PCV Blades - Large 630mm 18mm wall Pipe - Question

[BarkyJ]

Hi everyone

James from New Zealand here, I am part of the Backshed.com forum which is based in NZ/AU over this way, but I thought I would come on here and seek some support from you fine people.

I have picked up a large diameter PCV mains stormwater pipe, which I am planning to use to make some PVC blades for my windmill.
This is just a starting point.

I am wanting a 3m diameter blade setup, so each blade I guess would be about 1.35m in length, once you consider the central hub.
I am aiming for a TSR of about 7.5, and I am just trying to work out what the best shape is and width as you go from base to tip, would be.

I have seen the Warlock calculators, but they are more an aerofoil than a PVC pipe, and when I put in my specs into that, the base came out at almost 500mm wide, which doesnt seem right.

I downloaded the old version so it gave me the measurements.



What I am thinking is 1.35m long, tip end being about 60mm wide, and the widest part about 160mm wide, which then tapers back to the base, to mount onto the central plate.

After any and all help, would love some guidance on this. I am not trying to make 'the best', but aiming for ballpark decency, if I can achieve a TSR of about 7.5 that would be magic.

I have put the pipe into Solidworks, and then have mocked up a sketch for the approximate shape, which I can then cut out and make a printout for to trace around etc.



Measurements in mm obviously (sorry)

Would love to hear some input, thanks in Advanced.

[Adriaan Kragten]

We have tested a rotor with PVC blades in the open wind tunnel of the University of Delft already in 1978 and it isn't working. The modulus of elasticity of PVC is much to low and therefore the torsion stiffness is much to low resulting in stall flutter at already very low wind speeds. We have also tried aluminium but this gave similar problems at higher wind speeds. The torsion stiffness is increased if the sheet thickness is increased but to get enough stiffness, the thickness must be taken very large for aluminium and then the airfoil drag becomes too high for a cambered sheet airfoil with a small chord. Finally we ended with stainless steel and this works good for a tapered blade with an acceptable sheet thickness of 2 mm for a rotor diameter of 1.8 m.

The measuring results are given in the Dutch report R 343 D of the Wind Energy group of the University of Technology Eindhoven but this report is no longer available. Recently I have written report KD 616 in which the measuring results of the rotor with stainless steel blades are given in English. The geometry of the tested rotor is also given. In chapter 5, I give a slightly larger alternative rotor which can be manufactured easier than the tested rotor but which will have about the same characteristics. Report KD 616 can be copied for free from my website: www.kdwindturbines.nl at the menu "KD-reports".

A problem with cambered tapered blades made from a cylinder is that the camber is low at the blade tip and high at the blade root. If you make the calculations of the rotor geometry for different stations (see report KD 35) you need aerodynamic characteristics of cambered airfoils for many different cambers and the characteristics for only three cambers are available (see report KD 398). Only for 7.14 % camber, the drag/lift ratio is low enough to be used in a rotor with a rather high tip speed ratio. This is one reason why I use constant chord blades with 7.14 % camber for my smaller VIRYA-windmills with cambered steel blades. The other reason is that constant chord blades can be cut and cambered easily if the correct tools are available. If the design tip speed ratio is chosen larger than about 5 the rotor will be too noisy.

[BarkyJ]

Hi Adriaan,

Thanks for the response but I have no idea what you just said. Your level of understand is well over my head I am sorry.

I have seen a ton of people using PVC blades for DIY windmills, operating fine. Yes they might not be as good as proper blades, but they still work right?

Given my large diameter PCV pipe, I would have imagined it would have had more chance of success than those who use say 300mm diameter pipe...

Im not quite sure what to make of this now

[BarkyJ]

I found this page
windandwet dot com /windturbine/tube_blade/design dot php

In there you you specify the pipe sizes and you can print off a template to cut out the blade
Anyone seen this and found it helpful?

