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Feb 16, 2016, 06:00 PM
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technical stuff comparing fiberglass/epoxy to paper epoxy


This is a reference for this post: https://www.rcgroups.com/forums/show...2&postcount=78

This is just about wing skin stiffness in bending, without the core material.

I remember that, after some time, I couldn't interpret my old notes for experiments I did on this stuff. So I'll do a reality check on this stuff with other people's data. Unfortunately, I didn't find data on epoxy/paper, so I'm using data for phenolic and paper, which ought to be comparable. It's my understanding that the basic grade of phenolic/paper uses kraft paper, which is the paper I'm talking about. Can't be sure of it. Anyway, there's enough uncertainty here that we can't say anything except we may be in the ballpark, or we're not in the ballpark.

Here are the material properties I'm using:
Style 1581 glass cloth with epoxy:
(this cloth is much heavier than you'd use on a wing, but I'd expect that the bulk properties are similar.)
Elastic modulus, flexural: 3.6 mpsi (this seems much higher than other sources but is what the paper says, and may be explained by a higher than usual fiber volume) Seeing as it's nearly as stiff at 280F, the epoxy must be some special stuff
2.0mpsi is what I've usually heard
Flexural strength: the paper only gives ultimate strength, I've usually heard about about 30kpsi yield. This stuff is about 68 kpsi ultimate, but I don't know what it is in yield.
Resin by weight: 33 percent I think usually this number is closer to 40 or 50 percent! Long ago, in MAN Mike Lachowski suggested a rule of thumb, for bagged wings, of equal weight for the glass and the epoxy. The use of a press in preparing the samples may explain this.
Fiber volume: about 48 percent
specific gravity: 1.86

Typical phenolic laminate:
Elastic modulus, flexural: 1 mpsi +/- 10 percent depending on orientation
Flexural strength: 12-15kpsi, depending on orientation I'll just use 13.5
Resin by weight: unknown
Fiber volume: unknown
specific gravity: 1.35

So, if we're talking about stiffness in bending, it goes by the cube of the material thickness. If we have lighter stuff, we can afford to use more of it. So a correction factor for the stiffness for the paper would be about (1.86/1.35)^3 or 2.6X. So the elastic modulus of the paper/phenolic is really equivalent to 2.6mpsi or so. The glass stuff is 3.6, or in the ballpark. Given the possible variables*, I think it's plausible my samples really were equivalent.

When it comes to strength, it's the square of the material thickness, so our factor would be 1.9. So, for the paper we'd have the equivalent of 26kpsi. The figure for glass is 68kpsi. However, since I can't find anything in the paper about orientation of the samples, I suspect that figure is from either 0 or 90 degrees. The paper does say that all the layers were laid up with the same orientation, but that doesn't account for the samples. A rock or fingernail could be in any orientation, so all directions are relevant. The paper fibers seem much more randomly oriented, so they shouldn't lose as much off axis. Anyway, we're still in the same ballpark.

I didn't do any tests of strength, but I did accidentally step on the wing I made. The damage was much less than I expected and easily repairable. But of course that's not really valid data.





*I suspect the layup in the test is significantly better than on a real, vacuum bagged wing.On the other hand, I don't know how much pressure was used on the phenolic paper stuff. An advantage the paper may have had is that the individual fibers don't necessarily cross the neutral axis at short intervals. With the fiberglass, a lot of the outermost part of the thickness is air or epoxy, but that doesn't appear to be the case with the paper. The fiberglass samples used many layers. I used two, so this effect would have been significant for my samples but not for the samples in the paper.
sources:
phenolic paper properties
http://www.professionalplastics.com/...ic_X_CE_LE.pdf

fiberglass and epoxy
AFWAL-TR-82-4013 Mechanical Properties of E293/1581 Fiberglass-Epoxy Composite and of Several Adhesive Systems
www.dtic.mil/dtic/tr/fulltext/u2/a117902.pdf
pages 3 and 14 (by the numbers on the pages)
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Mar 08, 2016, 09:00 AM
Ok that's high enough
FabFlight's Avatar
Quote:
Originally Posted by lincoln
This is a reference for this post: https://www.rcgroups.com/forums/show...2&postcount=78

This is just about wing skin stiffness in bending, without the core material.
Hi,

I followed both threads with great interest. Perhaps this specification sheet could be of interest.
http://www.aetnaplastics.com/site_me...Properties.pdf

This manufacturer shows the compressive, tensile and bending strenghts of fiberglass-epoxy and various fenolics laminates, including paper based laminate.

I found it from this page http://www.aetnaplastics.com/products/d/Phenolic

Basically, paper is half as good for most mechanical properties, except in shear and specific gravity.
Dec 13, 2017, 09:43 AM
Gots me a good used Hobie Hawk
Steve Corbin's Avatar
Ok, so I bought some hot neon pink and blue poster board at the drug store, to use to stiffen and smooth a foam wing. Would saturating this paper with epoxy, squeezing out as much excess as possible, be a good thing for covering the foam wing? I'm always looking for alternative cheaper ways of doing things. I hate fiberglassing anything, even dihedral joints.
Aug 31, 2018, 05:17 PM
Registered User
Thread OP
Steve:
Sorry I missed this for 9 months! I would say the epoxy would help. I'm pretty sure that people used to cover foam wings with poster board without epoxy saturation.

https://rclibrary.co.uk/files_titles...ology_1971.pdf

I suggest poster board may be a bit on the heavy side. 60 lb kraft paper with epoxy is probably enough.


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