Suitable steel for all brackets etc.

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wsimpso1

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Linseed oil is one of the primary constituents of old style oil based paints. It polymerizes by taking up oxygen from the air and stays put then. Distribute it through the welded assembly, then seal it up, and it does three neat things - it lowers the oxygen content of the air sealed in the tube, and becomes a plastic that stays put, and it tends to seal any pinholes in the welds.

Some water vapor may be sealed in with the linseed oil and air. For rust to form, you need water, oxygen, time. With only a little water vapor, air of lowered oxygen content, and polymer sealed surfaces and welds, not much rusting can happen.

Tubes rusted on the inside were ventilated... The ones I have seen with internal rust were seriously ventilated someplace - cracks, skip-welds instead of continuous welds, bolt through holes without a welded in bushing, worn through paint then rusted through at the wear, etc. The proponents of not linseed oil treating the internals depend upon fully sealed assemblies. The amount of oxygen and water in a fully sealed tube is tiny and will not support much rusting. But if your seal gets compromised down the road, the linseed oil will slow the rusting, hopefully giving you a Condition Inspection or three to find the cracks and set about a repair. I do not know how successful either finding it in time or the repair will be with naked tubes.

Billski
 

Aviacs

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Since we are on the topic, I will re-post link from Kitplanes:

Stainless Steel
Norm Ellis, stainless steel, corrosion, American Iron and Steel Institute (AISI), nickel, chromium, ductility, aluminum 2024-T3, 7075-T6, Iron Pillar of Delhi, iron oxide, phosphates, Leon Gillet, Philip Monnartz, Sheffield, England, metallurgy, alloys, martensitic stainless, Christian...
Summary: "know the properties of the materials you intend to use, and use them appropriately for the application."

AFA that article had any useful info re: "Stainless" it is almost as if i had posted an article about "mild steel" and noted how iffy it would be for aircraft use based on paper analysis of A36 (commodity structural steel for buildings) or 1018. Or how bad rebar is too machine. Oh....wait....normalized 4130 is a low alloy, essentially low carbon "mild" steel, too! (see how the words can imply meanings slightly askew to the technical definitions?)

The author comments:
According to a friend who is a retired machinist, stainless steel is difficult to work with. He does not like the material, nor does an A&P friend. Aluminum, on the other hand, is easy to machine until it has been heat-treated.
Hmmm....people with minimal experience, (or whose work is normally "easier" substrates don't like it?
The point is valid for manufacturing - the proper cutting tools to efficiently machine some stainless steels in many conditions are more expensive and have shorter life, compared to similar cost tools cutting in aluminum, and often "milder" steels. Cycle times can be longer than in, say, soft leaded or sulphered steels, and certainly compared to aluminum ("shiny wood"). However, for one at a time customs as homebuilts tend to be, that is a much smaller factor. (Your time is "free"-ish, so use what floats your boat so long as appropriate).
I rather like machining 17-4 PH (which is widely used in aerospace) and similar stainless steels. Also like 4140 pre-hard (which is considered difficult) because it machines crisply, is stable, and makes strong tooling & machine parts without post HT. Contrary to the article, i find most aluminum machines better (more crisply, less gummy chips to adhere, fill the lands, and break tooling) in the heat treated condition compared to dead soft.

So back to the premise of the article: Use materials appropriately.
IOW or OTOH, if you think "stainless" might have an app in your build, find an alloy that meets the structural specs including toughness and specify it in the appropriate condition. ("hardness"). Then decide if the costs warrant any perceived benefits.

smt
 

Pops

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After welding the fuselage, I drill 1/16" dia holes in the tubing lengths and inject with Poly-Fiber tube seal oil. The amount is on the back of the can for how many CC's per foot and diameter of the tubing. Inject with needles used for large animals. Check at Tractor Supply or most farm stores. After drilling, I mark the location with masking tape, inject all the tubing and quickly weld the 1/16 dia hole up to seal. A short tube is hard to weld without blowing a hole in the puddle from the internal air of the tube expanding. If you have tubes under a foot in length its best to just drill a hole inside the cluster so oil from the longer tubes can flow in the short tube.
Takes about quart to do a Bearhawk fuselage. Picture of my fuselage with the marking tape getting ready to weld the holes.
 

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misterpeter

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Hi Pops and thanks for the pic. Are you gas or TIG welding? I often have to drill short tubes before welding, (depending on where and what has to be welded, of course) because of blowing problems, even with TIG. Having said that, with tubing that is under stress, care is also needed choosing where to drill each hole - where the stress is least - or not? What are your thoughts on ‘normalising’ or stress-relieving after welding? If you gas-weld ‘properly’, not such an issue, I guess.
 
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