On Jul 2, 5:55*am, Terry Walsh wrote:
"Probably the worst culprit is the
Grob 102/3

Despite all their other reported issues Grob is famous for having one of
the more durable Gel coats Don.

Terry Walsh

At 12:26 02 July 2012, Don Johnstone wrote:







It appears that a major cause of cracks in gelcoat is the difference in
the
expansion coefficient of polyester gel and epoxy resin/glass. For this
reason my club discourages the practice of taking a glider into air which
has a temperature of less than -20 deg C. Giving the airframe a cold soak
and then returning it to warmer air quicly will cause substantial cracks
but only in the gel.
Having said that the gelcoat used on the Slingsby Kestrel did not seem as
baddly effected as many other gliders. Probably the worst culprit is the
Grob 102/3

At 11:18 02 July 2012, John Firth wrote:
One of the best, most informative and authorative threads
I have read; reassuring!
John F
An old, no longer bold pilot. (PIK 20E)

At 23:21 01 July 2012, BobW wrote:
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

...

read more »- Hide quoted text -

- Show quoted text -

Yup. Grob gelcoat seems to be about the best I've seen. I'm partner
in a Janus C that had gelcoat literally blowing off of the top of the
wing. The bond had failed so completely that I didn't sand the
remaining gelcoat off, I SCRAPED/CHIPPED it off using a pnuematic
scraper (see the terrifying video on YouTube "Removing Failed Gelcoat")
(sorry, I can't provide a link right now). 5 gallons of PCL
PolyPrimer, lots of sanding, and gallons of PPG concept, followed by
more sanding and buffing. If it still looks good in 5 years, I'll
declare victory.