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?

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.

Regards

Fred

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?

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.

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.

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

MNForrest said:

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

Forrest

Click to expand...

That stuff is pretty abrasive, and it will hold a bit of moisture through capillary action, but it wont absorb it into the fibers.

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.

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.

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

Injected foam data point

Aluminum boat manufactures fill non primed cavities with expanded foam. I have two different size Lund boats we mostly use on salt water. I have never observed any corrosion beyond light surface hazing. Lund Boats uses 5052 H34. I agree with others that potentially trapping moisture is a concern. Some of my ?still in the box? kit parts have corroded just sitting in the attic.

Moisture

The following is from the Cessna Pilot's Association and is a good lesson in foam-filled aluminum parts. Seems to me that the belly is a lot more likely to attract and retain moisture than a trim tab.

From 1960 thru 1984 Cessna used a foam core inside the elevator trim tab and elevator trailing edge of all models of the Cessna 210. A foam core was also used on the elevator trim tab and elevator trailing edge of the 1968 thru 1984 206 and 207 models. The purpose of the foam core was to act as a lightweight spacer to help the surface retain its proper shape. Because the foam used was a closed cell foam it was initially felt that moisture retention would not be a problem.

After these foam cored surfaces had been in use for many years it became apparent that moisture was a problem. What would happen is that for various reasons a section of the foam would get delaminated from the skin it was attached to. Moisture would get trapped in the pocket this delamination created and the skin would begin to corrode from
the inside out. By the time the corrosion became detectable as visible spots of corrosion, paint blisters, unsealing of the edges the damage was sufficient that the trim tab or trailing edge would have to be replaced.

John Clark
RV8 N18U "Sunshine"
KSBA

John,
That matches most accounts of corrosion I've read; the foam delaminates and moisture gets into the space. Apparently there is more to it than just water trapping. Some accounts discuss chemical leaching with some kinds of wet urethane foam.

Corrosion

DanH said:

John,
That matches most accounts of corrosion I've read; the foam delaminates and moisture gets into the space. Apparently there is more to it than just water trapping. Some accounts discuss chemical leaching with some kinds of wet urethane foam.

Click to expand...

Ah, the wonderful world of the unintended consequence!


John Clark
RV8 N18U "Sunshine"
KSBA

<<Ah, the wonderful world of the unintended consequence!>>

You bet.

There are supposed to be some foams that do not cause problems with aluminum. Brian may have given us a lead on one. Maybe I'll give Lund a call and see what they use.

If not foam, the GE 120 or similar seems like the best bet for injecting an enclosed space; just spots spaced out across the panel, maybe an 1/8" hole for each.

When I was in the Marines, we occasionally used an Epocast product called "syntactic foam" for repairs in bonded-aluminum-honeycomb panels. It was a 2-part mix that expanded to fill voids.

A brief Google search for "epocast syntactic foam" yielded several part #s for Epocast products (single, 2-part, and 3-part mixes), many with Bell Helicopter Textron, Boeing, Airbus, and/or Sikorsky materials specs/qualifications. Obviously, you'll need to do more research into it if you're interested. I'm sure it's an expensive product, but a dedicated aviation product might be the best bet for this application.

I don't know if this is relevant, but I offer it as food for thought.

F-15's used to have a honeycomb structure inside all the control surfaces and wingtips. The honeycomb was bonded to the skins. As the planes went to altitude, the air pressure differential between the inside & outside the structures, in combination perhaps with maneuvering loads and maybe even expansion/contraction caused some of the air in the honeycomb to find a way out. As the aircraft descended, the structure started sucking air back in. Repeating the process caused moisture to accumulate and when several adjacent honeycomb cells filled with water the skin would pop loose in the affected area when the water froze at altitude.

Anyway, the point is that climbing and descending can lead to moisture accumulation where you don't expect it.

Also, I would be a little concerned with expanding foam as it can distort structures when injected into confined spaces.

I was at AutoZone buying primer tonight and I happened to notice a couple different spray cans of what I guess is rubber. One looked like bed liner, and another was similar but also mentioned noise reduction. I should have looked at them closer than I did, but both were from DupliColor.

Silent Running and Truck spray on Bedliners

Has anyone used this slient running stuff recently? Could it be used the the RV-12 and still be E-LSA compliant? On a related note, could you use spray on truck bedliner on the floors and in the baggage compartment?

I just started to think about this subject the other night when I flipped my fuse on it's side to deburr some holes in that area. This is also where the infamous 'cooling ramp' gets installed. This has been known to develop cracks. Most point to exhaust vibration as causing the problem and using turned-down exhaust tips seem to help a lot.
The Silent Running stuff sound interesting, but I see it is only rated to 250F. I don't know if that area gets hotter than that. And what happens if it does get hotter?

contractor foam

I know some of you will poo-poo this but I had a similar problem on my first plane (BD-4) and used some of that contractor foam in a spray can (find at home depot or lowes). I used it on unprimed 2024 T3 and 15 years later - no adhesion or corrosion issues at all. In fact this stuff is nearly impossible to remove from well prepared (clean) aluminum. when I was testing the only way I could get the stuff off was with a belt sander. I also put a blob of it in a can of water for a month and cut it open. The interior was bone dry.

For my -8 I've primed every part with epoxy so I'm comfortable using this stuff again. I think a few distributed dabs of this will be enough to dampen the vibes enough to minimize or hopefully eliminate any fatigue cracking.

FWIW,

Ken

RobByers said:

Has anyone used this slient running stuff recently? Could it be used the the RV-12 and still be E-LSA compliant? On a related note, could you use spray on truck bedliner on the floors and in the baggage compartment?

Click to expand...

I sprayed Silent Running on the floor and rearside firewall of my -8. I didn't feel comfortable using it in the ramp becuase of how it is sprayed, my ramp is already built, and I think a total fill would be much more effective. Haven't decided exactly what to do there yet as my ramp area and exhaust outlet is going to be highly unconventional. Am not flying yet so cannot report any results.

RobByers said:

Has anyone used this slient running stuff recently? Could it be used the the RV-12 and still be E-LSA compliant?

Click to expand...

Only if it is included in the plans.

<<I sprayed Silent Running on ......the rearside firewall of my -8.>>

You're in very risky territory. Remember, "self-extinguishing" means nothing when in contact with a 2000F firewall panel.

Postscript:

Silent Running publishes various data sheets on its website, inlcuding the results of test per ASTM Standard E162-02e, "Surface Flammability of Materials Using a Radiant Heat Energy Source".

It can be hard to find free web data about a particular standard, although the ASTM will sell you a copy for $43. In this case I got lucky and found an article describing the test proceedure. An 18" long sample is placed before a radiant heat source (think toaster oven elements), at an inclined angle of 30 degrees. The close end is 4.7" from the element. The element is regulated at a little less than 1250F. The test is run for a specified time period and the "flamability index" is mostly a matter of how far down the inclined sample flame is observed to progress. Put another way, how far away from the heat source must the sample be in order to become self-extinguishing?

In this case, all samples exhibited open flame to about halfway down the panel, and given the conditions we know the ignition temperature was less than 1250F.

If you must use Silent Running, apply it to the engine side of the firewall, not the cabin side.