Gelcoat Crazing and Structural Integrity

[Ken Kochanski (KK)]

Hi,

My first glass glider was a cheap, but well-crazed, 15 meter ship that
I flew for 5 years before trading up to a newer model.

Other then potential performance issues (if not sanded smooth) ... do
we have any cases where moderate/heavy crazing led to damage to the
underlying glass ... which grounded the ship at annual - or which
caused an in flight failure.

If you have crazing, but sand and wax the ship occasionally - does
this provide sufficient protection - say for 5 years - until you got
around to refinishing.

thanks,

KK

[JJ Sinclair]

On Mar 5, 2:21*pm, "Ken Kochanski (KK)" >
wrote:
> Hi,
>
> My first glass glider was a cheap, but well-crazed, 15 meter ship that
> I flew for 5 years before trading up to a newer model.
>
> Other then potential performance issues (if not sanded smooth) ... do
> we have any cases where moderate/heavy crazing led to damage to the
> underlying glass ... which grounded the ship at annual - *or which
> caused an in flight failure.

Hi Ken,
I have been grinding out gelcoat cracks for 35 years and never found
any of them that didn't stop at the fiferglass structure. On ships
with thick gelcoat (DG's) the 'working' back & forth on each side of a
large crack can leave an etched mark in the fiberglass structure, but
examination with a magnification glass shows no broken fibers, just
resin that has flaked away in the emmediate area adjacent to the
crack. I have found that painting on a fresh coat of resin will make
this 'etched' make disappear. I know of no inflight failuers caused
from simple gelcoat cracks. I believe it's a good idea to have your
local glider repair guy take a look to be sure your crack isn't a tell-
tail indicator of underlying structure that has moved (failed).


> If you have crazing, but sand and wax the ship occasionally - does
> this provide sufficient protection - say for 5 years - until you got

Yes

> around to refinishing.
>
> thanks,
>
> KK

[Bob Whelan[_2_]]

JJ Sinclair wrote:
> On Mar 5, 2:21 pm, "Ken Kochanski (KK)"
> wrote:
>> Hi,
>>
>> My first glass glider was a cheap, but well-crazed, 15 meter ship that
>> I flew for 5 years before trading up to a newer model.
>>
>> Other then potential performance issues (if not sanded smooth) ... do
>> we have any cases where moderate/heavy crazing led to damage to the
>> underlying glass ... which grounded the ship at annual - or which
>> caused an in flight failure.
>
> Hi Ken,
> I have been grinding out gelcoat cracks for 35 years and never found
> any of them that didn't stop at the fiferglass structure. On ships
> with thick gelcoat (DG's) the 'working' back & forth on each side of a
> large crack can leave an etched mark in the fiberglass structure, but
> examination with a magnification glass shows no broken fibers, just
> resin that has flaked away in the emmediate area adjacent to the
> crack. I have found that painting on a fresh coat of resin will make
> this 'etched' make disappear. I know of no inflight failuers caused
> from simple gelcoat cracks.
KK's question is an interesting one indeed (to me, anyway!). And JJ's
considerable anecdotal experience matches what my own engineering
background (aerospace - essentially ME with some extra finite element
and fluid dynamics thrown in) leads me to conclude...at least about
pre-carbon-reinforced FRP gliders. In essence, the FRP structure is a
relatively flexible, load-carrying, substrate 'prettily covered' by
relatively inflexible, weak, gel coat. (Imagine climbing rope covered
in chocolate.) Which material is more likely to crack? What mechanism
exists to propagate cracks from the stiff/weak material across the
boundary into the flexible/strong material?

Logic aside, massive amounts of FUD (Fear, Uncertainty, Doubt) permeate
the glider pilot and the A&P/IA communities. Happily (as owner of an
experimental, 1st-generation FRP ship, who has put all but 30 hours on
the airframe), I haven't been forced to actively deal with FUDmeisters.

OTOH, I/my club *has*. We had (~10 years ago) a G-103 fail annual for:
1) cracks radiating from the corners of spoiler boxes; 2) spanwise
cracks along the sharp-radiused leading edge bits; and 3) chordwise
cracks along some portions of the lower wing surfaces. The larger
portions by far of both wings were uncracked. The A&P/shop were
'relatively structurally unfamiliar' with glass gliders; they provided a
$12k bid to remove/repair-as-necessary *only* the cracked regions.
Implicit was any *real* structural cracks would result in considerably
higher costs. And, being a club ship, the possibility someone(s) had
actually overstressed the glass could not be dismissed out of hand.

Spurred by cheapness and theoretically-based suspicion, after obtaining
commitments of technical guidance from a German-trained glider repair
guy and a 'signoff-IA' familiar with his talents/work, I helped convince
the club/members to provide grunt labor ourselves. About 4 months later
(overwinter), we had 100% de-cracked, urethaned wings and a re-annualled
G-103; total cost ~$3000. Not a single crack had propagated into
glass...IOW, add one anecdotal experience to JJ's.

