Wood Blades vs. PVC Blades

[oak leaf]

Hi Y'all

I was reading in the Site News Section, and I have a question about wood blades vs. PVC blades.  What I am interested knowing about is, will the PVC blades work? Is it a durability issue, is it a flexing issue?  

I have made PVC blades. The first set I made were made from 12" diameter PVC water pipe ¾" thick. Each blade is 5' long 10 ¾" wide at the root and 7" at the tip. They were cut out of the pipe with an offset center line it gives it a nice twist. They move very well with little wind. But they weigh 20 lbs each. I did not want that much weight to hoist up when I am ready to fly my machine so I made the second set the same size as the first set but with 12" drain pipe that is 3/8" thick. They work as well as the first set at half the weight.    Here are some pictures of my blades.  Please let me know your thoughts.   I want to know what problems I could encounter with these blades.  All pictures are of the first set. The second set is the same but thinner, and no extra holes where I had changed mounting bracket.                              

Harlon

[TomW]

Harlon;

You will never get that sewer to work right if you keep stealing pipe for turbine blades!

Looks pretty nice.

Up here above 43 north latitude I think brittleness from cold would be an issue. I only did a couple sets when Zubbly and Woofer were doing them. Thin pipe and too thin and flexy for my liking. They were only 2 foot diameter but flexed very far back. Be sure you build in lots of tower clearance on that size I think.

I love these doer posts. Its what we are here for and all that.

Hope it works well for you. I think you may be infected now with the turbine virus. No known cure.

Tom

[wooferhound]

I'm not a prop expert, but the twist angle on the tips seems a bit too much. Actually it looks like the whole angle of the prop is too much to get any speed or power out of it. Looks like you have it flying, do you get much speed or sound? I know that if you just put it up there won't be any wind for a few days.

[oak leaf]

Yes I get a lot of speed, a red white and blue blur at 10 to 15 mph winds. I am going to try to get an rpm meter in the next week or two to check the speed.

Last night we had 25 mph winds when a storm blew in and the blades were moving strong, I placed a welding rod out from the test stand post to the blades and did not see a lot of flex in the blades.

I wished I had it finished to see what it would have put out. I hope to have my rotors done this week so I can put a load on the machine to see what it will be with a load. I have a lot still to do to get it running, the 107 deg temps we are having here is slowing me down.

Harlon

[veewee77]

My first observation is that in the last photo in the original post that the small, narrow strips that the blades are mounte to seems like it would be pretty weak. I would hope you would mount each blade to something a bit more substantial in the finished product!

Doug

P.S. Yup, guys! He's gotten the 'bug!'

[valterra]

As someone who's built a set of PVC, Barrel, and Wood blades, I can tell you that I my wood blade set outperforms the other two by a great margin.

Long story short, I made a video with my barrel blades of running a small 12v television from the 8-13 volts my Ametek generated on a solidly windy day.  Watching that video a year later,  I couldn't believe what I was looking at.  My crappily-built wood blades will generate 25-30 volts in the same conditions.  Plus they only weigh about 1 pound, and they are very well balanced.

You will probably notice over time that the flexing / vibration will cause the plastic to warp slightly at the mount point (screw hole) and that they will loosen and wobble more, which will loosen them more......  

Your mileage may vary.  Woofer has probably had more success than me.  Could be the pvc.  Could be my poor craftsmanship!

[wooferhound]

Another opinion looking at the last picture.

It really helps a lot to Angle the leading edge at 45 degrees or more. There are 2 ways to do it: cut it at an angle when you cut the blade from the pipe, or Cut it straight through the first time then cut the angle in after the props are cut. It is much simpler to do it the second way. As the wind blows by it Peals away from the leading edge in such a way that it pulls the prop forward. I think it's called shear?

Have you seen these plans for a two bladed prop made from a single piece of PVC

http://www.fieldlines.com/story/2003/11/11/172721/53

I have recently decided that it is better cut the angle of the trailing edge Opposite of the angle that is in the drawing . . .

[wooferhound]

Jumping in here with another Opinion . . .

PVC props are known to break at the hub so you need to pay closer attention to how you mount the blades on there for durability. Use much bigger Washers than that, and add smaller backup washers on top of that, followed by lockwashers and Loctited bolts. A backup plate made from the same pipe will greatly increase the strength of the prop at the hub. Here is a diagram I made for a 2 bladed pipe prop that shows little small backup plates for the pipe prop hub.



you can see how to adapt the design to make 3 blade props like yours.

[HaroldCR]

 Hi Guys. New here. Re: making blades, I was IN the wind business in the 70's-80's. Started out by buying a set of blades and governor and building my own 4KW machine. I had access to a machine, to get measurements and photos, so, my job was easier.

