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Old Oct 16, 2004, 10:07 PM
Registered User
Centerville, In. USA
Joined Jul 2001
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Carbon fiber rods vs.carbon fiber tubes.

Which are stiffer, cf tubes or cf rods of the same diameter? RCNutt
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Old Oct 16, 2004, 10:16 PM
Anything to do with RC
3D4ever's Avatar
San Diego Gillespie Field, California, United States
Joined Jul 2004
272 Posts
if you don't care about weight use the CF rods, but if weight is a problem use the CF tubes
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Old Oct 16, 2004, 10:29 PM
29 rods from you in western WI
Karl Bē's Avatar
Joined Jan 2003
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For the same diameter, rods are stiffer and heavier.

For material of the same weight, tubing (of a larger OD) is stronger.
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Old Oct 17, 2004, 05:48 AM
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DGrant's Avatar
United States, CA, Clovis
Joined Mar 2004
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I just installed a C/f rod on the wing of my Blade. I bought both tubes and rods(ya never know where you can use it). The scale told me the tubes weigh about half of what the rod does. These are about 4mm tubes/rods too. I wouldn't think that would be totally relative in all cases though, as the wall thickness of the tubes will change. If I remember right though.. a 48" tube weighs. around .445 or so... and the same size in rod weighs in at .835.....
I do know the rod is definately stiffer... if you don't mind the weight.. although I'd bet the tube would take anything we can throw at it too with these little foamy type plane..... I'd say get the tube.... even though the rod is stronger... if you break one of the tubes... the plane would probaly be totaled anyway... LOL.
I bought both the rods and tubes for just this purpose, to find out the diff for myself.... I only used the rod in the Blade because the tube(now that I've found it to be lighter) is being saved for a new plane... Good luck.
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Old Oct 17, 2004, 06:11 AM
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East Anglia, UK
Joined Sep 2002
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Years ago I actually had to do some calculations on this sort of problem.I cant remember the maths, but the answer surprised me, and I double checked it again and again.

It turned out that the volume of matrerial dictated the breaking point more or less (taking Euler buckling issues out of the equation) and definitely dictated energy absorption (shock load) but within that overall context you could use the same volume of material to make shapes of widely differing stiffness.

The I beam is one of the stiffest uses of a given material there is. Tubes come pretty close. Solid cores are far less stiff per unit weight.

So weight for weight a tube is a lot stiffer, but will break at similar stress to an equivalent weight rod. Thin wall tube is better up to the point at which buckling failure mode occurs. I canot remember the calculation for this point but there is a critical wall thickness to diameter ratio under which it not wise to go.

There is a tendency among those who have not studied engineering to think of stiff being the same as strong. Its not the case. Car springs are strong, but not stiff. An eggshell is stiff, but not strong. If you see what I am driving at.
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Old Oct 17, 2004, 07:12 AM
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power's Avatar
Eagle Lake, Minnesota
Joined Apr 2003
5,613 Posts
What Vintage1 said,


If you want really stiff go with wrapped carbon tubes. The cost is about triple but very stiff and not as strong as pultruded




POWER
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Old Oct 17, 2004, 08:04 AM
Registered User
Centerville, In. USA
Joined Jul 2001
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So, from what I have read here, I gather that weight for weight, tubing is best.I tend to agree with Dgrant in that if you break a tube you have crashed so hard it won't make any difference. I have never actually heard of someone snapping a tube or a rod when being used as a spar, as say in a high stress manuever. I wonder which would be best when used like an IFO uses a rod up front for shape and protecting the prop. The rod or tube would be in a bent position and it strikes an object directlly. Whereas a tube or rod doesn't strike anything directly. It just bends. So weight for weight, is tubing still the best? RCNutt
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Old Oct 17, 2004, 08:46 AM
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macr's Avatar
Newcastle, NSW, Australia
Joined Aug 2003
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A rod is definitely better for an IFO as you will have trouble bending a tube. I agree with vintage1 about the tube being stronger, but a rod will bend easier.
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Old Oct 17, 2004, 09:16 AM
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Newark Ohio
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drrant, I have a Blade on the way. where on the wing did you put the tube at, I should do that also.!!! Thanks
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Old Oct 18, 2004, 12:28 AM
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United States, CA, Clovis
Joined Mar 2004
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Hivoltage... I used rod....as I'm now saving the tube for another new project, and I only needed one 24" piece.. so the diff was in that length was less then 1/4oz.
I also installed mine after the plane was built. Right on top of the aluminum wire they say is thier "spar-system". Which is only 2 aluminum wires that you imbed into the wing, top and bottom. I don't know that I would even waste the time with those wires after having done one already.
I would take c/f tubing... less then 4mm as it doesnt need that much. I'm thinking 2 tubes less then 1/8" in o.d. Slightly imbed a tube in the top and bottom of the wing, 24" tubes centered in the span should be enough. I do emphasize "Slightly" though, as you don't want to cut into that EPP any more then you need too. Don't imbed the tubes though at all, as that weakens that foam alot. Really I'd just "press" them into the foam some, enough to make an indent... then c/a them with med. c/a. Gorilla glue does stiffen the foam a bit too.
You might get some flexing of the tips outboard of where your c/f spars are too... so you might want to just go the whole span with the c/f tube. I didn't .......well 2 reasons...one being the Blade is a learning thing....and it will be replaced..maybe with another Blade, or Blitz(I do have to say.. I don't know when, I've nerfed it in dozens of times with little to no damage).... and the second being cost factor. The tubes/rods are between $4.50 to $6 a piece.. for 48". So to do the whole span would cost me around $10 to $12. That's 25% cost of new kit.... just for a foam plane.. and the way I did it now... hey...it works.
You'll like the Blade though... once you get the weak spots taken care of.
If you like PM me about it.. I won't rattle any more on Blade in this thread. Thanks..
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Old Oct 18, 2004, 05:32 PM
Elfi Flyer
Doug Sipprell's Avatar
Rock Hill, SC
Joined Oct 2002
5,554 Posts
Really!

>>"Car springs are strong, but not stiff. An eggshell is stiff, but not strong. If you see what I am driving at."<<

Sounds like a practical yoke to me.....

Actually, a very good analogy. thanks


RDS
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