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Carbon Fiber Tube Fuselage

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sigrana

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Oct 19, 2010
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Op. Do you have loads determined ? If you could give me, i would design it in two variants - only shell and only tube. For tail section only.

As mine numbers shown already - it is safer and faster to make a shell and add only local reinforcements against buckling of large panels.

Tubes are nice and sound (done using pultrusion or other high fiber epoxy ratio), but joints are not..

One of example :

View attachment 103616
As I mentioned, email Jim Marske and ask him.
 

wsimpso1

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If one has not got the loads on something, how can one ever know if it is strong enough?

That is a serious question. You are proposing a structural scheme different from what is already out there, so you can not copy someone else's details, and you can not make a sample and load it appropriately... It would then be entirely guesswork, and guessing is BAAAAD.

Billski
 

TLAR

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If one has not got the loads on something, how can one ever know if it is strong enough?

That is a serious question. You are proposing a structural scheme different from what is already out there, so you can not copy someone else's details, and you can not make a sample and load it appropriately... It would then be entirely guesswork, and guessing is BAAAAD.

Billski
Oh okay I will stop then.
Thanks for your assistance in this matter.
 

wsimpso1

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Oh okay I will stop then.
Thanks for your assistance in this matter.
I was not asking you to stop. You do have to learn a little bit about how to calculate the loads in order to design an airplane. We actually have to make the weight come in on the safe side of minimum strength but not so heavy as to convert it into a ground vehicle.

Fuselage aft of the wing on a conventional airplane is pretty straightforward. FAR Part 23 (the old version, lots of them posted on the web) tells us tail surface loading we have to design for based upon wing loading and max g's. Know that number and your tail area, and you know how much shear load goes into the aft fuselage. Bending moment at any spot along the aft fuselage is the tail load times the distance from 1/4 chord spot on the tail plane to the spot you are interested in. Then the same FAR's give us a way to compute twisting moment too. OH, in composites the structure has to be designed to be that strength times 2.0...

What about the fuselage forward of the wing? Well, all that stuff forward times max g is the shear load, and sum of the weights times max g times the longitudenal distances to the spot in question is the moments there. The moment forward plus the wing pitching moment should be somewhat below the moment from the tail.

You should be doing all this for the vertical tail too.

The harder ones are making the cockpit part of the fuselage stand the 19g and 23g crash loads ...

Once you know all of these maximax loads, you can start really light and iterate the design until it passes your test. You could learn how to analyze the structures and design to pass the tests on the first try. You might even find out how light much of the fuselage really is...

Then there are the wings...

Billski
 

WonderousMountain

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Clatsop, Or
My experience level with Composites is low.

Sure, I read Tsia & Hahn, but until I get guild
worthy outcomes, it has to be done in metal.
this despite having a good case for composite
judging by the structure.

A three section space frame.
 

Bille Floyd

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Sep 26, 2019
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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.
Is this post, mainly about attaching cured tubes to each other , with
a composite, (fiber / resin) , the best ya get is a mechanical bond, and
not a molecular bond ; where as when the tubes and attachments
are cured at the same time ; ya get both a mechanical , and a molecular
bond ?

Bille
 

TLAR

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Please lock this thread.
I said I would stop, so end comments
 

wsimpso1

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Is this post, mainly about attaching cured tubes to each other , with
a composite, (fiber / resin) , the best ya get is a mechanical bond, and
not a molecular bond ; where as when the tubes and attachments
are cured at the same time ; ya get both a mechanical , and a molecular
bond ?

Bille
True, but you can approach the chemical bond strength by sanding with 200 grit sandpaper immediately before doing the bond or lamination. It messes up the resin molecules, leaving loose ends to bond to. You have an hour after sanding to get the surface covered with new resin or the effect is lost and you go froma strength of 5000 psi (partial chem bond) back down around 1000 psi (mechanical).

Billski
 

TLAR

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Sep 29, 2020
Messages
108
In my design there is no secondary bonding
Yes I can design an aircraft without knowing the loads. I don’t use a pocket protector I use common sense.
I am an artist with filler metal and am in the process of becoming an artist with carbon fiber
 
Last edited:

BJC

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TLAR:

You seem to have plenty of enthusiasm, energy and drive. I encourage you to make use of the many references cited on HBA to get acquainted with the fundamental physics involved in aircraft aerodynamics and structures. (The Glider is a good place to start.) Armed with those basics, your chance of successfully developing your envisioned aircraft will be much improved.


BJC
 

TLAR

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Joined
Sep 29, 2020
Messages
108
If one has not got the loads on something, how can one ever know if it is strong enough?

That is a serious question. You are proposing a structural scheme different from what is already out there, so you can not copy someone else's details, and you can not make a sample and load it appropriately... It would then be entirely guesswork, and guessing is BAAAAD.

Billski
Totally Ignore
I am an experienced aircraft builder and have the necessary skills to do exactly what I have proposed
 
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