Author Topic: Critique my blades, please  (Read 6602 times)

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Dave

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Critique my blades, please
« on: September 19, 2014, 11:08:38 AM »
Hi guys,

I came up with a process for converting 4” schedule 40 PVC pipes into 6” chord length airfoils based on usnps4. (6" pipes -->8.5ish" chord length ) I plan to use these in a 10 footer. The nice thing about it is that the blades are really easy to make in a couple of hours using only a table saw and a slap-together cinder block oven. So they're definitely cheap and easy. But I want to get your opinion on how effective they are going to be before I lock into this route.

A couple of downsides:
  -- no twist, although I'm fairly confident I can do that with some extra effort if necessary.
  --static width, I'm less confident  that I can easily fix this without more effort than would go into other methods
  --weight? There's not a ton of empty space in these blades (see below). I'm guessing this is still lighter than wood thought
  --strength? I think these should be about as strong as wood, but haven't done the math yet.

Any holes in this theory? Any advice against going ahead with this? Here's a picture. I cut the top part of the trailing edge on this one (victim of experimentation), but you get the idea.

gww

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Re: Critique my blades, please
« Reply #1 on: September 19, 2014, 01:48:21 PM »
I have just cut the pvc before and never had great luck.  I find making wood ones take a couple of days the way I do-it.  A couple hours would be nice.  I really like wood and question whether pvc will be lighter.  I don't believe it will be.  I hope they work for you.
gww

tanner0441

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Re: Critique my blades, please
« Reply #2 on: September 19, 2014, 01:52:34 PM »
Hi

I would like to see how much they distort and flex close to the root. the lack of twist is not too much of a problem as long as the blades offer the correct angle of attack to the wind. I had a commercial unit with air foil blades and the mount set the attack angle. My blades also had a slight concave shape to the side of the blade that was into the wind.

You could maybe look at forming your blade round a steel pipe if flexing is a problem, also fusing or welding along the trailing edge would stiffen the blade up. Having played with plastic blades the flexing with long blades was a problem with the tips getting worryingly close to the tower at times.

There is a freeware program Q-Blade it will let you model the performance of your blades. (once you get your head round it.)

Brian

Dave

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Re: Critique my blades, please
« Reply #3 on: September 19, 2014, 02:52:35 PM »
I really like wood and question whether pvc will be lighter.  I don't believe it will be.

I did some math. Solid PVC is 8.5x heavier per unit volume than Balsa Wood. However, a wooden blade is solid, while a schedule 40 pvc blade is a little less than 35% hollow. So, effectively, yes, this PVC design is about 6x heavier than the same shape in balsa. (Which blows my mind. I have never worked with balsa, so didn't know it was that light. Pine would be 1/2 the weight and Oak would be equal weight to PVC). Thanks for questioning that.

Dave

gww

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Re: Critique my blades, please
« Reply #4 on: September 19, 2014, 03:13:16 PM »
I used the cheepest pine that I could find and my last set is made out of,  wish I knew.   Could be some kind of ceader or pine.  I took beams off the cieling of my house.  I don't believe it is pine but I don't know what it is.  I would think to a point that weight is minute.  If it starts slower it will slow down slower.  I hope it works for you.
gww

Dave

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Re: Critique my blades, please
« Reply #5 on: September 19, 2014, 03:47:55 PM »
Having played with plastic blades the flexing with long blades was a problem with the tips getting worryingly close to the tower at times.

There is a freeware program Q-Blade it will let you model the performance of your blades. (once you get your head round it.)

For this method: Looking at the elastic modulus for PVC, you'd expect at most a 13” deflection at the end of a 5 foot blade (10 foot diameter) assuming 100lbs of force. This should be very similar to Balsa since the two materials have comparable elastic moduli (3 GPa vs 3.7). Keep in mind that these are only 35% hollow. So, they're closer to solid wood than they are to a typical 0.2" thick plastic blade cut out of a pipe.

I'll check out Q-Blade. I've just been using airfoildb.com to get the properties of various shapes (since these are essentially 2D blades: no twist or width changes). I liked the USNPS4 properties, and noticed that it's been used in Jacobs models, so figured it was a good start. Happy to entertain ideas on why certain airfoils are better than others for turbine blades, though.

SparWeb

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Re: Critique my blades, please
« Reply #6 on: September 19, 2014, 03:49:20 PM »
Dave,
Glad you could join us.
I like where you're going with this, don't stop now!

Bonding the trailing edges together with something that resists the flexing, temperatures, UV, and moisture will be crucial in making this blade a success.
Fortunately, strong adhesives for PVC are on the market.

Attachment to the hub at the root of the blade will require some thought.  If it was me, I would insert a flat plate of aluminum

Stop considering balsa.  Not a suitable material for making blades, unless used as the core of a fiberglass molded part.  I'd leave that kind of manufacturing to Vestas and Enercon.
WT blades made of wood need structural kinds of timber, such as spruce or cedar.

