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Old Sep 03, 2006, 01:50 AM
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How do you cap a rib with a carbon fiber strip? How much strength does it add?

It's something that has intrigued me for some time, and it's something I might try if it's useful enough, and cost worthy, but, how do you do it? Where do you get the strips for it?
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Old Sep 03, 2006, 04:07 AM
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Bonnie Scotland
Joined Feb 2006
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I've only seen carbon cap strips used in conjunction with carbon trailing edges, kevlar thread-wrapped carbon/balsa shear web/carbon spars and carbon D boxes, a technique pioneered and used by free-flight flyers for many years.

The carbon cap strips link the leading edge assembly with the carbon trailing edge, resulting in a structure of great lightness but immense strength, especially torsional rigidity. The cap strips themselves are simply cut from sheets of carbon of the right thickness.

I once had the pleasure of spending a weekend with Mike Woodhouse, a free-flight flyer at WC level, to see first-hand just how these structures are fabricated. Mike showed me how I could make a replacement T tail stabiliser/elevator assembly for a model I had which needed substantial nose weight. The foam/glass/veneer original weighed 80g and was pretty torsionally rigid. Mike's sample weighted 5g before covering and was bulletproof. One of the most worthwhile 1800km journeys I've ever undertaken .

Mike sells all the products needed for such structures from this Aladin's Cave.

John
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Old Sep 03, 2006, 04:30 AM
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If the balsa rib has medium density, it has compression strength of 1700 pounds per square inch (PSI) along the grain and 300 PSI cross grain. However, the tension strength is about two times more.

The strength of unidirectional carbon with epoxy depends on the layup process, wet with tow, wet with Uni-web, prepreg, compressed (or not), Pultruded Carbon, and Modulus (standard, medium of high). So, the carbon compression strength can vary from ~75,000 PSI to over 300,000 PSI. With very, very small cross section area (skinny) carbon compared to its length, the problem is buckling far before it uses its compression strength. So the skinny carbon needs its support to prevent bucking.

The team work with balsa and carbon makes a strong, stiff and light structure such as rib.

I suggest use manufactured carbon prepreg only 0.007" sheet and strip it with the rib thickness (1/16 or 3/32 inch) depending the size of the rib and spacing between ribs. Glue the balsa to carbon strip with epoxy or thin CA. See:
http://www.deltronix.net/cgi/acp_display.exe
Or
http://www.cstsales.com/carbon_fiber...nates.html#925

Prepreg carbon sheet 0.007" x 1/16" has a compression strength of about 95 pounds!!!
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Old Sep 05, 2006, 03:57 PM
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One of my Soprano wings came with a slight twist in it. I tried to take the wist out by the usual heating the covering method but it would not stay. The only way was to carefully strip off the carbon rib cap from one side of the rib and weight the wing down to a flat surface. I then CA'd the cap back on and it is still straight and in use today. It was amazing how much torsional strength was on that wing panel using so little carbon.

Mark
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Old Sep 07, 2006, 09:43 PM
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Thanks guys! Sounds like a done deal to me.

Ah, a related question.

None of the planes I've seen using this design had a hollowed-out rib. Is there some engineering reason why I shouldn't hollow it out when using a carbon rib cap?
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Old Sep 08, 2006, 03:27 AM
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Quote:
Originally Posted by Accu157
Thanks guys! Sounds like a done deal to me.

Ah, a related question.

None of the planes I've seen using this design had a hollowed-out rib. Is there some engineering reason why I shouldn't hollow it out when using a carbon rib cap?
The reason is probably the time, needed to hollow out the rib. If you would have laser cut ribs them mabye you would program that process.

And the effect(weight saving) is probably not that much, in a 100-150" design...

It's just not worth it!

cheers Jonas Ekman
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Old Sep 08, 2006, 05:36 AM
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"Is there some engineering reason why I shouldn't hollow it out when using a carbon rib cap?"

You don't get it? Read and understand post #3 again.

With very, very small cross section area (skinny) carbon compared to its length, the problem is BUCKING far before it uses its compression strength. So the skinny carbon needs its SUPPORT to prevent BUCKING. If you take away the SUPPORT (like balsa or rigid foam) the skinny carbon doesn't work to resist the compression load.

How much SUPPORT? It depends on how much you hollow out the ribs. That's why you test the sample or you learn to apply engneering math. If you want specific answers, then you must give specific numbers like specific dimensions (thickness, shape, height and length) and materials (balsa density, which epoxy and which carbon).
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Old Sep 09, 2006, 04:19 PM
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I guess there is such a thing as a stupid question? I understand it now.
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Old Oct 31, 2008, 10:32 PM
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That failure mode is called buckling BTW. When I see it in caps a few times I feel the need to correct.
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Old Oct 31, 2008, 11:21 PM
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Couldn't resist that urge, huh? From 2 years ago, no less.
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Old Nov 01, 2008, 12:01 AM
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Crap, I didn't see the date and there was no way I was that deep into this forum. I hate it when people do that. Oh wells. I swear this thread popped up next to the forum title.
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Old Nov 01, 2008, 12:06 AM
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sorry, no ill will. I just thought it was funny.
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Old Nov 01, 2008, 02:12 AM
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This reminds me of how much Ollie is missed around here.


T.D.
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Old Nov 01, 2008, 09:29 AM
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Yep, we miss Ollie. This is still an interesting read. From what I heard about the process during manufacture of these models... They make a balsa block, wide enough to make say, 100 ribs, with a CNC router that is the shape of the rib they need... say R-14... They use a sheet of cf the same size and glue it to the block, top and bottom. Then they slice the ribs off, ready to use. Now that have 50 sets of R-14 for their tapered wing. No tedious gluing of tiny strips to 3/32" ribs.

Jack
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Old Nov 01, 2008, 12:04 PM
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still the teacher

Well I learned quite a bit reading this ...soooo THANKS
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