Carbon Fibre Tubes

[Chris Walby]

Hi,

I want to use some carbon fibre tube inside another carbon fibre tube (wing tubes) as they are convenient sizes for the model.

The problem I have is that there is a seam on the inside of the outer (must be part of the extruding process) which causes the tubes to bind.

The inner tube needs to slide at least a foot into the outer and probably longer, so normal drill bits are too short. I have tried tipped masonry bits and they don't work. Looked at reams, but there is no easy way of making an extension for a ream.

Any suggestions as to how I can ream the seam out and what works well to lubricate carbon against carbon?

Thanks in advance

[Ron Gray]

I had a similar situation a couple of years ago and what I did was to use a bit of alloy tube which was a bit smaller than the carbon one and then stuck some sandpaper to the alloy tube and used that to 'ream' out the carbon tube. Worked OK but I was lucky to have the alloy tube to use! Messy job!!!!

[FlyinBrian]

Posted by Ron Gray on 03/07/2020 07:39:43:

I had a similar situation a couple of years ago and what I did was to use a bit of alloy tube which was a bit smaller than the carbon one and then stuck some sandpaper to the alloy tube and used that to 'ream' out the carbon tube. Worked OK but I was lucky to have the alloy tube to use! Messy job!!!!

Alloy tubes of various sizes are available from most B&Q stores, not too dear.

[Flyer]

If you must use the carbon tubes, could you use a rod of smaller diameter with some glass paper glued around it and use as a reamer? The carbon tube I've used does sand quite easily.

Ade

[Piers Bowlan]

Not too sure why you need carbon outers, I usually buy a K & S Ali or brass tube to fit my carbon wing joiner. Alternatively SLEC sell carbon tubes with phenolic outers specifically as wing joiners.

[Peter Miller]

Stockists of J.Perkins ca get you various carbon tubes and carbon rods which I have used in the past

[SR 71]

I make my own outer tube from fibre glass

[Lima Hotel Foxtrot]

My LMS Addlestone Models have lots of carbon tubes that are designed to nest. I suspect they come from JP.

Also, what is that interesting looking plane Peter Miller is building? It's not like his usual output.

[perttime]

What sizes are we talking about in this case?

[Chris Walby]

Off topic, but if I use phenolic tube it comes out heavier.

Carbon spars				213.00
Card/ali spars				331.53
			difference	118.54

OK its 118g on a 6Kg model, but its already heading over 36oz/sq ft wing loading and it does not need to be any heavier for no reason!

Thanks Ron/Flyer, I'll male some rod reamers and give it a go, its just the seam that needs to be removed

[Peter Miller]

Posted by Lima Hotel Foxtrot on 03/07/2020 08:30:37:

My LMS Addlestone Models have lots of carbon tubes that are designed to nest. I suspect they come from JP.

 

Also, what is that interesting looking plane Peter Miller is building? It's not like his usual output.

THat plane is my MAK 15 MP. A russian powered glider from the 50s.The plans are with Kevin and there is a build blog on this forum.

I can tell you that it is a builders model. It flies well and It will do aerobatics

The Red Kite didn't like it

 

Edited By Peter Miller on 03/07/2020 11:20:13

[Manish Chandrayan]

A dowel or alloy tube of suitable size with a slit to take one end of the abrasive cloth/paper.

Glue the end of the abrasive cloth/paper in the slit and tightly wind the remaining on to the dowel/tube to form a reamer.

The tightly wound end should just slide in the carbon tube to be reamed. Once inside, the open end will expand and give a snug fit.

Rotate to sand off the seam line. Carbon dust will get everywhere so take care

[Chris Walby]

Perttime

The outer tube ID is 14mm and I have a cunning plan, more cunning than Baldrick's plan.....Just noticed that Perma-Grit do a 14mm round tube file, I can then turn a plug and Loctite it in the handle. Screw a bit of bar into the plug and get the battery drill out!

Oorrr I feel a Perma-Grit order coming on!

[Martin Harris - Moderator]

Do be careful with the carbon dust - it's supposed to be very hazardous to the lungs and quite possibly carcinogenic. Best not to get it into the house or workshop...

[Bruce Collinson]

Mind the dust, try candle wax between them; we use it on carbon fly rods to prevent binding; maybe try a short section first, as the ferrule even on a 15' salmon rod is about 3". I'm told the worst thing you can do to a stuck ferrule is squirt WD 40 down it. Order new rod.

BTC

[perttime]

Posted by Chris Walby on 03/07/2020 12:46:43:

...

I have a cunning plan, ..

Gotta love cunning plans

Being cheap, I would have stuck with the dowel and sandpaper plans.