The weight of your PVC blade is not likely an issue, provided you keep the RPM down.
The "simple" PVC blades you may have seen before are just cut from a tube, so they are just a semi-circle.
If you can make a strong joint at your trailing edge, your PVC blade will be 10X stiffer in torsion than the other kind, and many times stiffer in flexure, too.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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Dave

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Re: Critique my blades, please
« Reply #7 on: September 19, 2014, 03:56:59 PM »
Thanks SparWeb! We seem to be thinking along the same lines. I was planning to just use EzWeld to bond the top and bottom parts of the trailing edge.

Also, thanks for the note on Balsa. Trying to compare PVC to balsa (assuming that was a viable option) sure makes it hard to beat. Like i said, i know nothing about balsa, so whether it is brittle, expensive, susceptible to rot, etc, is completely out of my area of expertise.

The biggest thing i was thinking with weight is the moment of inertia. I get that it will spin longer on the tail end of a gust. However there will likely be lots of little tiny gusts that just don't move it at all. I guess, though, that compared to the force from cogging in the generator, this is probably negligible. need to do some more math i guess. :)

gww

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Re: Critique my blades, please
« Reply #8 on: September 19, 2014, 04:40:58 PM »
My limited experiance is that weight was not as important as bearing stiffness and the blades being balanced when getting the blades to move.  I once made some ten foot really heavy blades for a motor that had a 6 to one pully on it.  If I removed the pully the blade took off very well.  I have a turbine made with a chrysler sealed bearing and could not remove any of the grease.  It had light blades but never worked well.  I have a trailer hub bearing that I removed half the grease and it did better.  I then removed the little rubber seal on the back of the bearing and it is very good.  I have had to adjust balance cause a turbine wouldn't start well.

I don't know about stress of heavy blades on the rest of the turbine but I think weight means little for starting in a steady wind as long as the blades are ballance and the bearings are lose.
Good luck
gww

Dave

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Re: Critique my blades, please
« Reply #9 on: September 19, 2014, 04:56:02 PM »
Thanks all for the encouraging replies. I think i'm going to move ahead and give this a try on the full scale!

To elaborate on why i REALLY want this to work, just let me say that in a single afternoon,  I was able to try 12 different methods (various shapes, sizes, ways of cutting, heating, molding, etc) in one foot sections. Compare that with other methods of making blades: It would take weeks to try 12 different options. So clearly, if this works, it could help a lot of DIYers get in on the fun. More people trying different shapes, sizes, and methods means faster innovation and improvements that we can all adopt.

I hope it works out, and, if so, I'll post pics etc. But to quickly break down what i'm doing here's the super simple overview.
1) cut a line down the length of the pipe
2) heat that sucker to about 350-400 degrees F
3) flatten it into a board. place between two pieces of plywood and stand on it for a while
4) now you have a 10'X14"x.25" thin PVC board (assuming you started with 4.5" schedule 40 pipe). you're going to be surprised by how sturdy the board is.
5) set the table saw to 8 degrees and taper both edges
6) heat the board again, this time targeting only half of the width (i set wet wood on the half i want to stay cooler)
7) carefully wrap the heated (flexible) end around a wooden "frame" (basically, strategically placed thin strips of wood)

Mary B

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Re: Critique my blades, please
« Reply #10 on: September 19, 2014, 07:43:38 PM »
Running a spar down the middle and using pourable expanding foam to fill around it would stiffen the blade a lot. Use cedar or spruce

Dave

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Re: Critique my blades, please
« Reply #11 on: September 19, 2014, 08:01:07 PM »
Thanks for the idea, MaryAlana. I'm definitely considering different options for handling a scenario where the rigidity isn't sufficient. However, I think you'd be surprised by how sturdy a half inch of PVC is, so I'm not convinced it will be necessary. (As I mentioned above: the elastic modulus is about the same as wood, so imagine a similarly shaped piece of wood with a few holes drilled down the length.) This is definitely not some flimsy cut-out from a pipe. We'll see how it goes and I'll be sure to post updates.

Thanks!

joestue

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Re: Critique my blades, please
« Reply #12 on: September 19, 2014, 08:25:52 PM »
I have no doubt you can develop a hollow pvc blade this way but I'm a bit wary of heating the pvc as hot as you have.
In any case I know you can bend that stuff pretty easily and with a bit of work I think you can push a pvc pipe inside a mold, heat it up perhaps with steam, close the mold down(which will flatten the small end) and then pressurize the pipe and it should expand to fill the mold.

In doing this the blade will be too thin at the root.
So a second operation could be to put another pipe inside the root (capped off of course) and then pressurize that pipe while heating it internally and it will form to fit. then you can then pull it out, spray it with pvc pipe glue, shove it back in and pressurize it a bit to make it fit.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

Dave

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Re: Critique my blades, please
« Reply #13 on: September 19, 2014, 08:47:03 PM »
Thanks for the input, joestue. I really think that folks are underestimating the strength of the PVC. I've got the 1 ft sections here, and let me tell ya, they are very very sturdy. Don't confuse it with cut-outs from a single wall of a pipe. I'll post updates when the full sized blades are done and give very specific measurements of strength, elasticity, lift, and whatever else folks are interested in. I'm even willing to intentionally break a few just to measure the material properties. Because--what the heck--they don't take much cost or effort to produce, which is the beauty.

