Extruded PVC blades

[gizmo]

Hi Guys

A few weeks ago a member of my forum ( thabackshed ) posted a message looking for help with a small windmill he wanted to bring to market. Wes is his name, and American who has a manufacturing business in the USA, Germany and China. I contacted Wes and found out more about his windmill project. Its based on those Chinese bike hub motors, similar looking to a F&P, and readily available in China. He's rewinding the motors to suit the windmill application.

Now the interesting bit. Wes has access to PVC extrusion equipment. He's looking at extruding his own blades for his windmill, and it looks like these extrusions will also be available to the home built windmill market. Basically sold by the length PVC blades we can use on our own windmills. But Wes needs help with the best profile to use. I've made a few suggestions, seen below, but I'm no expert or experienced enough with windmill blade profiles to give the Wes the advise he needs.

I've posted a message on my site, and had some good ideas and feedback. I thought I would post the message here too, so hopefully the combination of thebackshed and fieldlines member base can come up with the best design for Wes. The backshed thread is here, its on about page 3 that the current design shows up.

http://www.thebackshed.com/Windmill/FORUM1/forum_posts.asp?TID=1331&PN=1

This is where we are at now.

The blade shown will be extruded PVC, in this version the chord is 160mm ( sorry, its all in metric ), its 30mm fat across the middle, and thats a 22mm hole in the middle where we fit a length of steel pipe. Whats the steel pipe for? 3 reasons, 1: to give the blade strength, 2: to provide a mounting point at the hub, and 3: to allow us to twist the blade. Thats the best bit. The idea is the PVC blades can he heated and twisted around the steel pipe to give the correct angle of attack along the blade length.

This means we could buy the extruded blades in lengths ( plan is to have a few chord sizes available ), fit the steel pipe and using a heat gun ( ie the wifes hair dryer ), twist the blade to the correct angles.

Once twisted, the blade can be riveted or glued to the steel rod. There is also a winglet and end cap on the drawing board to seal off the ends.

We dont have a taper unfortunately, but thats not really an option for extruded blades and not as important as correct twist, we can live without taper.

So what Wes needs is advise on the best profile to use. Remembering we need to fit the steel tube up the middle, and we dont want a shape that will deform or buckle to easily when heated. We may need to add internal webbing to give the strength needed for twisting, would anyone have experience in heat bending plastics? And we dont want a noisy profile like those horrid Chinese fibreglass extruded blades.

What do you think? Its an opportunity for us to tell a manufacturer how to make windmill blades that will eventually be available for the aftermarket windmill guys, like us.

Glenn.

[A6D9]

I am by no means a pro on this matter but i had an idea for you...

instead of adding webbign for strenghts.  why not look into that foam they have inside garage doors for insulation and rigidness?

it has some flex to it, but it strong and will fill all the voids.

it comes in a liquid that you just put in and it would expand the whole blade.

[scorman]

Glenn,

Don't pop a blood vessel in your brain trying to get too fancy.

There are reasons why Bergey, Jacobs, and the original Wincharger (GOE222) from the '30s (replicated by DaveB in his 18footer) use a non-tapered, non twist blades ..they work!

here is a sample of some of the common airfoils successfully used:

I have an xls that calculates exactly why the above perform well:

http://www.otherpower.com/images/scimages/7526/windpower.xls

You can input amount of twist and see how it affects the lift and a comparison of 3:1 taper vs uniform cord width is presented.

I won't post them here, but look in my file photo uploads to see charts of lift/drag/AOA, polar plots etc for the above blade profiles.

AND, remember that all fixed pitch turbines are "stall limiting" aerodynamics ...the non-twist feature actually causes stall in high wind to be self limiting ie helps with  furling to prevent overspeed situation.

Note that a 6 inch width is fine also, altho many would claim that wider is better.

Note also that DaveB cut down his 18 footer to a 16 footer BECAUSE it was too powerful.

