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Old Feb 22, 2012, 11:59 PM
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Adding carbon tow

I need to add tow to my ducts to keep them from collapsing. I have seen pics of it twisted I guess this it to add strength. Wondering should I wet it out first with epoxy and tack to the end of the duct with C/A then twist it and wrap it into place. Then tack the other end to keep it in place till the epoxy dries. Thanks Dave
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Old Feb 23, 2012, 03:17 AM
Just fly it!
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No need to twist it. You want the fibers to be as straight and untwisted as possible.
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Old Feb 23, 2012, 05:54 AM
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Will it add more strength flat ? I thought the ducts I saw with it twisted was because they were trying to form a like rope to give strength, but I know nothing about composites just trying to learn as I go. I see you are the Master of the Composites. I was looking at your work on your site very nice planes you are making!! Thanks Dave
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Old Feb 23, 2012, 08:54 AM
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Carbon is strongest when it's straight. Twisting tow can give more thickness if you're looking for a fillet, but you could likely find something better for that application.
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Old Feb 23, 2012, 12:10 PM
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Dave-
Are they that flimsy or is the EDF that strong to collapse them?
Many intake ducts ( think F-18E) use stiffeners on the outside of the duct to keep them rigid. A simple balsa rib would also do the trick. Oriented perpendicular to the airflow on the outside of the duct, it increases the apparent thickness and hence panel stiffness. This is the sandwich panel approach.
Twisting the tow might increase the apparent thickness of the tow does not align the filaments for the highest loading.
Consider a blend of these ideas: after the initial lay-up, add a balsa rib with a touch of tow around the outside of the flange. This makes the panel into a sandwich panel and still allows the tow to take load well.
Clear?
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Old Feb 23, 2012, 11:17 PM
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No there not that flimsy, but they are pretty powerful and have collapsed on others. In the edf planes we try to keep them lite as possible like they do with gliders and so on. I wiill add the tow to what I have and see if I think they will collapse,I will even test them first before instaled
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Old Feb 24, 2012, 01:07 AM
Dean
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Quote:
Originally Posted by unclecrash View Post
Will it add more strength flat ? I thought the ducts I saw with it twisted was because they were trying to form a like rope to give strength, but I know nothing about composites just trying to learn as I go. I see you are the Master of the Composites. I was looking at your work on your site very nice planes you are making!! Thanks Dave
My inlets were more rigid when using the "rope" method. Think which bends easier. a carbon rod or a thin ribbon.
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Old Feb 24, 2012, 01:43 AM
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Assuming that everything is the same thickness, an untwisted fiber will perform better.

Here is a little example of how twisting can reduce strength:
Look at fabric style 7533 and 3733. They are both 18x18 and the yarn has nearly an identical yeild but the 7533 uses a twisted pair to make it softer. The breaking strength of the untwisted yarn is 350 lbs per inch and the twisted yarn is 250 lbs per inch. The untwisted yarn is 40% than the twisted yarn.

If you can wrap the inlet to the same thickness the untwisted tow will be stronger and stiffer than a twisted tow. The twisted tow may be easier to manipulate though.
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Old Feb 24, 2012, 02:36 AM
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I think that is why I see the guys twisting it. Because it will take a lot more tow to get the thickness they want if laying if flat. I will have to do a little test with it just for the heck of it. I will wait till I figure how long the ducts need to be when I get everything together to check balance once fan is placed I will trim ducts to size and use the extra pieces for tests
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Old Feb 24, 2012, 02:57 PM
Composites guy
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"Because it will take a lot more tow to get the thickness they want "

This is not a strength of stiffness critical appliaction really. An 1" x 1/8" thick piece of foam attached radially to the duct would likely work fine.

Tow is neat but likely not really needed in this case.

Cheers
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Old Feb 24, 2012, 06:52 PM
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I understand what you are saying. There are lots of other ways to make it work. Foam ,balsa, even cardboard, would probably do the job, or thin ply layed along the length.
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Old Feb 24, 2012, 09:07 PM
Composites guy
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Cardboard- I like it. cheap and light.

Let me call that: fibrous reinforced sandwich panel...

Hope it works out okay.
Scott
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Old Feb 24, 2012, 09:11 PM
Dean
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Quote:
Originally Posted by sarmoby View Post

fibrous reinforced sandwich panel...

Scott
LOL!! I'd much rather use the "high tech" product.
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Old Feb 25, 2012, 08:41 AM
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If you want "high tech" then get some small carbon sock and run foam "backer rod" inside it, and position it inside the duct, around the diameter. It's super high tech, light, strong and durable.

http://www.bestmaterials.com/Backer_Rod.aspx Sizes from 3/8 to about 1" are available at Lowes, HD, Menards, etc.

I like the 1/4 stuff so I have to order online. Soft cell is nice if you're closing it in a mold and need to compress it slightly, and normally you'd want closed cell so as not to trap excess epoxy.
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