Building, testing and operating an experimental aircraft

Showplanes Fastback: Canopy Bonding

13 Aug 2014

After the big Oshkosh show, Dad and I got busy on the Showplanes canopy. We had done plenty of prep work prior to this day, including bonding together the components of the frame and cutting and drilling the plexiglass bubble.

Bubble Drilled and Clecoed Bubble Drilled and Clecoed

Unlike the stock RV-8 canopy, the Showplanes canopy frame is designed to be bonded to the bubble, rather than riveted, although many builders are now strictly bonding the stock canopy as well.

There are drill holes in the plexi with which to cleco everything together while the bonding agent cures, while also allowing some epoxy to seep through and create a mechanical joint as well. Every other hole is countersunk for this purpose.

Pro-Set Adhesive Pro-Set Adhesive

For the bonding agent, we decided to use one of the excellent PRO-SET Epoxy adhesive products, used throughout the marine and aerospace industries. I had only heard of one other guy using this on RVs, but Scaled Composites uses PRO-SET in their layups, so that was a good enough testimonial for me.

Actually getting the stuff, however, was another matter. Either none of the standard aircraft tool suppliers sold it, or the HAZMAT shipping costs were outrageous, or the lead times were on the order of weeks. We ended up ordering from the East Coast boating supplier Jamestown Distributors and received great service, reasonable prices and quick shipping. Highly recommended.

PRO-SET ADV-276 PDF Datasheet PRO-SET ADV-276 PDF Datasheet

The product we selected was the PRO-SET ADV-176/ADV-276 super-toughened adhesive epoxy, as it had the mechanical properties we desired for the plexi-to-fiberglass bond. Since the Showplanes canopy has no mechanical fasteners, we wanted something with considerably better adhesion properties than the Sikaflex 295 UV that many builders use to bond their canopies. Comparing the data sheets, we found that the ADV-276 has slightly more than ten times the lap shear stregth of the Sikaflex, making it the clear winner for this application.

DIY Adhesive Applicator DIY Adhesive Applicator

To dispense the PRO-SET adhesive, you can either buy an expensive dual-component applicator, or you can make your own out of scrap wood and a pipe clamp. Dad decided to go with the latter. I was highly skeptical at first, but in actual practice the DIY Dispens-o-lator worked just fine.

Laying Down a Bead Laying Down a Bead

After preparing the surfaces of the canopy frame and the plexi, Dad started to lay down a bead of the epoxy while I followed behind with an epoxy spreader. The PRO-SET system comes with a neat epoxy mixing wand that takes care of the proper mixing ratio for you. Yellow resin and blue hardener combine to make a nice emerald bead, and you know you’ve got the right mixture.

Not Unwieldy at All Not Unwieldy at All

As you can see, the DIY Dispens-o-lator is not unwieldy at all, and is as simple to use as a real professional applicator – haha.

Spreading the Adhesive Spreading the Adhesive

We tried to lay down a thick enough bead so that the epoxy would spread to cover all of the mating surfaces, and also flow through the holes in the plexiglass. We put a bead on both the fiberglass canopy frame and the plexi as well, to ensure we got good coverage.

Clecoed for Curing Clecoed for Curing

Finally we mated the plexi bubble to the frame and clecoed them together for curing.

Five Hours to Pre-Cure Five Hours to Pre-Cure

We left the clecos installed for about five hours to give the epoxy a chance to pre-cure, then removed them and let the assembly set up overnight.

Epoxy Cured and Ready for Fiberglass Epoxy Cured and Ready for Fiberglass

The next morning the epoxy adhesive had cured and was ready for sanding and the fiberglass overlay. More on that later.

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