This page describes a simple formula for making wind turbine blades. There are several excellent formulas out there which are a bit more scientific, but the goal here is to keep things simple and inexpensive. I dont believe we need to use a terribly thick board, or have really steep angles at the root like most correct formulas would tell us. Here we make blades from standard 2" thick (usually 1.5" after its planed) lumber. The very steep pitch that most blade formulas would call for at the root require much thicker lumber. Such blades are a bit more work - the wood is more expensive and they may startup easier in low winds, although I don't believe that's an issue, especially with very free spinning dual rotor alternators. Machines we make with blades like I'm about to describe start turning in very low winds (below 5mph) and come up to speed quickly - they seem very responsive. They also seem reasonably efficient across the range of windspeeds. 10' diameter machines we use these blades on frequently produce over 1000 watts in winds below 30 mph and they seem quite reasonable in lower winds.
There are a lots of ways of doing things. I prefer to make blades from wood for the following reasons. It's easy to work with, a person can make a nice blade from wood with simple tools fairly quickly. It's fairly inexpensive. It has an excellent strength/weight ratio, and it stands up to fatigue very well. If its finished reasonably well and maintained it can last practically forever. I've seen 70 year old Winchargers running with their origional Cedar blades. Lots of different types of wood can be used. It seems like lighter weight 'pines' are the best bet - white pine, fir, Spruce ... we've used lodgepole. Lately Cedar has been our favorite, it's very light, very easy to work... it doesn't rot. We've made blade sets from single boards - lately we prefer to laminate up smaller boards, it makes things a bit stronger and we can use less expensive lumber.
Simple tools are required to work with wood. The whole job could be done with a drawknife, a hand saw, and a plane. A hammer, chisel can be handy sometimes too. A few power tools can speed things up a lot.
The blades should be tapered in their width (narrower at the tips than they are near the root). A bandsaw makes it quick to cut the width. A proper formula would suggest that this cut will be somewhat of a curved line, but I prefer to keep it straight, this makes the whole process easier and doesnt hurt our power output significantly I don't think. So.. a skill saw would also do this nicely.
The blades are also tapered in thier thickness (they are thinner at the tip than they are near the root). If the bandsaw is large enough, you can save a lot of carving by removing some of this material.
In this case the blades were too large for a bandsaw, so we used an old sawmill to cut the width, and the thickness.
When we get the blank finished (tapers cut), we start carving the front side. One nice way to remove material, especially near the root of the blade (the root is the area thats closest to the hub) is to curf it with a saw (a handsaw, or a sawzall... I've even seen folks use a chainsaw but that seems scary), and then chisel out blocks of wood.
For most of the carving a drawknife is the best choice.
Once we get close with a drawknife, a hand plane... or a power planer works nicely to get right down to the lines.
Once the front side is finished, we start on the back side. Now that the front is finished, we can plane down the back side so that the board is the same thickness as the 'thickest' part of the blade. We probably got close to this with the bandsaw...but usually there is a bit of planing to do before it's just right. As a rule, the thickest part of the blade (the fattest part of the airoil) is 1/8 as thick, as the blade is wide, and that spot should be located 1/3 of the way back from the leading edge. (in other words, if the blade is 6" wide, then the thickest part of the airfoil would be 2" back from the leading edge towards the trailing edge). The front of the airfoil (from the leading edge to the thickest part) needs to be nicely rounded, and the back part (from the thickest part to the trailing edge) is pretty much a straight line. The front of the blade should be rounded, the back of it should be fairly sharp. A drawknife, and a plane (or power planer) are nice for working the back side of the blade).
Once they're finished, we assemble them together on flat ground. The hubs that hold them together should be of at least 1/2" plywood - I prefer high quality baltic birch but outdoor rated plywood should be fine. I put 1 screw either the front, or back hub into each blade so we can adjust them a bit. We then measure tip to tip and make sure the blades are spaced equally. Once the distance from tip to tip is the same (within reason), we put in lots more wood screws through the hub. Once one hub is screwed down, we can put on the other hub. For 10' diameter blades, I like the plywood hubs to be 11 - 12" in diameter. Usually about 10 screws through each hub into each blade seems appropriate (so a total of 60 wood screws roughly).
