Carbon Fiber Tube Fuselage

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pictsidhe

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You HBA regular guys are so funny
Total lack of imagination
No innovation
Extremely limited in scope
On the contrary.
We can see the problems that you can't imagine.
Composites is not an easy field to master. It needs experience and advanced maths to really do it well. You have neither, yet you think you know better than the seasoned composite experts here?

You can't ******** or wish around physics.
 

Victor Bravo

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I think he hit the ignore button on several of us who have offered opinions on this thread, or perhaps Elvis has left the building altogether.

There's a very old aviation saying that God watches over fools and pilots.

Being a poster child for both groups, I always add "occasionally both at the same time".
 

wsimpso1

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Ambitions aside, I want to make a plane that substitutes CF tubes for the steel tube in a rag/tube airplane. Furthermore, I want to eliminate the rag (fabric) too by making a mold that merely has a single layer of CF or Glass that would serve as the outer skin of the aircraft.
I advise that you get your hands on a copy of Roark's and go Chapter 11 and Table 11.4 seems to be the applicable one, figure out your skin loading due to the difference in airspeed between inside and outside of the fuselage skin, and run the numbers for max stress and max deflection of your single ply panels between the tubes. You might be surprised. Then get an idea of the tube grid deformation and the stresses that puts on the composite panel between tubes. The stresses add to each other unless your panels go into elastic instability, then you have a whole new set of analytical problems. This whole subject drives the rib spacing on wings and max panel sizes on fuselages. Airplanes with low Vne have a much smaller problem with this, but in fast airplanes, thin skins can be quite the problem.

This same table is applicable to sandwich panels too, but you have to start with plate theory and make some adjustments to get the math converted for sandwich panels. Much larger tube spacing can then be used with sandwich panels. In high Vne applications, this drives thicker skins or sandwich panels. Which takes us to just making the fuselage of sandwich skin, and being finished.

Billski
 

TFF

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I think the original intent was to use the steel tube sizes and thicknesses assuming that carbon is stronger than steel. Thinking it should be stronger with some free weight loss.
 

pictsidhe

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Carbon tubes won't be stronger in compression. A carbon tube of the same dimensions as a steel tube will take much less compression load.
 

Lendo

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wanttobuild, I for one am happy to learn - anything!
The term for stiffness is Modulus and Carbon CAN BE very stiff and strong but when it reaches failure point there is little warning, it fails catastrophically. Seeing it happen brings home the reality. Looking for something that is less brittle would be in the Standard Modulus range for Elastic Modulus is about 230 GPa ( * 145038 = Psi) . A standard Modulus and a good Tensile Strength is something to look for and the Japanese 'Toray' has a solid reputation. It's a PAN based Carbonized Precurser and readily available in the States - not so here in Australia.

Now once you add resin the Elastic Modulus goes from 230 to 135 Gpa and less depending upon NUMEROUS factors, which are better researched to get a full understanding. Weave affects strength, lack of straightness in the Filaments affects strength, different resins, the different methods employed making the part, the list goes on and on, they all affect strength. Be assured the BIG BOYS do it best as they have the big money and best of equipment.

Now we get to design, I'm a big advocate of Warren Truss applications but they all need FEA (Finite Element Analysis) for best outcomes in strength and weight.

So lets see what you got!
George
 

wsimpso1

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I think the original intent was to use the steel tube sizes and thicknesses assuming that carbon is stronger than steel. Thinking it should be stronger with some free weight loss.
Sounds like someone has not been doing the homework... The concept of copying the diameter and wall exactly will fall short. If the builder is willing to design each tube to its load case and length, some weight savings is possible, but it will not be huge, although the cost and man-hours to do it will be huge.

Carbon tubes of the same cross section (diameter and wall thickness) may have higher tensile strength than 4130 tubes, but in fuselages, many of the tubes are sized based upon column buckling. Then there is the higher FOS required with composites, which drives still higher critical loads for each tube in the assembly. For long columns, critical load is entirely based upon bending stiffness and length:

Pcr = Pi^2*L*E*I/L^2

E for unidirectional carbon is lower than steel (usually about 22Mpsi vs steel's 30Mpsi), but when you wind a tube out of square weave cloth to +/-45 degrees, Q11 is more like 8.4 Mpsi. For longer columns, this means that for a carbon tube to be the equal of a steel tube, it will need to be bigger in diameter at the same wall thickness or at the same diameter have much larger wall thickness. In either case it can still be a little lighter until you get into joining tubes, where I suspect the weight advantage will finish going away.

