square vs round tube

[Guest micgrace]

I was thinking of changing my design from traditional round 4130N tube to square 4130N tube.

 

This has several pros and cons.

 

Round, lighter for same size, less gussets, much tube notching (yuk) and difficult to do compound angles. Much time spent in joint preparation. More variety of sizes.

 

Square: heavier for same size (corners)

 

slightly less strength than round, requires gussets to compensate. Much

 

less time in preparation. Straight cuts, compound angles not a problem.

 

Simple fittings. easier welding i.e don't have to chase the post

 

around. More expensive than round.

 

Could probably knock up fuselage (from scratch) in some 20 hours (square) versus est 100+ hours in round. excluding much more time consuming construction of fittings for round. Square (little more than 0.095" / .125" angle, channel bolted/welded on 4130N or 304SS as appropriate)

 

Of course, it's not as aerodynamic i.e. square cornered fuselage, somewhat heavier (50kg 4130N round, versus some 60 kg square) Only 10kg in it, NO loss of strength (provided gussetts are used)

 

And not the least, not traditional.

 

Any thoughts anyone?

 

Micgrace

 

 

[Guest micgrace]

Something interesting on the above post. Square tube yield 860MPa, round tube 660MPa

 

Why is this so? Square tube is round tube cold rolled. The extra forming process is responsible.

 

Anyhow,

 

gone back over my design and incorporated sq. tube. Soon I'll be moving

 

into our "purchased" home and then order materials and start. More

 

posts to follow when I'm on the scrounge for bits & pieces (engine/instruments/wheels/brakes/radio etc) the stuff that really costs the money

 

Micgrace

 

 

[pylon500]

In our field of aviation (low cost and light weight) I would have thought the heavier more expensive (and harder to find) square tube wouldn't stand much chance.I

 

tend to look more from a production point of view sometimes, and had

 

thought of square tube, but I would have to use enough of it to make it

 

worth while.I'm still thinking.....Arthur.

 

 

[Guest micgrace]

Main drawback with square is the reduced welding area available.

 

Although easily compensated for by gussets at the expense of some

 

weight.

 

However the production advantages (simplicity in construction) I believe to greatly outweigh this disadvantage the more I think about it.

 

Plenty of various sizes available from Performance Metals Australia. Optional test certificate if needed, full traceability.

 

Micgrace

 

 

[facthunter]

This is an old post so I hope you revisit it. Iam interested in getting information on welded steel truss fuselage design criteria so maybe someone can point me in the right direction. Need data on compression load analysis for steel tube sections , stress relieving techniques post welding, good design philosophy for a modern version of taylorcraft/auster piper cub style of construction. Iam very impressed by the in service reliability of the skyfox--gazelle structure (fuselage)considering the size of the tube used. I cant see any other structure being as easy to mount seats, seat belt attach points, U/C parts, engine mount sub frames,lift struts, wing attach points etc I know I'm old fashioned & crude but Ducati still use welded steel frames.So. Hopefully...