I think that what breaks canopies is attaching the plexi to the frame under load. In other words, if you have to pull the plexiglas bubble down to meet the frame, you are attaching it under a load. And eventually you are susceptible to a load induced crack. Usually happens at the attachment point, and commonly during OAT temperature extremes. I don’t think it matters what your attachment method is. Sika, or some other “glue” might spread the load out a little, but it is still susceptible to eventual cracking if attached under load, as evidenced by several Sika installed canopy cracks that we have seen. Let’s face it, the steel tube frames we use are considerably ‘variable’ as is the bubble. Many of us have spent countless hours trying to make these flimsy frames fit our canopy bubble. Making these two parts fit almost perfectly is difficult, but key in a stress free canopy install. The RV8 is probably more susceptible because of the length of the slider part of the bubble - much more expansion variation with temperature changes. Any load induced stress points are going to be exaggerated. Starting the attachment of the RV8 slider bubble at the aft end, and working your way forward will eliminate some of this misalignment stress (which I’ve talked about in previous thread), but not all of it. You can eliminate some of the remaining stress with the way you attach the plexi to the frame. If you use Sika, or preferably Silpruf, you might allow enough stretch in the adhesive to make up for the difference in coefficient of expansion. If you are using solid fasteners (I did on some of them), I would suggest screws vs pulled rivets. I used #6 screws and drilled a 3/16” hole (countersunk) for each one. This gave some “give” when the canopy expands more than it’s attachment structure. These screws were installed barely finger tight with a countersunk washer, and the screw was surrounded with some clear RTV to offer some ‘bumper’ protection between the screw threads and edge of the hole in the plexi.
In the four RV’s I’ve built this way, I’ve never had a canopy crack on an installed RV airplane. I’ve talked several times with Jeff Rogers, owner of Airplane Plastics (they supply canopies to Vans, and are near me), and these are things he recommends. Another thing he recommends - on SBS RV airplanes (RV6, 7, 9) with slider canopies, he doesn’t recommend any holes being drilled in the center frame tube on the slider frame - “nobody does that”. On my latest RV6, there was as much as a 3/16” gap between the center frame tube and the canopy bubble. Riveting, or using a screw would create a stress on the plexi at that point as well as the adjacent attach points. I used Silpruf to adhere this gap on mine by filling that gap. Silpruf was developed by GE to glaze windows on skyscrapers, and glass/plexiglass on boats. It’s pretty much the main thing used by Glastar builders to attach all their windows. It remains flexible after cure.
Having a canopy crack after your dream airplane is finished is one of the most distressing failures you can have. An engine problem isn’t good either, but you’ll pull the the engine and probably have a shop take care of it in most cases. A canopy crack issue will be on you, even if you didn’t build the airplane. And if you didn’t build it, you can still fix it if you want, and you don’t need any FAA inspection. It’s good to know what caused the crack/failure so it doesn’t happen again.
RV6/2001 built 2000/sold 2005
RV8 Fastback/2008 built/sold 2015
RV6/August 2022 build - Flying
Last edited by Scott Hersha : 05-04-2023 at 06:02 PM.