It is possible to buy braided graphite fiber tubes that will pull down to maybe 30 degrees from the long axis and get some increase in Q11. Have fun, these products come in three thicknesses for each base diameter, and are usually build over disposable mandrels. Resin fractions are never as low as you might like too. Have fun. The folks that make bicycle frames use machine wound tubes of prepreg strands at high angles to get high Q11, and then some of them are ground to OD size. You pay for that...

For shorter columns the buckling load smoothly moves towards Sc*A if crippling is precluded. All of this is covered in Shigley (2nd Ed, pages 113-119).

If crippling is not precluded, the buckling load drifts down further to the crippling load for thin wall tubing. See your aero structures texts or elastic stability texts for this particular issue. So, if the tubes are short enough between joints, the advantage might swing to carbon tubes if they have higher strength than 4130 tubes. My information on +/-45 degree fiber tubes - such as the amateur can make in a home shop by table rolling - has a Sc = 59 kpsi, which is less than the 63-66 kpsi. So again the cross section on amateur built tubes will have to get bigger than the steel tube.

If you are copying an existing steel space frame, the column lengths are a given. Remember though that you can increase buckling strength by putting clusters closer together, which means adding weight for more tubes as uprights, laterals, and diagonals.

There still might be some weight advantage here, but it will never be huge. Pick up a steel tube fuselage when you get an opportunity, and you might wonder why you would spend huge effort and big bucks trying to take some weight out of the thing.

Billski
 

wanttobuild

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A few things to say in the interest of safety:

Composite frames and structures are best envisioned and constructed as a single assembly of load-carrying members, consolidated and cured together at one time, for example, by skinning a core of the required shape. There is false economy and possibly serious danger in approaches that create or allow stress concentrations.

Joining extremely strong, stiff structural members (such as precured carbon fiber tubes) via a secondary bonding process requires great care and attention to joint design, load path, combinatorial loads, reinforcement, flexibility, adhesive selection, and bonding preparation. It may be wiser to intentionally take strength and stiffness away from the connections than to allow it or increase it.

If we're not yet nodding along in total agreement, please take that as your clue to consider the whole thing a very bad idea. It isn't a bad idea, just a very dangerous one because rigid frame construction 'looks great and works great'...right up until it suddenly doesn't.

The kinds of failures that can be expected are too often 'catastrophic surprises' unless the designer has knowledge of the principles and issues of moment frame construction, plus ability to test the structures under all required loading scenarios, including 'easily tolerated' vibrations at various frequencies (which can heat things up enough to challenge many resins.)

Here's why it's such a challenge to get it right: whatever load the tubes can carry, they can concentrate somewhere. They will carry that load right to their end, delivering all of it to the joint, which may be getting other loads as well due to the combination of forces and resulting deflections/rotations.

Since carbon tubes can transmit extreme tension and compression loads and usually a lot of torsion too, there can suddenly be a lot more load than might need to be absorbed by a less rigid system. On the bright side, the more 'thunderous explosions of carbon shrapnel' one has witnessed, the more informed one can become if still alive to learn from it.

Joint design: Either make it strong enough to carry all the forces and moments the tube can deliver (and free of abrupt thickness changes such as the step that results from the cut end of a tube that hasn't been tapered)... OR, design a connection with selected degrees of freedom, perhaps taking a page from the flexible bolted connection designs and semi-flexible, welded 'moment frame' connections common to structural steel building design. A safe joint will NOT be a mitered tube glued to another one. If the joint is of the 'strong' variety it will probably have to grow in size and thickness quite dramatically, perhaps as seen in the mount pictured below.

Bonding preparation: Commercially available precured tubes will usually have a VERY effective release agent still on them, especially those built on a mandrel. Solvent wipe, grinding, sanding, and solvent scrubbing may not do much more than move it from the inside to the outside. It's important to be extremely conservative in calculating the areas and allowable stresses you will accept for the bondlines.

Adhesive selection: secondary bonding is a crapshoot, even when testing has validated the basics of what we want to try doing. How much do we actually know about the tube, its resin system, processing, and resultant properties? How much of our bond will be electrochemical, rather than mechanical? What have we done to enhance the mechanical advantage and minimize force transmission through the bondline, and what is the nature of the predicted (and hopefully tested) mode of failure? Have we designed to accept or mitigate the weakest mode of adhesive failure, such as peel?

