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Canopy Replacement with Screws

RhinoDrvr

Well Known Member
Hello all,

Looking to engage the VAF brain trust on this one. I'm currently replacing the canopy on my RV-8 due to multiple (7) cracks in the plexiglas. I've been using the procedures outlined by Danny King and Paul Dye https://vansairforce.net/community/showthread.php?t=87210 in a thread from 2012. So far, I have successfully removed the old transparency with minimal damage, and have cleaned up the frame.

The problem I've discovered is that the original canopy was attached with #6 screws threaded into riv-nuts in the tube steel frame. As seen in this photo, the rib-nuts have a flange that I am concerned will induce a stress riser on the new plexiglas canopy, and cause another crack.

I've brainstormed 3 different courses of action going forward, but wanted to pick the VAF braintrust. Below are the options I've thought of, but I'd like to hear everyone's thoughts, even if (especially if) they're different than the options I've considered below.

Courses of Action:

1. Sika-Flex. I've been leaning against this option, just because I like the idea of mechanical attachment, and I know that historically Vans hasn't endorsed the Sika-Flex method. If that has changed, and Vans endorses the Sika method, I'd be open to giving it a shot. Also, I have wondered about the long term durability of the Sika given UV exposure. Not intending to start a Sika-Flex vs Rivets debate, but this is why I haven't already glued the canopy on and moved on with my life.

2. Oversize holes and screws. I've considered drilling the holes in the plexiglas up to 3/8" ( to ensure that they clear the .330 flanges on the Riv-Nuts), then using silicone tubing of 3/8" OD and 1/8" ID to serve as a bushing. Cut the bushings slightly proud of the plexiglas, and secure the canopy with screws. The hope is that the bushings would squish, securing the canopy, as well as keeping the plexiglas from interfacing with the Riv-Nut flanges given that it will flex with temperature changes.

3. Oversize holes and rivets. The other option is to remove the canopy skirt, and punch the Riv-Nuts out of the canopy frame. I'm not sure what size hole this would leave in the steel tube, but I'd likely need to drill the canopy for the same size 3/8" oversized hole, then use a soft aluminum pull-rivet into the holes in the canopy frame (which are oversized due to the Riv-Nuts). This gets rid of the flange interaction problem completely, but I don't know if I can find aluminum rivets that are large enough to work in the holes in the canopy frame.

So, the questions I have are;

1. Which of the 3 options (or other options I haven't considered) do you think would be best.

2. Does Vans endorse Sika-Flex as a canopy attachment method?

3. Is edge distance a concern with the 3/8" holes in the plexiglas? If so, I'd need 9/16" of edge distance, which seems like a lot of extra plexiglas. Not sure if I'll have that much available.

4. For Option 2: Is there a good source of silicon tubing with a 3/8" OD and 1/8" ID that is pliable enough for this application? Is a durometer of 50A appropriate for this application?

Thanks!
 

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If it was me I would glue it.
I’m not at that stage in my build yet, but that’s my intention.
I know there were debates about how strong and UV stable Sika-Flex can be, but many aircraft have been flying for many years, with thousands of hours under their collective belts, with bonded canopies. I know of none that have lost a canopy due to it being bonded. (Happy to be corrected on that!) it’s not a guarantee that you won’t get a crack in the future. However, to me, it reduces the chances of introducing stress risers with the drilling of holes.
 
At least one Sikka Crack

https://vansairforce.net/community/showthread.php?t=179925
https://vansairforce.net/community/showpost.php?p=1411053&postcount=25

Sikka might be a good option if it was always thick enough to shear, maybe 1/4" all around. Actual thickness TBD from an analysis. But thin application approaches no shear layer so the sikka or any other bonding method with a resilient material looses its ability to solve the problem. Based on this it would seem -just a guess - that a mechanical joint- soft to shear, but hard orthogonal to the surface to retain the canopy would fit and function better.

Appeal: We have lots of engineers with FEA skills. It is a wonder we have not gotten this systemic problem addressed yet. One of a few. It would be a good engineering project for a college team to address.

Sorry, I am a 7 guy and have some tiny crack forming around my fasteners too. Ironflight seems to have done a lot, maybe the most, research on this for the 8.

Good luck and please post the journey and create a good some good documentation for others.
 
