Building with Square Tube?

Dave, the greatest strength to weight ratio is to be had in round tube, thus it's common use in aircraft.

Rather than make the building project conform to you existing skills, develop the new ones; I also have a background using square tube/stick and wire to weld towers, etc.  Using oxy-acetylene probably won't be much of a learning curve since you should already know what the puddle looks like and the feed rate.  There are some youtube videos showing the OA process up close and they will help you get it right, i.e., not too shallow but not burned through either.  It's not hard for someone who understands welding basics.

I have an AC/DC welder, a mig and oxy-acetylene within a few feet of each other and I strongly prefer  OA to the other two for appropriate projects like 4130.  Of course, the 24 foot trailer I'm about to build will be square tubing and stick welded BUT it is not 4130.  If you buy some flat 4130 strip from Aircraft Spruce and stick weld it together and then test it to destruction in your vice, you'll see why tig and OA are the recommended ways.  It is very prone to cracking when welded that way--don't ask me how I know this!

By the way, you can find square 4130 here if you insist on it rather than round:

http://www.aircraftspruce.com/menus/me/steel_4130square.html

Jay

For someone with your experience learning oxy-acetylene welding should not be that hard.   By the time you tack a fuselage up you should be good enough to weld smaller parts and move on to the fuselage as you gain experience and confidence.  Fitting the tubing for a steel tube aircraft does not need to be as exacting as you might expect to get good safe welds.  Not only would you pay a weight penalty for the square tubing, but the cost may be as much as 3 times higher.  With this savings you could more than cover the cost of attending a SportAir Workshop and gain the confidence you need.

Dave,  Take a look at Pitcairn (sp?) fuselages.  EAA has a bare fuselage at Pioneer Field.  It uses a lot of square tubing.  There are a lot of other good features such as small diameter x tubes rather than heavy diagonals. 

Ross

Check out the prices on that square tubing$$$$ and the weight #####. Learn Oxy fuel welding. There's a reason why they taught us to weld clusters of round tubing in A&P school. Round 4130 tube is simply the best compromise of strength and weight. Plus anything that be made round is cheaper to produce than square. The round tubing is drawn over a mandrel, which is a fancy way of saying turned around a rod while the rod is being forced into the "mother ingot" Sounds perverted but that's how they do it.

How in the heck do they make square tube without welding a seam in it? Explains some of the cost I guess.

My Acro Sport II fuselage is a marvel of light weight and strength. Why mess with a good thing? I'm all for experimentation but sometimes a good thing is just that.

Dave,

I do not agree with some of the respondents who say there is an automatic weight penalty when using square tubing over round tubing.  Many years ago in the early 1970s, I designed a steel tube fuselage for a very light aircraft (it was an ultralight before that term had been invented--you can see a photo of the airplane on the Gallery page of my website www.sportaviationspecialties.com). 

I was interested in minimizing the weight as much as possible, so I put a lot of effort into the design of the fuselage, carefully analyzing the stresses on each member of the fuselage truss using rigorous engineering methods.  (Bear in mind, that this was before any computerization of the process--it was all done with a scientific calculator). 

What I found was this:

1.  In optimally-designed trusses, the individual truss members are subjected only to pure tension or pure compression loads, no matter whether the truss is bent one way, bent the other, twisted one way, or twisted the other.. 

2.  When a truss member is under a tension load, the best strength/weight option for tensile strength considerations is sometimes round tubing, as determined by its outside diameter and wall thickness.  However, because 4130 tubing is available in only discrete sizes, if a given round tube does not have an adequate cross-sectional area and the next larger size (either tube diameter or wall thickness) has excessive strength, it may be possible to select a square tube size/wall thickness that has a tensile strength in the middle.

