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.

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.

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.

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

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.

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.

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.
The-mrea.org 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.

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,

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.
Cheers.

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.

OK Patrick, some pointers if I may.
Your foil shape is not bad, but you need to move the thickest part of the blade towards the trailing edge, and it needs to be thinner at the tip, also round the transition from the upwind side to the downwind side at the leading edge, that will reduce the turbulence there. Washout or twist is vital, the blade will turn in the wind at 10 degrees, but will never "fly"
My blades used to spin at a speed that "filled the circle" ie in theory no wind passed through the disk covered by the blade without imparting its energy to the blade.
Your blade looks better than my early efforts, so good on you.
Suggest you will be happier with a permanent magnet motor than with an off the shelf alternator.

Thanks Nibs. I might be able to move the thickest part back by adjusting my router, but I don't know how I could get any twist while using a table saw to cut the blade. Also, I should be able to round the lower portion of the leading edge some which would make the effective angle of attack lower also.

I read about Bergey wind turbines using "pultruded" fiberglass blades that were the same chord the whole length and had no twist. They claim that they perform well, so I thought I could get away with something like that, only made out of wood (which is cheap and I have lots of) and using power wood working tools (that I already have). Low to no cost, just some time to figure it out and make them up.

Bergey also has issues with those blades causing noise and vibration but feel that the low cost makes them perform/sell well.
Whisper used to use two blades but most of those machines shook them selves appart and went flying off their towers.
Simple Wind Turbine Blades is a rather good blade design, these machines tend to work and hold up pretty well.

I bought a 400W HAWT on Ebay. It has the plastic blades. A few photos are attached. I think the airfoil shape, at least at the tip, looks remarkably similar to the ones I made. Of course the chord is a lot less and they have twist and taper that mine don't. Both the leading and trailing edges are sharp all the way down. This particular turbine uses 6 of these blades bolted to a steel hub powering a modified GM 10SI alternator PMA. Total rotor diameter is 60".

Ive just read you're posts and was wondering if these wooden blades would work with the generator I'm going to make. If the weight would have any effect. The easiest way to is to give you a web address and say I have bought two of these F&P washing machine motors to build my wind gen for $40 auzz. its http://thebackshed.com , let me know what you think .click homegrown then Projects and info- F&P smartdrive. This is the actual motor that drives the washing machine, pull it apart rewire and you have a generator, up to 3oow

I don't know if they would work or not. I'm not even sure they produce any power. I made the first one and put a 1/4" lag bolt through the center for an axle. It started to spin up when I pointed it into the wind. I mounted it on the front of my deck, but the wind hasn't been coming in from the right angle to make it do much of anything. Yesterday it was spinning slowly but the wind was at about a 75 degree angle to the blades - in effect it was furled. I think I might make up a weather-vane mount to allow it to align with the wind and then see what happens.

OK, I made up a quick and (very) dirty pivot for the propeller. I made a video and uploaded it to YouTube (my first one, yeah!). Comments appreciated. Here's the linky: http://www.youtube.com/watch?v=G7YJqd171iw

I lolled. I would have laughed harder if you had saved some signs from the Nader or Gore campaigns.

Patrick, how do you plan to test the performance of your blades vs. other designs?

I think I'll set up the Hornet HAWT I bought on Ebay, then switch out the blades for the wood and see how it does.

Haha that is awesome!
 

Perfect music choice!

This is cool, I'm inspired to make my own table saw/router blades. Regarding the angles that nibs mentioned I wonder if you could cut the blades into thirds and pin them with dowels at varying angles... Or if it would just fly apart into pieces ;)

Here's a video of the same blades but they are now turning on ball bearings and I used a better camera (30 frames per second instead of 15). I think the wind speed is about the same as the first video. YouTube - 100_0301.MOV

Hard to say since the first video FPS was so low, but I imagine the bearings made a big difference. What is the next step?

I've ordered a different router bit to help improve the blade shape.

In another test that I didn't video I rounded the lower front edge of the blades and set it back up on the pivot. They seemed to be more reluctant to start but when they did they were scary fast, nothing but a blur. I took them down because I didn't want the blades to fly apart and break a window or hurt someone.

I did quite a bit of research and adding the twist to the blades just doesn't seem to be worth the hassle. These blades are very easy to make once you get the tools set up and trying to add some "twist" to the middle half or third doubles or triples the work. I think greater gains could be had just by increasing the rotor diameter, which doesn't really increase the work.

I'm also trying to build a small turbine for the "5 watts for $25" challenge. It won't be a VAWT, but I think that's OK as long as makes some usable power.

Here's the shortened, streamlined prop on the new "P-47" pivot: http://www.youtube.com/watch?v=2qXSQ1nky8c

Its hard to tell. Does it appear to be an improvement?

