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  #11  
Old 10-25-2018, 12:56 PM
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czechsix czechsix is offline
 
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Quote:
Originally Posted by mturnerb View Post
The question to me is: does the oil canning need to be fixed?

Is the flexibility of skin/structure that allows oil-canning to take place a normal/natural/desirable part of the design? Not a rhetorical question - I just don't know, thus the question I posed above.
I would not say it's a normal or desirable attribute of the design...the 14 fuselage is shaped subtlety different from the other side-by-side RVs and has just enough curvature in the lower tailcone side skins to allow oil canning (a bit more or less curvature and it wouldn't happen...I'm sure Vans didn't anticipate this when designing it). Personally I doubt it will ever cause cracking in the aluminum structure...depending on how flexible the paint is, it may degrade the finish over time. I bought some J-channel and bonded it to the side skins with Proseal. No more oil canning, but I added half a pound to my airplane. Which I'm not happy about...but in the end I decided I was more unhappy about the oil canning than the added weight of the stiffeners.

If it bothers you, do something about it. If not, leave it alone. It'll most likely be just fine as is, but nobody knows for sure what the long term effect will be.

Just in case anyone is confused, the oil canning in the side skin panels has absolutely nothing to do with the Service Bulletin to address cracks in the bottom skin (which is the original subject of this thread). I already fabricated my own solution to that problem, but if I had not, I would certainly take advantage of the parts offered in the SB...
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  #12  
Old 10-25-2018, 01:16 PM
Tom Martin Tom Martin is online now
 
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Mark
If you have not tied the prosealed J stiffener to the bulkheads I would encourage you to do so. The vibration that cased the oil canning will have changed frequency and it will focus on the ends of your stiffeners.
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  #13  
Old 10-25-2018, 01:27 PM
greghughespdx greghughespdx is offline
 
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The service bulletin addresses cracks in the described aft fuse forward bottom skin, and not tail cone oil canning, first of all, just to be clear.

Quote:
Originally Posted by mturnerb View Post
The question to me is: does the oil canning need to be fixed? I'm currently flying a 38 year old Archer with 3K hours or so, and when I fly I can see the top wing skins distorting between rivet lines - presumably due to the effects of lift on the top of the wing. Not a crack in sight.

Is the flexibility of skin/structure that allows oil-canning to take place a normal/natural/desirable part of the design? Not a rhetorical question - I just don't know, thus the question I posed above.
Norma and natural? Essentially, yes. Desirable? We'd all prefer for it to never happen, of course, but we also need to think about the nature of metal airplane design.

Oil-canning of skins, as described and discussed on these forums in the past, is not completely uncommon in a metal airplane where inside/outside in-flight pressure deltas and skin temperature changes result in some minor movement/expansion/contraction of aluminum skins. While some builders have in the past installed things like stiffeners or adhesive sound-deadening foam to help minimize the effects of oil-canning, it is important to recognize a couple things:

1) The very nature of differential pressures and temperature fluctuations on the skins is such that some oil canning can happen, and this is considered acceptable and normal within reasonable, obvious limits. Oil canning on it's own is not generally a structural concern (unless, of course, some new and unusual permanent skin deformation is seen, in which case we'd look for a cause other than simple oil-canning). Aircraft skin crack formation will tend to occur where skin flexing occurs rapidly and repeatedly. So, an oil-can area that doesn't continuously/repeatedly flex, over and over again at a a relatively rapid rate, is unlikely to develop fatigue-related issues such as cracking. Hence, simple oil canning is not generally a fatigue concern.

2) The addition of custom stiffening structures not specified in the plans can possibly lead to unintended secondary consequences, depending on the details of how they are built and attached. For example, where builders choose to rivet or use adhesive to attach stiffening angles of their own design inside the skins without proper attachment to the aircraft structure, one could unintentionally create a whole new stress concentration area in the skin at each end of the stiffener, which could then cause cracks to develop in the stress concentration area. This type of behavior has been observed in the past on metal airplanes.

Point being, some oil-canning is considered typical, acceptable and normal, and Van's advises using caution if and when determining what custom changes you may want to design and apply. When we analyze our designs for fatigue-prevention, our FEA computer systems and engineers apply a wide variety of cause-effect analysis to help ensure a design is resilient.

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!
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Last edited by greghughespdx : 10-25-2018 at 01:31 PM.
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  #14  
Old 10-25-2018, 01:37 PM
Aluminum Aluminum is offline
 
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Thanks for your insight Greg!

