From Blade Calc to real timber
By commanda, Section Wind Posted on Mon Mar 28, 2005 at 09:48:42 AM MST
compromises....

I've stated previously that the true art of design is optimising the compromises for the specific application. But here I would like some input from those more experienced at making blades than I. So I've played with my generator, and have a set of power/rpm curves. I've researched my site, and know the average wind speed. I've done a household power use audit, and know how many watts I need to make 24/7.  I've put all this into blade calc, and settled on a set of numbers to meet my bottom line. Now, the chord width near the root of the blade gets really big. My question; How much will the performance deviate from that calculated by blade-calc, by cutting back the chord-width in the lower 1/3 of the blade? And am I correct in assuming that the front face is more critical than the back. That is, if I join, say, two pieces of 6x2 inch together for the lower half of the blade, should I do so at an angle, in order to get the front face closer to the calculated profile? These are the numbers from blade calc. Radius(m)   Chord(m)   B(deg) 16        1.037      42 32        0.519      23.1 48        0.346      14.3 64        0.259       9.5 8         0.207       6.5 96        0.173       4.5 12        0.148       3.0 28        0.13        1.9 44        0.115       1.0 6         0.104       0.3 Amanda

From Blade Calc to real timber | 17 comments (17 topical)

Re: From Blade Calc to real timber (3.00 / 0) (#1) by Flux on Mon Mar 28, 2005 at 05:24:09 AM MST

From my experience it makes little difference what you do with the inner third as far as output goes. I seem to remember you are using a F & P motor and you may need a bit more starting torque than anything that I have done. I tend to follow the ideal profile for the outer 2/3 and keep this width and angle to the hub. Props built this way seem a bit more inclined to operate over a wider tsr than when the ideal dimensions are taken too the root. I am certain that bladecalc or any other program does not take into account all the departures from theory that happens in real life so I shouldn't worry about saving a bit of wood at the centre rather than go to the predicted extreme. Flux

Re: From Blade Calc to real timber (3.00 / 0) (#2) by DanB on Mon Mar 28, 2005 at 06:33:36 AM MST

Hi Amanda - I made up this page over the weekend about some of my current thoughts on making blades. http://www.otherpower.com/blades.html  I dont think any compromises you make at the first 1/3 of the blade are going to affect anything noticably.

Re: From Blade Calc to real timber (3.00 / 0) (#7) by PHinker on Mon Mar 28, 2005 at 03:48:24 PM MST

Thanks, Dan!  Great little blade making tutorial.  Carving blades is the one part of the windmill manufacturing process that still has me a bit spooked.  One of these days I'll just bite the bullet and give it a whirl and your writeup will be very helpful. Paul [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#8) by pyrocasto on Mon Mar 28, 2005 at 07:12:39 PM MST

I'm spooked as well! I'm ordering my mags tomorrow probably, and I'm still a little lost on the blades. I guess I'll spend the week practicing on 2x4s and such. [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#3) by christopher on Mon Mar 28, 2005 at 07:04:15 AM MST

Hi, I think everyone else's comments are correct.  The first third of the blades are going to make very little difference.  I recently tried carving a 6' set more out of curiousity of what they would look like and perhaps gain a bit of earlier start up due to cogging of my mini conversion.  I do not think I will try this again it is very labor intensive.  My 10' blades only took me a day to carve out.  this 6' set took me a day per blade, three day total.  Best of Luck, Christopher

Re: From Blade Calc to real timber (3.00 / 0) (#4) by commanda on Mon Mar 28, 2005 at 01:32:24 PM MST

Thanks for all that people. That's a great write-up you've got there Dan. I think I've got it all pretty clear in my head now. Might push the diameter up a bit further to help the F&P with starting. Amanda

Re: From Blade Calc to real timber (3.00 / 0) (#5) by BruceDownunder on Mon Mar 28, 2005 at 02:27:01 PM MST

