Topic: carbon joiner /steel joiner strenth difference?

Hi all.

This is a simple question which might have a complicated answer.I have a rcm extreme(2.3m) with foam/balsa/solar film wings.Its rg15 and quite a fast glider and strong ish wings.It weighs 4lb without ballast and 5lb with.Its has an 8mm steel joiner about 8inches long.

Question..can i just simply change the 8mm steel to an 8mm carbon rod and save a few ounces.I was thinking for thermal days or is there more to it than that.Are the wings likely to break before either joiner bends/(steel) or snaps(carbon)  or is the carbon not going to give any indication then snap one day.I havent purchased any carbon yet so i cant judge the strength in my hands.

I know its no real indicator but i cant bend the 8mmsteel in my hands.....help anyone?

Hi all.

This is a simple question which might have a complicated answer.I have a rcm extreme(2.3m) with foam/balsa/solar film wings.Its rg15 and quite a fast glider and strong ish wings.It weighs 4lb without ballast and 5lb with.Its has an 8mm steel joiner about 8inches long.

Question..can i just simply change the 8mm steel to an 8mm carbon rod and save a few ounces.I was thinking for thermal days or is there more to it than that.Are the wings likely to break before either joiner bends/(steel) or snaps(carbon)  or is the carbon not going to give any indication then snap one day.I havent purchased any carbon yet so i cant judge the strength in my hands.

I know its no real indicator but i cant bend the 8mmsteel in my hands.....help anyone?

i'v been wondering this with my whisper too!, i will be following this thread eagerly

Steve

I am told but don't know for sure that in order to fly my AldiJ full of ballast the 8mm solid ( i.e. not hollow ) carbon joiners need to be changed to 8mm steel ones , This info comes from the maker . So I would say the steel is a lot stronger BUT if your just doing floaty things I would think the carbon one would be fine

Lee ( Not an expert )

I had one of them (Xtreme).  I reckon a solid carbon joiner would be fine (as strong as steel).  But that's just my guess, no data to back it up.

I guess one significant difference is that carbon can snap whereas steel will only bend which could be quite an important difference in some applications! I think we should be careful about the "carbon fibre is 5 times stronger than steel" type of headlines because it's material properties are so different and depend on the type of loading so I'm not sure you could be 100% certain swapping carbon for steel was OK or not until you heard a crack and a wing fluttered down!  Having said that, a lot of the spars are probably over sized anyway and so changing to carbon would be a low risk.

Are you putting your Whisper on a diet already Steve?!  As it's only a short joiner and the model has a low wing loading anyway I wouldn't bother to be honest.  The Luna has a short carbon spar but it is a much larger diameter than the Whispers, so that may (or may not!) indicate something.

Simon

There is NO comparison between carbon and steel joiners - steel is infinitely stronger and carbon WILL snap in an 'arrival' - steel won't, but it might bend depending on the steel (and the arrival)

A carbon joiner is 'strong enough' and will save other damage - I use one in my Jart and it's done just that - a steel on the other hand is bulletproof!

As a rough 'guesstimate' - an 8mm SOLID (never use tube - useless) carbon joiner is ok for something like a 60" model (only...)!

Put it this way,
   If you put the 8mm carbon joiner in 1 wing half and leavered it up or down, do you think the joiner would snap before it prises the joiner box/tube out through the foam and balsa skin?

  8mm solid will be fine

There is NO comparison between carbon and steel joiners - steel is infinitely stronger and carbon WILL snap in an 'arrival' - steel won't, but it might bend depending on the steel (and the arrival)

A carbon joiner is 'strong enough' and will save other damage - I use one in my Jart and it's done just that - a steel on the other hand is bulletproof!

As a rough 'guesstimate' - an 8mm SOLID (never use tube - useless) carbon joiner is ok for something like a 60" model (only...)!

I agree. Apart from the use of the word 'infinitely' which I know for a fact to be a slight exageration.............

I asked the same question regarding my Blade XL and was told by the manufacturer NOT to use a carbon rod.
I know the Blade XL has a reputation for bending it's joiner in high G turns but that just shows the forces involved.
A carbon joiner of the same cross sectional area won't bend, it will snap. I'd rather see a scary wing bend than hear an even scarier joiner snap. Stick with steel.

Just one more thing. Perhaps it's a good idea to think of a steel joiner as a 'safety valve' - Yes, it will bend and give the wing more dihedral, but that is effectively taking the sudden stresses out of the whole wing. A solid joiner and wing will not have that effect.

it's a well deserved reputation to be fair.........

Anyway, by my reckoning your steel wing joiner will weigh something like 170g. Not that much really I would say.  I doubt you would notice a huge difference if you did swap to carbon.

Put it this way,
   If you put the 8mm carbon joiner in 1 wing half and leavered it up or down, do you think the joiner would snap before it prises the joiner box/tube out through the foam and balsa skin?