It's not a matter of "IF it's possible" at this point--it's been done. It's just a matter of measuring the mechanical properties and determining if it's appropriate for the application.

joestue

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Re: Critique my blades, please
« Reply #14 on: September 19, 2014, 09:03:15 PM »
no doubt it is strong enough, i don't doubt that, but the modulus of elasticity of wood is something like 4 times higher.


Anyhow expanding a pipe isn't a good fit for a 3:1 ratio between chord length from tip to root.. if you can start with a tapered pipe, maybe buy sheets of pvc and roll them into a cone before inserting them into the mold and forming them to fit?

the solvents in the pvc glue will soften the plastic quite a bit.. i would look into buying pvc in 1/16th thickness and spraying it with pvc glue, rolling it up into a cone, then flattening it out in a mold while pressurizing the inside with air or something.
you may not even need to heat it up if your glue has enough tehydrofuran in it.. on second thought that is quite a stretch, but it will lower the temps needed to get it formed.
« Last Edit: September 19, 2014, 09:08:00 PM by joestue »
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

Dave

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Re: Critique my blades, please
« Reply #15 on: September 19, 2014, 10:44:58 PM »
Great ideas, joestue. When you talk about rolling it into a cone, that gives me an idea about how to potentially adapt this method to something similar. I'll work on that as a second pass. Thanks for all the input!

tanner0441

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Re: Critique my blades, please
« Reply #16 on: September 20, 2014, 04:11:39 PM »
Hi

I have had to, on several occasions, weld PVC it is very easy to char it, it has poor thermal transmission properties so heating has to be done slowly to allow the heat to permeate evenly. Would it be possible to make a negative mold of the blade profile so when you have heated the pipe you can then repeat the blade shape accurately. If the mould is stiff enough you could apply pressure with sash clamps while it cooled and this could be speeded by blowing cold air through it.


Brian

Mary B

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Re: Critique my blades, please
« Reply #17 on: September 20, 2014, 05:28:45 PM »
Cut your pipe in a taper from one end to the other and take out a thin triangle shaped piece, then build a mold to fit the final shape when heated and compressed...

Great ideas, joestue. When you talk about rolling it into a cone, that gives me an idea about how to potentially adapt this method to something similar. I'll work on that as a second pass. Thanks for all the input!

Dave

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Re: Critique my blades, please
« Reply #18 on: September 21, 2014, 11:57:01 AM »
Update: For one foot sections, it was pretty easy to keep the heat consistent and get a very manageable piece of plastic. For a full five foot section, it is proving to be very difficult to bake the entire pipe evenly without putting more work into the oven than into the blade.  :(

I think, for now, the PVC idea is on hold. But, hey, I guess it was worth a shot!


gww

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Re: Critique my blades, please
« Reply #19 on: September 21, 2014, 01:49:47 PM »
Maby some day you will get a flash of insite and get back to it better then before.
gww

richp0169

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Re: Critique my blades, please
« Reply #20 on: September 22, 2014, 10:24:31 AM »
Well if you could get a 3 foot section to work, I think this would be very good in a VAWT.

Nice work.

SparWeb

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Re: Critique my blades, please
« Reply #21 on: September 22, 2014, 03:02:47 PM »
Would it be sufficient to make your own oven?
Cut 4x8 plywood into strips, to make a box with a face that opens.
Line the inside of the box with aluminum foil or other such material.
Remove the heating element from an electric dryer (free for the taking behind every appliance store that has a messy back lot).
Duct the hot air into one end of the oven from the dryer - you need a decent fan for this.

Make the oven big enough to hold both the pipe and the folding brake at the same time.
When both are hot, remove the brake from the oven, clamp it to the work bench (still hot) and then take the pipe out of the oven and put it into the brake.  Since the brake is still warm it won't rapidly cool the pipe before you've had a chance to bend it.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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Dave

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Re: Critique my blades, please
« Reply #22 on: September 23, 2014, 10:58:50 AM »
Great idea to use a dryer. I'll get back to this and experiment with some other oven options soon. I agree that this would be a great option for vawt blades, so don't want to give up completely.

keithturtle

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Re: Critique my blades, please
« Reply #23 on: September 27, 2014, 08:53:55 PM »
I've used this approach to cure epoxy

http://www.privatedata.com/byb/rocketry/composites/ovens/Airframe%20Composite%20Curing%20and%20Post%20Curing%20Oven.html

but it will never get hot enough to reshape pipe.

This (costly) device will heat the pvc in long lengths to shape it

http://www.ebay.com/itm/Greenlee-851-1-2-4-Electric-PVC-Heater-Bender-/371054757446?_trksid=p2054897.l5672

but I think the work you'll put into pipe is not worth the effort. You can't acheive a perfect shape, like shaving wood slab, or building up a wing can.

Turtle
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