Rag and tube structure

Tom-D

Tom-D

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Tom-D
How many times can you sand blast a 4130 tube fuselage before it becomes too thin to re-use?

we are getting to the point these tubes have been cleaned 3-5 times, where do we say enough is enough?
 
With man made fabrics, I can't image enough recovers for this to be a issue, unless you're just trashing the aircraft left and right.
 
James331 said:
With man made fabrics, I can't image enough recovers for this to be a issue, unless you're just trashing the aircraft left and right.
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Not the point.. how many times can you blast those tubes ?
 
howmanylicks_thumb.gif
 
How do you inspect the tubular frames, control surfaces, etc. for cracks when they're covered by fabric? That's what would concern me if I were in the market for a Tri-Pacer or something.
 
You would have to know how much material is being removed per sand-blast, and what thickness provides a reasonable margin against failure.
 
I take it Tom knows the answer... personally, I wouldn't sandblast. I knew someone that pretty much ruined a '69 Firebird by sandblast removing the paint.
 
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You aren't going to get a cogent response here, Tom. Anybody who says that the fabric is holding the airplane together doesn't have a clue. Likewise a response that says that the need to know how much material is being removed from each sandblast and how thin is thin is asking the same question that you are. I don't think there is a good answer UNLESS somebody has a source to say what the thickness of a chromoly tube minimum diameter is and then you can measure it with a micrometer or a digital caliper. Until then all answers are pi$$ing into the wind.

Jim
 
Media type and application dependent, as an engineer I would want to dimensionally inspect the hardware, maybe sonic check it for min thickness and compare it to the low end of factory spec. If out of spec replace or prepare to run analysis for and justify new lower spec.
 
I would be more concerned about gross changes in the surface finish and cracks in the tubes near welds. 4130 is nasty stuff, it wont budge with sand unless rust or something else attacked it. *** measure tubes in these areas and be cautious, there is rust on the inside most likely.

Dye penetration spray and a black light is used for checking cracks in the metal working industry. Cheap, and I think fits the bill for a small aircraft project.

I am no A&P or aircraft builder, all the info above is just my experience working with steel and nothing else. Don't blame me for anything.
 
How many times that it is sandblasted is not the right question. What are good questions are:
1. What is the nominal thickness that is structurally sound? This requires research.
2. What is the thickness remaining? This requires measuring.
 
SkyDog58 said:
How many times that it is sandblasted is not the right question. What are good questions are:
1. What is the nominal thickness that is structurally sound? This requires research.
2. What is the thickness remaining? This requires measuring.
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Hmm hmm. Agreed.

There are too many other variables to simply say x number of media blasting cycles will compromise the structural integrity of a tube frame.
 
there is a "punch" test for suspect areas much like the one used for the cub main spar before most of them were replaced.
 
I asked the question to raise the awareness that these aircraft are subject to this problem.

I know of several old Stinsons being restored at home by owners that are not A&Ps.

when buying -- be aware, the only way to really know is to cut a section out and measure it.. sellers probably won't allow that.
 
hotprops said:
there is a "punch" test for suspect areas much like the one used for the cub main spar before most of them were replaced.
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When you do the test for corrosion in the AD, you must lay out a grid and punch every 1/16th" square. are you going to do this to every tube in a fuselage?
 
avongil said:
I would be more concerned about gross changes in the surface finish and cracks in the tubes near welds. 4130 is nasty stuff, it wont budge with sand unless rust or something else attacked it. *** measure tubes in these areas and be cautious, there is rust on the inside most likely.

Dye penetration spray and a black light is used for checking cracks in the metal working industry. Cheap, and I think fits the bill for a small aircraft project.

I am no A&P or aircraft builder, all the info above is just my experience working with steel and nothing else. Don't blame me for anything.
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If you were the new buyer or the A&P-IA doing the first annual after a full restoration what questions would you be asking about how the fuselage and other tubing was cleaned. and what would be acceptable to you?
 
Thickness issues are a problem with anything that gets refinished over & over.

How many blend, measure and treat type skin repairs are remeasured later? Basically none. The verbiage in these repairs is typically "report any measurement beyond X to engineering for further disposition". X is exactly what NDT measured at the time the blend repair was being done, so there is no additional material removal allowed in the repair drawing.

Typically all these are done via ultrasonic inspection.
 
Tom-D said:
when buying -- be aware, the only way to really know is to cut a section out and measure it.. sellers probably won't allow that.
Click to expand...

Ultrasonic thicknesss testing does it. https://en.wikipedia.org/wiki/Ultrasonic_thickness_measurement

We've had NDT guys come and do it for us on some stuff, and once sent out a 172's control yoke to get the lower end UT'd for loss of wall thickness due to internal corrosion as per SB. https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=17090
 
Sandblasting a tubular structure is tedious, and time consuming. I would think a chemical dip, would be better, and less invasive.
 
