| Similar to hi po rag and tube aircraft.
I read some years ago that fuselage frames for highly sressed rag and tube aircraft were ported into every tube segment before welding up. Once welded, the frame was totally filled with boiling linseed oil, allowed to cool, and then drained as a corrosion preventative.
Subsequently, they were filled with nitrogen to well above atmospheric presure, to purge oxygen and further avoid corrosion. Pressure was monitored via a guage somewhere on the frame, and served as a warning of frame fatigue. Dont recall whether the guage was visible in cockpit, or only ground accessible.
Suspect that this info came from an early kitplanes mag, from a welding series by someone who had been in the game for many years.
Not quite oil/coolant transfer through the frame but a neat means of detecting fatigue or cracking at an early stage. |