http://www.thebackshed.com/Windmill/AssemblyA.asp

How to improve those cheap Chinese blades



With the popularity of the cheap Chinese 200 watt wind turbines increasing, we are getting reports about the fiberglass blades making excessive noise. So I ordered in a set of these blades and set about improving them, not only to reduce noise but to increase power. This first article exlains how I modified the blades and what I discovered along the way. Then we hear from a forum meber Kevin and his modification to improve tracking and reliability.

The blades can be purchased for around $120 ( Available from Foundry& Fibreglass, click here for contact details, or you can find them on ebay ), so they are cheap enough, and are made from fibreglass, so should be strong enough. Length is 1020mm long, and with extensions could be used to make a turbine diameter of 2.2 to 2.6 meters, perfect for most small windmill applications and well suited to the common F&P alternators.

The blades have a nice airfoil profile, but there is no twist along the blades length, and the blade tips are cut square. This no twist and the square tip means the blades will make a lot of noise when running, and efficiency isn't as good as it could be.


A note on windmill noise. There is no reason a windmill should make excessive noise, and its usually a sign of poorly made blades.

I ordered a set of 3 blades from Foundy & Fibreglass and had a good hard look at them. After a bit of a think, I came up with the following modifications.

You will need a 4 inch angle grinder with cut off disc ( the new 1mm cut off discs ), electric drill with sanding disc, power file or belt sander, some wet and dry sandpaper, and personal protection gear ( face mask, gloves, etc ).

A word of warning. Working with fibreglass is a nasty business, the dust gets every where and makes you itch something fierce. I made the mistake of reshaping my set of blades in my workshop, so the dust went over everything. Now, weeks later, if I pick up a tool or move something in the shed I get itchy! Best to do this outside in the open where you can hose everything down after.
First up, flip the blade over so the back of the blade (lump) is facing up. On the leading edge, measure and mark a line 200mm from the blade root (center most part, where the bolt holes are).
Then at the blade tip, measure and mark 70mm from the trailing edge of the blade. Remember you can click on these photos for a bigger image.
Then using a long rule, draw a line between these two points. This will be our cut line. The wedge shape we have drawn along the leading edge will be cut off.

Next grab the blade in a vice with the leading edge facing up. Use blocks of soft wood in the vice so you dont crush and damage the fibreglass.

I tried a few different tools to cut the fibreglass, including hand saw, bench saw and hack saw, all didn't work very well. In desperation I grabbed the angle grinder with cut off wheel and it worked a treat! There is a lot of smoke given off, so make sure you have lots of ventilation. I used a pedestal fan to blow the smoke away from the work piece.

The Angle grinder cut was clean, and it only took about 30 seconds per blade.

Now we need to mark out the blade for reshaping of the leading edge.

At the blade tip, mark a line at the middle of the new leading edge.

Then using your rule, draw a line from this mark to the other end of the cut line, where it stops about 200mm from the blade root.
Looking back at the tip, the line wont necessarily travel along the middle of the whole length of the cut, but this is ok. On my blades, the line was about 3mm from the concave side of the blade.

Next you need to grab the drill and sanding disc, and sand the concave side of the blade back to our mark line. We are trying to keep the concave shape, and the round sanding disc makes this easy. Just take it easy at first until you get the hang of it. This also makes a lot of fibreglass dust, so make sure you have you mask on.

I've drawn a black line on this image so you can see where the blade concave shape was before I sanded it back to the line.

Next we grab our power file or belt sander and cut back at a 450 angle from the leading edge, as shown. It will help if you click on these photos to see full size. We cut this 450 all the way back to the start of our cut, 200mm from the blade root.

Then we do the same but this time at about 200. Dont go all the way to the blade leading edge, about 5mm back. As you can see, we have something that looks like a airfoil again.


Now we need to grab a sanding block ( or try the power file if you have the confidence ) and sand down the edges. We want a nice clean curve from leading edge to trailing edge.

On the leading edge, used a bit of sandpaper and your fingers to round off the nose. I've drawn a black line on the image above to show the best shape, and also shown is a unmodified blade for comparison.




We also need to clean up the trailing edge of our blade. From factory the blades have a rounded trailing edge. Use you sanding block to give the trailing edge more of a point by sanding the top and bottom surfaces, but dont go too far, leave the trailing edge about 1mm wide for strength.

I've also reshaped the blade tips. No real science in this, but I've seen this sort of profile on the large windfarm windmills, and I know those guys are very concerned about windmill noise, so I assume this shape works.
The finished blades are very fury with fibreglass, so I painted my blades and then sanded with 800 wet and dry paper to remove the fibreglass hairs, then another coat of paint.
This is the hub I made to match up the fibreglass blades with the F&P drive shaft on my windmill. I wont go into the details of this as its only a prototype design.
Next up, assembly, some test results and a fix.