3.  However, when that same member is under a compression load (which will most always happen in one loading condition or another), the limiting condition is often buckling strength rather than tensile strength.  Whether the tube buckles depends on several factors including how long is its unsupported length, the modulus of elasticity of the material, and the "moment of intertia" and  "radius of gyration" (engineering terms) of the tubing cross-section.  Here, the choice of square vs. round is not as clear.  Assuming we stick with 4130 steel, for a given compression load and a given unsupported length, the lowest-weight situation is obtained by selected tubing that has an adequate radius of gyration for its weight-per-foot.  It may turn out to be round tubing or square tubing of a given size and wall thickness.

4.  There are other practical considerations in designing tube trusses, as listed below.

a.  To reduce compression buckling problems, other truss members can be added to reduce the unsupported length (like a jury strut on a wing).  Or, local stiffening of the cross section near the middle of the unsupported span can be done.  However, the additional weight and complexity may offset their value.

b. Some truss members such as longerons span several bays and engineering analysis shows that the loads in various bays is different.  That would mean that if we were being rigorous, we would select a different tubing size for each bay.  However, since that is impractical and undesirable, the tubing cross-sectional shape and wall thickness are a compromise.

c.  For lightly-loaded structures, it is sometimes possible to select a tube size and shape that will meet the required load requirement, but will be too fragile to withstand normal handling, moving in/out of a hangar, etc.

d.  Cost is always a factor and thus a practical consideration is to minimize the number of different tubing sizes used in the design.

e.  The weight of a fuselage truss is sometimes not the deteriming factor in the total weight of the airplane.  Often greater weight savings can be obtained in other aspects of the design.

In the design I did back then, I wound up selecting square tube for the upper longerons (most often subjected to compression loads) and round tube for the lower longerons (most often subjected to tension loads) and for uprights and diagonal members.

And you are right: joint construction using square tube is much easier than with round tube.

Regarding the welding process you use, MIG is being used in aircraft manufacturing facilities (ex: CubCrafters and RANS) daily, so it can be successful.  However, in those facilities, the welders are well trained and perform the same operations day after day; thus they have a very carefully-controlled process.  Not casting any question on your MIG welding ability, but it is much easier with MIG than with oxyactylene to produce welds that look good but don't have good penetration..  I do encourage you to learn to do oxyacetylene welding.  Either that or learn TIG welding, understanding that additional considerations exist in welding 4130 using TIG.

So, I encourage you not to write off square tubing without doing some good engineering analysis.

Hope this helps!

Mike Huffman
Sport Aviation Specialties

BTW, square 4130 tube is available as seamless drawn tubing, same as round tubing.

Thank you Mike.  I really do appreciate your extensive reply.   That was exactly the kind of information I was hoping for.  I fiqured it was series of compromises for any one situation too.   I will take everyone's advice and attempt to learn OA better.  I can't really compare the working environment for building an airplane to that of trying to patch a rusted out pickup floor at significantly below freezing temperatures, which was a real PIA and my only attempt.  I can tell you I was not happy with the outcome.

I've been reviewing one set of plans (that of the STOL King) and I can tell you that if I do build that... it will have some mods here and there.  I have always had the luxury of over-building with very little penalties... it will hard not to do that on an airplane where every bit of added weigh carries a performance penalty.

The other question related to the use of something other than 4130.... any comments on this?

Thank you again.  Dave

Dave;

My airplane 'de joure is a Hiperbipe; a 1970's design with a fuse made almost totally of square tube...it's damn strong and to date has saved several pilots due to it's design. N774HB went through trees after an engine failure during a low-level acro routine and the pilot walked (climbed) to safety!

That said; learn how to weld every size and shape tubing well before building an airplane...take an EAA welding class, go to a local Junior College or High Skool, etc.

As to improving an existing design; if you do structural mods on an airframe do so remembering that the airframe load will be the same, mods to make the airframe "Stiffer" or Stronger" might move the load somewhere that you don't want it to be...be cautious about re-engineering something unless you are a re-engineer!

                                          JMPO

                                                         Chris