Composite sandwich, material type and laod calculations 1

[awparran]

Let's get some ground information on the table first.
I am NOT an aeronautical engineer. I have built two aircraft successfully and obviously lived to tell about it. Both were built from scratch using plans.

I am a private pilot and have been flying since high school (1966). I am checked out in Pipers, Cessnas, Grummans, a Trojan T-28', Grob 109's, etc.

I am designing and hope to build my own aircraft as a completion to one of my life's dreams. I have used wood, metal and composite materials in my pervious aircraft building projects.

Anyway since I am self-taught in a lot of diciplines, I will understand a concept if I take my brain out and let the information "soak in" through osmosis.

I like working with wood and foam. I am toying with the idea of making my ribs out of balsa sandwiched between, carbon fiber or kevlar. I have searched the Internet and found an article or two relating to model aircraft, but nothing on the scale of a full-size aircraft.

I was also thinking of just using foam with carbon fiber as a sandwich.

Either way, I can't find any material related to setting up the ribs, calculations, and/or examples of each.

I am also interested in spar design using carbon fiber/foam. When I enter "SPAR" in the GOOGLE search engine, I can find everything I didn't need to know about boat design.

So my question is where can I find information that I can "soak up" without getting an advanced aeronautical engineering degree at Purdue University?

Any input would be greatly appreciated.

Tony

 

[SuperStress]

Tony,

Okay, here's my disclaimer: I'm an aerospace engineer but I've never built any airplanes from scratch.

There's not a ton of public information on the web about sandwich construction, but Hexcel Composites site is probably the best. Some good information can be found starting at this URL:

http://www.hexcelcomposites.com/Markets/Products/Honeycomb/Sand_design_tech/default.htm

They used to offer this information in .pdf form, but now it appears to only be available in HTML. Some of their other pages offer a lot of information about other types of composites products; prepregs, etc.

My initial impression is that building ribs and spars out of sandwich composites isn't such a good idea. Sandwich structures are useful in situations where light weight and stiffness are primary considerations. Many production airplanes use Fiberglass/Nomex or Aluminum/Aluminum honeycomb in fairings, floor panels and control surfaces.

Honeycomb composites are not a good choice for primary structures where strength is your biggest concern. That's because in sandwich construction your skins take all of the bending loads and the core takes all the shear. Shear flow in the core is what builds skin loads. Therefore, in very high load applications, the ability to transfer loads into your high-strength skins (you mentioned kevlar or carbon) is going to be limited by either the shear strength of your core (neither foam and balsa are terribly strong in shear) or the quality of the bond between the core and the facings.

Also, for spars, at your wing attach joints you would have to find a way to transfer all of the shear out of the core into your fuselage structure.

Commercial manufacturers of honeycomb products have a heck of a time nailing down processes that make high strength panels consistently. All strength allowables MUST come from tests on final products. Hexcel's website also has a good brochure on testing of composite panels that you may find interesting.

I hope you find what you're looking for, but I'm not optimistic that the information you find will support your plans.

SuperStress

 

[MintJulep]

Many of the first-generation fiberglass gliders used either a basla or foam core covered in fiberglass/epoxy for the wing, so the concept has merit and history. I think most of these had a traditional spar of wood or metal.

Later fiberglass gliders omitted the core, layed the upper and lower skins in negative molds and glued the two skins together around a fiberglass box beam spar. Ribs I think are either all glass, or all foam. The current generation I think has half-ribs molded in a part of the upper and lower skins.

Assuming that when you think about a sandwitch rib you mean glass/foam/glass moving from inboard to outboard, this seems like a waste of material. The loading of a wing rib is not in a direction to take advantage of the stiffness provided by that type of construction.

 

[awparran]

Hi:

Sorry for taking so long to respond to the above.

I would like to thank you for your response. I will look into each.

Tony

 

[sreid]

Go to Aircraft Spruce. They have many books on home built aircraft including composites by Burt Rutan.

http://www.aircraft-spruce.com/

 

[USACompositeGuy]

My company, Carbon Fiber Solutions, is a consulting firm that specializes in composite engineering. We have designed components for kit planes (and other applications) and are currently working on an all composite certificated airplane. If you are serious about building a composite airplane from scratch we would love to talk to you about it.

I think that you will find that a trial and error approach to a composite airplane will not only be expensive but unsafe. I think that we would be able to give you the needed expertise to accomplish this build. We could be as involved or as uninvolved as you would like and could work at a pace to fit your budget. Visit our website,

http://www.carbonfibersolutions.com,

to see what our company is about. Feel free to contact us if you have any questions (contact info is on the website).

http://www.carbonfibersolutions.com

 

[awparran]

Hi:

THanks. I briefly checked out your site at work. I will look at it later when I am off.


Tony