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  #21  
Old 10-25-2018, 03:44 PM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
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Default RV-14/14A Service Bulletin SB 18-09-17 published: possible cracking of F-01478 skin

Quote:
Originally Posted by greghughespdx View Post

Also - as a related sidebar item - when Scott and I were discussing this topic he told me, "If you want to read about some other interesting oil-canning aircraft stories for perspective, try Googling 'oil canning B-52' and taking a look at some of the articles and photos." Interesting stuff!
The ripples in the B-52 fuselage skin are diagonal tension buckling. The weight of the fuselage ahead of the front landing gear provides a downward load on the structure and each section of the skin between the longerons and bulkheads is loaded in shear. Take a piece of paper, hold it vertically with your left hand on the left edge, your right hand on the right edge and pull down with your right hand while keeping your left hand still. The ripples in the paper occur at approximately 45 degrees and the paper on the diagonal from top left to bottom right is in tension. Keep applying load. Even though the paper has buckled it is in tension and will accept more load.

The buckling in the bottom fuselage panels of the 2 seat RVs results from applying up elevator which loads the top fuselage panels in direct tension and the bottom fuselage panels in direct compression. Take the piece of paper again, this time move both hands together applying a compressive load to the paper. It will buckle, and at that point the same force will continue to make the buckle larger. In the aircraft structure, this means the skin no longer carries any further load once the buckle starts, and the load is then carried by the longerons or stiffeners riveted to that skin panel.

So there are a couple of different kinds of buckling (oil canning if you like). In my view, compressive skin buckling is to be avoided.

Hopefully some aeronautical engineers on this forum will weigh in with further insight.
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  #22  
Old 10-25-2018, 03:57 PM
greghughespdx greghughespdx is offline
 
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We've conflated two different items here. This thread was specifically an announcement about the F-01478 skin cracking potential and the service bulletin/kit that has been released to address the issue. There are already other threads on the forums here discussing skin oil canning, so I'd suggest in the interest of not confusing people unnecessarily we separate these topics going forward, as they are not the same/related.
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  #23  
Old 10-25-2018, 04:24 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by terrye View Post

The buckling in the bottom fuselage panels of the 2 seat RVs results from applying up elevator which loads the top fuselage panels in direct tension and the bottom fuselage panels in direct compression.

So there are a couple of different kinds of buckling (oil canning if you like). In my view, compressive skin buckling is to be avoided.

Hopefully some aeronautical engineers on this forum will weigh in with further insight.
More than a couple even.... But once again for posterity..... The cracks addressed by this S.B. are not related to oil canning. That was an entirely separate discussion. Hopefully anyone commenting on the S.B. has read it.

Also, not disagreeing with you regarding your analysis that compression loads are applied on the bottom with up elevator input, but I disagree that you can suppose that that is what must have caused the skin cracking addressed by this S.B..

The type of skin cracks we are talking about here are caused by many load cycles. This type of load cycle is induced into the tail cone in many different ways, so suggesting that it is caused by loads induced by elevator forces is just a guess. It could have been simply the result of vibration induced cycles that acted on the end rivet of an unsupported skin stiffener.
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  #24  
Old 10-25-2018, 04:28 PM
TASEsq TASEsq is offline
 
Join Date: Aug 2017
Location: Melbourne, Australia
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Quote:
Originally Posted by greghughespdx View Post
We've conflated two different items here. This thread was specifically an announcement about the F-01478 skin cracking potential and the service bulletin/kit that has been released to address the issue. There are already other threads on the forums here discussing skin oil canning, so I'd suggest in the interest of not confusing people unnecessarily we separate these topics going forward, as they are not the same/related.

I only referenced that thread, as it discussed the oil canning issue and included the crack photo on the lower fuselage. The thread indicated that Vans is releasing a SB for oil canning as well. Since these are seperate issues, will there be a seperate SB to address the oil canning? (Or do we wait for cracks to appear from that? Will/could they appear from that?)
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  #25  
Old 10-25-2018, 05:04 PM
greghughespdx greghughespdx is offline
 
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Quote:
Originally Posted by TASEsq View Post
I only referenced that thread, as it discussed the oil canning issue and included the crack photo on the lower fuselage. The thread indicated that Vans is releasing a SB for oil canning as well. Since these are seperate issues, will there be a seperate SB to address the oil canning? (Or do we wait for cracks to appear from that? Will/could they appear from that?)
Not sure how to be more clear than this, but we do need to be clear and ensure we don't confuse people via discussion: The cracks on the lower fuselage skin (including the one shown in the photo) are not caused by skin "oil canning," and have been addressed using the new parts kit issued with the SB.

There has been no indication made by Van's that a SB is planned for "oil canning." That's because there is no issue related to "oil canning" which needs to be addressed.

Two different topics being talked about here, unrelated, and only one represents an issue that needs to be/is being addressed:
  • Skin cracks on F-01478, which were shown in a photo in the linked forum thread and are addressed by adding stiffening structure in a specific location - Addressed via this service bulletin.
  • Skins may "oil can" in certain conditions related to differential pressures, temperatures and certain loading - Not an issue, not being addressed, not a cause of cracking.

Thanks, all.
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Opinions, information and comments posted by me are my own unless stated otherwise. They do not represent direction/opinions of my employer.

Building RV-8A since Sept 2014 (N88VX reserved)
Dual AFS 5600, Avidyne IFD 440, Whirlwind 74RV, Superior XPIO-360
VAF build thread - Flickr photo album - Project Facebook page
Flying spam can, helped build/fly a few RV-12s and some other RVs
Hillsboro, OR (EAA 105)


Last edited by greghughespdx : 10-25-2018 at 05:06 PM.
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  #26  
Old 10-25-2018, 05:15 PM
TASEsq TASEsq is offline
 
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Understood - thank you.
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  #27  
Old 10-27-2018, 11:40 AM
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HAL Pilot HAL Pilot is offline
 
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So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?
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  #28  
Old 10-27-2018, 11:47 AM
Bavafa Bavafa is offline
 
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Quote:
Originally Posted by HAL Pilot View Post
So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?
This is a good question, something that I have been wondering about. What material would be best suited to fill this void. I have never bucked a rivet with such void in the sandwich pieces so I am wondering what percautions need to be kept in mind when riveting this piece?
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  #29  
Old 10-27-2018, 12:00 PM
mturnerb mturnerb is offline
 
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Quote:
Originally Posted by HAL Pilot View Post
So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?

Maybe a dab of proseal or similar to help keep water out?
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  #30  
Old 10-27-2018, 06:37 PM
StressedOut StressedOut is offline
 
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Quote:
Originally Posted by HAL Pilot View Post
So for those of us where the dimples are already created. It looks like now there will be a void where the original rivet used to be on the belly skin. For corrosion protection does it make sense to fill this dimple with something before installing the F-14148A SKIN DOUBLER? I think Spruce sells some kind of aluminium filler?
You could make some countersink washers by driving some AN426AD3 rivets into some scrap (countersink the scrap first, of course) and drilling out the heads carefully. If you're careful you can break the head off intact, deburr and use it in the dimple.

Proseal is commonly used on the faying surface of repair doublers on large aircraft repairs. It is used to prevent moisture ingression into the joint. Without an electrolyte, corrosion is hard to form.
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