  Wooden blades won't flex a lot if built correctly. They should taper from the hub end to the tip, slightly. Make them like an airplane wing profile, and narrowing about 1½" from the hub to the tip.

  For example, if you have a 2 X 6 piece of wood, you start about 4" from one end, and cut the angle of the FACE of the blade, from the leading (face)  edge of the 2" TO the trailing (back face) of the blade. This gives the correct angle. Then, you remove the "Back" wood to achieve the airfoil shape. You also remove about ½" from the leading edge starting about 2/3 of the way to the tip, TO the tip. Remove the 1" from the trailing edge at about 2/3 of the way to the tip, TO the tip. This will give a taper look to the board. The profile is shaped the same, all the way to the tip, it just slowly tapers, so you have a nice wing shape.

  It's easier to do than explain.

  The Bergey machines that I sold, use a composite blade. It looks similar to the PVC pipe shape, but, you need to fasten a block at the hub end, both front and back, to achieve a FLAT place to bolt the blades tight. If not, vibration WILL happen and the bolts will loosen or the PVC WILL shatter. Trust me, I've seen it all.

  On the photos of the PVC blade setup, that was at the beginning of this thread, the mounting will not last long, under load.  Best way to mount the blades, is, make a 3/8 thick plate and bolt THAT to the hub. Bolt the blades to that plate. Use fine threads and loctite. That is where the blocks come into play, that I described. Make the blocks fit the PVC exactly, and use 4 bolts per blade. Orient the blades so you have the face of the blades a little less angle than shown in that photo. You want the blade setup, to start to turn around 7-8 MPH wind. Generator should kick in around 9 MPH. Output will be very low at that speed.

  The Bergey blades have a "Twister" device rivited to the leading edge. It has a Bullet shaped weight protruding out, ahead of the blade, and at high RPM, around 1500 or so, it will help twist the blades a little, and cause the machine to start to Yaw out of the wind. I would guess those weights to be around 1 pound  Never weighed one, but, you could feel the weight. THIS IS NECESSARY to keep from having a runaway in higher wind speeds, above 40 MPH. There is a neat way to construct the tail, also. It will auto furl and straighten back out automatically.

  Then, there is the slip ring to configure. Enjoy the project, and I will try to offer advice. Please don't think I know it all, but, I have been into it pretty deeply.

[domwild]

Hi,

Wooden blades will definitely outperform the PVC kind. My PVC blades seem to stall and never overspeed. The stall noise is a loud whosshing noise. I will try to build a mill without furling because of it. The beauty of PVC is that the blades can be shaped much more quickly. The problem is the inside curvature, which leads to bi- or triplane style wing profiles. Low speed!

Problems:

1. Jerry suggested an aluminium sandwich setup at the root to stop the blade braking off there. That means two alu pieces at both sides of the root to distribute the load.
   
2. PVC becomes brittle due to UV. Have bought a primer, which sticks to tiles, PVC, etc. and that can be used to be the base for later reflective paint, arctic white or silver.
   
3. Suggestion is to tilt the prop axle a few degrees up and away from the horizontal, so there is a larger separation of the wing tips from the mast, when the blade is at the lowest point. The blade will bend towards the mast and hit it otherwise.
   

[elt]

> The problem is the inside curvature, which leads [...] Low speed!

Gives me an idea - I found a piece of 8" PVC 30" long and am going to make blades this week; I'll try wrapping them in saran wrap (don't have monokote) to flatten the front and see what happens.

- Ed.

[valterra]

"My PVC blades seem to stall and never overspeed."

That's another great point.  Even my wood blades, which ARE poorly made by any official standards (read my post about the mistakes I made)...  the one thing I can say about these blades, with the not-so-good airfoil shape, is that you can watch them accelerate.  You can "see" the lift in action.  I did NOT see that with my PVC / Barrel blades.

[Aule Mar]

Here a idea if anyone wants to run with it.  In doing some model airplane work I have cut open a piece of PVC pipe, warmed it up under heat lamps (three in a row) till it was almost too hot to the touch and then flatened it out between two boards. Once it cooled I ended up with a nice flat pannel of PVC material, that could then be glued using regular PVC cement.  Taking this idea one could make a flat pannel to glue onto back of the airfoil, making the blade stiffer, and conforming it to a more proper airfoil shape.

Could be a lot of work, but might be fun to experiment, and see what improvements could be gotten with PVC pipe.

Aule

[oak leaf]

Hay,

Thanks for the comments. I do plan to back up the blade bracket to keep it from flexing in high winds.

I will keep in mind what y'all have posted. But I want to try it I have too much time in these to not try them. I am prepared to change blades if it does not work out.

I need to figure out what the stall is that you are talking about. I have read something about it but I cant remember what it is. I think it is they will hit a top speed lower than desired to produce max power?

Thanks,

Harlon