In reply to (re-ordering for clarity's sake)...

On Tuesday, August 30, 2016 at 7:20:26 PM UTC-4, Bob Whelan wrote:
On 8/30/2016 2:42 PM, It was written:
Aerodynamics isn't the issue - gelcoat failure can progress until it's
a structural issue.

In the spirit of seeking genuine knowledge - and readily acknowledging
widespread *concern* that cracking/flaking gelcoat can "somehow" morph
into a "premature" structural issue - can anyone cite solid information
(data, links, etc.) shedding light on the mechanism(s?), location and
quantity of UV-exposed plastic sailplanes thrown onto the garbage heap
after failing structural tests, etc.? It's a serious - not rhetorical -
query.

There's plenty of engineering data, as well as
"common-sense/observational data", around indicating UV is a catholic
attacker/degrader of all manner of materials, but have sailplane
manufacturers, or (say) the LBA, or anyone else, set about obtaining such
data for GRP/CRP sailplanes?

Hard data eagerly welcomed!

Bob W.


....On 8/30/2016 8:11 PM, Papa3 wrote:
Failed gelcoat absolutely "prints" down into the glass/epoxy substrate.
We've just refinished two gliders in our club which were left to go beyond
minor crazing to full-on gelcoat failure. Once the gelcoat is totally
stripped, you're left with visible lines in the glass/epoxy. Under a 4x
loupe you can see that there is an edge to these lines where there are
small voids in the epoxy.

Long-ago memory sez I/we saw similar "lines" in?/atop? the first cloth layer
beneath cracked gelcoat on our club's G-103 wings that we refinished
(mid-90s-ish). At the time, and under the supervision of a highly-experienced
plastic glider repair person/DER (with "hand-in-hand" signoff authority of an
FAA-blessed A&P w. IA), I believe their working conclusions (after loupe-aided
inspection) we a) the lines were in/on the epoxy only and did not extend
into the cloth; and b) the outermost cloth was a thin, fine-weave, layer
intended less for structure than to provide a "relatively smooth" underlayer
for the gelcoat to interact/bond with.

Does this materially affect the integrity of the wing/fuselage/control
surface structure? That's beyond my pay grade.

Gerhard Weibel's take is as follows (from the Schleicher Technical Note on
"paint cracks"): Owing to the UV-radiation the gel coat of the paint
surfaces grows brittle and shrinks; at the same time the UV-light destroys
paint ingredients. So moisture (rain, dew) working in on long term will
wash the decomposed paint ingredients out off the paint. The paint starts
chalking and gets hairline cracks owing to the concurrence of
embrittlement and shrinkage. Furthermore, these hairline cracks gather
dirt which through its aggressive effect and its stronger heating-up from
sun radiation further precipitates the degradation of the paint. Owing to
this the intended protective effect for the fiber composite structure
against moisture and UV-radiation is no longer granted.

Certainly a good care with hard wax can slow down the above process
distinctly, but it cannot be stopped completely. For this reason a
repainting of the aircraft will always become necessary at some point of
time. However, we point out explicitly that paint cracks - even deep cracks
- do not represent damages to the aircraft structure if as of their first
appearance immediate correct maintenance and care is given furthermore to
the aircraft. As all the outside skin of the aircraft is dimensioned for
stiffness, there are no critical mechanical strength problems, even if some
cracks have gone down into the fiber composite structure and have already
attacked the resin matrix base. The unknown ageing effects caused by the
influence of moisture and UV on the unprotected fiber composite structure
are more dangerous.

P3


Thanks for Dipl. Ing. Waibel's thoughts! Opinions from those with his
background and experience ought not be taken lightly.

As I interpret the above, his thoughts seem in alignment with our thoughts
"from the 90's." Paraphrasing: taking care of a ship's exterior finish is a
good idea if extending the life of the factory-applied finish is a concern.

The last two sentences of his second paragraph passingly touch upon the
question raised in my earlier post. I infer from the closing sentence, that at
the time his thoughts were put on paper, he was no more "structural life
informed" then than I seem to be today...though he also (and, almost
certainly, rightfully) had serious respect for the power of UV to eventually
degrade the structure...key word being "eventually."

I'd love to see GRP/CRP glider-based engineering data allowing
"even-semi-informed" inferences to be made as to whether "eventually" is (say)
one year, or (say) 10 years.

My interest stems simply from being by nature disinclined to worry about
"stuff" that can be safely ignored and that isn't simultaneously "good for
one's soul." If it's good for one's soul and pocketbook to "keep after" a
ship's factory finish, have at it! But don't agonize over structure if it's
unnecessary. Life's full enough of real worries without inventing imaginary ones.

Bob W.