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Carbon Fibre Tubes

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Ron Gray03/07/2020 17:51:41
2235 forum posts
978 photos

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 Cotsford03/07/2020 18:00:59
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8646 forum posts
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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 203/07/2020 19:01:13
424 forum posts

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.

Erfolg03/07/2020 20:27:11
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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 Harris03/07/2020 21:19:04
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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

Chris Walby03/07/2020 21:36:28
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1277 forum posts
315 photos

Back to the OP

They are 14mm ID out tubes with a 14mm OD tube inside both outer tubes. If I manage to shear a 14mm OD carbon tube in normal flight I would be amazed as I won't be pulling high G or going that fast as its only a Focke.

I headed out to the garage and went low tech as suggested with a bit of aluminium tube, emery cloth and lots of elbow grease. The result is that I am within an inch of where I want to be, the only problem of doing it this way is its very easy to taper the first part of the outer tube which is not what I wanted or needed.

Thanks to everyone who posted.

PS - Teach me for not having a one piece wing. wink

Edited By Chris Walby on 03/07/2020 21:37:23

Bob Cotsford03/07/2020 21:45:06
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Posted by Richard Clark 2 on 03/07/2020 19:01:13:

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.

Sounds like a very good idea in that case, what was the phrase I used previously? Bad design?

Richard Clark 203/07/2020 22:23:23
424 forum posts
Posted by Bob Cotsford on 03/07/2020 21:45:06:
Posted by Richard Clark 2 on 03/07/2020 19:01:13:

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.

Sounds like a very good idea in that case, what was the phrase I used previously? Bad design?

Bad design is common. Even the horrendously expensive Bob Violett BVM Jets (turbines and EDFs) suffer from it. They use a separate vertical carbon tongue in each wing half which fit into a carbon 'whisker' impregnated nylon moulding bolted to a plywood former in the fuselage.

As a result that moulding is the only thing that resists the wings folding. It works most of the time but they have had to introduce 'modification notices' for several of their models advising you to buy their 'improved' mouldings AFTER wing folds on their demonstrators. Notably when they have increased the thrust of their EDF units by a couple of pounds - so it's a pretty marginal design. (Personally I think the design is nuts.)

Richard Clark 203/07/2020 23:02:50
424 forum posts
Posted by Ron Gray on 03/07/2020 17:51:41:

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.

My 'solid' modification added 60 grams to a near 5 Kg model. It is insignificant. Considerably less than an empty or full tank on the average glow plane.

Increasing the diameter of the joining tube will involve increasing the diameter of the tubes in the wings. A lot of hassle on an ARTF.

Thicker walls? You have to use whatever your LMS or online shop has got.

In fact I suspect the 'carbon' joining tube on the Black Horse Viperjet isn't carbon at all. Just cheap glass with just enough carbon filaments in it to turn it black. Also cheap polyester resin rather than epoxy.

Bob Cotsford04/07/2020 10:29:30
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8646 forum posts
483 photos

Spiral wrap or extruded would also make a big difference. Most of the tube I've seen advertised (or used in ARTFs) is extruded with the fibres parallel to the bore. It has little crush resistance so needs supporting as per Richard's mods.

Erfolg04/07/2020 10:59:01
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11781 forum posts
1340 photos

I know, or perhaps sense, that there are a few engineers or in my case ex-engineers on this site. The design and construction of beam arrangements, is pretty much second nature. Then there are contributors who are intuitive to the requirements.

Yet separating out good arrangements, from the poor, is not always as obvious and easy as we first think.

We have all seen in the past, the beam going through the centre of each rib, not the best way of making use of the available material, from a structural perspective. Yet from a kit makers perspective, the arrangement jigs the wing ribs to some extent.

It is pretty much the same with the various tubes. An "I" beam would be generally be better, than a tube. Yet tubes are readily available, and just as important can be incorporated into the wing very well. A continuous removable "I" beam is generally not available to us and the practical issue of getting the flanges to the outer surface would be difficult.

Much of the success of a design, relies on the detail as well as the materials etc.

The sliding tubes should pass through a number of wing ribs, the more the better (to a degree) to distribute the loads, via the ribs. In my HK Waco SRE, this was not the case, as built. It would be better that the tube was fastened (glued) to the ribs, as this would better transfer the loads, more evenly, as a principle. In practice this often defeats the objective of some tube arrangements.

If a tube fits within a tube, the arrangement of the HK Waco is to be avoided, where the inner tube, is short, in that it does not pass through a number of ribs (inside the tube). In this case a stress concentration took place at the end of the inner tube, the wing root rib acting as a fulcrum point.

CF and GF manufacturing lay ups come in many forms, You get spirally wound strands, longitudinal, braided weave and probably many more, that I am unaware of. They all have there characteristic properties, that differ significantly. That is before you get into the resin and processing and weight of the composite.

With metals, I was lectured, time and time again, processing history matters a lot. In other word what has been done to say steel, such as drawing, when cold, or hot rolling and so on.