The implementation of a tubular spar like yours is how I am mounting my experimental blades as well.

http://www.otherpower.com/images/scimages/7526/3blade.jpg

It allows easy rotation for fixing the pitch angle to accommodate different wind environments. Note that DaveB has a pitch around 10 degrees and TSR around 5.5, but they at Royal have a chart showing comparitive performance (don't know how they generated the numbers within that modeling program):

http://royalfabrication.com/performance.htm

Weight is always a concern.

Do the math and see that a 12footer only needs 4 foot blades with 2 foot exposed spar.The only real question I have about your implementation, is how far to the tip do you intend to insert the metal spar, since that added weight is not a benefit if blade is otherwise structurally sound ( fill end void with expanding foam?)

Stew Corman from sunny Endicott

[clflyguy]

Here is one that might be a little more efficient through the range of wind speeds

[FishbonzWV]

Hello Glenn,

I was following that thread and the capacitor thread on the backshed.

I have used HDPE for my blades and have heated them up to get a better profile.

I don't know about heating PVC but I do know that with the HDPE you have to heat it almost to the melting point for it to retain the new shape.

My blades are made from 1/4 inch flat stock that I cut to blade shape and then use a jacking screw to obtain the airfoil. They seem to do a fairly good job and I can change the TSR with the jacking screw.

I've been through numerous lengths, widths, and blade configurations.

I guess the profile is like the bergy. It's supposed to be a high lift type.

These are a few of them.

http://www.otherpower.com/images/scimages/1725/Plastic_Blades_001.jpg

http://www.otherpower.com/images/scimages/1725/Green_Blades_002.jpg

http://www.otherpower.com/images/scimages/1725/front6blades.jpg

http://www.otherpower.com/images/scimages/1725/Red_blades_001.jpg

One thing with these is that if they are too wide the sun will heat them up and they will curve toward heated side. But that can be a good thing, more tower clearance.

They lay back nicely though when the wind gets up.

I've had a mill on a test stand fall over and lay there overnight. When I picked it up one blade was bent at a 90 degree angle, when I returned home from work it had returned to it's original shape. :-)

I've also had a guy wire come loose and get wrapped around one of the blades.

http://www.otherpower.com/images/scimages/1725/Toppled.jpg

All it did was scuff up the leading edge and a file smoothed it right out.

I am very pleased with the HDPE blades.

Oh, they are also quiet.

Fishbonz

[SparWeb]

If the cross-section was symmetrical, the LE and TE could be squeezed, spreading the pads for the tube apart.  Insert the tube with epoxy on it, loosen clamps, shimmy the tube a bit to spread the glue, remove the clamps.  Set the twist, leave it to cure.

Digging a little deeper into aerodynamics...  Geez it's been a while...

The tube would be better placed at the 1/4 chord point.  Currently it's about at 1/3 chord.  Airfoils usually have the lift force (it is a pressure or vacuum on the surface) centered at 1/4 of the chord from the leading edge.  The point is called the "aerodynamic center", and all forces act around it, within just a few percent.

If there is a difference between the 1/4 chord and the tube that supports the airfoil, the tendency will be to twist the blade.  In your case, the lift makes the angle of attack increase, especially at the tip, and keeps the TSR down.  Not necessarily a bad thing on a windmill, but it's a killer if it happened to your airplane.

You would probably enjoy this book:

Theory of Wing Sections; by Abbott, VonDoenhoff, Stivers; Dover Press.  

It is still in print.  It's the seminal work of airfoil research that aero engineers still reach for first.

[SparWeb]

Uhh, just to clarify, I was thinking that the tube fit between two pads, top and bottom, not inside a closed cavity in the extrusion.  There was a picture like that in the Backshed files.  I wasn't looking at the picture when I suggested "spreading the tube pads".  I was thinking of the other diagram when I wrote that.

[gizmo]

Thanks for the feedback guys.

I've made a few changes, like moved the hole forward to 1/4 from leading edge, etc. Send the updated drawings to Wes to see what he thinks.

I would like to see a NACA4412 extrusion in the future, we made a set of fibreglass blades using that profile last year and they performed very well.

I'll post another message as things start to happen.

Glenn