Topic: carbon joiner /steel joiner strenth difference?

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!

I've seen many a floater type of model die due to getting too high in a thermal. When the model is a speck at 1000' you just can't see what it's doing up there. Attempting to get the model down inevitably ends up with excessive speeds at which point some 'up' is applied to try and slow it down. This tends to be the point at which either the joiner or the wing structure fails and the model then starts it's rather faster return to ground level!

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.
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!

Sorry if I sounded aggressive, wasn't intended to.
I'm all for taking a calculated approach, I've seen too many models die because "it looks right" really wasn't up to the job.
Need to take all the factors into account though. Stress calcs are a real love it or hate it thing - at least there's computer software available these days to help out.
In this specific case, the difference between a carbon tube and a steel tube or rod is that the properties of the carbon tube are non-uniform - all the fibres are going in the same direction. As the carbon tube gets stressed, it'll try & squish out into an oval, at which point the sides will split as the fibres become unbonded in the "least strength" direction - it shatters - Hope that makes sense, without going into shear flows etc. Solid carbon rod will be stronger, for only a few grams more weight. Voltij now has solid carbon joiners, and also steel ones to add a bit of weight (but not a lot) for big winds.
Phil.

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

Well the reason it is still alive is because it has more carbon tows in the fuz than an Erwin 5..not really, but you get the gist!This is not from crashing,I have never landed it real heavy..just that the beautiful paperthin glass fibre fuz cannot cope with the wings.As i said before ,the wings are quite strong for foam/etc...and it will full crow down from a thermal at 45 deg or more  without going too fast.

It will never be an f3f ship although i can make it sweat a little from what i have learned.I am a little closer to a decision,It will need more effort to check it before using it every time.I was going for a SOLID rod as a tube would crumble,I can break an 8mm tube with my hands.......

Right - that's it - too much 'square root of a jamjar' going on here

Forget stress calculations - forget the maths even - I've NEVER (not ever ever ever) seen a steel joiner break - I've not only seen, but broken carbon ones myself!

End of really - anyone of us can get a model back to the ground with a bit of extra dihedral - try it with only one wing 

I use lots of carbon joiners but they ain't ever going to be as strong as steel of the same diameter!

Hi Steve.

Like your logic...  so thought of a few more observed truths. 

The world is stationary and it doesnt spin- never seen it moving.

Havent seen electricity that doesnt exist.

Radio waves- piffle- its magic boxes with voices in them.

The sun goes out each night thats why it goes dark.

Sure you can come up with some more


Si


PS almost forgot - turns into and down wind- your choice?

Hi Steve.

Like your logic...  so thought of a few more observed truths.  

The world is stationary and it doesnt spin- never seen it moving.

Havent seen electricity that doesnt exist.

Radio waves- piffle- its magic boxes with voices in them.

The sun goes out each night thats why it goes dark.

Sure you can come up with some more


Si


PS almost forgot - turns into and down wind- your choice?


Lookee here matey - if you can break an 8mm steel joiner with a ten pound sledge and all your body weight then I'll cut my Erwin XL into bits and eat it!

How d'ya like THEM apples

Carbon WEIGHT for WEIGHT (if of the correct layup, pultrusion - whatever) may well be as strong  or stronger than steel but to weigh the same as an 8mm steel joiner you're going to need a bloody gert thick Gottingen section to accomodate it tosh!

steel is 5x density of c.f. - so that would be 18mm carbon rod  
which would be "quite strong"  

Phil.

It would be very strong - bit of a job to force between the skins of yer Fermin Death Machine though

Mnay years ago, shortly after i discovered carbon fibre rod, I used to have a Calypso F3a Power model , piped Rossi 61 and retracts.

I had to reverse the Piano Wire U/C legs after pretty much every flight, as they bent as they exited the retract block, and were always in danger of jamming in the wheel wells with all the nastiness that could cause.

Having heard that carbon fibre is a zillion times stronger than steel I figured I'd just make some carbon fibre U/C legs, so I did.

Fired it up  and carried out it out to the strip, having shown off my new bling legs to anyone who would listen, Pointed it into wind and opened the throttle.

The wheels stayed exactly where they were but the model set off as both legs snapped clean across at the edge of the retract block.

This kind of surprised me a bit, but I gues since carbon is so rigid there was a massive stress concentration at just that point, which caused it to fail.

I am a little more careful now where I use carbon.

But answer me this.

If a man speaks alone into a forest,

And there is no woman there to hear him.











Is he STILL wrong?

Tim

Ah well - the old 'carbon is stronger than steel' - yep it is - weight for weight, NOT size for size though 

Still you know that - now.................

Ah well - the old 'carbon is stronger than steel' - yep it is - weight for weight, NOT size for size though 

Still you know that - now.................

Indeed.....

So guys,After all this discussion,is it a yes or a no for thermal and no ballsat with the 8mm solid carbon joiner.I wont blame any of you if it breaks...Honnest!

Hi all.

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

Ok, so what if you could have bent it in your hands ?
What would you have done with your bent wing joiner ?
Eh ? 
Ray.

Ok, so what if you could have bent it in your hands ?
What would you have done with your bent wing joiner ?
Eh ? 
Ray.

Then i would have to make another out of carbon !Also reduce my visits to the gym as I dont want to be too strong..

Ok, so what if you could have bent it in your hands ?
What would you have done with your bent wing joiner ?
Eh ? 
Ray.

8mm solid carbon will be more than strong enough IMHO

So guys,After all this discussion,is it a yes or a no for thermal and no ballsat with the 8mm solid carbon joiner.I wont blame any of you if it breaks...Honnest!

my vote is ....

NO 

as others have said - the amount of weight you will save will make virtually no difference to the plane
if you really want something lighter for floating around - get a floating around sort of plane!

Phil.

So guys,After all this discussion,is it a yes or a no for thermal and no ballsat with the 8mm solid carbon joiner.I wont blame any of you if it breaks...Honnest!

Depends.................................

8mm solid carbon will be more than strong enough IMHO

Snapped an 8mm carbon joiner in a 'little' Jart - heavy but by no means terrible landing!

Pay's your money takes your choice - I still use carbon in that model as it'll save things in a nasty - not broken in the air.....yet!

Can anybody on this thread help me -  see general discussion?

Topic: Help- Carbon Joiner sizing?