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I’ve done this exact thing on my RV6 canopy (still under construction). To protect the plexiglass from the edge of the rivnut shoulder, I put a thin nylon washer over the rivnut shoulder with an ID of 3/8” (I think), so that the rivnut is inside the washer. This allows the plexi to ride on the nylon washer instead of the metal rivnut shoulder. I used #6 screws, but the hole in the plexi is opened up to 3/16” with just enough countersink to swallow the dimple in the skin, or in my case, the countersunk #6 SS washer. Screws are only tightened to finger tight. I put a little clear silicone in the hole before inserting the screw. My thinking is that this will give the screw a sort of ‘rubber bumper’ in the enlarged hole, so that the plexi won’t actually touch the screw, and gives the canopy a little room to flex with temperature changes. It also helps keep the screw in place. When you tighten the screws you want to make sure you are not putting any load on the plexiglass. If you can see the plexi deform, even slightly, by way of reflected light, that is putting a load on the plexiglass right at that hole. And make sure you polish the edges of your canopy and the countersunk holes before assembly. I got some of this advice from Jeff Rogers, owner of Airplane Plastics, the supplier of Vans canopies. He is fairly close to me (near Dayton), and I’ve been up there three times getting new canopies.

On mine I have slight gaps between the canopy frame and plexiglass in places because of spacers needed to make everything line up on my slider canopy, and also on my windshield. Plus there is about a 3/32 gap where the rivnuts hold the plexiglass off the frame slightly. These areas will all be filled with Silpruf - a siliconized adhesive, used primarily in window glazing on buildings and boats. It is what most Glastar builders use to attach their windows. It has excellent long term adhesive properties, and stays sort of rubbery, so the plexiglass can give a little. I haven’t done this step yet, but I’ve seen it done by a couple local Glastar builders, and it seems to work very well. Plus it looks good if done right.
 
I used screws on my -6 canopy but drilled and tapped the frame rather than rivnuts. Also used #6 screws and countersunk all. I dimpled the sheet metal to fit and used Tinnermans over the front bow. You could offset the holes if you want to drill and tap or weld the holes which would allow even more threads.

That said, if I did it again, I'd probably go Sika.
 
Canopy to frame bonding

I would recommend to use the Sikaflex bonding system instead of using screws.
On my -8, I used the Sikaflex 295 VU and the primer 206 G+P.
I attached the canopy by two screws one on each side at the rear of the frame just for correct location of the plexiglass on the frame and the rest bonding was by the Sika bonding system.
More details can be found here: https://industry.sika.com/content/dam/dms/global-industry/b/Pre-treatment_Chart_Marine_e.pdf

Ariel Arielly
RV8a - 4XOAA
Israel
 
+1 for Sikaflex

The stress on the RV-8 canopy due to temperature is extreme and is mainly from front to back rather than side to side. Jon Thocker related his experience with a canopy crack several years ago: He took off from his home base in southern Ohio on a cold winter day and climbed to 11,500 feet. He HEARD the canopy crack over the engine noise and observed a crack from side to side right over his head. The crack opened to about 3/8 inch wide. He returned to home base and put the plane in his heated hangar and observed the crack close up tight!

My canopy cracked in a similar fashion and I installed a new one with Sikaflex, filling a gap of about 1/8 inch between canopy and frame all around. No further problems.
 
Screws for me

I also used screws. about 300 hours, no issues including flying in 100+ and near freezing temps. Drill and tap. Put the heavy rubber type electrical tape between the canopy and the plexi. Put dimpled aluminum strips on the outside for the screw heads to bear against. Plexi is oversized drilled and countersunk.

RV-6A Slider
 
My 7A has been flying with a SikaFlex canopy for 14 years. I live in Michigan so it has seen a wide range of temperatures. I see absolutely no degradation in the Sika whatsoever. The primer is protection from UV exposure.
Highly recommended.
 
The first snag

So, I’ve got the new canopy out of the box, and the “nest the two canopies together” step isn’t working out as neatly as hoped. The front where it is flexible is almost perfect, but further aft (where most the canopies crack) the new canopy is significantly wider than I can get the old plexiglas to spread.

I assume it will be more flexible once I cut the flange off, but is this the expected amount of delta between the new and old canopy? Moving it forward and aft doesn’t yield any better fit than this, and the windscreen portion about matches what’s currently fit on the aircraft.
 

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Sika

I have replaced canopies on three RV-8s, using sika after the original was installed with mechanical fasteners. My advice is to use sika, making sure you have enough margin - gap between frame and plexi - to allow shear movement. The sika will flex, but it is a function of thickness, and a narrow gap will not allow for the necessary amount of movement. You might find it necessary to reduce the bow length to keep the frame-to- plexi margin, while getting the canopy to fit to the windshield. I would shoot for a 3/16 margin.
 
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