Some mod's to PVC blades

[henjulfox]

Hello,

Let me start by thanking everyone who posts at this site! Such an excellent source of down to earth information on renewable energy. I particularly appreciate the fact that the experts take the time to help us newbies.

I'm in Oregon with class 1 winds, in an established subdevelopment thick with beautiful old established trees. Therefore I'm just messing around with no real hope of harvesting much electricity. But I enjoy messing around...

First I put up an Air-403 on a 20' pole. Looks nice up there. I even saw it spin once!

Then I got a 1/6 HP DC motor, rigged up a mount and tail, and started looking at the PVC blades posts.

1. I tried 4" ABS instead of PVC. ABS has a wall thickness of 1/4", much thicker than PVC and hopefully stronger. I even took the chance on cutting 3 individual blades instead of the usual 2 together.
   
2. Then I took the basic design and expanded each line outward by 1/2 inch. I get about the same pitch (allowing for the curve of the pipe), but get more suface area facing the wind. The thickness of the pipe gave me some material to sand down with a belt sander to fine-tune the airfoil.
   
3. This left me with the usual narrow tip. Small area facing the wind, out where all the energy is. Looking at the site of "jerry's blades", I saw where his have the usual twist to lower the angle at the tip but keep up the surface area. It struck me that if the center line of the blade (trailing edge) were to twist down the pipe a little, barber-pole style, I would end up with a twisted blade. For a blade 2' long I moved the trailing edge about an inch at the tip and ended up with a blade only slightly narrower at the tip than the root, but with a steep angle at the hub and a very shallow angle at the tip.
   
   

Sorry, I can't give much in the way of dimensions. I did so much "fine tuning" with the belt sander that what I ended up with isn't real close to what I started with. Can't even tell you how it works - the usual wind-anchor effect is working well. Besides, I'm hoping a few of you folks that know what you're doing will either point out the error of my ways, explain that I missed a few posts that already tried my ideas, or improve on my attempts.

Would there be any words of wisdom out there on what length of blades I should try on a 1/2 HP PM DC motor?

-Henry

[stevesteve]

Hi,

Any pics?

[henjulfox]

Maybe these pictures kinda show what I carved out...

-Henry

[rotornuts]

You have the spitting image of the first set of pvc blades I ever built. 4" ABS. If you attach them right where your fingers are they should work well. I wasn't terribly happy  with the rpm they began to stall at. After that set I began to saw through the entire pipe in one go using a taper jig. It produces a sharp leading and trailing edge but no twist. The sawn through blades performed far better than the others despite no twist. Anyhow they look good. Enjoy. just be sure to bolt them up close to the trailing edge to give some additional angle or they be poor off the start and stall from too much drag at high rpm.

Nuts.

[ghurd]

The leading edge should be rounded.

The root looks square. How did you do that?

Heat? How did you heat it?

G-

[henjulfox]

Excellent! I didn't think I was the only one to have tried this... And yes, my fingers are on the bolt holes. I didn't think of cutting the whole blade with one cut, would save a lot of sanding.

Finally got a little wind. I would guess 10MPH. She does start spinning sooner than the air-403 but wouldn't go much over 12 volts with no load. Motor rated 90 volt, 1750 RPM, therefore I believe around 233 RPM. 12 volts dosen't do my 24 volt battery bank much good. I suspect I will have the same result as you, but I look forward to some good wind to see how it'll do. Thanks,

-Henry

[henjulfox]

By "the root looks square" I'm guessing you mean that it looks flat instead of rounded in the dimension where it's bolted to the hub. The whole blade is the arc of the pipe, no flat spots. Between the lousy picture and me slipping with the belt sander, the second picture makes it look like it's flat down the center line, but it's not.

I realize that blade-carving 101 says that they should be airfoil shaped, thus a rounded leading edge. While thousands of wind turbines can't be wrong, I miss the point to an airfoil. Seems to me that the lift generated would tend to bend the blade in towards the pole. Might it be that because the blades are at an angle, some lift bends them in but some also pulls them forward (rotation)?

In either case this weekend I do plan to rounding off the leading edge. Better minds than mine have determined that it's what works best. Thanks,

-Henry

[ghurd]

Got it. Just cut at an angle.

Sure looks like a bend.

G-

[rotornuts]

I've done alot of research on aerodynamics and I'm still fuzzy on whether a sharp or rounded leading edge would be best on these blades. Pipe blades are a highly cambered symetrical airfoil which has a narrow low drag operating range, this I believe is why they tend to self regulate or stall regulate. In any case the blades I cut the first time had a blunt leading edge and I think that contributed significantly to thier poor performance. If you go anywhere between sharp and rounded on the leading edge you should do better than me as long as you go sharp on the trailing edge to let the air flow reform with as little disturbance as possible. You may want to try and modify the pitch angle to match your generator. Like I said before It is my understanding that highly cambered profiles have a narrow band of low drag operation(perhaps 5 degrees) so if your generator is loading your blades at the wrong angle you'll have a less than optimum situation. Of coarse rpm and angles are related untill you apply a load then you'll run somewhat behind what your blades would like to run, so if at that time the appearant wind direction Vs. the rotor velocity produces too much angle the blades will stall. A bit more set angle on the blades may help in this situation.

Good luck.

[ghurd]

Uhh... What?

You mean the 'set angle' is like the proverbial 25' root and 7' tip?

And that is why 7' seems overly steep to me? As compared to the usual 4 and 5'.

G-

[Jerry]

Henry where are you located in Oregon? I'm in Salem.

                     JK TAS Jerry

[henjulfox]

Jerry of the famous Jerry's blades? Jerry, the dude with all the wind turbines on his roof? I feel honored and privledged to live in the same town! I'm on the south side of Salem.

-Henry

[Jerry]

KOOL. Hi Henry.

Have you been by the store? Have we spoke in person? I talk to alot of folks that stop by the store.  

                         JK TAS Jerry