carbon rod vs. steel rod - RC Groups
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Dec 11, 2010, 07:28 PM
planepainter's Avatar

carbon rod vs. steel rod

Hi guys. I am now doing the drawings for a major rework of my SIG Riser 100. I plan to build a new center section, apply "D" sheeting all the way to the tips, add a carbon boom and a lot of other great things. It will end up with a wing span of 132", the same as my OLYIII.
But I have a question: can I use a carbon rod or carbon tube as the joiner instead of the steel joiner rod that it has now? You see, when I am done it will take two rods, not just the one it requires now.I figure that if the carbon were strong enough to take a light winch, it would be a lot lighter than using steel rods.

I hope to do a log on it but I can't find my digital camera and might have to just go ahead with the project without the log...

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Dec 11, 2010, 09:47 PM
Registered User

Wing rods..

Planepainter, what is the diameter of the rods you want to use to replace the steel? I'm going to use a titanium rod for my oly III in place of the steel. It weighs about half. I have a little stash of good stuff, and if I have the size you need, I would share. Reason? cause without your great build log of the Oly I would never have figured it out. Those plans & dimensions are the worst. Thanks to you I'm sure many folks are happy you did that.. Randy
Dec 12, 2010, 12:49 AM
KE your cub.
Curare's Avatar
I think that while the weight is certainly a plus factor, the failure mode of carbon is what scares me.

Lets take a winch launch scenario. If you have steel or aluminium rod for a wing joiner and you overstress the joiner, the failure mode is that the rod will bend. so in essence you may be flying an aircraft thats all lopsided because you've bent the joiner, but you will probably still be able to land it, albeit a bit wonky by leaning on the rudder.

Now if you overstress carbon, it has a tendency to start to tear. First it's a little snap of one fibre, which then puts more load on the rest, another one snaps, and so on and so on. If you catch it you might be able to save the plane, but in my experience it's pretty fast, and whats worse is when it goes, unlike steel or ali, it stays broken.

The choice is yours, I've used steel because I trust it to get me home, even if it's shaped like a V!
Dec 12, 2010, 03:23 AM
Detail Freak
target's Avatar
I think there are a lot of other factors to consider.
We need more info.
It is true that breaking a CF joiner at launch can be catastrophic. That is worth consideration.
Dec 12, 2010, 04:30 AM
planepainter's Avatar
Randy, thanks for the compliment! I would love to do a log on this Riser rework if I can find my blasted camera. I went to Arizona for three weeks and when I returned the camera and charger were both gone. I think my daughter used it and it just grew legs!! But to answer your question, the current joiner is 1/4" o.d. It would be great to use titanium but I have no way to cut it. But if you have a couple of pieces about 8" long I would be happy to pay you for them.
I could also just use the one I have and add another ejector pin to match. I was hoping to save a few ounces of weight.
Curare and Target, I see your point about bending vs breaking. At 1/4" I suppose the danger is even greater of a carbon rod overstress? As a Riser 100 I have had this plane on a gentle winch and have never bent the rod. But I did break out a joiner box once. Lesson learned. I can't replace the joiner tubes because they are already embedded into both of the original wings I am going to use.

Dec 12, 2010, 04:10 PM
Registered User


Well, you are in luck sir. I have two pieces of 1/4" rod at 11 inches in length. Let me know for sure how long, and I will cut them down for you. The material does cut with a hacksaw, a GOOD blade, and a strong arm if you wish. Just let me know, and where to mail them to ya! Randy
Dec 12, 2010, 07:11 PM
Registered User
If it was me, I might well cut my own joiner from 1/4" music wire. The reason to use ejector pins is when the diameter is larger than available music wire sizes. Up to 1/4", the yield strength (in psi) on music wire is very high. OTOH, you don't trust the joiner box. So in that case a titanium rod will allow you more warning because it bends a lot more. (I know this from personal experience with my Anthem.) But, as I recall, the yield strength is likely to be 140kpsi, which is a lot less than the music wire. If you can, it would probably be better to reinforce the joiner box somehow and keep the steel rod. I used to have 3 or 4 bent 3/8" titanium rods. (Of course, they were undersized for that wing.)

Weight is probably not going to be a major consideration unless you change the airfoil. Older, thicker airfoils need a certain amount of speed to work right. If you go too light, it will be too slow, the Reynolds number will be too low, and it won't fly well. At a large span you can afford a bit more loading anyway.

