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Piggott Blades simplification possible?

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Menelaos:
Hi there, Hi Hough!

I have built many wind turbines so far but last week I visited "Fritzblitz" in Spain and we carved a set of rotor blades for a 2.4m machine. It was great fun and the blades turned out great :-)

Having them finished, I now wonder about some thinks and maybe some people here have already tried things in that fields or even Hough personally can tell me the reasons why he does thinks the way the are printed in the book.

So what did we do:

Forst we carved the windward side (Luvside) of the blades out of the blank peace of wood that we laminated togeter. Thats quite fast and easy. After that the thickness of the blades was marked on the leading and trailing edge. The blade was turned around so that the Lee-side was on top and again with a chiesel and later with a power plander the material was taken away. The next step would be to get everything to the right thickness in the 5 stations on that 1/3 line and when that works out, the shape of the profile towards leading and trailing edge is made...so far so goof and works out fine.

Now what I wonder about:

If I have a straight peace of wood with an even surface (which I can easily make with a power planer), why do I have to bother with the Lee-side so much? I could have all the 1/3 points on the same hight (level) directly on top of my plane peace of wood, only having to make the round shape on the leading edge and the "nearly straight line" towards the trailing edge.
Still this would not affect the angles on the profile on each station.

Of course, on the windward side, I would have to take off more material then  but that sould not be a problem, should it?

So what is the reason to take off material from that peace off woof from both sides at the thickest point of the airfoil instead of only one side?

My guess is, that the way it is done, allows more accuracy during the carving process...
My backround is different. As I am also one of the leading people of that german discussion bord with a small webshop, we were thinking about offering cheap wood blanks.

My idea was to keep the highest point (the 1/3 mark) of the airfoil on top of the even peace of wood (on the Lee-side) and have the windward side waterjet cut. Tins would end up in high accuracy and low costs. All people have to do the is the easiest part: carving down fron that highest point of the airfils towards leading and trailing edge and no more measurements would have to be taken- the thing would be basically fool proof.

I could also have it watercut the way Hugh does it aproaching from both sides but then the peace of wood would have to be turned around on the machine and more cuts would be needed which makes it more expensive....and what we want is a cheap way of doing it.

Or would there be any aerodynamic  disadvantages making it the way I suggested?

Would be nice to getting some help

Max

Menelaos:
höm...after reading my artice again I am not sure If I expressed myself understandable...

To make it more clearly again...what I suspect is:

The LUV side is made first up to a straight line on the leading edge beacause it is easy to rough it out by first makimng the orientation cuts with a saw and then roughing out with a chiesel or whatever power tool before planing it. I then have a straight reference area to mark out and cout down to the thickest point of the airfoil approaching from the Lee side... which is a lot easier that doing it the other way around as I would then have to watch two lines to keep level with.

If I can have the LUV-side waterjet cut with high accuracy, this would not be a problem anymore and the highest point of the airfoil that is on the Lee side can be on the same level throughout the whole profile and only the "roundings" towards the machine-precut leading and trailing edges would have to be made....

Am I confused or has it been done this way and works out without disadvantages other that accuracy problems when done without CNC technics?


Max

Flux:
There are many ways of making a blade.  If you have a specific profile in mind then you need to keep the profile width thickness and shape at each station. It doesn't really matter if the leading edge is a straight line and you get the width of sections by curving the trailing edge or wether you keep things symmetrical about the centre line and end up with curved leading and trailing edges.

If you are going for a non specific hand carved profile then you have an almost infinite number of options.

There is some merit in having a thickness just sufficient at each point for the required strength, but it is easier just to fit the thickness on a straight line joining the full board thickness at the root to the chosen thickness at the tip.

Similarly you can make the width of each section correct at each station and end up with a highly curved trailing edge or curved leading and trailing edges. I find it works just as well to make a straight line joining the tip to the root again. This leaves the inner stations less than ideal width but it works as well and may be better unless you use mppt.

