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Old 07-23-10, 10:44 PM   #1
nibs
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Default Wind turbine blades.

There appears to be some interest in wind turbine blades, so perhaps a few tips from an old turbine blade & ultralight aircraft prop builder may be useful.

No need to get exotic with materials wood is almost a perfect material.
wood can be stressed to within 90% of its modulus of elasticity an infinite number of times without failure. (B. Fuller). Wood is not very abrasion resistant or they would use it on more aicraft.

The most efficient design is a single blade, since it sets up minimal turbulence. It is virtually impossible to build a single blade that is balanced, so a two bladed prop is the next best for efficiency. The advantages to adding more blades is the fact that increasing the sq inches of blade surface helps the blade start in light winds. Once the blade is "filling the disk" (ie spinning fast enough that no air escapes the blades, in theory) the number of blades is irrelevant. Big wind turbines never spin that quickly.

I used 1/2 of a NACA (now NASA) 0010 foil for the downwind surface and a flat or slightly concave windward face. The windward blade angle at the hub should be as steep as the material will allow for blade starting since it is the slowest moving part of the blade. My angle of attack at the tip, on a 4' diam prop was about 2 degrees or less, on a 6' prop about .5 deg. but this would depend on the generator and loading, as well as the local wind charactaristics.

Simply fair the windward side out from the steepest part of the blade at the hub out to the 2deg tip. Use a protractor every inch out from the hub to make sure the angle remains the same on each end of the blade. I always carve the upwind side first, then shape the downwind side, again with a template used every inch to keep the blades symetrical. The blade will be narrower chord wise at the tip, I used to use selected 1/4sawn 2X4 or 2X6 for my blades with great success.

You should leave the trailing edge quite sharp or it will moan, I built one for a friend with a 10' diameter prop and left a 1/8" square trailing edge, installed it and left town for a week, PO'd the whole neighborhood. Sharpened up the trailing edge and was an instant hero.

The NACA 0010 foil is a symetrical curved foil with its thickest part 1/3 of the distance from the leading to trailing edge, this is (or was) considered to be the foil shape with the least wind resistance. Hope this helps, do not be afraid to carve your own blades, it is not all that difficult, and if you start out a bit crude you can refine the prop to increase its efficiency.

A one piece wood blade balanced on a shaft will not come apart as long as you used common sense and did not make the blade too thin at the hub.

A couple of tips in closing (the dishes are calling) if you have a runaway blade, you can stop it by tossing a length of rope into the blade from upwind, better though is to have a tag line on the tail so that you can pull the machine around out of the wind.

If there is interest in this, I will gladly answer questions, though you will have to do your own math, I spent many hours on the design of my blades but watching them fly was a joy.


Last edited by nibs; 07-27-10 at 12:01 AM.. Reason: Tip for keeping the blade symetrical.
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