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  #1  
Old 06-17-2021, 05:10 PM
avatty avatty is offline
 
Join Date: Sep 2015
Location: Walnut Creek, CA
Posts: 61
Default Has anyone tried shrinking an oil-canning skin with heat treatment?

A friend was looking at my fuselage build today. When I showed him a slight oil canning (lower front of fuselage side skin immediately aft of F-01406 bulkhead) he suggested it may be possible to shrink the skin locally as a fix. He said he has experience in heat-treating steel auto body parts to remove dents, but hasn't tried aluminum. His approach, at least, is pretty conservative, as he recommended using a hair dryer instead of a heat gun (or a torch) to avoid overheating the material.

I looked online a bit and found some references on auto body blog sites, one stating that you would not change the temper or hardness of the aluminum if you don't heat it past 550 F. Basically it seems you heat the site then cool it with a rag soaked in cold water.

Has anyone tried this? I'm not trying to start a debate or sound too uninformed, but it does feel like using a hair dryer and cold water on the fuselage at least should not cause too much havoc.

I maybe should mention that this is not a "hard" oil can, i.e., it doesn't snap back and forth - it assumes its proper, slightly convex shape with very little finger pressure from inside, but prefers to rest slightly concave (about 3/32" gap with a straight edge held vertically).

It may also be relevant that this method seems mostly used for automobile dent repair by helping to return the stretched-out material to its original thickness. Here it would seem to be an attempt to thicken the skin beyond its original thickness.
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  #2  
Old 06-18-2021, 07:35 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by avatty View Post
A friend was looking at my fuselage build today. When I showed him a slight oil canning (lower front of fuselage side skin immediately aft of F-01406 bulkhead) he suggested it may be possible to shrink the skin locally as a fix. He said he has experience in heat-treating steel auto body parts to remove dents, but hasn't tried aluminum. His approach, at least, is pretty conservative, as he recommended using a hair dryer instead of a heat gun (or a torch) to avoid overheating the material.

I looked online a bit and found some references on auto body blog sites, one stating that you would not change the temper or hardness of the aluminum if you don't heat it past 550 F. Basically it seems you heat the site then cool it with a rag soaked in cold water.

Has anyone tried this?
WHile they may be correct that 550* will not significantly temper the AL further (it is already heated treated), it will not permanently shrink the sheet. Yes, it will expand as it heats up to 550 and will shrink back as it cools to ambient. However, this process will NOT create a permanent shrinkage of the sheet. The only real way to shrink it without destroying the heat treatment of the AL is to shrink mechanically with a hammer/dolly.

Just ask anyone who heat treats steel. They rarely get the shrinkage in the places they wanted or expected. It often results in undesired warpage.

Though not sure, I have a feeling that heating to 550 WILL disturb the heat treat process. Though have not done the research to confirm. I know that 300-400* is used to temper hardened steel, so stands to reason the 550 on AL will temper and possibly remove some of the desired traits created by the hardening process (hardening & tempering). The designers of your plane were counting on the engineered traits and strengths delivered by the 2024 T351 sheets and changing those traits will impact things, likely in a way that you do not desire.

I would NOT recommend heating the sheet like this.

Larry
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Last edited by lr172 : 06-18-2021 at 07:45 AM.
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  #3  
Old 06-18-2021, 07:49 AM
Dave12 Dave12 is offline
 
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If it bothers you that much, have you considered just fabricating a piece of very light angle and installing it over the oil can, then fasten it however you choose? Rivet, bonding, good double sided tape might even work. With a bit of work you can make it look like it belongs there.
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  #4  
Old 06-18-2021, 07:52 AM
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bkervaski bkervaski is offline
 
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Quote:
Originally Posted by avatty View Post
A friend was looking at my fuselage build today. When I showed him a slight oil canning (lower front of fuselage side skin immediately aft of F-01406 bulkhead) he suggested it may be possible to shrink the skin locally as a fix. He said he has experience in heat-treating steel auto body parts to remove dents, but hasn't tried aluminum. His approach, at least, is pretty conservative, as he recommended using a hair dryer instead of a heat gun (or a torch) to avoid overheating the material.
Got a video so we can see?

