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  #1  
Old 10-07-2021, 02:24 PM
ronshort2000 ronshort2000 is offline
 
Join Date: Oct 2020
Location: CATAWBA, SC
Posts: 20
Default Securing foam ribs in elevator

I am about to start riveting the elevator and have been trying to decide how to handle the foam ribs in the elevator and trim tabs. When I did my rudder I used the 3M tape that Vans now recommends so I didn't have to deal with tank sealant. I am planning to do the same with the trailing edges of the elevators. The problem is that the plans call for tank sealant for the foam ribs in the elevator and the trim tabs. I purchased some sealant from Vans so if I use it then I'll have to have the elevator and trim tabs ready to go within the 2 hr working time, but that seems like a real hassle.

With all that said, since I am using tape for the trailing edge instead of sealant, is there anything that can be used for the foam ribs instead of sealant? I assume the only reason for the sealant is to make sure the ribs don't move so I would assume anything that holds them in place would work. I think I saw a couple people on youtube using some sort of spray glue, but I didn't see what kind.
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  #2  
Old 10-07-2021, 03:39 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Default

Engineering decisions are often more complicated than they look (especially through the eyes of a non engineer).

The fuel tank sealant was chosen for a number of different reasons.

Just a couple are -

The expansion coefficient (as effected by temp.) of aluminum is quite different from what it is for the foam. This makes choosing an adhesive that has good enough shear qualities (usually related to the adhesive having some flexibility to it, which tank sealant does)

The foam used is an open cell foam. For good adhesion, an adhesive that will penetrant into the cells is important. Thant makes using an adhesive that is quite viscous and can be pre applied to the surfaces of the ribs (like described in the instructions is important.

Having said that, tank sealant is probably not the only thing that would work, but until a test is done to compare the performance of an alternative, it is simply a guess that it will be.

At this point, the proof that it is a good choice is that with multiple thousands of airplanes flying with ribs bonded in the described way, I am unaware of any bond failures.
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  #3  
Old 10-07-2021, 09:31 PM
Taltruda Taltruda is offline
 
Join Date: Oct 2017
Location: Las Vegas, NV
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Default Foam ribs

Scott, while you are here, I wanted to ask how you feel these foam ribs will hold up in 40 years? One day these planes are going to be 40, 50 years old, and I wonder if the foam will still be there, or just shrunken down, broken free or turned to dust..
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  #4  
Old 10-08-2021, 12:16 AM
ronshort2000 ronshort2000 is offline
 
Join Date: Oct 2020
Location: CATAWBA, SC
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Default

Thanks Scott, I appreciate the response and explanation. Sometimes I feel that I am following instructions but don't really know why. This helps clear some of it up. I'll be using the tank sealant
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  #5  
Old 10-08-2021, 08:50 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by Taltruda View Post
Scott, while you are here, I wanted to ask how you feel these foam ribs will hold up in 40 years? One day these planes are going to be 40, 50 years old, and I wonder if the foam will still be there, or just shrunken down, broken free or turned to dust..
Like every other aspect of the RV-10 design (and the other models for that matter), only actual time in service can fully answer that question.

The foam used is basically the same as what is used as core material in a lot of composite construction. It has a good history in this application. I realize it is not an apples to apples comparison because in this application it is not encapsulated, but it is shielded from UV exposure, etc., so we have always had a high level of confidence in it.

As a bench mark, the first use of the foam in this way on an RV was in the left elevator trim tab of the prototype RV-9A. It was 22 years old this past spring and there is no signs of degradation at all.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

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Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")
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