  8mm solid will be fine

I'm not sure that test takes into account the complexities of the way shock loads may impact (literally!) carbon Adam.  I think it is entirely conceivable that a carbon joiner would take a progressively applied static load until it tore through the "weaker" material and yet could still be the first thing to fail catastrophically under shock loads.

I agree with the others, if a plane has been designed for a steel joiner I'd stick with it...

Simon

I have the 2.1 m Toplight Mega Aka Alex 2.1, Spyder etc.

This has a single 8.8 mm solid carbon joiner, and to be honest, I think its a bit on the flimsy side - I haven't dared to use ballast for example, even though I built a tube in.

My Erwin DS has two carbon joiners for light days, or two steel ones for ballast.

I would be wary of using a single 8mm carbon rod at 2.3 m myself, an arrival is one thing, but i would hate your first square loop to be your last.

JMTC

Tim

You guys don't know this plane.  Carbon will be fine for the type of flying it will be subjected to, believe me .  There's lot more that will give way on this airframe before an 8 mm solid carbon joiner!

That said, as Satinbed ses, the weight improvement will be small.

I'm not sure that test takes into account the complexities of the way shock loads may impact (literally!) carbon Adam.  I think it is entirely conceivable that a carbon joiner would take a progressively applied static load until it tore through the "weaker" material and yet could still be the first thing to fail catastrophically under shock loads.

I agree with the others, if a plane has been designed for a steel joiner I'd stick with it...

Simon

   Totally agree mate, but we are talking about a big foam vaneere wing here, I just can't see any serious impact making it to the joiner berore being absorbed. But then I do favor a more hands on approach rather than theory and calculations. Although I did DS my Erwin last week at 130mph with the two 6mm carbon joiners in. And those wings don't really absorb much shock.

     Id still think that if you did my experement above, and hit the carbon joiner with a hammer ( told you I have a different approach on theories) it would still destroy the wing root and not the joiner?

try a wooden dowel.......

tbf Adam, I think the weight of the wing joiner in question is an important consideration. As you know a 150 - 200g of weight isn't that much.  I think when you are dealing with those kind of differences it does become a question of whether it is really worth it in the first place.  My personal view is that a carbon joiner would be fine, but I'm not sure it's worth the bother.  I agree the wing root would probably get mashed in a crash.  

I agree with Woodstock that it is also very much down to the type of glider and also the type of flying it's likely to be doing. For most of the time it may well be just doing 'light' soaring with very small stresses being applied to the airframe. However there may also be 'that day' when you end up specking it in a massive thermal. Then when it's at 1000 foot and you're struggling to get it down could well be the point when you wish you'd got a steel joiner installed!

I don't want to knock this glider at all, it's a very nice plane for fairly gentle flying, and I really enjoyed it .

But, there's no way it's doing square loops and massive dives from spekked-out heights..  There are two pegs at the leading edge of the wings that retain same in the body of the fuse, they will break out long before anything happens to the rest of the wings..  And then, the wing tubes will prolly come out long before a Carbon joiner breaks. 

ah well go for it in that case!

Stronger isn't always 'better' - as I said - carbon snaps where steel bends - a bent joiner can be a right PITA if you can't get it out!

Don't confused gentle bending 'tests' with carbon - that's not the same as a shock load where a sudden failure occurs!

By way of a demonstration of the aforementioned - I have a large oily delta that has a wire noseleg - it punches holes in the fuselage if you bounce it on a landing, but if you tried to bend the leg back that far it would have the firewall out in all probability!

Those karate brick breaking mentalists are another demo of 'shock load' failures!

Hi,

The only problem I found using Carbon rods as wing joiner, is that carbon rods need more "pre-flight" checks than a steel one.
Carbon rod inside a steel / brass / aluminium tube have tendencies to be slightly cutted with time where the tube end and you can be sure the day your rod snap, that will be here.
Always check the rod for traces, marks etc on its diameter. And change it when you can clearly see a deep mark.

Saw some snaps of rods in flights after the glider went trough turbulences on the slope... Quick up and down movements of the wings, and the rod snapped right at the tube exit.
We were able to clearly see the rod being "attacked" by the tube, and that don't need to be deep.
Rare cases I guess, but something to be aware of, and can be fixed by a good look at the rod 

I have done a full set of design calcs for my latest project and will be using 8mm o/d x 6mm i/d c/f tube.  From memory the moment at the joiner is something like  880 lb inch and the safety factor about 2.7 even with the low design stress input of 100,000 psi.


I will run through my calcs tonight .   The word steel is a bit of a loose description for design calcs,  any idea of a grade or better still tensile stress and youngs modulus?


Si

Que? (In voice of Manuel from Faulty Towers)

As soon as i pressed the send button I realised that calculations were not  needed.

