On 7/1/2012 8:23 AM, JJ Sinclair wrote:
Remember the crack in the engine mount that failed and brought down the
airliner? How about the B-52 that taxied onto the runway, applied full
power and the left wing fell off! This all started as a crack near the spar
after an air-refuling mishap. Would you fly a wood sailplane with cracks in
th skin? No way, don't walk, run away from that puppy! Would you fly an
aluminum ship with cracks in the skin? That old girls been rode hard and
put away wet, right? Fear of cracks is in our DNA. Remember; Step on a
crack and break your mothers back?

So now you find a crack in the skin of your fiberglass sailplane. Bad news,
right? Actually no. Fiberglass sailplanes are covered with a thin layer of
rock-hard gelcoat that was placed over a flexible structure. I remember the
DG-400 at Minden, that had been flown extensively in wave conditions. It
was literally covered with cracks. The wings had cord-wise cracks every
half inch on both sides of both wings. This ship was flying regularly and
was considered airworthy.

Yeah, but I got a crack coming from the corner of my spoiler box, is my
wing going to fall off? Nope, when your wing skin was laid up in its mold,
the fiberglass cloth wouldn't fit tightly into the corners and around the
edges of your spoiler box, so filler and extra gelcoat was applied all
around the spoiler box to allow the cloth to smoothly overlap the box. The
corners of the box are stress concentration points and cracks will quite
likely appear there. How deep do these cracks go? All the way through the
gelcoat and filler, but they stop when they reach the fiberglass cloth
because they are gelcoat cracks migrating IN from the rock-hard coating,
NOT cracks in the fiberglass migrating OUT!

Once again, this is just my humble opinion, but it was formed after 40
years of grinding out your cracks and finding no structural issues. :) JJ


Thanks for bringing your "street cred" to this arena, and, for having the
intestinal fortitude offer an empirical, repair-based opinion, JJ! Even if it
gores FUD-based oxen...

FUD = Fear Uncertainty Doubt

My own aerospace-engineering-degreed opinion/conclusion mirrors yours. (FULL
DISCLOSU 1) I never made my living in the airplane structural-analysis
field; 2) the following discussion assumes "first generation glass" ships,
simply because they're the "floppiest" of the composite birds, due to the
relative lack of stiffness of glider-specific, structural fiberglass compared
to carbon. What follows blends critical-thinking and empirical observation,
underlain by a reasonably decent engineering understanding of the materials
involved and typical physical properties. It is a GENERAL discussion. The
devil is always in the details. YMMV, of course...)

There are LOTS of 1st-generation, non-carbon-reinforced, gliders out there in
used glider land. Likely, most have experienced gel-coat cracking at various
times in their lives, regardless of whether they originally arrived with "the
good gelcoat" or "the less-good gelcoat." Probably, by now, most have had
gelcoat cracks at some time in their lives.

The key element - as JJ noted - is the underlying structure is MUCH more
flexible than is any sprayed-in-mold gelcoat. Think chocolate-coated vinyl bar
stock. Yeah, it's an awful analogy, but you get the idea...what's underneath
will bend - without breaking - far beyond what unmelted chocolate will
withstand in its crack-free state. Want another analogy? Think plastic paint
stirring stick. How do you clean 'em once paint on 'em has dried? If you're
lazy like me, you simply bend 'em back and forth to crack the paint film, then
peel. I've never yet broken a plastic paint stirrer.

Gelcoat (or paint or any other coating atop the glider's structure) is present
for essentially 3 reasons: 1) aerodynamics (maximizing laminar flow runs
requires a smooth surface); 2) looks (few people would purchase an un-coated
composite glider even if it was the laminar equal of competitors, because to
most eyeballs, uncoated would look "unpretty"); and 3) UV protection (UV
degrades essentially everything!). Rank 'em in whatever order is important to
you...

In a nutshell, there tends to be two schools of thought concerning gel-coat
cracks. One tends to be FUD-based, one does not.

If cracks per-se concern you, then limit your future-ownership-searches only
to ships in pristine, uncracked condition. Be prepared to pay accordingly.

If you're comfortable with input such as JJ's and the thought process
underlying posts as this, your selection will be considerably larger, the
asking-price range considerably less exotic, and ship performance
little-degraded, in sport XC terms.

Dick Johnson had a saying: "Air has fingers, but no eyes." He meant exterior
looks were unimportant viz-a-viz surface smoothness, when considering ONLY
laminar flow. He also very kindly measured clean and "buggy" performance
numbers for just about every 1st-generation ship you might find for sale out
there. Number freaks - and many wannabee-XC pilots - obsess over the
differences; for all practical purposes few weekend sport pilots will ever
have the ability to detect 'em. *Discussing* numerical performance differences
is great fun, but of little real-world effect on one's ability to go XC and
have huge amounts of fun doing so.

