sqare tubing vs. round

In the interest of generating some discussion on the topic I would make some observations. First I'm assuming you are referring to welded steel construction as opposed to aluminum tube fastened with sheet metal gussets.

First square steel tubing would be a lot heavier than round stock because of the amount of material required to form a square cross section compared to a round one.

Also I think the square stuff is a lot more expensive for the same reason, more material required to form the square cross section a similar sized member.

Perhaps some of the engineers could chime in here, I don't think the square tube has the same torsional stiffness as a similar sized round tube because a square tube is essentially made up of 4 flat sides that can flex under torsional loads whereas a round tube translates torsion into a tension load around the tube.

I hope I didn't tell you any lies, but these are my thoughts on the subject.

I'd like to share my experiences shaping round tubing here if I may. First of all forming the correct shape on the end of a steel tube for welding does not require any expensive tools. The easiest way to do it is buy a good bench grinder. Put a 1/2 inch wheel on one side of the grinder and a 3/4" wheel on the other. Dress a full radius on both wheels and use the wheel that matches the size of the tube being matched and grind away. When fitting fuselage parts for a truss type fuselage fit the longer tubes first, then if you screw up and make a tube too short you can fit it somewhere else without wasting that piece of steel.

Rob.

I may someday attempt to scratch build an "afford a Plane" ultralight which has a square tube aluminum structure.

If interested, look up

affordaplane.com

I've got two teenagers that drive cars, so it may take a while to start.

Bill

Bert Howland did some work along these lines with his ultralight-type planes (H-1 through H-5). He used TIG-welded square aluminum tubing for his fuselage structures, and if memory serves the fuselage for his H-3 weighed all of 18 lbs!!! Search for him in the Sport Aviation archives, there were some interesting articles. Sadly Mr. Howland is no longer with us but there are some examples of his various designs flying, and I think some of his plans are still available from somewhere. The H-3 is a very pretty little plane!

If you are talking about a welded steel-tube truss structure, square tubing has indeed been used successfully.  As you mention, the joints are often easier to fit because the cuts are straight.  And, it would not necessarily be any heavier.  Here's why:

A truss structure gets its strength from forming triangles.  One of the unique properties of a properly-designed truss is that, when it is loaded in bending (in any direction) or torsion (in either direction), each of the individual pieces of tubing is loaded either in pure tension or pure compression.  That is, there are no bending or torsional forces on any of the truss members.  That is why a truss can be made very light using thin-wall tubing, but still be very strong and very stiff.

So, from an engineering standpoint, what does it take for each of the tubing members to withstand the tension and compression forces.  Well, first, an engineering analysis would need to be done to establish the magnitude of those forces under the design loading conditions.  For the tension loading condition, it is then a simple matter to calculate the strength of a particular size of tubing based on its cross-sectional area and the tensile strength of the steel.

However, the compression loading condition more often determines the tubing size required.  The ability of a piece of thin-wall tubing to withstand compression loads relates to how prone it is to buckling, rather than pure strength.  So, what controls buckling?--answer: the unsupported length of the tubing between tube clusters and the cross sectional shape of the tubing.  Obviously, the longer the unsupported length, the more prone it would be to buckling (that's why wing struts often have a "jury strut" about halfway out--to stabilize the strut under negative-g loads.  For example, a long unsupported length under a given compression load would require a larger diameter round tube to support the load without buckling.

From an engineering standpoint, a better way to say it would be this: the longer the unsupported length under a given compression load, the greater the cross-sectional "moment of inertia" or "radius of gyration" would need to be.  These are arcane engineering terms, but suffice it to say that a 1" x 1" x .035" wall square tube has a higher moment of inertia than a 1" diameter x .035" wall round tube.  So, if the unsupported length and the compression load required a higher moment of inertia or radius of gyration, I might be able to use a square tube instead of a round tube of the same dimensions.  Yes, the square tube will weigh more per foot than the round tube.  However, a proper engineering design would involve trading off the required buckling strength against the weight and it might turn out to be more advantageous to use square tubing. 

Some years ago, I designed a welded-steel-tube truss fuselage for an ultralight, where minimum weight was absolutely paramount.  It turned out to be more advantageous to use square tube for the members of the truss primarily subjected to compression loading (upper longerons) and smaller round tube for members primarily subject to tension loads (lower longerons) and short cross members.

Of course, engineering design is always a compromise between not only strength and weight, but also cost, ease of fabrication, serviceability, and other factors.  It is easily possible to design a light aircraft that will handle all its required operational loads just fine, but be so fragile that it cannot be easily moved in and out of the hangar without damage!

Wow! I still feel obligated to point out that square 4130 tube is a bit limited in the selection of sizes and costs about five times as much as a comparable round tube.

Just more proof it's all about trade offs. Remember the old expression "cheap, quick, or good" You can have a combination of any two!

Hi John

As Mike correctly pointed out, from an engineering point of view the important parameter is the moment of inertia of the section, and since we are interested in getting the maximum strength we should compare the round and square sections for a given weight.  I think the best way to understand it is by looking at an example:

Let's take a square section 50mm X 50mm with a wall thicness of 2mm.  A round section of the same weight and wall thickness would have a diameter of 63 mm.   Both sections will have the same tensile and compression strengths.  But the round section will have a moment of inertia that is about 20% higher than that of the square section.

Bottom line:  For a given weight and wall thickness, the round section will be substantially stronger in bending, torsion and buckling.

Hope this helps.