Have you a design in mind for the two bladed props, i would like to have a go at carving them with a handheld router, or any step by step design using a router would be handy. No matter how much i search it always returns using a cnc router. At present i have a homemade affair made from scooter parts for the bearings,motor,gearing,chain etc and some homemade pvc pipe blades. Startup seems problematic along with the neighborhood kids throwing things at them,hence the move to a wooden design
many thanks

Prototype 3' 4-blade downwind prop for the 5W for $25 challenge. I have about $15 in materials in this one, mostly in the metal parts (bearings and bolts). Now I just need a 12V, 5W generator for $10. Anyone know where I can get one?

YouTube - 3' 4-blade Downwind Table Saw and Router Prop

Darin used a car fan blower motor for his bike... Something smaller would be better though. Search ebay for small PM motors I guess. Maybe a hair dryer motor or something...

Sorry I do not get on the interweb much in the winter, as I have to go to a hotspot. We used to call a blade with no "twist" a drag machine, as the wind hits it and turns the blade, the classic example is the American Agricultural water pump, with its blades that almost fill the circle, it will turn relatively slowly but quite relentlessly. If you are building a constant pitch blade, the more you fill the circle with muliple blades with a low aspect ratio the better.
If you want a blade that flies you need washout or twist.
My 4' blade was carved from a single 2x4 first drill a center hole to mount it, then about 2 inches out cut on the leading edge draw a diagonal line from the upwind side to within 3/8's of an inch of the downwind side. This will be the leading edge, grind or cut the wood off using the trailing edge (other edge of the 2X4) as a guide leave about 1/8" of wood at the trailing edge. Round the leading edge by eye so that the whole looks like an airplane wing, use a protractor every inch to keep left side = right side. this will give you a pretty good jumping off point to experiment from, keep it balanced by sliding it on to a shiny metal screwdriver shank. When I get home in the spring, I will try and post a drawing of the blades I used to produce.
Yesterday I saw a turbine blade made from what looked like 4" white plastic water pipe, it was cut in such a way that it had washout, and was turning in a modest breeze. It looked kind of easy once you cut the tube lengthwise in the right shape, each blade was bolted to the hub with two 1/4"(+/-) bolts, once you got the design right, you could easily make a three bladed prop.
cheers and sorry for being gone for the winter.

Prop with Twist
 

After Nibs latest post I decided to make a prop with twist. I used a bandsaw instead of the table saw for this one. If you want to duplicate it you will also need a rasp (rotary rasp optional), a hand saw, and some way to sand (I used a random orbital sander, but you could do it by hand). It took about 5 times as long to make this one as the one with no twist due to all the hand carving required.

Here's how to do it, step by step.

1) I ripped my board to 3" wide so it would fit in my bandsaw. You could go wider if your saw can handle it.

2) Mark the line for the leading edge on the top of the board (3/8" at the tip to full board width at the hub end. Flip the board end-to-end and do it again.

3) Set up a low fence on your bandsaw table. I used one of the cutoffs from ripping the board down to 3". Leave about 1/8" between the blade and the fence (this will form the trailing edge as the blade is cut).

4) Start with the line on the top of the board lined up with the bandsaw blade and the bottom edge of the board pressed against the fence. Turn on the saw and feed the board into the blade following your drawn line. Make sure you keep the bottom of the board pressed against the fence and the table and tilt the board as you feed it in. Stop when you reach the inside edge of the top of the board (where your drawn line ends). Turn off the saw and remove the board. Repeat for the other end of the board.

5) I used my router to get a start on the airfoil shape, but it's only good for the first few inches in from the tip of the prop on this design. You can do it all by hand with a rasp or a rotary rasp (see Step 7).

6) Draw a line squarely around the board where your bandsaw cut ends. Use the handsaw to cut into the board at an angle until you meet the bandsaw cut. This removes the waste piece. Repeat for the other end of the board.

7) Use the rotary rasp (or hand rasp) to carve the general airfoil shape into the back of the blade. You need to remove a lot of material near the hub, less toward the tip. When you get it roughed in, use the straight hand rasp to smooth it out.

8) Use the random orbit sander (or hand sanding block) to smooth out the rasp and saw marks.

9) Mark the center of the hub and drill the hole. I used a Forstner bit to fit 2 ball bearings to mine.

11) Balance it per Nibs' post.

10) Fly it.

I put mine up in the place of the 4-blade and it seems to fly well. I don't think the twist is right on this one, though (the blade angles don't match some online calculators that I tried). I'll post a video of it once I get some steady wind.

I can't upload all the pics in one post so I'll have a continuation . . .

Prop with Twist Part 2
 

Attached are more pics.