Whether carbon and epoxy particles are carsinogenous, or not, they are pretty nasty just mechanically. Better keep them away from your insides. I prefer to keep them from embedding themselves in my skin too.

[Richard Clark 2]

Posted by perttime on 03/07/2020 14:39:35:

Posted by Chris Walby on 03/07/2020 12:46:43:

...

I have a cunning plan, ..

Gotta love cunning plans

Being cheap, I would have stuck with the dowel and sandpaper plans.

Whether carbon and epoxy particles are carsinogenous, or not, they are pretty nasty just mechanically. Better keep them away from your insides. I prefer to keep them from embedding themselves in my skin too.

While a tube, carbon, brass, light alloy, or glass, is fine in the wings you should use a solid rod as the wing joiner. A tube joiner will flatten into an elliptical shape under strain which weakens it in the vertical plane. Thus weakened it flattens even more, weakens further, and eventually breaks.

[Former Member]

[This posting has been removed]

[Erfolg]

A lot of +50cc Extra type models (that I see) use a single Carbon spar as a push fit through what appears to be light ply wing ribs, with no trouble.

On the other hand my HK Waco SRE had a aluminium inner wing tube, inside a Carbon outer. Much to my surprise the carbon tube shattered, leaving a wobbly wing. There was a lot more wrong with the model, but that is bye the bye. My repair just used an outer aluminium tube.

At the end of the day, there seems to be CF tube and then there is CF tube.

Many of my open gliders used either a GF or CF boom (fishing rod, deep sea type of size), which never broke without my arrivals. On that basis both materials can be great

On that note, I would just observe that like any other component or assembly, the material in itself is not always the answer, it is the detail of how it is used. Also I would be concerned about the potential binding/seizing issue raised.

Edited By Erfolg on 03/07/2020 16:56:58

[Chris Walby]

Off topic again, and I can see your point about load points on the tubes RC2, but IMHO why do E-flight, Seagull, Black Horse, Chris Foss, Motion RC and Century all use wing tubes (either ali or carbon fibre)? These are models I know have wing tubes, in fact I can't think of any 6Kg models that use solid wing joiners.

I shall don the appropriate PPE and carefully remove the seam inside the wing tube (not using WD40) and try either candle wax or Teflon spray.

[Ron Gray]

Simple answer - weight. Size for size a solid bar will be stronger than a tube of the same diameter but a lot heavier. You can increase the diameter of the tube by a small amount and with the correct wall thickness you will have the same strength as the bar but a lot lighter.

[Bob Cotsford]

Surely with the tube contained within a sleeve, itself held by ribs, the tendency to deform is greatly restricted, and in any case at the sizes commonly used it takes a great deal of commitment to generate enough G to collapse the tube? If inadequately sized then yes, it will fail while the solid rod would just bend or break the wing at the end of the joiner. Either way it was bad design or bad flying!

[Richard Clark 2]

Chris and Bob,

Often the part that goes through the fuselage isn't constrained in an outer tube. My 90mm 55 inch span EDF 10 cell approx 4 Kilowatt Black Horse Viperjet weighs nearly 5 Kg (their claimed weight didn't originally include the batteries) and the way the wings flexed just holding it up by the tips I would never have trusted the carbon joining tube in flight, least of all with a high G manoeuvre. So I found a close fitting carbon rod and epoxied it into the provided tube.

The better fly fishing rod manufacturers go to great lengths to produce what they call a high 'hoop strength' in their rods.

[Erfolg]

Bob, I think I know where you are coming from. The but is, I have seen a number of F3J and F3b type gliders with solid SF joiners which appear to be about 25 mm square solid joiner.

It does seem there is no one answer or correct way of doing things. However some of the concerns are worth noting. An example is the the observation that an arrangement can bind solidly.

I must admit that a wing loading of 35 oz per foot square initially has me worried. Then I build small models, if the model is big, it is possibly quite low.

[Martin Harris - Moderator]

Posted by Erfolg on 03/07/2020 16:56:00:

On the other hand my HK Waco SRE had a aluminium inner wing tube, inside a Carbon outer. Much to my surprise the carbon tube shattered, leaving a wobbly wing. There was a lot more wrong with the model, but that is bye the bye. My repair just used an outer aluminium tube.

I wonder if the reason it shattered was due to the aluminium tube deforming and applying local pressure to the CF? I've had ARTFs with large diameter ali tubes sliding into cardboard tubes which I would assume really only act as guides, the loads being transferred to the wing structure locally at the ribs i.e. the tube is not acting as a significant part of the structure. In fact my well over 10 years old all weather hack electric model [Extreme Flite Extra] uses a relatively small carbon tube joiner and I certainly haven't held back from applying high loads to these models so I don't think an adequately sized tube is a total no no...

Edited By Martin Harris on 03/07/2020 21:20:22