Quote:
Originally Posted by greghughespdx View Post
Aircraft skin crack formation will tend to occur where skin flexing occurs rapidly and repeatedly. So, an oil-can area that doesn't continuously/repeatedly flex, over and over again at a a relatively rapid rate, is unlikely to develop fatigue-related issues such as cracking. Hence, simple oil canning is not generally a fatigue concern.
Has it been determined beyond doubt that the particular case of skin cracking addressed by this SB is vibration fatigue as opposed to a hard landing? A single cycle is certainly sufficient to cause cracking if the amplitude is high enough. This seems like the place where the crumpling forces would concentrate between the landing gear mount and the tail surfaces.
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  #15  
Old 10-25-2018, 02:31 PM
StressedOut StressedOut is offline
 
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Quote:
Originally Posted by bkervaski View Post
Anybody affected have pics of a crack so we know what to look for?
I'm very interested in seeing a picture of the actual cracks as well. I see the service bulletin adds a single shear splice plate connecting the forward and aft center stiffener across the F-01408 bulkhead. It also has a couple of tension fittings that connect the F-01486-L/R stiffeners directly to the same bulkhead.

Were the cracks located on the skin at these three stiffeners? Greg, perhaps you can post a picture?
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  #16  
Old 10-25-2018, 02:45 PM
mturnerb mturnerb is offline
 
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Quote:
Originally Posted by greghughespdx View Post
The service bulletin addresses cracks in the described aft fuse forward bottom skin, and not tail cone oil canning, first of all, just to be clear.



Norma and natural? Essentially, yes. Desirable? We'd all prefer for it to never happen, of course, but we also need to think about the nature of metal airplane design.

Oil-canning of skins, as described and discussed on these forums in the past, is not completely uncommon in a metal airplane where inside/outside in-flight pressure deltas and skin temperature changes result in some minor movement/expansion/contraction of aluminum skins. While some builders have in the past installed things like stiffeners or adhesive sound-deadening foam to help minimize the effects of oil-canning, it is important to recognize a couple things:

1) The very nature of differential pressures and temperature fluctuations on the skins is such that some oil canning can happen, and this is considered acceptable and normal within reasonable, obvious limits. Oil canning on it's own is not generally a structural concern (unless, of course, some new and unusual permanent skin deformation is seen, in which case we'd look for a cause other than simple oil-canning). Aircraft skin crack formation will tend to occur where skin flexing occurs rapidly and repeatedly. So, an oil-can area that doesn't continuously/repeatedly flex, over and over again at a a relatively rapid rate, is unlikely to develop fatigue-related issues such as cracking. Hence, simple oil canning is not generally a fatigue concern.

2) The addition of custom stiffening structures not specified in the plans can possibly lead to unintended secondary consequences, depending on the details of how they are built and attached. For example, where builders choose to rivet or use adhesive to attach stiffening angles of their own design inside the skins without proper attachment to the aircraft structure, one could unintentionally create a whole new stress concentration area in the skin at each end of the stiffener, which could then cause cracks to develop in the stress concentration area. This type of behavior has been observed in the past on metal airplanes.

Point being, some oil-canning is considered typical, acceptable and normal, and Van's advises using caution if and when determining what custom changes you may want to design and apply. When we analyze our designs for fatigue-prevention, our FEA computer systems and engineers apply a wide variety of cause-effect analysis to help ensure a design is resilient.

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!
Greg: thanks for this explanation. Exactly what I was hoping to hear. I will of course do the SB as cracks are clearly an issue, but I don't feel the need to add stiffeners etc for oil canning given that it's not a structural concern.
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  #17  
Old 10-25-2018, 02:46 PM
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Thermos Thermos is offline
 
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Quote:
Originally Posted by greghughespdx View Post
Normal and natural? Essentially, yes.
<snip>
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!


50+ years of oil-canning on this airplane, no fuselage failures yet. And you definitely hear the fuse flexing when you go over bumps on the taxiway.

Dave
(former BUFF crew)
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Last edited by Thermos : 10-25-2018 at 02:58 PM.
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  #18  
Old 10-25-2018, 03:05 PM
greghughespdx greghughespdx is offline
 
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Quote:
Originally Posted by Aluminum View Post
Has it been determined beyond doubt that the particular case of skin cracking addressed by this SB is vibration fatigue as opposed to a hard landing?
Yes, that has been analyzed and determined.
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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
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  #19  
Old 10-25-2018, 03:14 PM
greghughespdx greghughespdx is offline
 
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Quote:
Originally Posted by StressedOut View Post
I'm very interested in seeing a picture of the actual cracks as well. I see the service bulletin adds a single shear splice plate connecting the forward and aft center stiffener across the F-01408 bulkhead. It also has a couple of tension fittings that connect the F-01486-L/R stiffeners directly to the same bulkhead.

Were the cracks located on the skin at these three stiffeners? Greg, perhaps you can post a picture?
Yes, and that is what the SB 18-09-17 addresses specifically. The thread linked by TASEsq above includes an example photo.
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Van's Aircraft - Director, Community/Media
Van's web site | Instagram | Facebook

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)

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  #20  
Old 10-25-2018, 03:36 PM
Allan Stern Allan Stern is offline
 
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Default Cracks

I have just finished my tail cone so I can turn it on the side to drill out the rivets and install the fix.
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