Is it really less work to make the existing wing a d-tube and also make those other parts? If you started from scratch you could use a better airfoil. Either way, though, I'm sure you could end up with a nice flying glider.
Dec 13, 2010, 07:17 AM
LSF 4685
Good morning all,

Well here I go, thinking out loud in an area in which I have no particular expertise....

I would calculate the wing loading using the steel joiners, then recalculate it using the CF or titanium joiners and see what the difference is. On a 132" span ship I would guess it wouldn't make a lot of difference.

Couple this with the info I recently read in Dave Thornburg's "Old Buzzard's Soaring Book". Dave says every ship is different. First get very familiar with its flight characteristics. Then experiment with using ballast. Dave says ballast can improve the flight characteristics in any wind, from zero to a bunch. He says to figure out what the different balance requirements are for your ship in several different wind environments, then make up ballast packages for each and have a system in place in your ship to easily change that ballast in the field. Wind conditions will change as the day goes on at the flying field.

Given all of this info from a guy who is a legend in the sport, I can't see that an ounce or three in the wing joiner of a large ship that is probably lightly loaded anyway, will make a huge difference.

Bottom line? Is it worth worrying about?

Dec 13, 2010, 12:27 PM
planepainter's Avatar
Dave, I hear what you are saying about a few ounces between titanium and stainless or music wire. If I had built the original Riser 100 stock it would be ok. But when I originally built this plane it was my first attempt at building a sailplane. I over built it and it turned out somewhat heavier than it should have been. In other words it went something like: "trailing edge much could that weigh?...brass tail much could that weigh?...6-20 allthread to hold the removable tail option... how much could that weigh?...I think I'll just leave in the lightening holes, those removable cut-outs don't weigh much...".
And so what I ended up with was a sailplane that you could fly into a brick wall and see no damage to the plane. But I have since learned my lessons.
This plane is going on a diet and if I could use a lighter weight solution for the joiner rods among other things it would make a better floater. JMHO.

Last edited by planepainter; Dec 13, 2010 at 12:41 PM.
Dec 13, 2010, 01:55 PM
Kurt Zimmerman ≡LSF 4461≡
kzimmerm's Avatar
One of the things I was having issues with were piano wire joiner rods. I found a local distributor of Drill Rod, which is case hardened steel. I found it to be much harder than piano wire as well as it comes in a variety of diameters. Something you may want to look into.

I had 1/4" diameter joiner rods in my Viking which I replaced with drill rod and I'm much happier.

Good luck.
Dec 13, 2010, 06:26 PM
Registered User
In 30 years of flying, I have never seen a metal wing rod break but have seen several carbon wing rods break. Metal bends. Carbon fiber breaks.
Dec 14, 2010, 08:47 AM
Kurt Zimmerman ≡LSF 4461≡
kzimmerm's Avatar
What ChuckA said!
Dec 14, 2010, 08:51 PM
+1 for Chuck...experience, too, shows this to be true. I had a homebuilt 2meter ship die due to this..just a small nick can affect a rod. Good luck with the mods on the Riser and have a great holiday season!
Dec 14, 2010, 11:29 PM
The King Moonracer of balsa.
Windependence's Avatar
Just out of curiosity, those of you who have had carbon wing joiners fail, have they failed during the launch phase or during normal flight. Here is why I ask. Wouldn't the answer to this question be partially dependent on whether or not the plane was electric launch? If my memory serves me correctly Planepainter's Riser is a string launched configuration. So in that circumstance I would agree with advice of staying with a steel joiner rod. Primarily because to get the same strength as the steel you would need to use a bigger carbon rod and going bigger is not an option. But if this were a e-launched setup then wouldn't a carbon rod of the same size be more than adequate?

Once you remove the stress of the winch or hi-start I would think a carbon rod would be able to deal with the occasional loop and stresses of a TD plane like the Riser. I'm not looking for an argument here, this is more of a long winded question.

Last edited by Windependence; Dec 14, 2010 at 11:38 PM.
Dec 15, 2010, 10:29 AM
planepainter's Avatar
Wayne, I think you are right. The stresses of an electric launch are less than that of a winch. The point is well made though that carbon can crack under load and metal bends. But that begs the next question: wouldn't the wing explode first before the carbon? I can't imagine that the stresses under load would peak for the carbon before the wing.... JMHO
I am talking woodies here- not molded sailplanes.


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