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  #61  
Old 01-23-2018, 09:51 PM
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fl-mike fl-mike is offline
 
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According to these guys, it was/is blown and had quite a few variations in shape over the production runs.
http://www.warbirdinformationexchang...hp?f=3&t=64878

It was no doubt thicker, but was first generation acrylic too.
So, who has access to a 51?
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  #62  
Old 09-16-2018, 03:24 PM
Gusmax Gusmax is offline
 
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Location: Sherwood Park, Alberta
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The 2 person sailplanes I maintain have huge canopy's on them and they are all rigidly glued to the composite frame with epoxy glue, just like the beautiful fastback kit being produced by Showplanes. I contacted Showplanes to inquire about cracking and this was their response; "There has been over a 100 sold and shipped to all parts of the world. I have not had any customer report back with an issue of cracking from cold or hot temps. " If you study the pictures from their website, you will see that they drill the RV8 canopy for clamping clecos and it looks like larger holes between the clecos to act as epoxy pins when it squeezes out.

Dan mentioned earlier that the epoxy/glass (not carbon) frame has a closer thermal expansion rate to acrylic than the steel frame. Maybe there is something to this??

Phil
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  #63  
Old 09-16-2018, 09:56 PM
BillL BillL is offline
 
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Quote:
Originally Posted by SgtZim View Post
Good discussion. . . If you oversize the holes and avoid contact and stress between two holes - i.e. the plex has shrunk so that the outsides of those two holes now just touch the rivets or fasteners, the next two holes in each direction have already had that slack taken up and those two holes will be under stress + the subsequent holes even more stress. To never have any loads imposed on the plex, you would have to support it on floating fasteners or something that has the exact same coefficient of expansion. I'm thinking you'd be better off with "tighter" fit between fastener and plastic so stresses are all taken up hopefully evenly and at the same time?
This is certainly a factor combined with the fact all the holes (and restraint) are near the edge.

Quote:
Originally Posted by scsmith View Post
If your strip has flaws along the edges that are representative of typical canopy edges, this is a useful test. I wonder if there may be an effect of non-uniform stress distribution. Try bonding a big square of plexi along one edge to the steel bar. This replicates the large portion of the canopy that is not strained by the frame.
Quote:
Originally Posted by AlexPeterson View Post
. . .
Bill's interesting test of attaching a strip of acrylic to a steel angle might have a different outcome if the strip were wider, yet still attached in the same way. The resulting force on the fastener will be proportional to how wide the strip is. Our canopies are really wide, as analogized to this strip test. . .
I really did not polish the edge and had a normal edge distance on one side, but there was 1+" of extra on the opposite side. A bit of stress analysis could say what the width vs force/stress really is, but definitely more than a narrow strip. Just happy I have aluminum frame on my 7.

Alex, nice picture there, I taught an applied mechanics lab as a grad student. I used photosensitive material and stretched the sample then used polarized light to reveal the lines of constant stress. All this way before FEA. The industry is light years more advanced now.

On another thought: the canopy might creep when hot and in compression, thus aggravating the stress increase when colder. This might happen more with steel than aluminum. I still wonder about external loading like latches.
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