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Apr 18, 2004, 11:51 PM
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What are the IFO Carbon Fiber rod thicknesses?

They are CF rods right? Not tubes or bars?

I’m hoping to understanding what thickness of CF rod to use for certain diameter bends. So I’d like to understand the rod thicknesses for the wing and the tail sections so as to have a better idea of what thickness works better for smaller and larger arcs.

Please specify the thickness and if it is for the wing or the elevator/aileron or the rudder. Thanks!
Last edited by 1Way; Apr 18, 2004 at 11:54 PM.
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Apr 19, 2004, 04:36 AM
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I only have one size of solid rod on hand. It is 0.06 inches in diameter. It can be easily bent to a radius of three inches without over stressing it. Extrapolating from this test: 0.03 diameter carbon rod should be capable of bending to a radius of 1-1/2 inches, 0.04 to a radius of 2", 0.05 to a radius of 2-1/2", 0.08 to a radius of 4", etc. These are just rough but fairly practical guide lines and not based on any strict criteria.

What size to use in a particular model depends on the weight of the model, its maximum flying speed and, of course, the specific structural configuration. For small very light models of the simplest structural configuration the rod size can be based solely on bending radius criteria. As the models get bigger, more complex structures or proportionately much larger rod diameters are required to meet strength criteria.

Tubes should be used instead of solid rods where stiffness rather than flexibility is desired as in fuselage stringers.

Rectangular rods should be used where flexibility in one direction and stifness at right angles to that direction are desired. As the wing gets bigger and the aspect ratio higher, the radius of bend can be used for the thickness of the rectangular rod but the height of the rod can be greater to give the wing more stiffness in spanwise bending.
Last edited by Ollie; Apr 19, 2004 at 04:44 AM.
Apr 19, 2004, 10:07 AM
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Ollie - So to verify, if I had a .06" diameter rod of CF, I should easily be able to bend it into a six inch diameter arc or circle (= 3” radius). I'm not sure that the stiffness is linear with the thickness in terms of factors, like “twice as thick so it's twice as strong” because twice the diameter of a rod renders about 4 times the profile area or 4 times the mass, so it might be a doubling or halving the standard factoring. I accept and understand all your points and assistance; you’re the first person to give me something to go on about this CF rod issue, much appreciated.
Apr 19, 2004, 10:33 AM
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The force required to bend a rod goes up as the square of the diameter but that is not the same as a safe bending strain. Because the diameter of the rods is very small compared to the diameter of the bending circle, the relationship between rod diameter and bending circle size can be safely assumed to be linear (proportional) for a constant bending strain as size varies as long as the ratio of rod diameter to circle bending diameter remains constant.
Apr 19, 2004, 10:02 PM
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Apr 20, 2004, 05:13 AM
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Perhaps further explaination is in order. When a rod is bent, the fibers on the outside of the bend are in tension and stretch. The fibers on the inside of the bend are compressed. For elastic materials like carbon fiber, the stress ( force per unit area) is proportional to the strain (change in length per unit length). The constant of proportionality is call the modulus of elasticity. The modulus of elasticity for carbon fibers is very, very high (21.5 million). That means that the strain (elongation or compression) is very small for the applied load. In other words it is very very stiff. For pultruded carbon fiber rods the tensile strength is around 320,000 pounds per square inch and its compression strength is around 275, 000 pounds per square inch. That means it is very, very strong. Because the strength in compression is less than the strength in tension, the material on the inside of the bend is where the failure will begin. When a rod is bent, the fibers on the inside will shorten (be strained in compression) and the fibers on the outside will lengthen (be strained in tension). The strains in tension and compression will be very close to equal and opposite if the diameter of the rod is much, much smaller than the diameter of the bend. When the strain exceeds the elastic limit, the carbon fiber snaps suddenly (its brittle). It is important that the allowable strain not be exceeded for the worst case design load, when designing a structure. Because the strain is proportional to the load up to failure, it is equally important that the load not exceed its corresponding maximum. Unlike steel that yealds when the elastic limit is exceeded, carbon snaps suddenly. For this reason larger safety factors are appropriate in the design of carbon structures compared to steel or aluminum structures.

For more information on pultruded carbon fiber rods see:

I hope this clears up any misunderstanding or confusion.
Last edited by Ollie; Apr 20, 2004 at 05:22 AM.
Apr 20, 2004, 08:22 AM
Registered User
Wow, Ollie, that was outstanding. I appreciate it and it only aided me, while no previous discussions were particularly confusing. It was a bit “technical” but such is engineering. I pretty much followed you, although I am not familiar with some words you used like “pultruded”, I’ve seen it before and also short (or fine?) fiber something or another, but I do not understand these differences. If you can help me out on that stuff, that would be great too.

The point about the maximum stress load and it’s breaking point was stated very in depth, but the point does bare special attention. More simply, because steel is more brittle verses flexible, it tends to break quicker than CF in the “bend verses break” quotient (i.e. considering the same sized rod for example).

So because carbon fiber may perform great at high bend rates for long relatively long periods of time, still it might violently snap at any time, given sufficient weakness or excessive force. And when it does break, if it breaks in half, both halves still have all that stored energy in it for release but having way less rod to swing so the snapping reaction can be very violent and certainly fast. I’d rather replace the CF rods upon a crash than the plane or it’s expensive motor/prop/flight gear housing. (The CF rods are for a prop protector slash landing gear setup)

I’ve been scanning some CF supply sites and read a suggestion to protect the CF by sleeving it with a thin wall of metal tubing and gluing it (vinyl epoxy was it?) to the rod to help protect it. Whatever it was, I think they said it was the same sort of substrate that was used to create the rod in the first place. I wonder what might be the best method or materials when using such a thin rod as I’m planning on using (1.5-3mm). Or if I really need any “thin wall tube protection” since it will just be replaced if it breaks. Each rod will have a minimum of two connections to the model, maybe three or four, so even if it does break violently, both ends may remain attached to the plane.