I believe it's a good idea to have your
> local glider repair guy take a look to be sure your crack isn't a tell-
> tail indicator of underlying structure that has moved (failed).
>
At the risk of putting words into JJ's mouth, experienced glider repair
people tend to be comfortable doing such precautionary checks using the
non-invasive 'tap method' assisted in some cases with selective gel coat
removal. Point being, that the mere presence of gel coat cracks on
1st-generation FRP ships is NOT sufficient reason to
panic/ground-the-ship/throw-massive-sums-of-money-and-labor-at-it/etc.

The reason for the above's '1st-generation FRP' proviso is
carbon-reinforced/all-carbon ships are stiffer, reducing the relative
stiffness mismatch between the structure and gel-coat. They're also
newer, with less history to draw upon. If you own and/or know the
history of such a crazed bird, and have any issues (your own doubts, or
someone else's FUD input) with them, and your finances aren't unlimited,
take it to 'someone JJ-like' for assessment be*fore* assuming the worst.

(I am not a professional glider repairer, have never played one on TV,
nor have I ever met/worked-with/spoken-with one named 'JJ'. It simply
goes against my grain to spend glider-bux in the manner of Jimmy
Buffet...i.e. foolishly! That noted, I thank JJ for having the courage
to publicly state what too often gets the 'skeleton in the family
closet' treatment. FUD forever!!!)

Regards,
Bob - Flame Suit On - W.

[Eric Greenwell]

JJ Sinclair wrote:
> On Mar 5, 2:21 pm, "Ken Kochanski (KK)" >
> wrote:
>> Hi,
>>
>> My first glass glider was a cheap, but well-crazed, 15 meter ship that
>> I flew for 5 years before trading up to a newer model.
>>
>> Other then potential performance issues (if not sanded smooth) ... do
>> we have any cases where moderate/heavy crazing led to damage to the
>> underlying glass ... which grounded the ship at annual - or which
>> caused an in flight failure.
>
> Hi Ken,
> I have been grinding out gelcoat cracks for 35 years and never found
> any of them that didn't stop at the fiferglass structure. On ships
> with thick gelcoat (DG's) the 'working' back & forth on each side of a
> large crack can leave an etched mark in the fiberglass structure, but
> examination with a magnification glass shows no broken fibers, just
> resin that has flaked away in the emmediate area adjacent to the
> crack. I have found that painting on a fresh coat of resin will make
> this 'etched' make disappear. I know of no inflight failuers caused
> from simple gelcoat cracks. I believe it's a good idea to have your
> local glider repair guy take a look to be sure your crack isn't a tell-
> tail indicator of underlying structure that has moved (failed).

Schleicher has a "Maintenance Instructions" for paint cracks here:

http://www.alexander-schleicher.de/tm/allgWA/WA_Lackrisse_E.pdf

Some highlights:
* all cracks should be inspected regularly
* paint cracks, even deep ones going into the resin top skin, may not be
an immediate problem
* steps must be taken to prevent further damage from moisture and UV
* paint cracking is a problem in the USA and Australia, but not in
Europe, and the repair experience accumulated in those countries should
be considered.

It seemed to similar to what JJ was saying, but from the manufacturer's
viewpoint and experience. Perhaps Schmepp-Hirth and DG have similar
instructions, and these might be useful to show an inspector balking at
approving an annual inspection.


--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

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* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[Markus Gayda]

Eric Greenwell schrieb:

> Schleicher has a "Maintenance Instructions" for paint cracks here:
>
> http://www.alexander-schleicher.de/tm/allgWA/WA_Lackrisse_E.pdf
>
> Some highlights:
> * all cracks should be inspected regularly
> * paint cracks, even deep ones going into the resin top skin, may not be
> an immediate problem
> * steps must be taken to prevent further damage from moisture and UV
> * paint cracking is a problem in the USA and Australia, but not in
> Europe, and the repair experience accumulated in those countries should
> be considered.

LOL - sorry for the "lol", but when i hear that paint cracking is no problem in
Europe... then Germany must be on the moon.
Most older ASK-21s (ours are from 198x and 1990) are showing bad cracks. The
first got already a new gelcoat and the 1990 one is due any year now.

Not for cosmetic reasons but for protection against moisture - here in Germany
it sometimes rains..... :-)

The old Mini-Nimbus we have (one of the last ones build) did get a new gel-coat
in 2000. And that was REALLY neccessary. You were nearly able to hurt yourself
on the cracks.

So Schleicher is telling total BS there.

just my 2c... had to get that out....

CU
Markus

[BB]

Schleicher has a "Maintenance Instructions" for paint cracks here:
http://www.alexander-schleicher.de/tm/allgWA/WA_Lackrisse_E.pdf

another interesting tidbit:

" we suggest to spray the sanded surfaces first with polyester
fillers, to sand then again, and to respray them finally with a white
paint system on a polyurethane basis.."

i.e. ditch the gel-coat.

John Cochrane