I think you'll find most 14's have oil canning, it's typically not a big deal. I think it's just the way the J stiffeners slightly twist the skins. I have one panel about midway down the emp that will occasionally "pop" with temperature changes or if I push on it.
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  #5  
Old 06-18-2021, 11:22 AM
Bavafa Bavafa is offline
 
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This is a an area that is common in a RV14 and it is due to the large area with one J-stiffener. I talked to VANs about it and installed additional J-channel, just prosealed and tied them to each bulkhead with a couple of rivets to the bulkheads. There is absolutely no oil canning now including if you push on it with hand. It is a simple add on if it bothers you much.
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  #6  
Old 06-18-2021, 12:19 PM
avatty avatty is offline
 
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Location: Walnut Creek, CA
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Thanks to everyone for the input. I admit to shuddering a bit when I reread my post, particularly the hair dryer part.

Probably the main reason this bugs me is that I just completed joining the fuselage sections, and found it challenging to get the bulkhead flanges correctly angled. Some drill-outs and refluting corrected all of the problems but this last one, which admittedly is slight, but it's recent.

Thanks in particular to Bavafa for the stringer suggestion. So long as the stringer is tied to the bulhead and frame it hopefully would avoid any issues similar to the cracking addressed in SB 18-09-17. (Bavafa, you may be interested to know that the hair dryer solution came from a mutual acquaintance at KCCR.)
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  #7  
Old 06-18-2021, 05:17 PM
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KeithB KeithB is offline
 
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If itís evident on the ground, it can get more evident in the air - this picture revealed my issue, though it would be much less evident after painting. It is created by twisting of the structure J-stiffeners as described. I bought J-stiffener, installed segments between bulkheads with proseal as described above (although I floated mine and didnít tie them to the bulkheads).
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  #8  
Old 06-19-2021, 09:54 AM
JDeanda JDeanda is offline
 
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Location: Ventura, CA
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Default Nope

Long story mercifully shortened, aeons ago, we were told at A&P school that if structural aluminum parts, (like, say, a 2024-T3 skin), have been heated enough to boil water, we should be very suspicious of those parts. I donít think Iíve ever seen anybody heat structural aluminum aircraft skins to fix a dent. Iíve torch annealed nonstructural fairings, but never structure. Iíd be afraid that if it was heated enough to get the desired effect, youíd affect the existing heat treat in unpredictable and undesirable ways. Adding a stiffener or just living with the oil canning look like better alternatives to me. Just my two cents.
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  #9  
Old 06-21-2021, 08:20 AM
fixnflyguy fixnflyguy is offline
 
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Default Do not attempt to heat it!

As others have mentioned, add a stiffener if you need..DO NOT try heating. It wont work and you will change the T3 temper if you get carried away. I'm an aircraft structures specialist by day job, and have worked with shear wrinkle resolutions on many different airliners. Take a look at an old 737 fuselage behind the wing trailing edge or a B-52 bomber fuselage side skin and you will be astonished at the shear wrinkles (looks like oil cans). Your scenario will hurt nothing, and a simple stiffener may be able to give you the visual fix you are looking for.
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  #10  
Old 06-21-2021, 10:11 AM
avatty avatty is offline
 
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Location: Walnut Creek, CA
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Default

Thanks again to everyone, I agree my suggestion was full of hot air. The best option right now is to build on, any stiffeners can be added later, if need be, and I'll put away my wife's hair dryer.

The photo by KeithB certainly is interesting, Keith, how far back did you install stiffeners?

And, hopefully as a minor concern, how commonplace is this and does anyone think there is any concern beyond cosmetic?
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