One of the bugaboos of acrylic canopies is fastening these massive—and flexible—components to rigid canopy frames when the thermal expansion coefficients differ significantly from metal. Cracks often develop in tightly drilled holes secured with screws or rivets—so adding some form of flexible canopy fasteners is a good idea if you can. Oversized holes are one way to mitigate the issue; adding a rubber grommet to an even larger hole is another. In the case of the screws holding the windshield to our F1 Rocket roll bar, the build manual recommends using #6 screws, Tinnerman washers, and a rubber grommet made from soft automotive vacuum line.
The hose is cut into washers approximately as thick as the Plexiglas, and the holes in the glass are upsized to about 5/16 inch. The roll bar (steel) is drilled and tapped for the screws. The screw/washer/grommet assembly is installed, and the soft rubber grommet deforms to fill the void. This allows the Plexiglas to expand and contract without touching the rigid screw, preventing stress buildup that can lead to cracks.
Figuring out ways to lower the loads on your canopy is a great way to help it last the life of your airplane!
Paul, does the hose also form sort of a gasket for weather resistance?
In my opinion, not really – it might prevent water from getting in at the screw hole, but wouldn’t do anything between the holes. And in the example shown, these are windshield screws that will be covered by a laminated-in-place composite fairing that can’t be removed without destroying everything – and that will seal the joint much better anyways.