Composite Construction Question - Airplane Guys
 

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

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.

Re: Composite Construction Question - Airplane Guys
 

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.

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.

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.

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