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  #21  
Old September 26th 19, 04:11 PM posted to rec.aviation.soaring
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Default gel coat

On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote:
Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a √©crit¬*:
To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.

p3


These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it).

These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface.

Bert
Ventus cM "TW"


If the structure is not compromised we fill the "million pinholes" using polyester surface filler applied with a foam roller. This works filler into the voids and avoids the bridging that results if filler is sprayed.
Laminating another layer, without peeling the top ply,can result in undesired shape change, and when sanded smooth, can leave defects that still require detail filling.
FWIW
UH
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  #22  
Old September 26th 19, 05:59 PM posted to rec.aviation.soaring
Papa3[_2_]
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Actually no - these are not the normal voids. Being a sergeant in the "pinhole patrol army", I'm very familiar with these voids.

The pitting I'm talking about is 100%, directly related to the failure of the gelcoat. The crazing lines literally transfer down into the outermost layer of the substrate. When you put a 6x loupe on the crazing lines after the gelcoat is sanded off, you see that there is a) slight discoloration and b) tiny chunks of epoxy coming out of the glass matrix.

I have pictures, but not under magnification. Trust me - the key guy who looked at this is a materials scientist who works for one of the top electron microscopy vendors specializing in materials failure analysis.

Cheers,
P3



On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote:
Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a √©crit¬*:
To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.

p3


These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it).

These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface.

Bert
Ventus cM "TW"


  #23  
Old September 27th 19, 01:40 AM posted to rec.aviation.soaring
Charles Longley
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How many wing failures have composite gliders had over the years?
  #24  
Old September 27th 19, 04:04 PM posted to rec.aviation.soaring
Roy B.
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On Monday, September 23, 2019 at 11:50:30 AM UTC-4, BobWa43 wrote:
Is gel coat crazing purely a cosmetic problem or does it pose a threat to the structural integrity of the underlying fiberglass?.


About 28 years ago I refinished a glider (an ASW-17) with Simtec Prestec - which was then a somewhat new product for glider refinishing ( it was originally sold as a coating for radomes). The glider is now in Canada. Can anybody tell me today how that product has held up over time (compared to the original Vorgetlat and Schwabelack)?

ROY

  #25  
Old September 27th 19, 08:01 PM posted to rec.aviation.soaring
Bill G
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Excellent Question
  #26  
Old September 27th 19, 09:32 PM posted to rec.aviation.soaring
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I have a 1988 AS-W20 that had modifications done (tailwheel fairing, Canopy refit) that required recoating with Prestec. They are essentially perfect today. No crazing or yellowing.
  #27  
Old September 27th 19, 10:58 PM posted to rec.aviation.soaring
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On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
How many wing failures have composite gliders had over the years?


Exactly.
  #28  
Old September 28th 19, 12:16 AM posted to rec.aviation.soaring
Papa3[_2_]
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On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
How many wing failures have composite gliders had over the years?


A fair number, but I can't think of any that were due to normal flight. For example, at least 1 or 2 Slingsby Vegas came apart, but I believe both of those were due to exceeding design limits. Same for a Zuni. A fair number of others came apart due to flutter.

P3
  #29  
Old September 28th 19, 01:02 AM posted to rec.aviation.soaring
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Default gel coat

On Friday, September 27, 2019 at 7:16:17 PM UTC-4, Papa3 wrote:
On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
How many wing failures have composite gliders had over the years?


A fair number, but I can't think of any that were due to normal flight. For example, at least 1 or 2 Slingsby Vegas came apart, but I believe both of those were due to exceeding design limits. Same for a Zuni. A fair number of others came apart due to flutter.

P3


A control surface that has been compromised(easy to do given light structure) could lead to flutter due to reduced stiffness and you could lose a ship..
Glider structures are over built in some places for durability, but not everywhere.
UH
  #30  
Old October 9th 19, 01:37 PM posted to rec.aviation.soaring
Charlie Quebec
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A very good AN on this subject, including expert studies showing propagation of cracks into epoxy layer. Worth a look.
http://www.doc.glidingaustralia.org/...100&Itemid=133
 




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