Piano Wire- design stress (to failure ) is about 320,000 psi

T300 Carbon fibre - Design stress 100,000 psi although 150,000 psi is considered more acceptable to many.     Actual failure of 512,000psi gives a safety factor of between 3+ and 5+ to 1.

Based on ultimate strength carbon is consideably (60%) stronger but allowing safety margins means that a straight replacement will not be as strong as steel

As said earlier, a piano wire joiner will bend, carbon will go bang!

If you are able to stress the wing to the failure point of either joiner, the result would more than likely end in catastrophe!

Look, this is a forum:  you can't just go around using hard facts and data to spoil a nice "discussion"...

(the above is meant in jest, hope it's viewed that way ..))

Look, this is a forum:  you can't just go around using hard facts and data to spoil a nice "discussion"...

(the above is meant in jest, hope it's viewed that way ..))

It does make a very refreshing change though............................

Hi,

The only problem I found using Carbon rods as wing joiner, is that carbon rods need more "pre-flight" checks than a steel one.
Carbon rod inside a steel / brass / aluminium tube have tendencies to be slightly cutted with time where the tube end and you can be sure the day your rod snap, that will be here.
Always check the rod for traces, marks etc on its diameter. And change it when you can clearly see a deep mark.

Saw some snaps of rods in flights after the glider went trough turbulences on the slope... Quick up and down movements of the wings, and the rod snapped right at the tube exit.
We were able to clearly see the rod being "attacked" by the tube, and that don't need to be deep.
Rare cases I guess, but something to be aware of, and can be fixed by a good look at the rod 

A very good point! 

As soon as i pressed the send button I realised that calculations were not  needed.

Piano Wire- design stress (to failure ) is about 320,000 psi

T300 Carbon fibre - Design stress 100,000 psi although 150,000 psi is considered more acceptable to many.     Actual failure of 512,000psi gives a safety factor of between 3+ and 5+ to 1.

Based on ultimate strength carbon is consideably (60%) stronger but allowing safety margins means that a straight replacement will not be as strong as steel

As said earlier, a piano wire joiner will bend, carbon will go bang!

If you are able to stress the wing to the failure point of either joiner, the result would more than likely end in catastrophe!

Go-on then.... I have a titanium 4ALV joiner on my Simprop. What are the numbers for that? 

Go-on then.... I have a titanium 4ALV joiner on my Simprop. What are the numbers for that? 

Have you typed it into google?

WOW ,thanks guys for the discussion.I was only going to use the carbon on thermal days anyway but all the points have been a great help.Yes ,this glider would probably break elsewhere on the front dowls in extreme high g turns.I have been trimming now for 2yrs..I know ,things happen slowly with me,and have got the cg back to 90mm and will try 92/95mm next.It does go well for what it is and lands on the spot every time with the full crow.

I am not interested in what would break on a crash as i think there would be nothing left any way so ,I modify gliders to fly not crash!To be honnest as long as it was way out on the slope(safe) i would be proud to break the joiner with a high G turn as it would mean i have got the glider setup perfectly!

I would not use ballast with the joiner and would save a good 1/4 of a pound at least.This might mean the difference between hanging on and getting back or hitting the hilside.....I am still undecided...keep going guys..thanks.......

I guess one significant difference is that carbon can snap whereas steel will only bend which could be quite an important difference in some applications! I think we should be careful about the "carbon fibre is 5 times stronger than steel" type of headlines because it's material properties are so different and depend on the type of loading so I'm not sure you could be 100% certain swapping carbon for steel was OK or not until you heard a crack and a wing fluttered down!  Having said that, a lot of the spars are probably over sized anyway and so changing to carbon would be a low risk.

Are you putting your Whisper on a diet already Steve?!  As it's only a short joiner and the model has a low wing loading anyway I wouldn't bother to be honest.  The Luna has a short carbon spar but it is a much larger diameter than the Whispers, so that may (or may not!) indicate something.
Simon

si, i was just thinking of those really light wind days when the whisper would just about fly, when a ligher carbon joiner may be the difference between a nice flight and a long walk down the hill, lol 

Steve

Have you typed it into google?

No I bought it off ebay and plugged it into the wing? 

The Jart has a single solid carbon joiner and it's fine in that high-stress application.  Although of course it's a smaller span plane.  I also fly my 2 metre Erwin with 2 carbon joiners and it's been fine.  I think the wings would fold where the joiner ends before the joiner itself snapped.

Personally I'd feel totally confident flying your plane in a thermally way with a carbon joiner.

Put some numbers in place. assume 2.3m span,5lbs AUW, 200mm chord and 20g loading,

Ignoring weight of wing 100 lbs gives Moment at wing centre = 1125 lb inch or 94 ft lbs.

