fiberglass mesh in foam wings

[John Ferrell]

As I recall:
I believe Dave Guerin originated this process. Examination of crash damaged and failed wings revealed that carbon fiber will eventually peel a thin layer of foam loose since the foam flexes and the carbon is rigid. In time, the carbon is just along for the ride. The fiber glass window screening with its loose & wavy weave has a little give to it. I have not observed any delamination of the screening in spite of noting that different rolls of screening seem to be harder to wet with epoxy. 

If you are building a wing with a thin airfoil some kind of spar-like structure is cheap insurance. I have had wing failures with the screening in place that held together well. They were caused by off - field landings.

Originally I think Dave carried the screening all the way to the tip cap and lapped them over the edge. Experience indicated this was not really necessary, the epoxy works well as an adhesive in this application. I think the whole process originated with the very thin Desafio wings but that is another story. It really is not necessary to carry the screen to the tips but it is easier to do a neat job this way. If you put a piece of sand paper a little wider than the screening on a piece of aluminum angle it will only take a few swipes to recess the excess screen into the foam. Don't mess with the sheeting, it is thin enough! In my shop I use the same sanding bar that I use to true sheeting. 

I doubt thicker wings benefit from this treatment.

Someone (up Chicago way I think) puts glass between the foam & sheeting. Sounds like a great idea but if I did it it would be heavy.

I have seen Prophecy wings that were glassed after sheeting and painted. I expect they were a little heavy but they felt like they were indestructible.

Remember, this is as I recall, and YMMV!

John Ferrell    
http://DixieNC.US

  ----- Original Message ----- 
  From: Ed Alt 
  To: discussion at nsrca.org 
  Sent: Tuesday, December 21, 2004 7:39 AM
  Subject: fiberglass mesh in foam wings


  I'm getting ready to sheet a pair of Temptation wings and have questions about using the fiberglass screen mesh reinforcement strips they call for.  First, how necessary is it to even use any?  I can see where it obviously adds strength and stiffness in those sections directly under the mesh, but at the same time, it will also create a raised-up section of sheeting directly adjacent to the mesh strip.  It seems possible that there would be no bonding of sheeting to foam for about 1/4" to 1/2" wide all along those seems, at least not if a thin layer of epoxy is used.  It also seems that at the point where the sheeting does begin to bond with the foam again, it would be subject to shearing forces that might tend to pop the foam beads out over time due to bending loads on the panel.  Maybe not? I have no experience with this method, so it's all speculation.  It seems that a fairly heavy application of epoxy would be needed to bond foam to mesh, mesh to sheeting.  Is it possible that the same or better strength result could be accomplished with a slightly heavier wood selection and still be as light or lighter than adding mesh and extra glue?

  Asuming the mesh is really necessary:  The 2nd question is about the adhesive to use if the mesh is used.  I normally use either Pro Bond or Elmers polyurethane glue, which does a terrific job of bonding several bead layers deep into the core.  I think it probably stiffens the structure alot as a result.  However, it's not clear to me if this would be a good adhesive with the mesh applied.  In theory, it would do a good job of filling that gap I'm worried about along the mesh edges.  Has anyone tried this and are you happy with the results?

  The last question is about how far to run the aft reinforcement strip.  Do you really need to run it all the way to the tip?  At some point, you really get diminishing returns as you get further out in the panel. Sorry for so many questions!

  Ed
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