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Scared to Squeeze

AAflyer

Well Known Member
ry%3D400

I just made The Big Cut on my -8 canopy (Todd's) and my hope that the back end will fit more snugly against the frame has been dashed. While the front end fits quite well, I still have a big gap about a foot forward of the aft end of the frame. Many of you are familiar with the squeeze pressure these clamps (pictured) exert, so can someone please advise me if I'm risking a crack by pulling the canopy to the frame? As you can see, even with the clamps in position, they only close the gap from about 1" to 1/2". Any suggestions to help me close this gap? "Just squeeze it, clamp it, and glue it?" "Whoa, don't force it!?", Heat gun? Build up the frame in this area? Spacers? :confused:
ry%3D400

ry%3D400

ry%3D400
 
I'm not a builder. And I didn't sleep very well last night, either.:rolleyes:

But I wouldn't want to build in any stresses in the plexiglas canopy, I'm thinking. Wouldn't that built-in stress just be a crack waiting to happen?

I'm sure a real plexi expert will chime in soon.
 
For what it's worth...

Suggestions - this is what I did... sorry I'm not an expert...

Work with the canopy upright with the frame on on "high" supports so you can stand inside...



Work in a warm shop with additional heat under the canopy... You'll be sweating under there, and not just from the stress...

The canopy will relax somewhat as it gets warm...

Don't use "big" clamps, your picture sends shivers up my spine. Use LOTS of little (weak) ones working from the front back...





You will hopefully find that the warmed canopy will slowly conform to the shape of the frame. It will take several iterations of clamping, adjusting up / down, side to side and re-clamping to get it to conform.

You will probably have to re-trim to fit several times...

Remember the spacers...

When you think it's ready to sika...

Leave it cool to room temperature to better equalize expansion between the frame and the canopy...

Then iff everything looks good, (it might move as it cools) sika the inside, between the spacers / clamps if they are in the way. Sorry no shots of this, my hands were pretty icky...



Once cured, remove clamps and final sika the inside...

Turn over and sika bottom...







Just started flying so no longevity report, but so far so good...

Good luck...
 
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I had exactly the same problem with my Vans Canopy. I forced it in place , glued and rivet it. But I cut the rivets so as not to apply to much pressure....a few monthes later as I was working on the skirt I a heard a deplaisant noise, and saw a 2 inches crack just where the canopy was forced to be in place.
 
Ron,
Thanks for your detailed response. I appreciate your 'shivers' …I did discover that using lots of little clamps really helps spread the pressure out, and my shivers went away. Today, after a dozen sessions, I've gotten the glass to it's final shape, I think. I noticed you painted the ducktail of your glass…seems rational, and might be an option go me, too, as I trimmed too much of the canopy from the aft end, and it doesn't quite reach all the way to the tip of the frame. i'll need to add some fiberglas here, I guess, when I figure out where the skirts fit.
 
Maybe you have looked at this, but sometimes you can bend the aft portion of the frame up to meet the glass rather than pull the glass in towards the frame. It all depends on how everything goes together.

Don
 
Actually that's not paint on the plexi "ducktail" as you call it, but an aluminium cover on the frame to try and keep the draft from the back of the canopy from coming into the rear seat. That and some strategically placed weatherstrip and some foam have worked pretty well.

Check out the first picture of my previous post.
 
I had the exact same issue with my canopy in the area where it tapers towards the back. I had about 1.5" gaps on both sides in that area. I added 1/8" spacers around the perimeter, which helped a bit to lessen compression forces and I used Sika Flex to glue it on the canopy frame and fill the gap. It seems that this is a common issue with the RV-8 canopy. After gluing it on, I had a crack appear about three months later, right in the middle of the area that I had to compress it the most. I ended up building a "enhanced canopy skirt" to cover the cracked area. I have not flown my project yet, so time will tell how well my glued, "stressed" canopy will work out. Seems that there are several RV-8 canopy cracks in that area.
 
I had to cut off a couple of inches of the tail end of the canopy, which dramatically improved the flexibility of the remaining aft portion. Use the skirt to get an idea of how much you can trim.
 
I can't tell from the picture but are you trying to pull the plexi in to the reduced section of frame? Once you get back to the weld the plexi shoulded touch the frame for several inches.


I just did a Todds with Sika and it conformed nicely.

If you post this question over on the -8 section there is a lot of experience watching.
 
Did it!

IMG_4201_zps1f724386.jpg


Not the neatest result, but plenty secure. I made the rookie mistake of trying to smooth out the fillet after pulling off the tape. Don't do that! Luckily, as advertised, the smeared Sika does NOT stick to unprimed areas, so these blemished were easily rubbed off with my fingers.
IMG_4222_zps42c7aad5.jpg

Now working on fitting the skirts. The aft end will need a little "coaxing" to fit snugly.
 
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I think the census is use lots of small clamps to check the load is spread. If you need a BIG clamp maybe some more trimming is required (particularly around the ribs). Above all please learn from my small mistake - dress every edge to a S M O O T H finish - my link explains why!
 
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