I Beam or C-Channel Question 1

[Burner2k]

This is quite a basic conceptual knowledge an experienced structural engineer should have by now but recently I realized that my understanding may not be a solid as I want it to be.

If I need to select a cross-sectional shape for a beam like Spar, which one would be better?

An I or a C-Channel? I want to get an understanding from engineering as well as manufacturing (if possible).

For engineering (considering bending), I calculated some basic geometric properties like Area Moment of Inertia and Area of Cross-section taking arbitrary dimensions for both shapes. Both the area & MI (Ixx) turned out to be same. So why would one be chosen over the other and under what circumstances? I hope I haven't made any mistakes during data entry!

Simply put, if the above is asked in an interview (I-beam or C-channel), what would your answer be and why?

Secondly, would a composites beam of having layup like the below be possible to manufacture?

Note: I have just drawn 2 plies for illustration purposes. The thick black lines refer to tooling surface. Recently one person told me that such a layup would be difficult to fabricate since ejection of tool would be difficult. I have very little exposure to composites manufacturing. Just wondering if experienced folks could pitch in and provide their thoughts about the above (like perhaps are there any newer methods which would make it possible to fabricate)!

Would like to learn more about composites manufacturing, at least the parts designers/engineers should be aware when encountered with situations like above. Would appreciate any good pointers.

 

[dik]

An I or W beam is more efficient than a C Section. Greater strength per unit weight and they come in a greater variety of sizes. Can you consider using a rectangular HSS section with a thinner wall; they are often the most weight effective.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MintJulep]

Depending on the location of the load vs. the shear center beams may or may not twist as they deflect.

Depending on your need, you may want to prevent that, or use it to your advantage.

"Secondly, would a composites beam of having layup like the below be possible to manufacture?"

Yes, but not with tooling as you have shown.

Generically, a process called "pultrusion" is capable of making a great range of constant cross section FRP shapes, in stock or custom profiles.

If you need a varying cross section then there are a lot of processes that can be used, and even more things to consider.

It is possible to tailor the performance by selecting fiber materials, material weight, material weave, lay, strand orientation, and more.

You can have fiber and resin only, or you can embed other reinforcement.

You can layup over and encapsulate a foam, or wood, or balsa, or other core.

 

[rb1957]

why are you selecting a shape ?

Depending on where you are, and what lessons your company has learnt you'll use either
1) a C-section spar and straps to pick-up the the L/E (or T/E) structures, or
2) an I section with integral flanges for the L/E and T/E structures.

Neither approach is implicitly right or wrong

another day in paradise, or is paradise one day closer ?

 

[Burner2k]

Firstly, thanks for posting the replies.

An I or W beam is more efficient than a C Section. Greater strength per unit weight and they come in a greater variety of sizes.

dik, can you please elaborate on the above? The section dimensions I have taken in my OP figures are just for demonstrative purposes. I am getting same area of C/S & Ixx values which to be tells me that both sections will have same stress values (bending) for given applied moment. Further since the areas are same, wouldn't weight be the same as well. Am I missing something here?

MintJulep, thanks for your wondering reply especially about manufacturing part. Any suggestions on a book which I can pick up for some basic composites fabrication (like I mentioned basic knowledge for designers).

RB1957, would it be possible to explain more on the strap with C-Spar design?

 

[rb1957]

a strap is a strap ? For the Front Spar, you have a C-channel flanged aft picking up the wing skins (upper and lower). Now you have to attach the L/E structure ...
1) a spanwise strap that would pick up the aft flange from the C-channel and project a flange forward for the L/E structure, or
2) an spanwise angle that attaches to the vertical face of the C-channel and projects a flange forward for the L/E structure.

another day in paradise, or is paradise one day closer ?

 

[MintJulep]

A lot of learning resources available here:

https://acmanet.org/what-are-composites/

 

[SWComposites]

Suggest you purchase and read this book:

https://www.amazon.com/Airframe-Structural-Design-Information-Structures/dp/9627128090

I section and C section spars have been used; a lot depends on what needs to be attached to them.

Putting tooling in the middle of a composite laminate makes no sense.

 

[Ng2020]

Elastic properties aside, a major consideration is whether you want to shift manufacturing expenses and complexity to fabrication, or to assembly

If you really want a composite i-section, back-to-back channels would be laid up on male tooling (on a female tool the plies 'bridge' at internal fillet rads). Put a 'noodle' to fill the void formed by the junction of the back-to-back webs and flanges, and cocure with a separate flange layup... Or something along those lines.

Otherwise you could lay-up a channel section on a male tool, cure it, and live with a few extra parts and many additional fasteners in the assy.

 

[Burner2k]

Sorry for the late reply but thanks again to all who have answered.

Ng2020, we are going with C-Channel mainly due to fabrication constraints just like you have mentioned.

SWComposites, I have a copy of the book...need to re-read it again to get some more clarity.

RB1957, thanks for your reply. Still not able to picture the design arrangement you have mentioned. I will try to read Niu Airframe Structural Design and look-up images.

 

[WKTaylor]

B2K...

Perhaps SWC meant to link to another MCY Niu book... Composite Airframe Structures: Practical Design Information and Data [3-Ed]
ISBN-13: 978-9627128069
ISBN-10: 9627128066

A few other useful references, off the top of my head...
SAE AIR4844 Composites and Metal Bonding Glossary
Practical Analysis of Aircraft Composites
Care and Repair of Advanced Composites
Essentials of Advanced Composite Fabrication and Repair
Analysis and Design of Composite Metallic Flight Vehicle Structures [Abbott]

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

maybe I'm thinking more on a metallic approach, but should work just the same in composites. Either you have a complicated shape with flanges going this way and that, or you have several simple pieces ...

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

SWC stated it clearly/simply... but here's another look at that perspective...

When thinking structures... with few exceptions... 'metal is simple and easily defined/analyzed'. BUT analogies to 'simple metal structure' CANNOT apply to most built-up laminated/sandwich composite structure.

OK... if building a composite structure... with individual longerons-stiffeners/skins/frames assembled with fasteners/etc... IE: 'black aluminum' structure... there is a lot more similarity to metal structure... but what's the 'value-added' point for using composites, then?

LEARN about the world of composites first. There are very good intro courses in composites for assembler-technicians, structural-mechanics, QA/QC/NDI-NDT techs and design/analysis engineers.

Some Fud-4-Thot...
Learn the rules like a pro, so you can break[use] them like an artist." -- Pablo Picasso
The young know the rules: the old know the exceptions." --bumper snicker

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

I accept that the idea is more of a metal design concept ... but the same design guidelines apply ... either
1) a few complex shapes, combining the flanges into one shape (which would be an I type of section ... most likely ... you can still have the L/E structure pick up aft of the Front Spar web, but that would be "unusual"), or
2) several simple shapes, the front spar would be a simple channel (easier to make in metal or composite) and a separate simple strap for mounting the L/E structures.

Like I said previously, some company's DNA requires you to go one way (based on "lessons learned").

another day in paradise, or is paradise one day closer ?