Summarizing - the preceding mostly addresses issues arising from the relative
stiffnesses of 1st-generation-glass glider *structure* vs. that structure's
protective coating.
- - - - - -
What follows seeks to address two concerns - oft expressed - implying: 1) a
direct causal possibility that gel-coat cracks propagate directly INTO the
fiberglass, and 2) crack-enabled UV degradation is imminently life-threatening
to Joe PIC.

Some things to bear in mind: 1) ALL the structural fiberglass is 100% encased
within the resin matrix; 2) any NON-structural fiberglass is similarly
encased; 3)I've yet to hear a plausible theory for how a gelcoat crack can
propagate across the interface into the underlying (relatively
soft/non-brittle) resin substrate; 4) "all-fiberglass" composite ships don't
routinely suffer from fractured wings, regardless of gelcoat condition; 5)
cracks that don't propagate into the substrate will not allow any UV to
propagate either; 6) 1st-generation glass ships are designed to stiffness
criteria, not strength criteria.

That last is significant to the extent that 1st-generation composite glider
wings are considerably stronger than they need to be in a pure G-load sense,
simply because were they not, no one would buy the ships because their
(flutter-limited) Vne would be ridiculously/unusably low. I suspect somewhere
on YouTube is a video clip or two of German flutter tests of gliders. Even to
paid test pilots, the footage is impressive!

But back to "propagating cracks" and "the UV concern"...

Propagating cracks - Ask any experienced glider repairman how many crack
situations they've seen that they believed to have propagated DOWN/into the
structure from the gelcoat, as distinct from the other direction. If I read JJ
correctly, his answer appears to be "Zero." When I asked another well-known
western glider repairman the same question, his answer was, "Zero." Further he
knew of zero gliders relegated to the scrap heap from UV degradation...as
distinct from "refinish cost/relative value" considerations. His glider
build/repair experience then spanned nearly 3 decades.

My conclusion is the cracks you need to worry about do NOT come from routine
assembly/flight loads.

I know "UH" sometimes is on RAS, and would welcome him sharing his experience
in this matter.

UV - Pretend you know of a 1st-generation glass ship missing "huge areas" of
gelcoat atop both wings...meaning, UV CAN directly access the structure. In
time (years? decades? testing definitely required...) the structure would
degrade to where strength reduction would become measurably detectable. That
said, I've never seen such a "structure-exposed" ship since first setting eyes
on a composite sailplane in 1972.

If you happen to know of or own such a ship, simply spraying a UV barrier
would eliminate the UV issue (though the ship would still look cruddy and pay
an aerodynamic penalty from surface roughness).

My conclusion is "the UV concern" is in fact a non-issue with the slightest
application of common sense.
- - - - - -

My bottom line?

If it's good for your soul, limit yourself to, and do your best to propagate,
crack-free gliders. They ARE beautiful!!!

Meanwhile, take care of any glider you're responsible for to the best of your
ability...but don't lose sleep over gelcoat cracks you know are not directly
due to a structural overstress condition.
- - - - - -
Anecdotal tale...

Back in the early '90's my Club had a G-103A not get signed off for an annual
because the (unfamiliar with glass gliders) A&P "got nervous about all the
wing cracks." And there were LOTS of them. The small-radii leading edges had
beaucoup spanwise cracks from root to tip, and, from nose to several inches
back, on top and bottom. Both spoiler boxes had at each corner long cracks
radiating well over a foot in length. The remainder of the top surfaces had
various long, random, straight-to-arcing chordwise cracks bunched in various
spanwise locations. Both bottom surfaces, from root to nearly tip, had closely
spaced, mostly straight, chordwise cracks from nearly the leading edge to
nearly the trailing edge.

JJ explains above why spoiler box corners typically radiate cracks. The
differences between top and bottom surface chordwise cracks were likely due to
the undersurface being mostly in tension vs. compression for the top gelcoat.
The leading edge cracks were likely a result of gelcoat and filler being
thicker in that region.

The shop quoted $12K for partial gelcoat-removal/inspect/refinish of the
wings. By the time Club-labor sanded-to-inspection-depth the entire surfaces
of both wings (unsurprisingly, no "into-resin-cracks" found, anywhere), shot
on requisite, thin, pin-hole filling polyester "stuff" (I've forgotten what it
was, and we later sold that ship for one with a higher payload, despite it
getting slightly lighter due to our work), and had an auto body shop shoot
both wings with polyurethane, we had less than $4K into the refinish. Looked
great. Flew the same. Nary a crack ever showed through the polyurethane.

Again, YMMV. Written from the perspective of someone who believes information
trumps FUD...

Bob W.