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  #31  
Old 01-10-2018, 01:02 PM
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,154
Default Judoka question

Sika thickness is referenced frequently here to be desired at 1/8". I don't know how that is validated nor the Temp range corresponding. Perhaps someone has talked to Sika ????
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RV-4 fastback in process w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #32  
Old 01-10-2018, 01:46 PM
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WA85 WA85 is offline
 
Join Date: Apr 2010
Location: Huntsville, AL
Posts: 168
Default

I have three cracks in my RV-8 canopy after 315 hrs. I used Sika Flex with a 1/8" bond gap and sanded all the edges with 1000 grit sand paper to remove any stress risers. I did all this in a 100 deg garage in during a sweltering Alabama summer. I used a stock Vans canopy, which had about 1" gap between the plex and the aft portion of the canopy frame "hoop", starting at the canopy frame aft cross over brace and going equally around the aft end of the canopy frame hoop to the other side of the brace. Despite all sorts of machinations of trying to reduce the gap, I ended up clamping the plex to the frame with 1/8 spacers and then sika flexing it together, which forced the plex to fit despite having such a large gap. It wasn't a month later until the first crack showed up after an accidental ding in the plex, it cracked instantaneously. It was obvious that where the plex was under stress, it was going to be prone to cracking. I put a fiberglass Targa strip over the first crack and forgot about it. One day while flying faster than I should have in moderate turbulence with the A-P, I experience an a bit of wild yawing rolling event and added a second 4" crack, 1" in front of the cross over brace on the RH side....which got stop drilled and covered with a NASA meatball. About a year later, another 4" crack showed up 1" aft of the cross of over brace on the LH side....which got covered up with a US Flag decal. From what I have seen on other RV-8s and my own experience, most cracks happen around the aft cross over brace of slightly forward of it. A possible solution is to trim and re-fit the canopy frame hoop aft the cross over brace so that there are no gaps between the plex and the frame hoop. It seem that making the perimeter of the hoop about 1" wider around ends would help. Not sure if anyone has done this, but when I replace my plex after it cracks to the point of not being airworthy, I plan to do just that. I have heard all pros and cons of riveting vs Sika Flex, but in the end I think most RV-8 canopy cracks are due to the poor fit / pre-load of the plex...regardless of riveting or Sika Flex, which with a little enticement, will crack.
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  #33  
Old 01-10-2018, 02:29 PM
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chrispratt chrispratt is offline
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Posts: 796
Default A related suggestion

Hopefully this is not too far off topic, but 5 or 6 years ago, after hearing of a rash of cracked canopies that occurred in cold weather with the aircraft parked in the hangar, I decided that when my RV-8 is parked I would not completely close the canopy, i.e. I do not latch the canopy when parked in the hangar.

I figure there's no need to put stress on the canopy when it is just sitting especially with the rapid changes in temperature we can experience in Texas. Perhaps it has helped, I have no way of knowing for sure, but so far no cracks. If you currently are latching your canopy when securely parked in a hangar, consider leaving the canopy just slightly open. It may make a difference.

Chris
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  #34  
Old 01-10-2018, 04:16 PM
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rv8ch rv8ch is offline
 
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Location: LSGY
Posts: 4,317
Default floating canopy

Scott - how did you get your canopy to float in the air like that? Are you an illusionist on the side?
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  #35  
Old 01-10-2018, 04:43 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
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Quote:
Originally Posted by Larry DeCamp View Post
....have Jeff ( airplane plastic) stress relieve it. That removes initial stresses just to fit in the frame....
Larry, can you elaborate a bit about this?

If I understand what you did, you put the canopy into a fixture as it would be when it's complete, with the same forces and stresses and deflections, and then shipped this off to Jeff, who did something (?) to it and returned it, still in the fixture.

Thanks,
Dave
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  #36  
Old 01-10-2018, 04:55 PM
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chepburn chepburn is offline
 
Join Date: Apr 2006
Location: Ottawa , Canada
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Default

Quote:
Originally Posted by AlexPeterson View Post
I believe there is one zero missing from each coefficient.

4130 Steel: about 7 x 10E-6
Acrylic: about 40 x 10E-6

Both with in/in/R units.
I have fixed the results in the first post. It changes things.

A LOT.

Thanks for the correction Alex.
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  #37  
Old 01-10-2018, 05:10 PM
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,154
Default Thread drift to answer David, hopefully useful !

Using a wooden form that defined the bubble perimeter and top surface at panel, roll bar and fastback. Jeff blew the canopy to conform to these objectives. As stated here by Scott, this method ( not a female mold ) will usually over low the side constraints attempting to achieve depth dimensions. I wanted straight sides conforming to the top longeron profile AND some straight vertical wall at the fastback to match the natural evolution of the fastback cone. Of course, all this can be achieved with clamps and static stress levels when you Sika as noted previously. So, I screwed steel angle to the wooded form to constrain the plexi canopy where I want it. Took it to Dayton. While it was in the oven for 1hr at 290F, went to lunch with Jeff( name dropping here), and when cool, the overblown curves were as straight as Angle iron. Jeff has the fixture now for RV4. E mail me for picks and I will forward. decamplarry@gmail.com
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  #38  
Old 01-10-2018, 07:36 PM
Aluminum Aluminum is offline
 
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Location: San Jose, CA
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Quote:
Originally Posted by chepburn View Post
Except..... my canopy is still fine... and so are thousands of others..... So I have to conclude that there is some basic error in my calculations.. but, that being said, Danny's idea is still a good one.
Error or not, one simple piece of actionable advice I learned from your analysis is to not fit the canopy when it's hot!

The ideal temperature of all parts when drilling or gluing would be closer to mid-40s, given the temperature range they'll be exposed to when flying.
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  #39  
Old 01-10-2018, 07:43 PM
Kyle Boatright Kyle Boatright is offline
 
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Location: Atlanta, GA
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Quote:
Originally Posted by Aluminum View Post
Error or not, one simple piece of actionable advice I learned from your analysis is to not fit the canopy when it's hot!

The ideal temperature of all parts when drilling or gluing would be closer to mid-40s, given the temperature range they'll be exposed to when flying.
Exactly. Factory guidance has us heating the plexi so it is less brittle and doesn't crack during installation. This causes problems when the canopy is exposed to much cooler temperatures during use.
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  #40  
Old 01-10-2018, 08:24 PM
David Paule David Paule is offline
 
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Location: Boulder, CO
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Thanks for the description, Larry.

Dave
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