Building PA-18 fuselage with square tubing

During the rebuilding of wrecked cubs and citabrea's and the long hours of replacing damaged tubing. Grinding fishmouths to fit just right, I was curious why square tubing was not used instead of round tubing. It would have been so much easier to work with. I have been thinking of building a PA-18 fuselage from scratch with square 4130 tubing. Anyone out there have a comment on using square tubing.
Charlie

tubing

more weight for same strength, easier to fit simple angles, but complex angles ( diff. x, y, z ) angles near impossible.

Square tubing

Flysurge,
I see were compound angles would be a real problem, but square would be stronger if the same dimensions were used. A smaller size of square tubing would have the same strength as a larger diameter round tube. You would think that you could cut weight down, not increase it.
Charlie

have you looked at the cost of square 4130? I just bought 60 ft to rebuild some PA16 doors. It was close to $3.60 per ft. for 5/8". I can attest for the complexity of the angles. Piper used 11/16" seamed square tubing on the doors of the short wings and they are almost all rotted out and no two are the same. When I rebuild a fuselage I cut all the sheetmetal off the door frames and replace and then end up building an entirely new door or at least the bottom half and it is considerably more time consuming than fitting round. The cost would be the biggest deak to me plus the availability of the sizes and wall thicknesses.

Charlie,

I am a mechanical engineer and I strongly encourage you not to build a fuselage out of square tubing unless you plan to increase the size and or wall thickness considerably. Square tubing is stronger than round tubing but only in two planes, perpendicular to the flat sides. Round tubing is much more capable of handling loads in 360 radial degrees.

Tubing

Ksecub,
Thanks for the information on the square tubing. I have been wondering about it for years. :roll: I am under the impression that I could use thinner walled square and smaller outside dimensions because the square would be stronger than a larger round tube. I see what you mean by the 360 load factor with round tubing, but wouldn't this be uninportant with a warren truss constructed frame? Another mechanic told me that the old Navy N3N fuselage was made of square tubing. I appreciate your input.
Charlie

NASCAR uses round tubeing for rollcages etc. They should know.

Tim

For a given strength and mass, the round, thin-wall tubing will be usually considereably lighter. Also, if you plan to use rectangular tubing, I srtongly suggest that you carefully compute the 'shear flow' throughout the structure as you do your structural calculations.
JimC

Square Tubing

The Fairchild 24 uses square tubing for longerons.