As someone who collects carbon tubes, I fully understand the appeal of this idea. As someone who would advocate a stressed skin, continuously curving monocoque shell with gently increasing thicknesses at load points, I can tell you with credibilty: the latter is lighter, stronger, and much faster/easier to build. Especially when you include the required number of failures along the path to a decent carbon spaceframe.

I'm sorry I won't have time to weigh in on anything I'm working on or answer specific questions. This post is to help keep people alive and on a productive path. There are still many awesome ways to use carbon tubes, and plenty of reasons to keep inventing for them. Please be careful.

The hard point is a absolute work of art.
 

wanttobuild

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Thanks for your comments George!
Check out Synergyaircraft.com
I have stated the design fundamentals, and it is doable.
I have great admiration for those that can do the numbers, but a mtow of 500#ish, is really an ultralite. I have seen alot of crappy ultralights.
I stated I will disregard weight gain, labor and cost.
Boy, you start talking about a design that doesn't exist physically, people start talking about crashing and burning, I really get a big kick out of it all.
I have got a very nice education here on HBA.

I would like to say for those of you following this thread, DONT fly an aircraft that you are unsure of.
Composites, (reinforcement and resin) are totally awesome materials, there aint no doubt it, BUT, BUT, BUT, if you don't know the FUNDAMENTALS don't use composites.

Ben

Did I state that I love carbon fiber and tow?
 
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pictsidhe

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As Billski and others have detailed, using 'black steel' isn't really a good idea.

I am not planning on black steel, I am playing with Coroplast. Yes, the stuff made from impact modified polypropylene.
It has lower strength, stiffness, strength to weight than steel/aluminium. Yet, by using monocoque construction, I can make a lighter rear fuselage and tail than rag and tube. It's not what you have, it is what you do with it. If I tried using PP tubes, it would be a disaster.

Sadly, it isn't all roses, other parts of my plane will be a little heavier as I need to reinforce them with nasty traditional materials.

I've analysed a statically indeterminate 3D truss with a pencil and a dozen sheets of paper. I can see the appeal of FEA, but it isn't essential.
 

wanttobuild

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As Billski and others have detailed, using 'black steel' isn't really a good idea.

I am not planning on black steel, I am playing with Coroplast. Yes, the stuff made from impact modified polypropylene.
It has lower strength, stiffness, strength to weight than steel/aluminium. Yet, by using monocoque construction, I can make a lighter rear fuselage and tail than rag and tube. It's not what you have, it is what you do with it. If I tried using PP tubes, it would be a disaster.

Sadly, it isn't all roses, other parts of my plane will be a little heavier as I need to reinforce them with nasty traditional materials.

I've analysed a statically indeterminate 3D truss with a pencil and a dozen sheets of paper. I can see the appeal of FEA, but it isn't essential.
Wow a helpful post. Thank you.
Wish you all the best for for your project!
 
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wanttobuild

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I think the original intent was to use the steel tube sizes and thicknesses assuming that carbon is stronger than steel. Thinking it should be stronger with some free weight loss.
That would be nice but I'm a real chicken when I get oh say 3ft or higher off the ground.
The "Tubes" vary in section from plain old round, square and hat shaped, or I could say whatever shape the fiber needs to smoothly cross from one tube to the other.
I don't have the education or desire to calculate wall thickness, I will just slap on some more.
It aint gonna be ground bound either!

I don't believe I mentioned the aircraft will have an inefficient round carbon spar.
Wish I could have it rotate 90deg for storage, but I cannot get my mind wrapped around the concept.
 

wanttobuild

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wanttobuild, I for one am happy to learn - anything!
The term for stiffness is Modulus and Carbon CAN BE very stiff and strong but when it reaches failure point there is little warning, it fails catastrophically. Seeing it happen brings home the reality. Looking for something that is less brittle would be in the Standard Modulus range for Elastic Modulus is about 230 GPa ( * 145038 = Psi) . A standard Modulus and a good Tensile Strength is something to look for and the Japanese 'Toray' has a solid reputation. It's a PAN based Carbonized Precurser and readily available in the States - not so here in Australia.