Similarly in the ideal world the front and back surfaces should follow a specific profile. Most people make the front face flat and that doesn't fit any known aerofoil that I have come across, but ClarkY and some of the NACA are nearly flat over most of the surface.  Having made an approximation on the front, you can again make approximations on the back and as long as you have a thick part about 1/3 the way back and a sharp trailing edge it will work. ( unless you have means of plotting power curves you will never know how good or how bad it is, but blade matching will make more difference)

The old Lucas Freelite blade was cut with a flat front face of constant pitch, a chord that was a straight line from tip to root and a back surface that was part of a circle and was undoubtedly cut on a spindle moulder. It was simple and cheap to mass produce and worked well enough, especially considering that it was a very fast blade and these tend to be more critical.

If you can find ways to simplify things to suit your construction methods it will be absolutely fine. If you want to get into high tech methods of production then I would have thought it would pay to start from a specific profile known to be good for wind power rather than go to lengths of automating the production of a non existant hand cut profile.

Flux

Menelaos:
Hi Flux,

That helps a lot...anyway I am not new on airfoils and I know that the one describes in Piggotts book is far away from optimum. If you have time, one can start with rounding both sides of the leading edge in order to reduce stall and carve a little sagging on the front side...but those are Thinks that take time...for a few % of efficiency and for most folks it would be easier to just make the prop a few centimeters lager to compensate for that.

Over there in the US, people carve blades like hell. Over here they are scared of doing that as they feel it might be to complicated...Germans are strange when it comes to precission. It has to be perfect, not just close and they feel they cannot do this by hand...so they do not give it a try at all...

I personally like building props from wood, it is relatively cheap and looks nice and if something breaks, it can easily be fixed with epoxy or you just carve a new blade an thats it.

We do offer Hughs book and had it translated into german by greenstep.

Fpr people that are scared of making laser parts just like magnet discs and welding parts like the chasis, we offer cheap parts for that and also for the blades we would like to give some help. We do not want to offer completed blades. People want to have the feeling that it was them having made the blades :-) We just want to give some help on the critical parts.

A simplification by having the highest points of the airfoils on the same level (but of course keeping the angles right) and precut the the front section of the blade with a cnc machine will help those guys a lot.

I aleays remember when I made my first set of blades. I had an experienced guy teaching me and giving hints, Fritzblitz. He is also a member of this discussion bord, living in the middle of nowhere in the mountains of spain. I am not sure it would have worked out that well without his help. Some thinks you can read again and again and I would not understand it until I did it.

Precutting the with waterjet and moving the highest points of the lee side to the same level already deterines all the measurements needed. All there is left to do is to go don from that highest point towards the trailing an leading edge without having to meassure anything anymore. Everybody should be able to get a straight line done on the trailing edge and round the leading edge according to the right airfoil shape.

I feel that if precut sets were offered, many more people would give it a try.

The pre machining must not have do be complicated! In this case we would laminate the wood and precut it to the desired shape, then have it waterjet cut which is only needed on one side so that it is not nessessary to turn the piece over during process. It then can be made on a cheap 2 Axis machine.

It needs one single cut for the drop section on the front side and then the cuts that give the shape of the profile when viewed from the top. This really goes fast and should not be too expensive but it saves a lot of time and helps unexperienced guys.

Do not get me wrong here, we are not a commercial site: kleinwindanlagen.de

We are a discussion bord like this one with some specials that cost a lot of money. We opended a web shop to support these costs and the money does not go into private pockets- we do it in our free time after working to help people with their constructions.

I am saying this becaus we do not feel that we will sell hundrets of sets of blades. I already thought about building one of those machines and I have finished the plans as well....

http://www.youtube.com/watch?v=LmtMQeoKCu8&feature=related

But I the realised that the efford is enormous and that there are easier ways...
I want people to have it easy finishing the blades with a minimum option of getting someting wrong but not to get a finished product.

I now have already build some blades and for really experienced people like Fritz who have built something like 50 Blades, it takes him about 6 hours to carve a complete set of 3 blades for a 10 feet turbine that then only needs paint to be finished. It would probably take me 2 days although I have experience...but it only takes me about halve an our per blade to do what is left after having it pre machined the way I suggested with a minimum risc left to fv(k it up ;-)

Thats how I approach it...

Max

fabricator:
I personally don't think it would matter at all, the thing to do would be to care a set that way and test them against a "normally" carved set.
Water jet wood carving is a new one on me I didn't know they did that.

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