Tom-D said:
How many times can you sand blast a 4130 tube fuselage before it becomes too thin to re-use?
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I would think the bigger concern would be that of repeated "peening" by zillions of tiny hammers (the blast media). Thus hardening to the point of embrittlement, or distortion due to uneven stretching and shrinking under the "hammers".
The latter is what happens to most cars that are sandblasted.
 
bluerooster said:
Sandblasting a tubular structure is tedious, and time consuming. I would think a chemical dip, would be better, and less invasive.
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The dip is an acid, how would you know it didn't enter the center of the tube, or how would you get it out?
 
Would it be safe to assume that it could only get in, and out, the same way linseed oil gets in, and the excess out?
Now, if the tube is sealed so that you can't get linseed oil in or out, then the acid shouldn't be able to get in either. And if it does, you have other problems to worry about, (cracking, corrosion, etc)
Anyway, the dip is a multistage process, with a dip/strip, rinse/wash, neutralize, oil bath.
 
bnt83 said:
Thickness issues are a problem with anything that gets refinished over & over.

How many blend, measure and treat type skin repairs are remeasured later? Basically none. The verbiage in these repairs is typically "report any measurement beyond X to engineering for further disposition". X is exactly what NDT measured at the time the blend repair was being done, so there is no additional material removal allowed in the repair drawing.

Typically all these are done via ultrasonic inspection.
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Most of this type of inspection methods are not found in the field. plus the expense of having the test done.
 
bluerooster said:
Would it be safe to assume that it could only get in, and out, the same way linseed oil gets in, and the excess out?
Now, if the tube is sealed so that you can't get linseed oil in or out, then the acid shouldn't be able to get in either. And if it does, you have other problems to worry about, (cracking, corrosion, etc)
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Lots of these old aircraft were hand welded with a torch, the welds are not really sealed up. plus the fact that many were not oiled inside. and yes many have problems that are never discovered.
 
In answer to your first question; I suppose you could get a piece of 4130 tubing of the same size and thickness, and sandblast it to the point of failure and find out.
I'll lay 80 that it will take a while.
 
There are already a few good replies to the original questions. However I cannot help but address this issue from an engineering perspective, well, because I am an engineer. The original question can be distilled down to this: what is the minimum acceptable wall thickness of the tubing? Assuming we are talking about 4130 chrome-moly tubing the answer to this question is found by referring to the specification MIL-T-6736 and other specs referenced in that document. The tolerance is +/- 10% of the nominal wall thickness. Because tubing is available in more than one wall thickness you would need to determine exactly what OD/ID tubing was specified by the original manufacturer. As other replies to this post suggest, there are methods to measure the wall thickness of the tubing in a non-destructive manner. However it would take very aggressive sandblasting to remove enough material to cause an out-of-tolerance condition unless the surface was already eroded by rust or perhaps a bad weld joint.

As important as it is to be worried about the erosion of the wall thickness, there are other factors to consider. Metal fails mainly because of stress, and therefore it is important to understand what factors contribute to stress. It is easy to understand that an extremely physical force such as a very bad landing can exert stress on the airframe. A fundamental, yet insidious, cause of stress failures results from microscopic surface defects which are sometimes referred to as “stress-risers”. This brings us right back to the original topic because sandblasting the tubing can create stress-risers that compromise the strength of the structure far more that the likely erosion of the wall thickness. It all comes down to the use of the proper media (and pressure) when blasting the tubing. I cannot stress (no pun intended) how important it is to understand this point. Most aircraft welders and A&Ps will be aware of this, however if you take your stripped down airframe to your local automotive paint shop for blasting they might not have a clue. “Sandblasting” is a term commonly misused and certainly you don’t want to use sand as a media in most cases. “Media-blasting” is a more generic term. There are dozens of different types of media that can be used to strip paint and prepare the surface for the application of a good epoxy primer. The main idea is that you don’t want to use media with sharp edges but rather the media should have blunt edges that “peen” the surface rather than cut the surface. If you have a badly rusted area, that might requires some more aggressive media (or chemical process), but the bottom line is that you don’t want the final surface to have microscopic stress-risers. BTW, if you use the correct media to remove old paint you won’t be removing any the metal from the tubing. Hope my long dissertation wasn’t too boring!
 
Rob58 said:
There are already a few good replies to the original questions. However I cannot help but address this issue from an engineering perspective, well, because I am an engineer. The original question can be distilled down to this: what is the minimum acceptable wall thickness of the tubing? Assuming we are talking about 4130 chrome-moly tubing the answer to this question is found by referring to the specification MIL-T-6736 and other specs referenced in that document. The tolerance is +/- 10% of the nominal wall thickness. Because tubing is available in more than one wall thickness you would need to determine exactly what OD/ID tubing was specified by the original manufacturer. As other replies to this post suggest, there are methods to measure the wall thickness of the tubing in a non-destructive manner. However it would take very aggressive sandblasting to remove enough material to cause an out-of-tolerance condition unless the surface was already eroded by rust or perhaps a bad weld joint.

