gel coat

On Friday, September 27, 2019 at 4:16:17 PM UTC-7, 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

P3 - I was the owner of one of the Vegas that crashed (N4312B, in 1987 in Hemet) - the root cause was elevator flutter resulting in failure of the elevator linkage. No overspeed, no structural failure, just flutter followed by loss of pitch control causing the pilot to bail out (unfortunately at low altitude, with serious injuries).

Cheers,

Barry

Charlie Quebec's AN posting is very informative. Thanks for posting.

Crazing Gelcoat is just like a wife who drinks too much.
You've got to get rid of it and its going to cost alot of money.

The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

Crazing Gelcoat is just like a wife who drinks too much.
You've got to get rid of it and its going to cost alot of money.


Nick:
More like a stalking ex-girlfriend. If you don't do it right it's going to come back.
ROY

On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

The AN is an interesting read--and an alarming one given the warning that:

"Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
....
(4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
(5) Transfer into and through the foam layers of sandwich skins.
(6) Transfer from skin layers into spar caps."

However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:

"It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"

That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?

Chip Bearden
JB

On Monday, October 21, 2019 at 3:37:59 PM UTC-4, wrote:
On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

The AN is an interesting read--and an alarming one given the warning that:

"Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
...
(4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
(5) Transfer into and through the foam layers of sandwich skins.
(6) Transfer from skin layers into spar caps."

However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:

"It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"

That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?

Chip Bearden
JB

I can see no basis to an expectation that a crack propagating into an outer skin would continue through a core foam and then start again through inner skin.
I've seen imprinting of gelcoat cracks in the outer layer of skin laminate, but have not seen one that progressed beyond the outer(usually 110) layer.
The real message is that if the glider is in gelcoat failure(deep cracking, cupping, peeling) real attention needs to be paid.

FWIW
UH

I concur with Uncle Hank’s assessment. After grinding off all gelcoat, I have seen what looks like a ghost of the crack that was there, but close examination with strong light and magnification, showed some resin degradation in the outside layer, but I could not see broken fibers. That said, I believe deep cracks should be ground out, filled, contoured and re-finished.
Refinishing the whole ship is the hardest work I have ever done, my hat’s off to anyone willing to take on that nasty job!
JJ

I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

On Wednesday, October 23, 2019 at 1:45:51 PM UTC-7, Charlie Quebec wrote:
I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

What, exactly, is "dry penetration testing?"