When it comes time to attach a plastic or fiberglass wing tip to the end of a metal wing, we sometimes grab a blind rivet to do the job. This same fastener that is used to rivet our metal skins together might not be the best for this purpose. Close examination of using these rivets to bond soft and hard materials together reveals some problems.
Backside of bulb-type rivets showing the “petals” expanded for grip.
When a blind rivet is pulled during installation, the blind end expands into a small ball that prevents it from pulling through the material. For strong materials like metal, this system works great. For softer materials like plastic and fiberglass, the expanding ball can pull through the hole, defeating the grip of the rivet. The force of the expanding ball can also crack some materials.
One method to prevent this problem is to use metal backing washers for reinforcement. This works fine as long as you have access to the backside of the material. Otherwise, you may need to glue these washers in place prior to rivet installation.
Look carefully at a bulb-type rivet, and you can see the “petals” that will expand from the side as the rivet is pulled.
Every builder should know there is a unique style of rivet that can alleviate the need to install washers when working with soft materials. These rivets are made entirely from aluminum, which makes them softer. But more importantly, the expanding ball is replaced with large protruding “petals” that provide a greater surface area for holding. The petals extend automatically during the regular riveting process.
Bulb-type rivets appear like normal blind rivets prior to installation.
These rivets can be found at McMaster-Carr with the name “Wide-Grip Blind Rivets for Soft Materials.” However, they are also known as bulb-style rivets, and this term works much better when searching the McMaster-Carr website. They are available in 1/8-, 5/32-, and 3/16-inch diameter sizes. You need to determine the grip length you want (the thickness of all materials to be held together) to arrive at the exact part number.
Keep a box of these handy, and next time you need to attach plastic, fiberglass, or rubber (engine baffling), you will be using a great fastener for the job!
As the founder of HomebuiltHELP.com, Jon Croke has produced instructional videos for Experimental aircraft builders for over 10 years. He has built (and helped others build) over a dozen kit aircraft of all makes and models. Jon is a private pilot and currently owns and flies a Zenith Cruzer.
I am sure the author probably already uses this idea, and for simplicity of the article just neglected to mention it. I would suggest as an additional “layer” of joint integrity that a flat washer be added to the back side of the mating surfaces prior to installation of the blind rivet. If you are in a completely “blind” situation – i.e. no access at all to the pulled side, not much you can do. But, if you have continued access during the process, simply add a washer prior to pulling the rivet. If you have access to the space before riveting starts, but not after, simply bond in washers (in any manner that holds them in place until the rivet mandrel “captures” them), and then finish the pulled rivet.
This would have been a great fix for my RV-10 airbox if I had known about these rivets last year. Several of the original set rivets had worked their way through the fiberglass and I repaired it using a backer plate. The wide-grip rivets would’ve made the repair significantly simpler.