Carbon tube at 100,000 psi requires a modulus of 0.000938 which is easily covered by an 8mm x 6.5mm tube at 0.00173.

In plain numbers the cf tube joiner will cover about 40g before it reaches its safe design limit of 100 ksi.  Remember that the tube has a 5.12 safety factor too.

PS cf tube 27g per m, so a 6 g joiner rod see 8.0mm Carbon Tube - Carbon Rod, Strip & Tube

I have done a full set of design calcs for my latest project and will be using 8mm o/d x 6mm i/d c/f tube.  From memory the moment at the joiner is something like  880 lb inch and the safety factor about 2.7 even with the low design stress input of 100,000 psi.

So please tell me why one of the 2x 8mm carbon tube joiners on my Voltij failed in flight when doing a fast cross-wind knife-edge pass in a 53mph wind on Lundy? (as witnesses by some folks on here).
And if you provide the full static & dynamic stress analysis, with shear flow diagrams, and take into account the limitations of unidirectional carbon pultrusions, and stress concentrations due to surface nicks, and shock loadings, we may be impressed.

Meanwhile - I'm with Tom - but not sat in the same bed - 8mm steel vs. 8mm carbon = not enough weight difference to bother with on a 2.3m plane.

Phil.

Good point ... but I really don't think the poor chap is going to be pulling high-g stunts in 53mph winds while thermalling his plane!

Good point ... but I really don't think the poor chap is going to be pulling high-g stunts in 53mph winds while thermalling his plane!

getting down again from great heights when thermalling can pose some interesting "structural challenges" 
(ask me, no, don't ask me!)
anyhow - its not exactly a thermal floater
X-Treme - Buy online from Puffin Models>
which is why he wants it a bit lighter
Phil.

So please tell me why one of the 2x 8mm carbon tube joiners on my Voltij failed in flight when doing a fast cross-wind knife-edge pass in a 53mph wind on Lundy? (as witnesses by some folks on here).
And if you provide the full static & dynamic stress analysis, with shear flow diagrams, and take into account the limitations of unidirectional carbon pultrusions, and stress concentrations due to surface nicks, and shock loadings, we may be impressed.

Meanwhile - I'm with Tom - but not sat in the same bed - 8mm steel vs. 8mm carbon = not enough weight difference to bother with on a 2.3m plane.

Phil.

You're right Phil, a static loading analysis is meaningless on a complex dynamic situation like this.

Simon

hit us with some wing loading calcs Phil.  You know you want it!

I couldn't believe it when I saw this thread was still alive !

I couldn't believe it when I saw this thread was still alive !

I can't believe an RCM "Extreme" is still alive!

Phil,

That seems an aggressive response when all I am trying to do is offer some figures for carbon versus steel.  I am no master of aerodynamics or stress calculations but set myself the task of designing a model from calculations rather than the usual monkey see monkey do approach.
I have done a bit of research and can use my limited knowledge in both fields to provide base level calculations for my own purposes. So please help me out if I have got it wrong.

On here to gain knowledge and occasionally be amused and entertained, not after keyboard battles. 

Intrigued by your Voltij failure myself on several levels. 

if as you say the model is flying in a knife edge- all lift from the fuselage why would the wing joiners be stressed at all?

If however you consider that flying parallel to the slope with the 53mph wind perpendicular to same you are effectively pulling a load I don’t see it.  As before the fus is providing the lift and the model is Yawing to the wind in a trimmed condition to maintain heading. 

My understanding of maximum achievable G loading is the ratio of Cl max at and Cl at maximum speed.  Eg travelling at max speed cl0.02 then instantly applying enough up elevator (or turbulence) to put the wing at its maximum AoA Cl of say 1.0.  This would give a load increase – G of 1/0.02= 50g.   Since the limiting factor is the wings ability to provide lift, I don’t know what other forces are being applied.

Do you think that the wing is magically affected by some unknown force to such an extent that the joiner is overstressed?  Don’t know the setup of the joiners but if two 8mm are used in tandem to cope with flight loads then they should share the total load between them equally. If one fails due to an overstress condition the second would instantly see twice the load and both would fail.

Trying to learn more about carbon fibre and composite materials so cannot comment about their exact performance characteristics but it seems that modelling experiences have made empirical allowanced for imperfections in design manufacture and use by reducing the design figure from 512 ksi to 100-150 ksi. 

You mention shock loadings- Looked at flight loads only,  I haven’t even considered shock loads- that’s a crash isn’t it?  I would really appreciate some guidance on the point or shock loads you are suggesting.

A piano wire joiner would weigh about 80g, a carbon tube less than 6g.  A massive weight saving on a component basis and about 3% of the models weight but next to nothing on a wing loading basis.

 If carbon-composite construction is so bad we had better keep looking skywards, there is a lot of it travelling to and fro in many modern aircraft.

Chill man!