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  #1  
Old 12-18-2008, 08:41 PM
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Default Aluminum-safe Silicone and Foam

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?
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  #2  
Old 12-19-2008, 05:18 AM
fred gassit fred gassit is offline
 
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Default

The stuff we use in our aircraft (commercial) is General Electric RTV 120 silicone adhesive sealant. The stuff is neutral cure (no acetic acid) and is used in electronics and aerospace widely. It comes in the standard tubes and is readily available.
The engineer I work with says any neutral cure sealant/adhesive is ok to use.

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Fred
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  #3  
Old 12-19-2008, 05:45 AM
lancef53 lancef53 is offline
 
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I would be real careful with anything silicone if you have not yet painted, unless you like the look of fisheyes in your paint job.

This sounds like a neat idea, I wonder if it would also help insulate for cold weather flight?
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  #4  
Old 12-19-2008, 07:18 AM
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I acquired some expired DC 3145 RTV, this is high dollar non-corrosive milspec RTV.

When I was doing my baffles, I bought some of the Permatex Copper High Temp RTV at the auto parts store, it was the same price as the regular Red high temp RTV.

The Copper RTV is labeled as "sensor safe" and does not have the acetic acid smell, so it's probably a good choice to use on bare aluminum.
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  #5  
Old 12-19-2008, 07:52 AM
Steve Sampson Steve Sampson is offline
 
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Dan, I would be nervous about doing that for another reason.

My concern would be that you would create an area that would hold dampness and encourage corrosion in that way. Clearly not a problem if every part is completely sealed.....but is it...


Just a thought. Steve.
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  #6  
Old 12-19-2008, 07:59 AM
MNForrest MNForrest is offline
 
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Default Go Pink?

Can you stuff the space with pink fiberglass insulation? Benign. Cheap. Wont hold moisture. Drill a hole at one end and start stuffing.

Forrest
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  #7  
Old 12-19-2008, 08:08 AM
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Quote:
Originally Posted by MNForrest View Post
Can you stuff the space with pink fiberglass insulation? Benign. Cheap. Wont hold moisture. Drill a hole at one end and start stuffing.

Forrest
That stuff is pretty abrasive, and it will hold a bit of moisture through capillary action, but it wont absorb it into the fibers.
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  #8  
Old 12-19-2008, 08:48 AM
MNForrest MNForrest is offline
 
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Default One more...

Would you need to fill the space completely? If you found an expanding foam that would be safe, could you just inject the foam in a few places, spaced evenly. You end up with "hockey-puck" sized foam vibration dampeners. Less worry about holding moisture.
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  #9  
Old 12-19-2008, 09:10 AM
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Default Proseal before riveting...

I know this does not help you at this point and I apologize. However, for those that may be cruising this thread and have slow build 8 fuselages there has been some discussion about using proseal between the skin/ribs to reduce or elliminate vibration oriented cracking. This was related to .016 elevator and rudder skins but the principle is the same.
I do not know if this would be practical on the quickbuilds or even possible at any stage of QB construction let alone where you are at.

My biggest concerns with any type of "gap filler" would be adhesion issues, if the void was not completey filled, or if it shrinks, and the stuff popped off it would no longer do the job, and moisture entrapment as you are most acutely aware.
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  #10  
Old 12-21-2008, 02:36 PM
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Dan

I recently used this on the firewall and floor of my -8 and it did do a good job of deadening the vibration in the skins (thump test). I can tell you it adhears very well to aluminum, because it is a real pain to scrape off the areas I did not mask so well. It's not a foam and may not be what you are looking for, but ..

http://www.silentrunning.us/products.html
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