At the end of the day, we have model aircraft, where theory, is one thing, carrying out theory is another, practice not always closely matching theory. That skill levels do vary, from my rough and ready, to artists with a scalpel and balsa. What decides me generally is what I have got and what I can readily get.

In general I have found that observing old bridges, often provides a clue as how to deploy material, Victorian Builders can be good for this. Yet you also see the situation where the view is taken, lets keep it simple, and forget the economical use of material, as the benefit if frugality is limited. Modern casalated beams, indicate the principle of the "Parallel Axis Theorem", where the "I" beam is the idea in its purity.

So with respect to our CF tubes, as long as it works, does it matter if the arrangement is not the ideal solution? As everything is pretty much a compromise.

Richard Clark 204/07/2020 16:35:19
424 forum posts

As Bob and Erfolg say it's all down to good design. Which is rare in the average ARTF, which in any other field would mostly be thought of as very 'low end' products, (It's even worse in ready made and quite expensive (£1000 plus) so-called 'scale' model ships of the Nelsonian period, mostly made in the Philippines, which are really horrendous 'caricatures'.

So what do I do? First, I very rarely buy an ARTF (three in my last 20 years of model planes). I only bought the balsa with a little plywood BH Viperjet because glass and/or carbon fuselage ARTF EDFs are very rare. I don't particularly like the Viperjet as a plane but it was easy to obtain. Gluing a solid carbon rod inside the original wing joiner tube was a simple and obvious solution to its floppy wings.

Generally I build from scratch or use plans and kits, both of which I modify to suit my own ideas. Doing that is more difficult with an ARTF where it's not easy to get inside.

Take wing spars. I always put a web with vertical grain between them. Between, not stuck on the outside as it uses more material, so is heavier, and the glue joint could shear under stress.

If it's a two piece wing (which I try to avoid) I mostly use brass wing tubes as model plane carbon tubes have near zero resistance to splitting, as Bob said. And I don't just glue them to the ribs, I pack out with hard balsa to the spars, or if they are not in line with the spars, to cross grained sheet I put internally between the spars and the LE.

If a scratch built planw I always taper the spars between the root and the tip as the stress gets less as you go outwards and there is no point in making things heavier than they need be.

On a one piece wing, which I prefer I use two ply joiner, one at the front, the other at the back and one extends further than the other to avoid too much of a discontinuity where the joiners end. Also I don't make them too thick, which causes a greater discontinuity.

Tailplanes. A built up one, with or without sheet covering, but preferably with, is far less likely to flutter than a solid sheet one.

Incidentally, when I first met my wife-to-be (in a pub) some years ago she came back with me. She fondled the three pointed star on my car's bonnet and said "That's an engineer's car". Sure it is

Stuart C04/07/2020 18:18:22
136 forum posts
4 photos

Took my wife to LeMans for her first trip in my "Engineer's Car". Got as far as Portsmouth, when she blurted " GET ME OUT OF THIS ! ". Passenger comfort was not part of the design spec.

Bruce Collinson04/07/2020 20:49:13
543 forum posts

But does that actually happen unless the inner, joining, tube is significantly smaller o/d than the outer tube I/d? I imagined that the tubes acted together a bit like a spoked wheel, only the uppermost spokes are actually in tension, the others merely prevent the wheel from becoming elliptical (ok they’re all in tension to resist the tendency to become elliptical, but I imagine the uppermost spokes might be under greater tensile load).

if the tubes are a close fit, doesn’t the outer tube prevent the described deformation of the inner, leaving it as the perfect efficient beam? Or am I succumbing to the tendency to believe everything I think?

And another reason, The perfect cross section is a circle, Max area for min circumference. An ellipse has a greater circumference for the same cross sectional area. Unless the outer tube is forced into an equivalent ellipse by the inner tube, which feels a bit unlikely, then the outer will prevent the deformation.

I'm not a real engineer, you know.

BTC

Erfolg05/07/2020 14:29:40
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11781 forum posts
1340 photos

If seeking a solution that most closely matches the loading conditions of wings, something that looks like a "I" type beam, which could also be a box beam, matches the requirements. In general there is very little turning, or bending along the axis of the NA of the wing.

In the real world of modeling, tubes work extremely well, for a number of practical reasons. In bending they can be pretty near to the "I" type beam solution (when optimised). Tubes are more often than not of the shelf items. Their properties can often be known with some certainty, although probably not in the modeling world. Incorporation into a wing is often much faster to do than the conventional spar arrangement

IMO the success or failure when using tubes will be the detail, of how you use them. You rarely see a wing these days fail. In part due to current practices no longer mimicing FF practices, where weight and ease of transport and so often was the design criteria. Being honest, the +50cc aerobatic models look flimsy built to my "Battleship" engineering eyes. Yet I am obviously wrong, as they survive the extreme manoeuvres they are subjected to. UC on the other hand, seem to fall of at alarming rate, as part of the landing process.

Just do what you think is OK, and I am confident that all will be well.

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