Friday, November 24, 2006

AV - 'Line' Oil


Back before Randolph's there was Lyon Paint Co., from somewhere in Ohio (as best I can recall). Lyon was one of the first companies to specialize in AIRCRAFT lacquers and enamels.

Structures fabricated of welded steel tubing were typically given a dose of linseed oil before being sealed by a threaded plug (near the tail), bolted plate (at the front, typically behind the engine mount attachments) or welded plug (all manner of struts; N-strut, cabanes, lift-struts, etc).

Why linseed? Because it is a 'getter' for oxygen; the linseed oil (ie, made by pressing flax seed) plasticizes as it oxidizes (ie, absorbs the oxygen), turning into a thin layer of varnish.

No oxygen means no rust.

So how did plain old-fashioned linseed oil become 'line oil?'

It didn't. It was always LYON oil. Specially refined linseed oil with a neutral pH (ie, neither acidic nor alkaline). Back then, common linseed oil as used for finishing furniture, improving the flowability of oil-based paint and so forth, was never meant to be used on steel and its pH was not a factor when applied to wood.

So Lyon's 'aircraft-certified' linseed oil became the standard for doping the interior of steel tubing. Nowadays, any high quality linseed oil will serve since all are now close to neutral with regard to pH.

Lyon Paint Co. used the head of a lion as their logo. If you'll examine air-race photos from the 1930's you should be able to spot their logo.


Sealing the interior of welded tube structures assumes the structure can be closed; sealed off from the atmosphere. When that was not the case the interior of the spar, strut or tube was painted, typically with an anti-corrosion paint, diluted about 5:1, applied by flooding (ie, filling and then pouring out) or by 'sponging' -- using several small pieces of sponge tied to a length of marline or rib-lacing cord. The sponges were saturated with paint as they were pulled into the tube and then pulled through.

Sponging was the preferred method since the interior of seamless tubing always has some residue of lubricant used in the forming process. Sponging served to 'brush' the anti-corrosion paint onto the surface. Of course, you couldn't sponge a tube if it had any interior obstructions. Flooding was the preferred method for shorter sections, the paint usually preceded by one or two floods of solvent followed by an air-blast. Such messy little chores often fell upon the shop gopher (which was me, fifty years ago :-)


The need to protect the interior of the tubing reflects the propensity for mild steel to rust. The standard procedure when repairing or recovering an early fuselage or landing gear was to pull the plugs, usually a socket-head set-screw installed in a weldment, and see if there was any liquid 'line-oil' left inside. Most airframe manufacturers cited how much 'line-oil' was used (usually about a pint) and where to pour it in , after which the fuselage was tilted and rotated, the plug(s) removed and any residue allowed to drain out.

Thanks to a whiff of chromium 4130 is less prone to rusting than 1025 and during WWII many structures did not receive 'line-oil,' although some critical parts such as engine mounts and landing gear yokes were pumped full of dry nitrogen under pressure via a Schraeder valve (think of an industrial-grade tire valve) and fitted with a simple pop-up pressure gauge. Any drop in pressure was good evidence of a cracked weld.


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