I'd like to place some skin-damping material between the cockpit floor panel and the belly skin on my RV8. For those not familiar with the structure, 8's have a "double floor" of sorts. The space between the floor and the belly skin tapers front to rear; it is about 2 inches just behind the gear boxes and tapers to zip at the spar. Some owners have developed skin cracks in this belly panel. Vibration no doubt, but exact cause/source debated. There's a good thread elsewhere on that subject (the cause, noise, and soundproofing) relevant to all RV models. Let's keep this one specific to injectable damping materials.
Why injectable? The area is fully assembled with a QB fuselage. No access into the enclosed space. No way to bond in a conventional damping sheet. Same is true of course if retrofitting a flying 8.
It would be a simple matter to drill a few small access holes in the floor and inject an appropriate material. The top candidates are pour-in-place foam, silicone/RTV, urethane rubber compounds, or even proseal. The foam would fully fill the space and bond the skin and floor in shear. The various "rubbery blob" materials might damp gross skin vibration much like a fingertip on a drumhead. Don't know which approach is better.
The lightest might be the foam, perhaps 2 to 4 lb density. However, some urethane-based foam has been shown to cause aluminum corrosion (Google "urethane foam aluminum corrosion"). Seems you can eliminate the problem by first coating with an epoxy primer, but that's hard to do in this case. If your airplane is slow-built and epoxy primed before assembly, you're in luck.
There may be aluminum-safe urethane foams. Apparently there are also polyester based foams which don't cause aluminum corrosion. More research needed.
I've cast some urethane rubber parts for past projects. Mixes like proseal, less viscous, cures to a known Shore hardness.
Silicone/RTV may also have a corrosion issue in some cases. Not sure about that, and also not sure how well it will cure in the enclosed space.
Ya'll know about proseal <g>
Anyone with thoughts or information about specific products?
Why injectable? The area is fully assembled with a QB fuselage. No access into the enclosed space. No way to bond in a conventional damping sheet. Same is true of course if retrofitting a flying 8.
It would be a simple matter to drill a few small access holes in the floor and inject an appropriate material. The top candidates are pour-in-place foam, silicone/RTV, urethane rubber compounds, or even proseal. The foam would fully fill the space and bond the skin and floor in shear. The various "rubbery blob" materials might damp gross skin vibration much like a fingertip on a drumhead. Don't know which approach is better.
The lightest might be the foam, perhaps 2 to 4 lb density. However, some urethane-based foam has been shown to cause aluminum corrosion (Google "urethane foam aluminum corrosion"). Seems you can eliminate the problem by first coating with an epoxy primer, but that's hard to do in this case. If your airplane is slow-built and epoxy primed before assembly, you're in luck.
There may be aluminum-safe urethane foams. Apparently there are also polyester based foams which don't cause aluminum corrosion. More research needed.
I've cast some urethane rubber parts for past projects. Mixes like proseal, less viscous, cures to a known Shore hardness.
Silicone/RTV may also have a corrosion issue in some cases. Not sure about that, and also not sure how well it will cure in the enclosed space.
Ya'll know about proseal <g>
Anyone with thoughts or information about specific products?