Now once you add resin the Elastic Modulus goes from 230 to 135 Gpa and less depending upon NUMEROUS factors, which are better researched to get a full understanding. Weave affects strength, lack of straightness in the Filaments affects strength, different resins, the different methods employed making the part, the list goes on and on, they all affect strength. Be assured the BIG BOYS do it best as they have the big money and best of equipment.

Now we get to design, I'm a big advocate of Warren Truss applications but they all need FEA (Finite Element Analysis) for best outcomes in strength and weight.

So lets see what you got!
George
George, thank you for the toray. Now I have more information to consume. They have a large plant down in Decatur, Al., right down the road from rocket city (Huntsville).

This subject has certainly created a lot of conversation, and I like to glean pearls of wisdom. I will ease up on the regulars, if they ease up on me.

I will try to make you a presentable drawing for your leisurely review and would welcome your comments, I like to learn too!
Ben
 

stanislavz

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I advise that you get your hands on a copy of Roark's and go Chapter 11 and Table 11.4 seems to be the applicable one, figure out your skin loading due to the difference in airspeed between inside and outside of the fuselage skin, and run the numbers for max stress and max deflection of your single ply panels between the tubes. You might be surprised. Then get an idea of the tube grid deformation and the stresses that puts on the composite panel between tubes.
Could i ask to provide numerical example or just plain comparison rag and tube / vs composite skin on same tubes ?

Or - due to fabric be "eleastic" fabric do not translate stress to tubes ? I do not think this is right. You may have trampoline vs trampoline frame with plywood on top. They both transfer same stress from load to frame
 

Rik-

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I advise that you get your hands on a copy of Roark's and go Chapter 11 and Table 11.4 seems to be the applicable one, figure out your skin loading due to the difference in airspeed between inside and outside of the fuselage skin, and run the numbers for max stress and max deflection of your single ply panels between the tubes. You might be surprised. Then get an idea of the tube grid deformation and the stresses that puts on the composite panel between tubes. The stresses add to each other unless your panels go into elastic instability, then you have a whole new set of analytical problems. This whole subject drives the rib spacing on wings and max panel sizes on fuselages. Airplanes with low Vne have a much smaller problem with this, but in fast airplanes, thin skins can be quite the problem.

This same table is applicable to sandwich panels too, but you have to start with plate theory and make some adjustments to get the math converted for sandwich panels. Much larger tube spacing can then be used with sandwich panels. In high Vne applications, this drives thicker skins or sandwich panels. Which takes us to just making the fuselage of sandwich skin, and being finished.

Billski
I agree. Carbon Fiber is not magic, yet everyone is so enthralled by it that they think it's the next magical solution to every problem. Yes it has its place where it can shine but in a lot of places it is more of a placebo than an benefit.

The plane I was thinking of is a "Slow and Low" STOL plane so the speed and pressure differentials are not great enough to cause the for mentioned concerns.

Thanks!
 

wsimpso1

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Could i ask to provide numerical example or just plain comparison rag and tube / vs composite skin on same tubes ?

Or - due to fabric be "eleastic" fabric do not translate stress to tubes ? I do not think this is right. You may have trampoline vs trampoline frame with plywood on top. They both transfer same stress from load to frame
Not likely. This is a lot of work to scheme up, put together the analysis, debug the programming, all on something with ZERO value for me while my project waits in my shop. Get your hands on Roark's Table 11.4 Case 1a seems to fit for fuselage except through the wings. Super position of 1a and 1d can be applied to wings and fuselage through wings. Have fun.
 

stanislavz

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Not likely. This is a lot of work to scheme up, put together the analysis, debug the programming, all on something with ZERO value for me while my project waits in my shop. Get your hands on Roark's Table 11.4 Case 1a seems to fit for fuselage except through the wings. Super position of 1a and 1d can be applied to wings and fuselage through wings. Have fun.
Thank you. Just one question more - do we have similar load on frame with rigid skin as with flexible one ?
 

wsimpso1

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The lifting loads due to moving air outside and stationary air inside are the same regardless of skin material. There is probably some difference in drag with shape due to shape changes.

Make just a 2 ply skin, and the max panel size is pretty small before stresses go too high. So you can add plies or make the skin a sandwich panel. You get strength and stiffness at much less weight with sandwich panels. Well, it does not take much of a sandwich panel to allow you to skip the internal structure entirely.

which is where BoKu and I have been going...

Billski
 
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