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Old Jan 04, 2013, 11:02 AM
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Niko03's Avatar
United States, WI, Mt Pleasant
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Question
Carbon reinforcement for Woody Wing Spar

Hi All,

About to start my Oly III and was getting the last of my supplies. I found this at CST, Is this the carbon for the wing spars that all the kids are using these days?
I was a little confused between this and the poltruded stuff. What are the differences and the use differences?

Thanks

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Old Jan 04, 2013, 11:15 AM
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Here ya go...http://www.cstsales.com/intro_pultrusions.html depends on what your demands are going to be for the O3 your building... Winch, slope or Sunday cruiser?? I'd go with .007 if it's just bungie launch, .014 for a stiffer spar and .030 for a Viagra spar go w/ the poltruded stuff if possible IMHO. Whichever the thickness you decide to go w/ take that much out of the rib notch and NOT the spruce spar thickness as I've seen some mistakenly do... I know you know that though
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Old Jan 04, 2013, 11:28 AM
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Here is a spar I'm currently building for a winchable 4 meter Build log in progress here if you would like...http://www.rcgroups.com/forums/showthread.php?t=1796345


* edit sorry Niko, just realized that you had already been to the build log...
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Old Jan 04, 2013, 12:42 PM
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Thanks Woody,

I take it the poltruded is stronger. I was thinking .014 and they don't make that thickness in poltruded. More important to me is not having to mess with the width and they don't show a poltruded at a .375" width. The regular carbon laminate they do have in the perfect size .014 x .375. No Winch and I'm not flying too crazy. Think that will still work?
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Old Jan 04, 2013, 12:48 PM
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absolutly will be great... the stuff to really stay away from IMHO is that DB stuff from tower... I hope I don't get bonus points for that. Post some photo's of your O 3 build in the woodie thread if you get a chance, we'd love to see it being born...
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Old Jan 04, 2013, 12:55 PM
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Thanks
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Old Jan 04, 2013, 05:54 PM
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You might find a writeup I did several years ago useful. It describe how to size and use tapered carbon fiber strips. I don't know if tapered strips are still available. If not, you can laminate strips of thinner carbon fiber to approximate what is provided by the tapered strips.
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Old Jan 05, 2013, 01:59 AM
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Four pages on the Charles River RC page are good reading on sizing the carbon:

http://www.charlesriverrc.org/articl...spar_test1.txt
http://www.charlesriverrc.org/articl...spar_test2.txt
http://www.charlesriverrc.org/articl...par_sizing.txt

http://www.charlesriverrc.org/articl...rengthcalc.htm

Take home message seems to be that the carbon cross section should decrease as a square along the span to zero, and the shear web should decrease linearly to zero. That way the tips can flex, and the wing will be lighter
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Old Jan 05, 2013, 04:50 PM
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There's a ton of great info out there, but the main thing to keep in mind is how you will be flying and launching. If you're building a woody and just want to strengthten the spar so it can take a gentle to moderate winch, then .014 should be fine. If you're trying to build a woodie that can take a full-pedal killer launch with a zoom at the end, that's an entirely different matter.

The other think to keep in mind is the spar is a system of components that work together. The upper caps are in compression, the lower in tension, and the web keeps them separated and (hopefully) keeps the upper spar cap from collapsing and breaking. It is imperative that all the bits fit together well when building, otherwise the carbon is just a waste of your hard earned money. And finally, you see kevlar thread or tow wrapped around the spar. This is to add strength to prevent the spar caps from coming apart vertically, and are good to have near the root, and maybe where the tips plug in, if the tips are removeable. Think of 2 pieces of wood put together with a little glue, and then pried apart with a screwdriver. This is the same kind of load your spar is under, and the wing joiner is the screwdriver. Now take those same 2 pieces of wood, and wrap them with kevlar, and again try to pry them apart with the screwdriver..... see the difference?

And my disclaimer: I am not an engineer, nor an aircraft structural designer. Just a modeler for 35 years, and an A&P mechanic for 26 years. I know what works for me, and I hope it help you in your quest!
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Old Jan 07, 2013, 02:39 PM
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I have a question on this topic if I may as I know there are some very knowlegable people here. Not intending to hijack this thread either.
Can I substitute a bottom 2mm thick x 5mm wide lower spar with carbon fiber alone on a 1.7 meter guppy 2 electric glider. Or does it need to be a lamination with some wood? I'm still using spruce on the top spar. It is 3mm x 5mm.
Thanks for your input. Th
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Old Jan 07, 2013, 03:54 PM
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tension/compression question

Concerning my electrified Mirage wing center plank:

I wanted to add some strength to the spar, but minimize the weight added.
I decided to apply carbon sheet to only the lower spar cap using epoxy.
I then wrapped the entire spar lightly with kevlar thread with a heavier wrap at the joiner boxes.

My thinking is that (if the spar contains good shear webs and is wrapped with Kevlar) a spar is more likely to fail in tension than compression.

Do you believe this to be true ?

Craig
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Old Jan 07, 2013, 04:02 PM
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Wood and CF have greater strength in tension than in compression, roughly 2:1. You should have put the CF on top.
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Old Jan 07, 2013, 07:15 PM
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Quote:
Originally Posted by John Walter View Post
Wood and CF have greater strength in tension than in compression, roughly 2:1. You should have put the CF on top.

The simple demonstration proving this is to take a stick of wood, doesn't really matter what kind, clamp one end down, and pull on the other end....putting it in tension. Pretty hard to break. Now, with the same setup, simply push on the stick, putting it in compression. It bends very easily, and with just a little more force, will break.
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Old Jan 07, 2013, 07:45 PM
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Most spar failures start with compression buckling of the top spar cap.
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Old Jan 07, 2013, 08:16 PM
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Quote:
Originally Posted by John Walter View Post
Most spar failures start with compression buckling of the top spar cap.
I thought that good sheer webs and a good kevlar wrap would make compression buckling much less likely to occur.

Craig
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