PVC blade vs WOODEN blade angles

[amiklic1]

I got software to determine the tip and root dimensions of the PVC blades (made of tube), but I don't know is the tip anngle that has to be entered in the table the same as on the wooden blade (of the same rotor diameter).

Do I just need to put 3 degrees for the tip and calculate root angle by 6 deg at half the way down?

[gotwind]

Hi Amiklic1.

Everyone here will advise wood - I agree.

I have made a lot of blades with UV treated 'grey' 6" PVC conduit - They work ok on my 4 foot prop, but you will never create the correct aerofoil shape, that a well made set of wooden blades would.

Altons wind calculator is supposed to be very good and accurate.

http://www.alton-moore.net/wind_calculations.php

I would welcome seeing a 4 foot diameter wooden blade step through guide - like the 10' one on this site - danb's I think. The scaling down of the dimensions of the 10 foot dimensions don't work i've heard - they need to be re-calculated from scratch.

Possibly work the calculations to the popular Ametek 30 motor.

I know it wouldn't be a 5 minute job to prepare such a document, but i'm sure a guide/page would be very useful to wooden blade newbies wanting to get away from the 'quick fix' poor performance PVC route.

Thanks

Ben

[coldspot]

"the popular Ametek 30 motor."

I'm sure that many people would love a easy carving guide for this.

I know how bad I need one and have yet to try carving wood other than

My trim blade attempt at one. (That works about like PVC ones but still

haven't had a very close comparision do to lack of proper hubs).

Someday I'll do that equally to see.

I know they would work on smaller that Ametek size mills and be easy to build.

So if any of the expert carvers out there can help out

PLEASE do so!!

Thanks

[Flux]

This is not really my field but as I see it the angle used in blade calculators is the line joining the leading edge to trailing edge of the aerofoil. No decent aerofoils have a truely flat drive face, but many have a form that is flat from the trailing edge up to about 30%. Beyond that there is a slight curvature ( perhaps up to 4% of maximum thickness. If you make them flat they work well enough and that is how most make wooden blades.

With sections cut from pipe the characteristics will be so totally different that the line from leading edge to trailing edge will no longer be useful for the angles predicted by calculators.

I suspect that TDM's and small iron cored alternators have a lot of iron loss drag and need lower tsr and higher torque to turn in low winds.

If you scale Dan's type blades and scale all dimensions it will scale with the same tsr. If someone has tried it and it doesn't work then it is almost certainly because you need a tsr lower than 6 or 7, not because the blade is wrong.

Even with dual rotors with no iron loss, the bearing drag is more of an issue with small blades and tsr of about 5 will give better results. Hugh's 4 ft wooden bladed rotor works very well with the dual rotor machines but for draggy, lossy iron cored machines you may be better off going as low as tsr4 for results in low winds.

To make higher tsr blades from pipe you will likely need zero angle or negative at the tip, based on the line joining leading to trailing edge and nothing from a calculator is going to be much of a guide.

Flux

[ghurd]

My personal experience with PVC, is when the tip angle is low, and wind speed is high, the tips flex back and go negative.

The longer the blade, the sooner and worse it flexes.  It can be quite dramatic.  It is very easy to see changes from the side, especially in gusty wind.

With everything changing, I would not expect a calculator to work too well.

It is not quite as bad with a lower TSR because the tip region is heavier.

Not very scientific, but I wouldn't go past about 15" per blade, and have a feeling that is too long in big wind.

Being short, easy, and cheap... I just make several sets until I find what works best.

G-

[Flux]

Yes I agree with that, keep them for small slow rotors and they will be ok.

Trying to increase diameter or tsr will result in bending and totally unpredictable behaviour. I think they survive in higher wind by becoming inefficient.

Flux

[Norm]

   Have you ever tried nesting a shorter blade

inside for reinforcement?

   I think I've seen this posted where someone used this way.

   Really thinking about it I think wood is the

better deal....

   I'm waiting for someone to come up with a

real good design that will be a perfect match

for those mean cogging stepper motors and 5.5 mph

winds that we have here....LOL!

            ( :>) Norm.

 

[ghurd]

It would work with thin-wall.  Thick-wall doesn't have much give that way, so it would take some effort to get it tight.

A 4 blade set 10" long, 20" diameter, from 4", with a lowish TSR, will do it.

Seems I had that one going well before 5.5MPH.

[luckeydog]

alternative to pvc but using pvc as a mold or like I did using a steel pipe. I have some photos on how I built a blade using fiber glass foam and carbon fiber. the results on a ametek 50 motor were very good. it performed better than I expected. all though this blade is only 22oz and is lighter than aluminium and stronger than steel. I built another blade and it is even lighter than the first only 14 oz. and is bigger than the first. i am still testing this blade and will post the results. you can see a step by step of the processes involved in a carbon fiber composite wind turbine blade. http://www.hydropowercar.com/content.php?content.23

P.S. if you are looking for a cheep way this is not it. due to that war in Iraq there is a shortage of carbon fiber and the cost for this material has gone through the roof.