Carbon or balsa shear web for I-beams

[SkyPyro]

As most of you already know, a kind of spar for flat plate foamies is the I-beam, usually utilizing 3 strips of .007 (I think) carbon ribbon. The shear web piece of carbon seems like it wouldn't be all that useful in that situation, though. The fibers are oriented in the wrong direction. Would something like 3/32 end grain balsa be better here? I think I'll experiment with that on my next foamy, but I'd like to hear some input before I go wasting materials.

Thanks,

Dan

[davidfee]

Uuhhhh, yeah, the vertical grain balsa would be better as a shear web. Are folks seriously using .007" carbon as spars? What is the carbon attached to? Can you post a link? This does not sound like good structural design. Sounds like "It's carbon, so it must be good."

-David

[chlee]

Another option might be to make carbon weblets (thin vertical stringers under the sparcaps) as in the SuperGee and Kahu designs:

http://www.charlesriverrc.org/articles/kahudlg/chriskaiser_kahudlg.htm#Wing

Somebody more knowledgeable than me might explain how this compares to a balsa shear web. But I have heard that these weblets greatly increase the spar caps' buckling load, and they are very easy to install.

- Chung

[davidfee]

Yes, the weblets will help a great deal... I'm just stunned by the idea of someone using .007" carbon as a spar cap without a support structure. Kinda like asking a flat sheet of typing paper to support a compression load.

[SkyPyro]

Uuhhhh, yeah, the vertical grain balsa would be better as a shear web. Are folks seriously using .007" carbon as spars? What is the carbon attached to? Can you post a link? This does not sound like good structural design. Sounds like "It's carbon, so it must be good."

-David

THANK YOU! All along I've been thinking "that doesn't make much sense," especially when people started having wings fail. This is why I've always stuck to carbon tube. Here are two examples:

http://www.rcgroups.com/forums/showthread.php?t=108333

http://www.rcgroups.com/forums/showthread.php?t=126471&page=3&pp=15

Roll down to post 12 on the first one. A lot of people have been using I-beams in 3DX's, Ultimate Bipes, planes that Derek has designed and other flat scratch builds. I think that what you said is right, people think that almighty carbon fiber is always better than flimsy balsa.

Chung- I am pretty sure it is because the vertical balsa fibers connect the top and bottom spar, instead of just the resin in the carbon fiber. This means the carbon spars can't move compress, stretch, or move left or right independant of each other. That translates into no vertical movement, either.

Thanks,

Dan

[davidfee]

Dan,
Ok, I read most of the info you referenced. The guy has a good point about the I beam being stiffer than a tube of the same weight. However, his implementation isn't the best. You need adequate bonding area to get sufficient shear strength and .007" web width just doesn't give you much bonding area. Add to that he's using a weak adhesive (CA), it's no wonder they are seeing compression failures.

A side issue is the fact that you have an interface between an extremely stiff material (carbon) and a very flexible one (foam). You will always have problems when this transition is abrupt. However, I know that the builders of these flat foam 3D planes are going for simplicity... so over-designing the structure is pointless.

If I were building one, I'd use flat carbon spar caps with a 1/16" vertical-grain balsa shear web, bonded with laminating epoxy. The carbon must be roughed-up with sandpaper first! Then on the outside I would apply ~1oz glass on the bias, overlapping the foam and carbon spar to distribute the loads to the foam skin.

If I were making a kit, I'd use kite spars since they are cheap and light enough.

-David

[davidfee]

Actually, since we're on the subject of shear webs, the "best" shear webs I have seen are vertical-grain balsa with +/- 45deg bias carbon cloth laminated on each side. The vertical grain balsa gives the compressive strength and the carbon gives the shear strength. Way overkill for a foamie, but... as long as you're thinking about the loads and stresses it doesn't hurt to look at other structures.

An alternate web I have seen is to laminate sheet balsa into "plywood" with the grain going +45deg, vertical, -45deg... or even just +/-45 deg.

The compressive load between spar caps is arguably quite small compared to the shear loads. For this reason, rigid foam is often used as a shear web to reduce weight. The foam can be capped with the +/-45deg carbon (or wrapped in a braided tube). You would not use EPS foam... you'd use Rohacel for example.

Many ways to skin a cat...

-David

[SkyPyro]

David-

Thanks again for your input. This is very interesting, I haven't even thought about laminating the sides of the shear web. Still somewhat new to figuring out this composite stuff :) . Even though its overkill, I think I'll still experiment with your ideas. It wouldn't be all that difficult to do any of them, and the same basic design could be used on larger, faster, wilder planes. Foamies are just the beginning ;) . I'd imagine the weight of the I-beam would end up lighter than the tube, as well.