[electrondady1]

hello James, how are things down in N.Z.
i wasn't paying much attention because i like vertical mills but those plastic blades used to be used a lot on this forum. i believe user wooferhound  used to make them . also the term zubwoofer may lead to something.

[BarkyJ]

Hey Electrondady1

Thanks for the info. I did a search and found this post with the Zubwoofer plans. Very interesting.
https://www.fieldlines.com/index.php/topic,146280.msg1000228.html

Things are great in NZ as always Going into winter, so cooling down now

[BarkyJ]

Using the windandwet link below, entering the details, and then following the instructions, I came up with this....
It looks massive...
Thoughts?

[Adriaan Kragten]

Hi Adriaan,

Thanks for the response but I have no idea what you just said. Your level of understand is well over my head I am sorry.

Have you studied my report KD 616? In this report I describe exactly the problems we had with a 2-bladed rotor with a diameter of 1.8 m, a design tip speed ratio of 6 and PVC blades with a thickness of 3 mm. This rotor was already stalling at a wind speed of 3 m/s. Stalling means that the lift force on the blade is decreasing the blade angle and so increasing the angel of attack. The lift is then increasing which results in more decreasing of the blade angle and suddenly the lift becomes very large and the blade bends backwards like an umbrella. The airfoil drag becomes very large for large angles of attack and the rotational speed goes down a lot. Lift and drag acting on airfoils is explained in chapter 3 of my report KD 35.

This stalling problem is mainly caused by a too low torsion stiffness. The torsion stiffness increases strongly (with about the cube of the thickness) if the sheet thickness is increased but if you increase the thickness for PVC that much that the torsion stiffness is large enough, the thickness becomes very large with respect to the chord and this results in a drag/lift ratio which will be too high for a design tip speed ratio of 6. The drag/lift ratio can be reduced if you transform the cambered sheet into about an Epler airfoil (Gotingen 804). We have tried this for a 4 mm aluminium blade and it works but correction of a cambered airfoil into an Epler airfoil is a lot of work.

[Bruce S]

BarkyJ;
First Welcome to the forum.
Next , noticing that you're going the 3-bladed route and the TSR you mentioned. You should do just fine.
I hadn't thought about making PVC blades once our City shutdown my request to install a HAWT  , so it took me a little while to fire up the brain cells.

I'm sure by now you've read the other link on zubwoofer blades, so kudos to you. I'll only mention that in the later part of the post(s) others went on to paint there's to lessen weather related issues.
These probably will not work as well as 3 good carved blades, but they should get you "sumting man".
It sure will be interesting to see the numbers once it's up! From the looks of the pipe you could possibly get about 5 blades from it? would be good to have a spare or two blades.
Others on here are way better at blades than I am, and I'm sure they'll be able to help even more.

Cheers
Bruce S

[BarkyJ]

Hi Adriaan,

Thanks for the response but I have no idea what you just said. Your level of understand is well over my head I am sorry.

Have you studied my report KD 616? In this report I describe exactly the problems we had with a 2-bladed rotor with a diameter of 1.8 m, a design tip speed ratio of 6 and PVC blades with a thickness of 3 mm. This rotor was already stalling at a wind speed of 3 m/s. Stalling means that the lift force on the blade is decreasing the blade angle and so increasing the angel of attack. The lift is then increasing which results in more decreasing of the blade angle and suddenly the lift becomes very large and the blade bends backwards like an umbrella. The airfoil drag becomes very large for large angles of attack and the rotational speed goes down a lot. Lift and drag acting on airfoils is explained in chapter 3 of my report KD 35.

This stalling problem is mainly caused by a too low torsion stiffness. The torsion stiffness increases strongly (with about the cube of the thickness) if the sheet thickness is increased but if you increase the thickness for PVC that much that the torsion stiffness is large enough, the thickness becomes very large with respect to the chord and this results in a drag/lift ratio which will be too high for a design tip speed ratio of 6. The drag/lift ratio can be reduced if you transform the cambered sheet into about an Epler airfoil (Gotingen 804). We have tried this for a 4 mm aluminium blade and it works but correction of a cambered airfoil into an Epler airfoil is a lot of work.