However, after reconsidering all this, I think I could be benefited by making design efforts to reduce the stress loads on the CF rods to help reduce the stress ware problems. Perhaps stress it enough to keep it pretty resistant to movement from subtle forces, but at the same time not push the stress envelope very much.

One of the charms of my ideas is ending up with a lighter plane, which will in turn require slightly smaller CF rigging and slightly less loaded forces in the bent rods to absorb the lighter shock forces upon impact. I don’t know if you read my other threads, but I am seeking this info towards developing a robust bumper/landing gear system for a model plane. I want to fly way more than I want to become a repair technician (if you know what I mean). And I like building and creating a new design rather than wrecking and fixing things, though I’m sure I’ll have my fair share of repairs to do. Thanks much for your time!
Apr 20, 2004, 08:41 PM
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Update, I'm carefully reading your link, it is fascinating. May have more questions after reading since I don't really understand their construction methods very well, but I like what they are saying.
Apr 20, 2004, 10:22 PM
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Ollie - Interesting read, but found myself needing a primer to understand what he is talking about. So I searched the web for "carbon fiber rovings" as a starter for understanding the difference between that and pultruded. I found a realy great site about FRPs, Fiber Reinforced Polymers (see link below). Helps you understand complicated ideas with more simple explanations, very concise and to the point and well done. If you are interested, it covers the basics of the Polymer matrix variations and associated benefits, as well as the various fiber reinforcements too!

I'm only part way through it, but so far I highly recommend the read so as to become better acquainted with some of this technical and engineering jargon. It’s a pretty quick read and seems to have basically nothing but solid info, not a bunch of (superfluous or unhelpful) stuff to weed out as you go along. Enjoy
Apr 23, 2004, 09:20 PM
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skysthelimit's Avatar
The ifos are very nice planes you must have the mini if you are runnging .6 thousands.
Apr 24, 2004, 12:06 AM
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For basic definitions of strength of materials, see:
When questions of how much a structural material can withstand, how much you can bend it or how big an structural element should be, the only true and useful answers are to be found in the subject of strength of materials. When the question is a "how much" type question then measurement, math, and strict definitions are needed to get to a truthful answer. Qualitative descriptions just won't take you all the way to the answer.
Apr 24, 2004, 01:48 AM
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Ollie - ... And besides, I like learning new things in my spare time. So I really appreciate your attempts to help out, but I think you are making things harder than need be. I'm putting a combo prop bumper slash landing gear system on an RC airplane. And if the bumper slash landing gear rod breaks, then ok, just replace it, and hopefully that will not happen very often.

I don't think I need so many engineering answers at all, I just seek a few standard fair practical answers,

like this for example.

"The IFO (large size) uses a 2.75mm main wing rod. It is bowed into approximately 3' diameter arc and it is surprisingly stiff in this configuration. My plane weighs in a 13 ozs and I've been abusing the heck out of it for the last 3 years, including many nose ins, yet the carbon rod seems to work just fine, so far ... "

See, real world experience is what I am seeking, not theoretics. If it works ok for them, then I have a very good reason to believe that it will work for me, as long as I make whatever adjustments necessary for different plane weights or different sized arcs, etc. Sorry if I mislead you into thinking I needed more than that, my reason for this thread was to find out
What are the IFO Carbon Fiber rod thicknesses?
If you don't know, you surely have been trying none the less, and I do appreciate that.

As an update, I now have the smaller IFO sizes, but I am more interested in the larger stuff.

BTW, I saw a video that included the makers of the IFO! at an RC show of some sort, and they were displaying a model that was very similar to what I want to do, pretty cool deal.
Apr 24, 2004, 02:01 AM
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skysthelimit - No one here said they have an IFO that I know of. However, IFO's and other planes use carbon fiber rod or support spars (etc) for such things as prop "bumper guard" protectors, for landing gear, and for wing spars and fuse supports. I am simply on a tight budget so I do not want to buy too much of the wrong size material, so I want to get as close as I reasonably can in estimating the proper thickness of CF rod prior to purchase. I also plan on incorporating secondary shock absorption at the low end of the shock absorption spectrum to help with softening all the little bumps while also lessoning the standard ware and tare on the stiffer main bumpers for improved MTBF figures (=longer lifespan for the main rods).

Although on the surface, it looks like Ollie is really trying to help, I think he is making things way harder than need be. It's pretty obvious to me that CF is the material of choice for this sort of application, but a few things are not settled yet. The most important question is, what thickness should I use, but I am getting close to answering that question as one poster already reported the thickness specs of his mini-IFO, so now I just need stats on the large one and I'd be good to go! Once I get close enough, then I can buy some just over, at, and under sized for practical comparisons, instead of just buying one of everything, I simply would rather have a better idea prior to purchasing.
Apr 24, 2004, 03:18 AM
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I still request the following information.
What are the IFO Carbon Fiber rod thicknesses?
I don't need the mini-IFO, I need the full sized large version, and because I have the mini version info, I mostly need the larger rods, especially the wing and fuse, TE, and the aileron specs would be nice too. Thanks
Apr 24, 2004, 05:44 AM
Registered User
1 Way,

Of course there is more than one way. There is the artistic approach of,"That looks about right." There is the trial and error approach,"If it breaks beef it up and, if it doesn't, lighten it." There is the easist approach,"Copy it." Then there is the engineering approach of calculating the answer before expending any resources on materials or construction. It's your choice.

If the COMPOSITES forum comes to pass, it will be interesting to hear the various approaches debated.

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