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  #351  
Old 11-06-2012, 07:31 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,968
Default Potentially a VERY good idea!

Quote:
Originally Posted by Erik.37m View Post
... they would call in an outside vendor to do a "Helium leak test". Those tiny molecules can pretty much find any opening.
Erik
This is a really good idea, actually. Hellium sniffers are really expensive, but not out of the question for a shop to acquire to do QC on a lot of tanks. The good ones are ball-park $20K. But small hand-held ones for leak checking are about $4K.

So, if Van's really wants to do some quality assurance on QB tanks when they come in, before shipping to customers, they could buy one of the $4K sniffers and leak-check every QB tank. Also, slow-build builders could send their completed tanks in to get tested for a fee.

Now, helium is pretty hard to keep in a tank. It may be that all our tanks would ooze helium. But presumably, it would be a matter of degree, and testing a few would establish a threshold where it is OK vs. one that would leak fuel vapor. It may also be that with some experimentation, a different gas (you can buy sniffers for O2, N2, CO, CO2, variety of others) could be found that would be a more realistic leak rate that would provide a good go/no-go test.

One might argue that it would be more straightforward to train the QB builders to make them better in the first place, but in 3 years, we haven't seen any evidence of that.

Perhaps some vendor testing of some tanks that have been in service and got blisters, and not, would go a ways toward convincing Van's that there is more of a problem that needs addressing? We would need to find a vendor, and arrange for some good samples to test as an experiment.
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  #352  
Old 11-10-2012, 05:03 PM
g zero g zero is offline
 
Join Date: Jun 2007
Location: palm coast fl.
Posts: 997
Default Blisters

Working on my slow build 8 fuel tanks , I was thinking as I was applying sealant to the rivets on the amount of coverage on each rivet . Dan H had some photos showing the " sponge " holes in dried sealant . If rivets are covered with a thin layer would the fuel permeate thru as a gas ( not liquide ) . With that thinking I sealed each rivet with a 5/8 " diam - sealant thick enough not to see any signs of a rivet .
Will report back after a year of full fuel with paint .
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  #353  
Old 11-10-2012, 06:59 PM
nineninefour nineninefour is offline
 
Join Date: Aug 2012
Location: Dallas, TX
Posts: 33
Default

I'm curious about behavior of Proseal in thicker layers. Most 1-, and 2- part sealants will shrink and tear in thick layers, particularly where the thick material becomes thin, even when it stays adhered. Usually we see this with small mechanical stresses or thermal stresses, both of which could be present in these tanks. Any observations?
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  #354  
Old 12-26-2012, 10:12 AM
pa38112 pa38112 is offline
 
Join Date: Mar 2008
Location: Clarksboro, NJ
Posts: 1,003
Default

My plane was Painted in 2004, QB tanks. 2 blisteres appeared last week after 8 years in service. Tempature has been in the 40's. They were next to each other on the botom of the tank. The tank was empty and open to atmosphere for a week before these appeared because I was fixing a small leak on another rivet. I cut off the paint, and there is no sign of fuel.
After fixing my leak, a new leak appeared on the rear flange. My guess is that the QB tanks don't use pro-seal brand sealant, or that they don't abide by the shelf-life dates. This causes the material to not fully cure and degrade when exposed to oxygen. There is then off-gassing that causes the blister. Heat will accelerate the degradation...
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  #355  
Old 02-25-2013, 06:02 PM
BobB BobB is offline
 
Join Date: Feb 2013
Location: Tabernash
Posts: 14
Default Paint Bubbles

I don't know if my RV-7a is a QB or not, but I have bubbles.
SN is 71963. Has anyone explored whether the tank caps have not been releasing positive pressure? Don't see a way to attach photosto this message.
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  #356  
Old 02-25-2013, 06:15 PM
BobB BobB is offline
 
Join Date: Feb 2013
Location: Tabernash
Posts: 14
Default Bubbles

Here are photos of some examples.





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  #357  
Old 02-25-2013, 07:15 PM
Mike S's Avatar
Mike S Mike S is offline
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 16,096
Default Welcome to VAF!

Bob, welcome to VAF

The fuel caps are sealed, tank venting is done with a dedicated line, should be at the wing root, or on fuse close to the wing root. Look for a bit of 1/4" tube sticking out a short distance, usually has a slash cut on the end.

Good to have you aboard, Sorry about the blisters.
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  #358  
Old 02-25-2013, 07:31 PM
BobB BobB is offline
 
Join Date: Feb 2013
Location: Tabernash
Posts: 14
Default Bubbles

Thanks for the reply. Obviously, I didn't build this a/c. I will clean up the bubbles and repaint the areas. Hope the bubbles don't come back.
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  #359  
Old 02-26-2013, 08:56 AM
MPB MPB is offline
 
Join Date: Aug 2012
Location: KSNS
Posts: 14
Default re: Bubbles

For what it is worth I had the same problem on a motorcycle fuel tank. I stripped it, cleaned it real well and re-painted it. The problem seemed to be fixed for about a year and then the bubbles came back in the same spot. In hindsight I should have been more diligent figuring out why they started in the first place. just my 2cents...
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  #360  
Old 02-26-2013, 08:58 AM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,347
Default

Quote:
Originally Posted by MPB View Post
For what it is worth I had the same problem on a motorcycle fuel tank.
Mike, was the tank welded steel, blown or roto-mold plastic, fiberglass, what? Was the blister on a seam or elsewhere?
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