Hello friends.   I set up a 3 blade plastic blade set yesterday made by a Dennis Lathan down south Aus. way. They got going very quickly with the F&P ,having no trouble in the cogging with intermittent wind strengths of around 9-15 Kph-anything less,forget about it! The blades were very quite ,but seemed to get to a point where they won't go any faster-but that could be the gusts not being constant enough. I've come to my own conclusion that we have to have big blades and a geared-up drive for these F&P units-rpm has to get up around the 350 mark before you see any good power. All the best Bruce

Re: From Blade Calc to real timber (3.00 / 0) (#6) by Ungrounded Lightning Rod on Mon Mar 28, 2005 at 03:39:50 PM MST

How much will the performance deviate from that calculated by blade-calc, by cutting back the chord-width in the lower 1/3 of the blade? Look at it this way:  If you threw away ALL the power from the inner third of the blades you'd only lose about 11%. So don't sweat making them a bit less than ideal.  It's good to keep the angle about right (so they don't actively steal some power from the rest of the blade and use it to pump air).  And their gentler angle and greater length makes them a major contributor of startup torque to break cogging.  But feel free to fall back from the calculated chord length or otherwise drop the efficiency a bit to make fabrication easier.

Re: From Blade Calc to real timber (3.00 / 0) (#9) by rotornuts on Mon Mar 28, 2005 at 10:51:24 PM MST

Just a trivial fact you may find interesting. the blade portion of a maple seed has its maximun cord width located on average at just below 40 % blade length. Nature as well doesn't seem too concerned with the inner third.

Re: From Blade Calc to real timber (3.00 / 0) (#10) by bobn on Mon Mar 28, 2005 at 11:43:05 PM MST

How astute mr rotor nuts. Standing ovation may be in order. so ya think nature got it correct a million years ago?? Do ya think the guys that designed the Spitfire or the Thunderbolt with there eliptical blade planform were correct?? Not to mention whale tails or bird wings??? BobN ps now we gotta work on your definition of pitch

Re: From Blade Calc to real timber (3.00 / 0) (#11) by rotornuts on Tue Mar 29, 2005 at 03:12:10 PM MST

There's a possibility nature may be on to something there, trial and error ya know, as for the other guys? Maybe they just got lucky. PS, let's work on that definition of pitch. Nuts [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#12) by pyrocasto on Tue Mar 29, 2005 at 07:49:15 PM MST

Not a bad idea! Next time I see a bird as road kill, his wings will be my blades! :) ;)

Re: From Blade Calc to real timber (3.00 / 0) (#13) by rotornuts on Tue Mar 29, 2005 at 10:56:51 PM MST

Chris, I had a look at your site, looks good. I couldn't help but notice the rather well stocked woodshop and was curious - IS THAT YOURS. [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#14) by pyrocasto on Tue Mar 29, 2005 at 11:08:54 PM MST

Haha. Yeah. Should help me out a little for building blades, but when it comes to metal work I'm dead in the water. :| [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#15) by rotornuts on Wed Mar 30, 2005 at 04:03:08 PM MST

I'm wondering what your up to. That shop is set up for production work and thickness sanders like what you have there are not the average do it yourselfer item. In fact there not average at all and rather expensive. I drool. I like your radial arm saw as well. Is it an old dewalt or is it from the origonal saw co. Oh and did I mention how much I like the table saw and the drum sander and the wide open spaces and.... [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#16) by pyrocasto on Wed Mar 30, 2005 at 07:48:13 PM MST

Unfortuanantly I dont know about the radial saw, I'll have to check. I'll admit tools of this caliber are fun, but VERY dangerous. We've had quite a few get hurt(working of course) from slip ups. Here's a pic I did for my class though: http://photobucket.com/albums/v430/pyrocasto/?action=view&current=Shop-panoE.jpg [ Parent ]

Re: From Blade Calc to real timber (3.00 / 0) (#17) by ghurd on Thu Mar 31, 2005 at 08:23:18 AM MST

You have to be able to make a wind mill in there! Or a boat, or a small housing development, or... I make most of my stuff on the kitchen table. G- Ghurd.info [ Parent ]

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