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nibs 07-23-10 10:44 PM

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.

WisJim 08-24-10 07:29 AM

The reason 3 blades are used is that they are better balanced under running conditions with changes in wind direction. As mentioned, a single blade is theoretically most efficient, but not practical. Two blades is simple to build, but gyroscopic effects make it harder on the turbine generator and other components in gusty winds. A three blade design negates this problem. Any more blades just get in the way for a wind turbine design.

nibs 08-24-10 01:43 PM

The other advantage to a three bladed blade, is that it will start spinning a little bit sooner, taking better advantage of puffs of wind.
In practice the difference between bearing loads on two or multi blade props are minimal. I tested one of my units by letting it motor for two years, left the diode out of the circuit, so that when there was no wind, the blade still spun at a minimum of 275 rpm., there was no discernable wear. The generator I was using had good quality shielded ball bearings.
In my experience, both with wind turbines and small aircraft props, the difference between two and three blades is minimal and not worth the added complexity of the extra blade.

Patrick 10-25-10 08:48 PM

Nibs, have you made any 4-bladed rotors? If so, how did they do? Thanks.

nibs 10-29-10 04:33 PM

Have not made any 3 or 4 blade props for aircraft or for wind turbines, I suspect that the only positive gain would be an earlier start to the spinning, due to increased torque at the hub. Some folks seem to think that a 3 blade will be a bit more stable in 'flukey' winds, but imo not worth the extra work. The fewer blades you have, the better the efficiency.

Patrick 10-29-10 06:43 PM

Thanks. I was thinking about making some simple blades with power woodworking tools but the blade chord would be pretty small so I was thinking that maybe using 4 blades (with 2 boards crossed) would help.

Ryland 10-29-10 08:30 PM

Three blade wind turbines run smoother and tend to last longer because the blades are creating a balanced gyroscope, air planes don't have the same issue's as wind turbines because they are not pivoting on a tower. offers a wind turbine design class that is more about why things are done the way they are done instead of how to do it your self, they also offer a class on building your own wind turbine, but I've taken the wind design class along with a few of their other classes and feel that they are all well worth the time.

nibs 10-30-10 11:23 AM

This is my second crack at a response to Patrick, the computer ate the last one.
Patrick, please provide a bit more info on the generator you plan on using. How many rpm does it need to start developing the voltage you need? How much power are you trying for? What is the average wind speed (and max wind speed) at your site?

My experience in building as many as 600 small turbines, indicates that a two bladed prop will perform admirably if well designed and carefully carved. A three or four bladed prop is much more difficult to make and will give a very slight improvement in early startup, and may or may not prescess a little more slowly.

If you google NACA 0010, you will find a foil shape, use this shape (half only) for the downwind side of the blade, the upwind side can be flat. Carve as much angle into the prop near the hub as poss, radiating (wash or run out) to an angle at the tip that is almost perpendicular to the wind. The actual angle can be calced based on the parameters of your site and generator.

Hope this gets through this time,

nibs 10-30-10 11:31 AM

In response to Ryland, my goal here is to have people making their own blades, if folks begin carving simple blades, which they get to work, they may well be inspired to try multi bladed props, to discover for themselves the various design configurations. A discussion of two bladed or multi bladed prop superiority is not my intent. opinions vary. If we can get effective props generating electricity at lots of sites they will raise awareness. Prop carving does not need to be a black art, two bladed props are simple to make and effective.

Patrick 10-30-10 01:15 PM

2 Attachment(s)
I don't have a specific alternator in mind. I was just thinking of an easy way to make some airfoil shapes. A variation on the CAP 21 "icecream cone" airfoil came to mind as something I could make with a table saw and a router table. Here are pics of my first prototype. It has no twist and a 10 degree angle of attack.

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