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Author Topic: wood vs. carbon spars vs. loaded sail area  (Read 1903 times)
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greenbulletblues
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« on: November 23, 2010, 11:18 AM »

Hi all.  I question if this post should maybe be in the beginners section, but this about building so here goes.  I recently made a scaled down proto-type kite with wood spars, and in a very light wind, the bridled spars broke, obviously too much sail area vs. the dowel size.  Iíve got kites with carbon spars and have had those break before.  My question is specifically about SLK.  Is there any rule of thumb on the square footage loaded sail area of a kite in relationship to the size and type of spars to use?  I have found ways of extending the length of wood spars, are there extenders for carbon spars?  Does anyone know the approx. load handling of carbon vs. wood spars?  Sorry for all the questions, but I am new to building and have had sooooooooo many questions. To better answer my questions, my current interest is in making large (8-12 ft.) SLK in unusual shapes to combine the graphic with the shape.
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mikenchico
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« Reply #1 on: November 23, 2010, 11:48 AM »

David Lord did a pretty in depth study of scale factors on spars in regards to "What size spar will I need to make the kite 50% bigger?" You would need to know the sizes on a similar design though, still a good read for anybody designing or modifying kites.

I gathered all the info on spar stiffness & weights available from every source I could find into one MS Excel worksheet, it's still not complete and I have no info on Wood since I've never found a source and wood is so variable it would hardly be relevant. You can grab it here, Microsoft has a free viewer available if you don't have Excel. None of the work is mine and I've tried to give credit to those who put in the actual effort to get all that info.

Your best bet really is to head over to the KiteBuilder Forum (just Google it), post a sketch with dimensions if you can and ask for some help. Somebody may have built something of a similar enough design to give a recommendation, if not you'll get lots of experienced input, there's an active bunch of very helpful & friendly builders that visit there daily.

Oh and Steve right here at GWTW has any of the Carbon or Fiberglass spars you might need and yes there are ferrules available for any of them to make up a rod of any length you need.
« Last Edit: November 23, 2010, 11:54 AM by mikenchico » Logged

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sbrown
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« Reply #2 on: November 23, 2010, 12:03 PM »

Longer and/or additional bridle legs can help support longer spars too.
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mikenchico
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« Reply #3 on: November 23, 2010, 12:12 PM »

I've never seen a "Rule of thumb" on spar size vr square footage, especially since kites have so many variables also, some fly on the wind, soaring kites like delta's and genki's, others fly with their face against the wind like Rokkaku's and Eddy's. We pretty much use the TLAR (that looks about right) method in conjunction with experience gained from trial & error.

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« Reply #4 on: November 23, 2010, 12:27 PM »

I find that fiberglass spars, although heavier, "bend" more, rather than snap like wood or carbon. my 2 cents  Smiley
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Lex B
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« Reply #5 on: November 24, 2010, 10:51 AM »

trial and error; and ofcourse a little luck will help you get pretty far.
And doing so with wood, will cost you a lot less then carbon ;-))
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Old Greebo
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« Reply #6 on: November 24, 2010, 01:20 PM »

When I made my first 2-line kite, 30 years ago, I used aluminium tube for the spars.  Readily available (then, and in the UK) in shops catering for model-makers.
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Fore Check
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« Reply #7 on: November 26, 2010, 03:57 AM »

The suggestion to step over to Kitebuilder is a good one.  People (like me) love this stuff, but we *do* require some more information.  Suggested bits would be a sketch of the design or a photo of the prototype and the desired size.

I've done a lot of reading and background work for my own satisfaction and use regarding the Lord spar charts, scale factors, and engineering properties of spar materials as I'm always playing with different designs at different sizes on my "drawing board." 

There are many more factors to the suitability of a spar in any application than simply the material it's made from (although, that is what it ultimately boils down to.)  How much flex is needed or desired?  How "tough" does it need to be.  How much concern do you have with weight vs. cost? 

Lord (and others) have done quite a bit of work that is very useful in sizing fiberglass and pultruded carbon spars for kites - the most important being a a nice volume of bending tests for a range of sizes that can be used to evaluate the average modulus of elasticity for the materials.  That data can be used to predict the deflection of other sizes and shapes of the same material, and to verify the predicted suitability of a proposed spar in a given application.  I don't have the equipment to do it, but it would be nice if the same type of data was available for a large sample of wood dowels/spars.  Knowing how variable the mechanical properties of wood spars is compared to fiberglass and carbon, however, the sample size would need to be quite large and the average moduli would need to be appended with standard deviation data to be truly useful.

The application itself is another key.  Two spars, one fiberglass and one carbon (for instance) may have the same deflection but one will be "better" than the other.  A good example would be the spreader on a large delta.  The fiberglass spar will be heavier because it will be bigger than the carbon spar of the same deflection.  But it will also be "tougher" and withstand more bending than the carbon spar will before failure.  A common delta kite generally *likes* to have a higher bending allowance on the spreader to enjoy a larger wind range and greater stability in typical, non-beach winds.  On the other hand, you may want a carbon spar that is light and stiff so that the same kite will be more responsive in light winds. 
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Ca Ike
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« Reply #8 on: November 26, 2010, 11:29 AM »

Wood spars can work well but the biggest issue is the grain orientation.  If you plan on using wood spars take the time to find dowels with the grain oriented along the length of the dowel.  This will give you the most strength you can get from a dowel.  Also most dowels are some type of pine since it is cheap and easy to get.  I would try to find some Ash, Oak or Walnut since these are stronger woods inherently (i use these for joint pins in wood projects where strength is critical).  The best dowels i find come from model shops that cater to high end doll house building and these are the only places i can find dowels in wood other than pine.  IMO for best weight vs strength from my experience its Ash or Oak.  From my own non-scientific tests using hanging weights, for every 5 degrees the grain is off angle to the dowel cut you will lose 10-15% of the strength and ANY knot grain warping in the dowel makes it 0 stregnthas it will always fracture at the knot grain with very little force.
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sbrown
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« Reply #9 on: November 26, 2010, 11:31 AM »

Ever consider bamboo?
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« Reply #10 on: November 26, 2010, 02:38 PM »

IMO at 8 foot or larger bamboo would be the best natural sparring material short of aircraft grade spruce and at the cost of spruce carbon is cheaper.

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"Living is easy with eyes closed, misunderstanding all you see" John W Lennon

"People do not quit playing because they grow old, they grow old because they quit playing" George Bernard Shaw
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