away. This is a mistake I'll be careful not to make on the other blades.  Also there was a small void in the lamination at the tip I filled in with epoxy as well.


Here is another angle.  I like this photo because it shows the "bowl" at the root where the drop is greatest.  The front side shaping is complete.  I've started rough cutting of the back side...


Here is the unfinished back side...  Give me a couple more days and my first blade should be finished.


That's all so far.  In a few days I hope to have the first blade complete.


Rob

« Last Edit: May 04, 2006, 06:50:02 AM by (unknown) »

SamoaPower

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Re: New blade contruction
« Reply #1 on: May 04, 2006, 04:47:48 AM »
Great pictures Rob - thanks.


What airfoil are you using? What is the L/D and Cl?


Good luck with the project.

« Last Edit: May 04, 2006, 04:47:48 AM by SamoaPower »

SmoggyTurnip

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Re: New blade contruction
« Reply #2 on: May 04, 2006, 06:31:39 AM »
Great post - nice pics.

I am about to start my blades as well

and have lots to think about.

How much epoxy did it take for those blades?

Where did you get the cedar?

Was it expensive?


Good luck with the rest of the project.

« Last Edit: May 04, 2006, 06:31:39 AM by SmoggyTurnip »

willib

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Re: New blade contruction
« Reply #3 on: May 04, 2006, 07:22:53 AM »
looking great so far , keep up the good work !!

i'm also curious where you got the cedar?

it must be a local thing , because around here ,they dont sell cedar 2x4's.

anyway looks great!
« Last Edit: May 04, 2006, 07:22:53 AM by willib »
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Shadow

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Re: New blade contruction
« Reply #4 on: May 04, 2006, 09:05:50 AM »
Good job! Blade looks to be coming along fine. Everyone has their own method of doing most everything. But what I try to do is build all 3 at the same time, whatever I do at stage one with the first blade,I lay it down and do the same to blade 2, then blade 3. Second stage same thing. My thinking is I can keep all 3 blades more the same as I move along through the different stages. Rather than going back and saying 'Now what did I do on that first blade'? I'm constantly comparing all 3 blades as I'm building them.Good luck.
« Last Edit: May 04, 2006, 09:05:50 AM by Shadow »

force9BOAT

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Re: New blade contruction
« Reply #5 on: May 04, 2006, 10:49:47 AM »
You got me.  What do you mean by L/D and CI?
« Last Edit: May 04, 2006, 10:49:47 AM by force9BOAT »

force9BOAT

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Re: New blade contruction
« Reply #6 on: May 04, 2006, 11:04:44 AM »
I bought 12 eight foot 2x4 cedar planks from Lowe's Hardware Store in Bellevue, Washington for about $60.  I cut each to five feet long and then used three of the extra 3-foot segments to provide more cord width at the root.


One quart of West Marine epoxy is more than enough to laminate all the planks.  That costs about $50.

« Last Edit: May 04, 2006, 11:04:44 AM by force9BOAT »

force9BOAT

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Re: New blade contruction
« Reply #7 on: May 04, 2006, 11:05:40 AM »
Thanks all for the nice coments.  
« Last Edit: May 04, 2006, 11:05:40 AM by force9BOAT »

SamoaPower

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Re: New blade contruction
« Reply #8 on: May 04, 2006, 12:56:08 PM »
Sorry I didn't explain Rob. Since you used Alton's calculator, I thought you knew about the parameters.


Usually blade design starts with choosing an airfoil from the hundreds of published ones and obtaining the parameters L/D (Lift to Drag Ratio), Cl (Coefficient of Lift) and Angle of Attack. If you use Alton's typical defaults, you will end up with just that - a typical (read mediocre) blade. That is, if you carve an airfoil profile that matches those parameters. Be aware that you can do much better (or worse) with a different airfoil, even one that looks only slighty different. So, the airfoil shape you carve can't be arbitrary if you want good results.

« Last Edit: May 04, 2006, 12:56:08 PM by SamoaPower »

seanchan00

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Re: New blade contruction
« Reply #9 on: May 04, 2006, 05:40:44 PM »
Nice going Rob. It must be hard work with 4 inches of wood to cut into shape. First time I see and understand how you guys laminate the wood for blades. Thanks for the photos. I am using 3 pieces of 2 by 7 each 5.5 feet long to make mine at 11 feet diameter and I had to compromise quite a lot at the root for the drops.


SamoaPower, could you elaborate a little on how to change the L/D and CI to inprove on Alton's Calculator? If I need high torque and fast rpm in a poor wind site what do I change?


SeanChan.

« Last Edit: May 04, 2006, 05:40:44 PM by seanchan00 »

SamoaPower

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Re: New blade contruction
« Reply #10 on: May 04, 2006, 07:16:41 PM »
Sure, SeanChan.

Rotor design has little to do with the quality of wind at a given site except possibly for starting issues which shouldn't be a problem with an axial flux alternator. The real issue is rotor power output with efficiency and swept area as the main factors. Swept area is simply a choice as to how large a rotor you're willing to use to reasonably match the alternator power available. Some say that if you want more rotor power, simply make it larger. I prefer to first look at the efficiency angle to see what can be improved there.


Airfoil L/D mainly impacts rotor efficiency. I have a chart that relates TSR and L/D to rotor efficiency. I don't recall where I got it - too many years ago. A few sample numbers are: For a TSR of 6, an L/D=8 gives an efficiency of 12% at best. L/D=13 - 28%, L/d=33 - 43% and L/D=100 - 50%. Another important point is that L/D=100 gives a flat efficiency curve between TSR's from 4 to 14 where lower L/D shows a peak at TSR 2-3 with decreasing efficiency at higher TSR. This is significant because a fixed pitch rotor doesn't always run at a constant TSR. Maximum L/D for a given airfoil occurs at a certain Angle of Attack (AOA) to the apparent wind, usually around 4-5 degrees. Cl is the Coefficient of Lift which is taken at the AOA of the best L/D. Remember, all of this is for a given airfoil and obtained from wind tunnel measurements.


A favorite airfoil of mine, the Wortmann FX60-126, has a maximum L/D of 100 at 5 degrees AOA which gives a Cl of 1.08 and an efficiency of about 50%. If you compare these to the defaults in Alton's calculator you will see quite a difference. Try plugging them in to the calculator and look at the output power.


Hope this helps.

« Last Edit: May 04, 2006, 07:16:41 PM by SamoaPower »