DG-300/303 owners...

Earlier, Steve Davis wrote:
You have described a $20K repair on a product
which might not be worth $20K right after the
repair.

I assume that you're replying to my post of 1:39 today - though it
doesn't appear so in the Google view of r.a.s. Please let me know if
otherwise.

Yes, in the right hands that might be a $20K repair - I know a lot of
folks who can manage that, and I'm sure they're salivating over the
fallout of this situation. But if there's lots of them to do, a
relatively modest investment in tooling can easily cut the costs down
to around a third of that, possibly less. For example, it's virtually
a no-brainer to build a scarf-router to precisely mill out the
required chunk of wing spar. And the pre-fabbed skin repair panels are
easy, the layup is dirt simple and takes about an hour to do four;
with cure cycles you could probably yield 8 per day from a single wing
mold set.

I remember when the G103 team was here fixing spar spigots. There was
a lot of hand-wringing and gnashing of teeth around a repair that
requires hacking huge chunks out of the spar stub. But in the end,
they just lined them up and churned them out, and when they were done
they all fit fine, looked like new, and no problems since.

And, yeah, if it was some dogmeat glider only worth $20K last month,
this might all be moot. But the fact is that DG300 have typically
commanded around twice that, sometimes much more for young and well-
equipped examples.

My suggestion for the rods was to try
a $2K or less repair which would allow current
users the peace of mind to continue flying their
gliders.
My understanding of the DG 300 is that
it has a very stiff wing, presumably even with
undulations in the spar caps.

A for effort, but I think that the modulii mismatch between the
Graphlite and the fiberglass is probably too big to make it work
practically, even if the DG is reputed to be "stiff." The axiom of
such things is that as much as you might prefer it otherwise, stresses
are transferred not to the strongest parts but to the stiffest. If you
put any Graphlite into the spar, its stiffer modulus will make it try
to take on all of the stress, or fail its glue bond trying. So you'd
basically have to put in enough Graphlite to take all of the load, and
scarf it in shallow enough to yield enough bond area to get all the
load out of the fiberglass into it. And when you're done with that,
the surgery is likely no less traumatic than if you'd just done a
repair-manual scarf.

Another thing to consider is that if you execute an innovative repair,
you are probably on the hook to validate it with test it to
destruction, or at least to the somewhat-draconian EASA standard of
6.3*1.725. Whereas with a textbook repair, you may be justified in
only testing to design load or not testing at all.

Since Graphlite
ships in two and three ft. dia. spools it must be
plenty flexible and it is far stronger than the
equivalent amount of fiberglass rovings.

Yes, that's the way we've been buying Graphlite ribbon (not rod) for
the HP-24 and Glidair projects. Its flexibility is relative, though;
with a Young's Modulus of about 23 million it is much stiffer than the
equivalent profiles of pultruded fiberglass and is in fact 15% to 20%
stiffer than the equivalent hand-laid carbon tape or roving. For new
glider wing spar designs I think it is certainly the greatest thing
since sliced cheese, but it doesn't always play nice with others.

My
suggestion would be to saw several kerfs of
varying lengths and depths through the undulation
area extending for some length on either side of
the area possibly to the end of the spar stubs.
Graphlite rods could be epoxied into the kerfs,
like rebar in concrete, and they would take the
load from the rovings they butt up against.
I suppose fiberglass cloth could be wrapped and
epoxied around the spar butt to prevent the rods
from popping out, if that could happen, but I think
you would have a much stronger than designed spar
with a very stiff wing.

As I wrote, that might work, but it inflicts trauma on the same order
as the textbook repair, involves a pretty big modulus mismatch, and
has no track record. I'd like to see it tried, though, it'd be an
interesting experiment.

I have heard of someone cutting
a kerf in wooden spars and putting in the Graphlite
rods
to improve the strength.

Yes, I've heard of that too - I think it was on a Bowlus Baby
Albatross.

Thanks, and best regards to all

Bob K.