Hello,
Yes I have skim read your report and looked at the diagrams, but your materials are different, your lengths are different, the thickness of the pipe is drastically different, your TSR looks to be different, so I am struggling to correlate what I am asking with your report.

630mm diameter pipe with 18mm wall section. 3 Blades. TSR of 7.5. To be attached to a Dual Stator F&P setup, which is aiming for 1KW peak. Max RPM of 500RPM in a 10.5m/s wind. 3m total span, so aiming for about 1.35m blades
This is what I am trying to accomplish.

The plans I have cut out of paper and put on the top of the pipe, seem way to big.
My original design, which was based on some recommendation from others, some looking at what other people had done but with different materials etc, was much narrower.

I guess I am just trying to get some guidance as to what I should attempt first. If I go with the paper plans above, then my material is going to run out fast, and they just seem massive.

Thanks

[BarkyJ]

BarkyJ;
First Welcome to the forum.
Next , noticing that you're going the 3-bladed route and the TSR you mentioned. You should do just fine.
I hadn't thought about making PVC blades once our City shutdown my request to install a HAWT  , so it took me a little while to fire up the brain cells.

I'm sure by now you've read the other link on zubwoofer blades, so kudos to you. I'll only mention that in the later part of the post(s) others went on to paint there's to lessen weather related issues.
These probably will not work as well as 3 good carved blades, but they should get you "sumting man".
It sure will be interesting to see the numbers once it's up! From the looks of the pipe you could possibly get about 5 blades from it? would be good to have a spare or two blades.
Others on here are way better at blades than I am, and I'm sure they'll be able to help even more.

Cheers
Bruce S

Thanks for the Welcome Bruce.

Yeah the Zubwoofer blades certainly look different, but I am likely not able to use them as it would mean I need 3m of material in a single length, and my material is 1.5m long. It also means I would either have to be 2 blade or 4 blade, which from what I have been guided with, is not going to be right for a TSR of 7.5.

From what I understand, if a single blade has a TSR of about 13, then with 3 blades its 13/sqrt(3) = 7.5. I just dont know exactly how to design a blade with this TSR, given 3 calculators I have used so far have suggested massive blades like what I have pictured above.

Hopefully someone can guide me down the right path.

Thanks

[frackers]

I'm just outside CHCH and used to run a 3m mill with carved wooden blades - not as difficult as you'd imagine!!
I covered the making in my photo gallery after loosing a set of blades when the mill jumped off the mast!!
http://gilks.ath.cx/gallery3/windy
I've given up on wind power now - nor'westers are too much for the mill, I sleep easier at night with just 6kw of solar

[Adriaan Kragten]

The procedure how to design windmill rotor blades is given in my report KD 35. I think that a design tip speed ratio of 7.5 is much too high for a 3-bladed rotor using PVC blades. The model we have tested had two blades and a design tip speed ratio of 6. If we would have used three blades, the chord would have been 2/3. Increasing the tip speed ratio from 6 to 7.5 would result in even smaller chords if you follow the formula for the lift coefficient of the design theory. So you will get very slender blades. But if you use PVC pipe with a thickness of 18 mm, the thickness of the pipe will be very large with respect to the chord and this makes that the drag/lift ratio will be much too high for a tip speed ratio of 7.5. If you still wants to use this PVC pipe, the design tip speed ratio should be reduced to about 5 and the cambered airfoil has to be modified such that the nose has a nice aerodynamic shape and that the tailing edge is sharp.

[kitestrings]

My experience has been that you start to reach a point of diminishing returns as you get closer to the hub.  The design calculators generally show a theoretical blade profile that has a very steep angle and much wider chord length.  Except for getting the spinning from a start, I'm not convinced there is much to gain, and the added material comes at cost and complexity (where things meet at the hub).  It can also be a source of noise in higher winds.