I'll keep you updated on what I do.

Dan

[Salto]

On this question of I beams verses carbon tubes for spars in foamies, the I beam may well be stiffer than a tube of the same weight, but the simple I beam made from three flats has very low torsional stiffness. For faster foamies this is an important consideration. If the wing is not adequately torsionally stiff it may flutter, plus the ailerons will loose effectivness if the wing can twist.

I've made several flat plate biplanes and used two carbon flats epoxied to the foam opposite each other. This works OK on a biplane where the interplane struts and rigging provide the torsional stiffness, but for monoplanes I think the tube is the optimum solution.

On David's point about the dangers of using stiff materials in conjunction with flexible ones, it's interesting to note that with foamies the stiffest solution is not usually the best one. For example the common practice with EPP slope soarers of using two carbon rods top and botton of the foam core with no shear web at all is not as bad as it sounds. If the "spar" structure was completed with a shear web and maybe some bias wrapping, any load in a crash would be taken entirely by the spar which is by nature a brittle structure.

If however, the "spar" has only the flexible EPP to take any shear loads (no web), then the wing can deflect and allow the foam to take some of the load. The foam here is also a nice energy absorbant material, which helps dissipate the energy of the crash. Crash resistance is thus better than it would be with a traditional stiff spar construction. If you're carrying the weight of the foam around, it may as well contribute something structural!

Graham.

[davidfee]

If you use a tubular spar at the center (or ~25%) of a flat panel wing with no other structure, you will not gain a lot of torsional rigidity. There just isn't enough stiffness in the skin. If you want stiff and light, you'll have to give up crash survivability.

All I can say is... I design to fly, not to crash.

-David

[Salto]

David,

I've found on 3mm Depron flat plate wings, say on a model like David Theunissen's Extra 330 (approx 700mm 27" span), a 3mm dia carbon spar embedded somewhere near 25% chord adds considerable torsional stiffness.

In my above post, I was simply comparing this to three 3x1mm carbon flats arranged in an I beam which would add very little torsional stiffness to this kind of wing. The Depron wing by itself isn't very stiff because it is a flat plate, plus Depron is not a very stiff material. Something is needed to beef it up, and a tube of equal diameter to the foam thinckness provides bending stiffness plus useful torsional stiffness.

I agree with you that it's preferable to "design to fly", but the people who build these foamies (and I don't exclude myself here) tend to have a different approach!!

Graham.

[davidfee]

Glad to hear you get some torsional stiffness from the tube. I just imagine the foam flexing around the tube... so the gain is less than it might be... but certainly greater than nothing.

-David

[Magne]

I built a 3DX according to the drawing, including the 3-piece I-beam with 1/4" x 0.007" carbon spar. Simple to make, but it delaminates horribly. Will not use it again. I think the added weight of a tube is well worth it, particularly with the added benefit of torsional rigidity. The fuselage of my 3DX twists ridiculously, in fast rolls you can see that the tailplane "lags" behind the wings at least 15-20 degrees. Looks kind of funny. (or silly.) If I was to use a shear web from end grain balsa, I would make it about 3/16" wide (for 1/4" wide spar caps) to increase the bonding area. Ikarus Shock flyers, made from 3mm depron, dont use spar caps at all, but use two pieces of slightly thicker carbon strips 3mm wide, one attached right at the wing leading edge (glued in place, or just taped in place with tape wrapped around the leading edge.) The other carbon spar is attached at the main wing trailing edge, at the hinge line for the ailerons. This also gives the wing some torsional stiffness, as you use the bending stiffness of the two beams relative to each other to prevent twisting of the wings. Seems to work well.
Magne

[Capt. Crunch]

Bill,

I ran the Joe Wurtz spread sheet on your wing and found:

With 40% CF, or GF as Joe uses it, it has not enough strength to about 10" out from the root, then OK. It says if you use 100% chord GF top & bottom you're OK.

Or if you Double the GF out to 21" (or use 100%) you're ok. Another thread http://www.rcgroups.com/forums/showthread.php?t=237952 seems to say your .014 CF as a shearweb will add considerable strength to the CF skins in Joe's SS, i.e. moves the Blue line even more above the Red line in the graph.

I've attached his SS with the graph and his notes on using it.

Hope this helps more than confuses.

Cruncher