There are many schools of thought on finishing blades. I simply coat them with a very thick coat of linseed oil. If we do this at least once a year, it seems to hold up well, and after time it builds up to form a fairly thick, rubbery finish which seems quite tough and permanent. Most folks would paint them with something... I think linseed oil is fine, but time will tell.
If the alternator turns very freely (it should) we can balance the blades on the machine. It must be a calm day (no wind) unless you do this on a stand inside your garage. Small bits of lead work nicely, and wood screws go right through it. I like decoy weights. Balance the blades by first mounting them to the alternator and bolting them down. The heavy side will rotate round to the 6 O'clock position. Raise the heavy side back up to the 3 O'clock postion, and place weight on the opposite 9 O'clock position. Add, or subtract weight (or move the weight in and out) untill it seems balanced, then screw the weight in place. In practice I tend to repeat this process a few times before it's perfect, but it's fairly easy and goes quickly. Once you have it perfect, mark the blade and the alternator so that they always go together the same way. The alternators are never perfectly balanced so we're balancing the whole unit here.
Here is a drawing of a very simple blade, this is what I've been using lately. The drawing is for a 10' diameter blade, but this same setup can be scaled up and down. The drawing on top is looking straight at the side of a blade from the leading edge. You can see the waste that we'd cutoff with a bandsaw to get our desired thickness. At the tip (radius=5') the pitch is 3 deg, the width is 3", and the thickest part of the airfoil is 3/8". (remember, its always 1/8 as thick as it is wide) At the half way point (radius = 2'6"), were twice as wide (6"), we have twice the pitch (6 deg) and the thickest part of of the airfoil is twice as thick (3/4"). For a 10' diameter blade set, if we start carving this at about 13" from the root, then it will fit in a board which is 1.5" thick, and 8" wide. At the very root of the blade, I like to taper them down a bit (6" in the drawing) - mostly for appearance. We then cut a 120 deg angle so that 3 blades will fit tightly together.
In the drawing above I've detailed 'sections of the blade' - one at the tip, and one at the half way point. If we build the blade like this, then the whole front surface of the blade is perfectly flat, so from these two locations you can build the whole blade - any line on the front of the blade will be a straight line, and the result will be a flat surface. I do it this way to keep things simple - and... we can make them very quickly with a planer and a jig. In the picture above you can see how the thickest part is 1/8 as thick as the blade is wide, you can see that the thickest part is located 1/3 of the way back from the leading edge.
This simple layout seems to yield a fairly good/strong and lightweight turbine blade. It can be scaled up and down to some degree... for example, if this were to be a 14' blade set, then at the tip we'd still have pitch of 3 deg, but we'd be about 4.25" wide. It would be about 1/2" thick (.53" actually). At the half way point we'd still have 6 deg of pitch, but we'd be about 8.5" wide, and a bit over 1" thick. A 14' blade will still work nicely from a 1.5" thick board, but we may have to make some compromises near the root.
It's a very simple way to make a blade that seems to work fine. This represents my current thinking about it all anyhow! There are more scientific formulas which may be a bit more efficient. Check out Hugh Piggott's website for some other ideas and some excellent discussion about making blades. His book 'Windpower workshop' discusses 'proper' blade design and 'why'/'how' wind turbine blades work brilliantly. Check out Windstuffnow for some further discussion and a nifty 'blade calculator' which works along the lines of Hughs ideas.
There is lots of discussion and some debate about the 'best' way to do things... I tend to think, if we build very free spinning alternators that we dont need to get very complex with it all. Many good wind turbines out there have very simple blades that have the same width, and pitch - and thickness from the root to the tip and they work fine. Other machines have lots of fancy twist and taper. What I suggest is perhaps somewhat of a compromise that seems to work well and seems easy to build. It need not be a huge project. A couple days with hand tools... less than a day with power tools in my opinion, and its lots of fun!
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©2003-2004 by FORCEFIELD
