Composite Construction Question - Airplane Guys

[kach22i]

The question appies to hovercraft, but the concept is the same to airplanes (I think). Help answer this guys question.

Link:
http://www.hoverclubofamerica.org/forum/index.php?showtopic=1018

Quick question for you guys. I am undertaking a UH-13P with my INdustrial Mechnics & Pneumatics class. One of the things we've discussed is adding a VERY thin layer of carbon fiber between the layers of foam. One of the companies helping me with the project is recommending we use what they call a V Web. It is a piece of carbon fiber that is not woven. It is very light weight and thin. They are going to bring in what they call an impregnator (spreads the epoxy on the material) and it should get the excess epoxy off the material. Then vacuuming bagging the layers of foam together.

[Joeaksa]

The addition of any additional fiber should stiffen the assy up. As long as not too much fiber (weight) is added it should not be an issue.

One problem is that carbon fiber is expensive so you do not want to just start spreading it around everywhere.

Might want to get Thom (widebody911) in on this thread as he has a lot more carbon fiber experience than most on this bulletin board.

[fastpat]

Originally posted by kach22i
The question appies to hovercraft, but the concept is the same to airplanes (I think). Help answer this guys question.

Link:
http://www.hoverclubofamerica.org/forum/index.php?showtopic=1018

Generally, carbon fiber is very directional, adding stiffness in one direction, but nothing in another. That's what makes using it so attractive in wing spar caps, in composite wing structures. Wetting out CF isn't easily done, you used to be able to buy a small training kit for CF , S-glass, and the other structural composite materials from Aircraft Spruce & Specialty which is THE supply store for these thngs, along with Wicks Aircraft Supply.

[javadog]

He doesn't give enough information about the part he is designing. I don't know why he is using multiple layers of foam, nor whether the part is flat, or has curvature. etc.

In general, the strength of the composite structure comes from the outermost layers; the "skin," if you will. The core material mainly serves to keep the outer layers a certain distance apart. There may be little or no stress and strain near the center of a composite strucure. In such a case, adding a layer of carbon cloth would be a waste of material.

Not knowing any more about what he is trying to accomplish, I'd say no.

JR

[Tim Hancock]

Originally posted by javadog
He doesn't give enough information about the part he is designing. I don't know why he is using multiple layers of foam, nor whether the part is flat, or has curvature. etc.

In general, the strength of the composite structure comes from the outermost layers; the "skin," if you will. The core material mainly serves to keep the outer layers a certain distance apart. There may be little or no stress and strain near the center of a composite strucure. In such a case, adding a layer of carbon cloth would be a waste of material.

Not knowing any more about what he is trying to accomplish, I'd say no.

JR

Didn't open your link, but Javadog's statement above is spot on.

Carbon fiber can be bought in bi-directional cloth which posseses strength in both directions, or can be purchased in unidirectional tapes or tow that consist of strands all run the same direction to provide high tensile strength. Same goes for Kevlar and fiberglass.

[hardflex]

Sounds like he took some sketchy notes, and couldn't remember all the details.

He is talking about using Graphite unidirectional tape in a V configuration, and also mentions impregnated fibers, we call it prepreg for short, with the correct amount of resin for saturation.

Javadog is right. make it with two layers of carbon in a V, both sides of the skin, and foam core in the center.

[kach22i]

You guys are right about a middle layer typically not being used because the top layer is in compression and bottom layer in tension............as a general rule. I think this why the suggestion of a middle layer of CF has raised the question in the first place.

I should note that there is a composite replacement for marine plywood which will hold a screw as the exception.

Coosa
http://www.coosacomposites.com/

Coosa Composites manufactures structural panels made of high-density, polyurethane foam reinforced with layers of fiberglass. The no-rot and light-weight advantages of foam combined with the structural properties of fiberglass make Coosa panels an excellent replacement material for wood and other traditional core materials. Coosa panels are used throughout the marine, industrial and transportation industries in structural and non-structural applications.

Coosa produces panels in 4x8 ft up to 5x12 ft and can pre-cut and kit parts for those that prefer an extra level of customized service..

As far as I know, no one is using Coosa in aircraft construction, but they are using it in hovercraft construction (large and small).