Eric Greenwell wrote in message ...
I wasn't clear enough. The problem I thought would be eased was the
ultrasonic inspection problem. So, is the spar cap attached to the
outer wing skin, or is there actually foam between the outer skin and
the spar cap?



Is the spar cap on the Duo Discus bonded to the inner skin or the
outer skin? If it is bonded to the outer skin, it would not have the
foam to go through, which should ease the problem considerably.

The problem is that the bond in question attaches the top spar to the
web. Any problems with this bond will drastically affect the spar
strength by allowing the top spar to peel off the web. In the DG-style
of assembly with carbon cloth between the spar and the web the bond
between the spar and the cloth is equally as critical as that between
the web and the cloth.

In the model world we prevent spar peel failures by assembling both
spars to the web and then wrapping to lot with Kevlar thread or an
epozy-wetted woven carbon tube before building the wing round the
complete spar. This gives a 30G wing. I accept that this is overkill
for full size, but my teeth still itch a bit at the lack of any
binding round both the spars except on the protruding stubs at the
root.

I flat out would not fly a glider with foam between the web and the
top spar and would hope that such a structure has never been made or
flown.

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I don't think anyone said that there is foam between the spar cap and
shear web. Foam within the shear web adds a great deal of stiffness,
however.

The spar cap of German designs is built into one wing skin while the
spar web
and opposite spar cap is bonded into the other skin. When the two wing
halves are mated the one spar cap is glued to the spar web and cap of
the
other wing half. It's pretty much a blind operation and everything had
better well match or there will be a poor or no bond.

The Genesis 2 and the LAK-17a both have the spar completely built
outside the
wing. The spar is totally wrapped in glass fiber insuring it's
integrity.
It will not debond. Following is a site which has pictures of a
Genesis spar.

http://www.aviation-salvage.com/airc...enesis%202.htm

On the Genesis wing it appears that the spar is bonded to an inner
skin which is a foam sandwich with an outer skin. The Genesis spar in
the picture had a metal fence post cut through it and it looks like
Graphlite rods have been bashed into more of a round shaped bundle. I
believe they are spread out into more of an I-shape than the picture
would suggest. At least with this design, if the spar is not bonded
to the skin there is still a very strong spar.