As important as it is to be worried about the erosion of the wall thickness, there are other factors to consider. Metal fails mainly because of stress, and therefore it is important to understand what factors contribute to stress. It is easy to understand that an extremely physical force such as a very bad landing can exert stress on the airframe. A fundamental, yet insidious, cause of stress failures results from microscopic surface defects which are sometimes referred to as “stress-risers”. This brings us right back to the original topic because sandblasting the tubing can create stress-risers that compromise the strength of the structure far more that the likely erosion of the wall thickness. It all comes down to the use of the proper media (and pressure) when blasting the tubing. I cannot stress (no pun intended) how important it is to understand this point. Most aircraft welders and A&Ps will be aware of this, however if you take your stripped down airframe to your local automotive paint shop for blasting they might not have a clue. “Sandblasting” is a term commonly misused and certainly you don’t want to use sand as a media in most cases. “Media-blasting” is a more generic term. There are dozens of different types of media that can be used to strip paint and prepare the surface for the application of a good epoxy primer. The main idea is that you don’t want to use media with sharp edges but rather the media should have blunt edges that “peen” the surface rather than cut the surface. If you have a badly rusted area, that might requires some more aggressive media (or chemical process), but the bottom line is that you don’t want the final surface to have microscopic stress-risers. BTW, if you use the correct media to remove old paint you won’t be removing any the metal from the tubing. Hope my long dissertation wasn’t too boring!
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Nope, interesting read.. which leads to the most important question you as a buyer or A&P-IA must ask.

What media was used the clean the structure.?

What other questions would you ask, prior to buying a freshly restored tube structure?
 
Tom-D said:
when buying -- be aware, the only way to really know is to cut a section out and measure it.. sellers probably won't allow that.
Click to expand...
You can ultrasound strategic/problem prone areas to gauge thickness if you are concerned.
 
Dan Thomas said:
Ultrasonic thicknesss testing does it. https://en.wikipedia.org/wiki/Ultrasonic_thickness_measurement

We've had NDT guys come and do it for us on some stuff, and once sent out a 172's control yoke to get the lower end UT'd for loss of wall thickness due to internal corrosion as per SB. https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=17090
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Exactly. This is what the recent SB on the Beech 18 spar suggests. Ultrasonic testing to gauge the thickness of the tube spar. In that case, the question being addressed is more a matter of how much erosion has occurred inside the tube rather than on the outside.
 
Tom-D said:
What other questions would you ask, prior to buying a freshly restored tube structure?
Click to expand...

Tom, you would want to ask questions such as:
  • was any type of non-destruct testing (NDT) performed? Examples would include dye-penetrant testing, magnetic particle testing; etc. Of course a visual inspection alone can detect some types of defects. If these tests were performed were there any flaws detected?
  • Is there any record of repairs or modifications that involve welding, drilling or other structural changes. These should be recorded in the aircraft logs.
  • Naturally any type of damage history would be factored in to an aircraft purchase decision.
A few further comments: a tubular steel airframe structure is very strong and can often remain safe and airworthy even after suffering a lot of abuse. Much stronger than an aluminum monocoque design in many respects. Depending on the design a tubular steel fuselage is really over-kill in terms of the necessary strength to gain certification (I can't seem to find a reference to which aircraft type you are looking at). There is a weight penalty, but the idea of the protection provided should a forced landing occur is reassuring and validated through numerous accident reports over the years. It is one of the many reasons that I own a few Bellancas (but I fly a Cessna too). I certainly respect the fact that you are asking a lot of good questions!
 
Internal corrosion is what kills tubes. Sometimes a sandblaster will blow a hole in a tube where corrosion had eaten the metal from within.
 
avongil said:
Dye penetration spray and a black light is used for checking cracks in the metal working industry. Cheap, and I think fits the bill for a small aircraft project.

I am no A&P or aircraft builder, all the info above is just my experience working with steel and nothing else. Don't blame me for anything.
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No blame, just a correction. If you're using a blacklight, it's fluorescent penetrant with developer.
 
Rob58 said:
Tom, you would want to ask questions such as:
  • was any type of non-destruct testing (NDT) performed? Examples would include dye-penetrant testing, magnetic particle testing; etc. Of course a visual inspection alone can detect some types of defects. If these tests were performed were there any flaws detected?
Click to expand...

  • Dye penetrant is not going to tell you how thick the metal was. That was his original question.
 
Fearless Tower said:
Exactly. This is what the recent SB on the Beech 18 spar suggests. Ultrasonic testing to gauge the thickness of the tube spar. In that case, the question being addressed is more a matter of how much erosion has occurred inside the tube rather than on the outside.
Click to expand...
It's easier to simply strip a short section of tube, measure, then subtract that number from the tubes original size, to see how much material has been lost.
 
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