You might want to look at Hugh's (old site), he has some good tips on blade design, and a calculator/tool.  I also like Alton Moore's.  Good luck.

I've given up on wind power now - nor'westers are too much for the mill,

Sorry to hear this Frackers.  I enjoyed seeing your perseverance with the turbines.

[BarkyJ]

I had seen Alton Moores page, and that comes out with a similar result as the paper cutout I did, a very fat bottom end, but it does have a much more slender tip.
I will look into that more.

Sorry who is Hugh and what is his old site?

Because I am using 36 pole F&P stators with black rotors, there is little to no cogging at all, so the startup spinning should not be an issue at all, so I am guessing the big wide base near the hub, is not going to be required.
So I am sort of tending back to my first image in this post, which is a more gradual slender profile...

Thanks guys

[DanG]

Beware sun heating, the softening & random relaxing of the plastic blade - even schedule80 pipe needs to be factor derated down to 22% load at 140°F - paint them bright white,  full sunshine in calm/still air can bake stuff to near 200°F. ..

[kitestrings]

Hugh Piggott is one of the early pioneers of homebuilt wind turbines and axials.  He has several books on the subject and teaches classes. Very knowledgeable fellow and, lucky for us, approachable.

his old site (with lots of good info).  Some of the blade stuff is near the bottom.  Note the blade design calculator (xls).
www.scoraigwind.com/

scoraigwind.co.uk/

[SparWeb]

Hi BarkyJ,
Glad you could join us (everyone's welcome!)

I don't think you will have too much trouble, as long as you don't expect too much.  The neighbours of yours with carved wood blades, extruded aluminum blades, are going to get better performance, especially since they tend to put on longer blades than some F&P's can handle (depending on the windings) but will they last a while (depending on the Nor'westers).

Despite being cheap and simple, you can take care of certain details to help overcome the fact that you are starting with a curved slab:  round the leading edge, bevel the trailing edge to sharp, round the tip's end, and blend all that nicely.  They will spin better and with less noise.

Ignored by the discussion so far is the hub.  Will you be making an aluminum plate with straps to the blades, or something different?  Holding the plastic blades requires quite a few bolts - the longer the blades the more bolts are needed.  If there aren't enough bolts, they just bear into the plastic and make the holes into ovals.  Your blades can see radial forces up to 10 times their static weight when spinning (at normal wind-turbine speeds... at runaway speeds all bets are off).  Of course there's also vibration and bending, so overall it's best to be careful.

[BarkyJ]

Thanks Kitestrings

Hi SparWeb
Yes the plan is to have either a steel or alloy plate which the blades bolt to, including runners which go some length up the blades themselves.
I am actually contemplating getting a plate cut with these runners built in, rather than having to bolt on runners.
I might even have something on the face too, to tie them together.
Still working out that details, but yes, they will be well supported.

The plan is to start with these PVC blades, but I likely will progress to an alloy extrusion blade eventually, but we shall see what happens.

Cheers

[SparWeb]

That sounds great.  Go with two plates, to "sandwich" the PVC blades together, and then you end up with lots of resistance to bending, and bolts that won't "pry" as much.

PS, I have just recently arrived in the world of Solidworks, after a brief fling with Inventor (Autodesk).  Still just trying to stay out of trouble.

[MattM]

Maybe consider some near root flaps out of sheet metal.  Sheet metal is pitiful for full length blades, but for simple flaps there is much less weight than PVC for the surface area.  And because they don't require much stiffness, you can get away with extremely lightweight (<26 gauge steel or <.032 aluminum) flaps.  Your pitch can be easily adjusted as it takes no effort to reform them.

You connect the flaps in this simple design (see my jpg attached below) near where the green arrow is pointed.  The hem on the leading edge is just to keep you from cutting yourself.  The flap extends in front of the existing blade.  It's simple.  It's cheap.  Best of all it works.