On Apr 18, 12:30 pm, Steve Davis
wrote:
'....and the pilots who fly them regard' might need
to be changed to 'the
pilots who flew them regard' unless an inexpensive
method of inspecting
and repairing them is developed by someone.

Hmmm... I wonder who that "someone" might be. Whoever they are,
they're pretty brave to get wrapped up in this mess.

On the topic of inexpensive, that will have to be relative at best.

From correspondence and conversation with various engineers and
composites technicians, it appears that the scarf ratio for composite
repairs is determined by the ratio of the shear strength of the epoxy
to the tensile strength of the fibers. For a spar repair in E-glass,
it seems to come out on the order of 40:1, and perhaps 15% greater for
S-glass, let's say conservatively around 60:1.

My guess, based on my experience with wet fiberglass layups, is that
the degree of fiber "ondulation" will vary linearly through the depth
of the spar cap. That is, the worst "ondulation" will be at the
extreme outer fibers of the spar cap, and that there will be no
ondulation at the inner (last laid) fibers, and half way through that
depth the ondulation will be half as bad as the worst. The shame of
that is that the extreme outer fibers of a cantilever beam are the
ones with the greatest stress.

Anyhow, if the "ondulation" varies as I guess, part of the inspection
and repair process will be to assess what degree of "ondulation" is
acceptable, and how much spar cap has to be ground away to get to
acceptable fiber.

Suppose, for example, that the "Ondulated" fiber were to extend down
through 8mm of spar cap. Then you (or, more likely, the repair tech)
would have to grind out a scarf that extends spanwise through
8*60=480mm, call it a half a meter of span plus probably the full
length of the spar butt, call it a full meter. After grinding that
out, you'd have to build up the material removed by laying in new
straight rovings.

After executing the spar scarf, you'd have to repair all the
collateral damage inflicted on the wing skin when trenching down to
the spar. Probably the easiest way to do that would be with a
prefabricated patch panel, made in the original wing mold, that
encompasses the sandwich directly over the first half-meter of spar
plus 50mm or so chordwise fore and aft of the spar. The repair tech
would fit this patch panel, splice the inner skin, and execute an
outer skin scarf around the perimeter of the patch panel. After that,
gelcoat, sand, and polish to hide.

That is just my own half-informed guess at what the spar repair
entails. Your actual mileage has already varied. The response to my
own emails to DG has been on what I would call the chilly side. Their
position on this matter seems to be holding firm as follows:

* Supplementary explanations of the problem and surrounding issues
(such as the one I posted earlier) are unhelpful, since the
explanation posted on the DG Web site clearly addresses all aspects of
the issue. Beyond that, only "experts" are qualified to understand the
problem.

* Photos of affected spars are unhelpful because only "experts" are
qualified to read them, for everyone else they are just frightening.

* The inspection must be performed by DG-trained workers.

Regarding some of the repairs I've seen suggested, such as splinting
the spar with Graphlite rod, I think that those are non-starters at
best. I think that the only reasonable repair schemes are those that
restore the structure to its as-designed strength and stiffness.
Repairs that substantially alter the stress distribution through the
structure could well cause other unknown and unexpected problems.

Thanks, and best regards to all

Bob K.
Disclaimer: I'm the guy behind:
http://www.hpaircraft.com/hp-24

I'm an amateur - don't try this at work!