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12-21-2008, 03:49 PM
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Join Date: Mar 2005
Location: Bainbridge Island, WA
Posts: 333
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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.

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Brian Vickers
Bainbridge Island, WA
RV4
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12-21-2008, 03:55 PM
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Join Date: May 2005
Location: Santa Barbara, CA
Posts: 1,324
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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
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12-21-2008, 05:41 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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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.
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Dan Horton
RV-8 SS
Barrett IO-390
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12-21-2008, 06:26 PM
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Join Date: May 2005
Location: Santa Barbara, CA
Posts: 1,324
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Corrosion
Quote:
Originally Posted by DanH
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.
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Ah, the wonderful world of the unintended consequence!
John Clark
RV8 N18U "Sunshine"
KSBA
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12-21-2008, 07:03 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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<<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.
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Dan Horton
RV-8 SS
Barrett IO-390
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12-22-2008, 07:54 AM
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Join Date: Jan 2006
Location: PA
Posts: 264
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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.
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Ken
Helicopter mechanic (A&P)
USAFR KC-10 Boom Operator, on final approach to retirement
My RV-9/8/7 dream may be on life support, but it ain't dead yet!
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12-22-2008, 12:30 PM
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Join Date: Aug 2005
Location: Estacada, OR
Posts: 787
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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.
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Richard Scott
RV-9A Fuselage
1941 Interstate Cadet
Last edited by Mike S : 12-22-2008 at 01:02 PM.
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12-22-2008, 05:44 PM
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Join Date: Mar 2007
Location: Ankeny, Iowa
Posts: 194
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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.
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Chanler Childs
RV-8A
Empennage complete
Building wings
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08-27-2009, 11:15 AM
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Join Date: Jul 2009
Location: Patuxent River, MD
Posts: 14
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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?
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Rob Byers
CDR USN
Navy Test Pilot 
Aerospace Engineer
RV-12 SN 120244
N512RB Reserved
Vertical Stab, Rudder, ASTs done
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08-27-2009, 01:48 PM
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Join Date: Jan 2005
Location: The Beautiful NJ Shore
Posts: 409
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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?
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Greg Piney
RV-8 2547
Empennage Done!
Beginning Fuselage Final Assembly!
(Tub finished, on to Landing Gear).
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