Exactly right Graham, but the force I am trying to fix is not a flight
load but just a wing surface plate skin stiffness for my wide butt. If
I were to use carbon fiber, the joint needs to be tapered in to where
the next rib is. That rib will relieve some of the discontinuity of the
carbon/glass interface. The loaded outer skin compression over the rib
should take the discontinuity.
I too believe the best fix is to use E or S glass and epoxy as a
band-aid fix. The real fix is to add ribs and change the glass layup
sequence to add more surface stiffness for those Stilleto heels and
boney butts.
For those following this, anytime that you add carbon fiber to a glass
surface, all the load is transferred to the strongest element i.e. the
glass fiber, and the epoxy is for a better word, a bonding agent. If
you put a layer of carbon on glass, the carbon then will take all the
load until the point where the epoxy bonds surrounding the carbon fail,
then the glass would pick up the load (probably to failure). In the
case we are discussing above, this may result in a complete
de-lamination of that stiff carbon sheet which is only held on by the
filler/foam or whatever between it and the glass skin or worse yet a
buckling of the skin and failure. Glass epoxy structures are
interesting animals in that in the event of an extra layer of carbon is
put over the glass, the difference in stiffness (modulous of elasticity)
will cause a stress riser at the edge of the carbon layer. This can
cause numerous problems. The outer skin of the Europa XS is for shape
and stiffness. It is not a heavy weave like the Classic wing. Under
loading, the upper skin is under compression and as the glass takes up
the load, a sudden junction of very stiff carbon can cause the skin to
wrinkle or even debond under compression. The area over the rib is
transferring some of it's shear load to the rib and is somewhat less
stressed, so in my opinion, this would be an acceptable place to make
the transition. (Look at the wing in loading some time and you can see
every rib at 3 gs as the glass upper surface in compression slightly
bends under the compression load.) In my education on aircraft
structures, it was always drummed into our heads that the common failure
mode in aircraft is a compression failure due to buckling. Glass
structures are designed to take the compression and sheer loads together
through the orientation of the fibers. Most of the strength of a wing
is that shell. The tension loads that are easily carried by the lower
wing skin, through the strands of glass oriented at anywhere from 0-30
degrees, allow some of the linear stress to be taken by the shear load
of the D tube and transferred by contact and ribs to our extremely
overbuilt (thankfully) main spar and of course the upper skin.
As I said and Graham has eluded to: Without testing on the panel, we
will never know for sure. The theoretical solutions for analyzing
glass/epoxy structures is beyond my shops time and money. That is why
my bandaid is bid cloth over the skin tapered to the first rib from the
root. The thicker, now double sandwich will make that hollow leading
edge a little stiffer and the glass over the top of the filler may
preclude those cracks in the filler made by the depression of that skin
on the forward D section.
Just my opinion.
Bud
----- Original Message -----
From: Graham Singleton<mailto:grahamsingleton@btinternet.com>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Monday, February 02, 2009 1:55 PM
Subject: Re: Europa-List: Wing Walk Tape
<grahamsingleton@btinternet.com<mailto:grahamsingleton@btinternet.com>>
ALAN YERLY wrote:
> If you add carbon fiber rather than E glass, it will make it even
> more stiffer as the carbon will make the layer nearly rigid..
>
> I do not know without building a panel if the carbon is more
effective.
>
> Bud.
Bud
trouble is the carbon will take all of the load because of it's very
high modulus, that will tend to create a sharp discontinuity in
strength
at the edge of the carbon.
Graham
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