Acrylic thickness
A couple more complicating factors:
The thickness of acrylic sheet stock as it comes from the manufacturer can vary slightly across it's surface. Part of our business is making acrylic aircraft windows (including RV12 rear windows for the local builders btw) and for say a typical 3mm thickness we have seen occasional variations from about 2.8mm up to 3.2 in the one sheet. Generally not a problem because many of the windows we make are relatively small so we can avoid any issues. But for the bigger canopies who knows, this might be contributing to the cracking problem.
Something else to consider is that when a canopy is blown, especially the longer narrow types such as the RV8, there is a lot of stretch and thinning of the acrylic happening at the highest point. Starting off with say 3mm acrylic could easily see it reduced to around half that thickness in some cases (think of inflating a balloon). Add in slightly thinner than spec acrylic to start with and this could go some way toward explaining why some installations fair better than others.
I always cringe a little when I think of the compression forces being applied when rivets are used to directly secure acrylic. Not disputing that many get away with it for years on end but it's not ideal. With bolts many hundreds of psi can easily be applied when over-torquing nuts and bolts. That's why good practice is to tighten to a firm fit and then back off a full turn of the nut. It only has to be lightly tightened. Also the hole in the acrylic should be half as big again as the bolt diameter with each bolt centred in the hole i.e. 4mm bolt - 6mm hole. This is another disadvantage of rivets because they try to expand to fill the hole as they are pulled.
Personally I have gone with Sikaflex for my canopy and screen. Trying to give myself the best shot at a trouble free installation. Time while of course tell!
Clive Whittfield
Auckland
New Zealand
A couple more complicating factors:
The thickness of acrylic sheet stock as it comes from the manufacturer can vary slightly across it's surface. Part of our business is making acrylic aircraft windows (including RV12 rear windows for the local builders btw) and for say a typical 3mm thickness we have seen occasional variations from about 2.8mm up to 3.2 in the one sheet. Generally not a problem because many of the windows we make are relatively small so we can avoid any issues. But for the bigger canopies who knows, this might be contributing to the cracking problem.
Something else to consider is that when a canopy is blown, especially the longer narrow types such as the RV8, there is a lot of stretch and thinning of the acrylic happening at the highest point. Starting off with say 3mm acrylic could easily see it reduced to around half that thickness in some cases (think of inflating a balloon). Add in slightly thinner than spec acrylic to start with and this could go some way toward explaining why some installations fair better than others.
I always cringe a little when I think of the compression forces being applied when rivets are used to directly secure acrylic. Not disputing that many get away with it for years on end but it's not ideal. With bolts many hundreds of psi can easily be applied when over-torquing nuts and bolts. That's why good practice is to tighten to a firm fit and then back off a full turn of the nut. It only has to be lightly tightened. Also the hole in the acrylic should be half as big again as the bolt diameter with each bolt centred in the hole i.e. 4mm bolt - 6mm hole. This is another disadvantage of rivets because they try to expand to fill the hole as they are pulled.
Personally I have gone with Sikaflex for my canopy and screen. Trying to give myself the best shot at a trouble free installation. Time while of course tell!
Clive Whittfield
Auckland
New Zealand