Question(s) about Steel Tube Fuselage Design

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GarandOwner

Well-Known Member
..............First, the curved shape is inherently less stable that a straight tube so you'll need to use curved column design requirements. This can get a bit tricky but is doable.................
I don't know that I would agree with this statement, curved shapes are not necessary less STABLE, it depends on your configuration. While you don't want every member in your design to be curved (mainly due to cost of manufacturing and human error in forming), in some applications a curved tube can actually increase strength. Think about this, the strongest structure out there is an arch.

If you have transverse loading, then a bent tube IS better, because some of the load is transferred into an axial loading. If you have pure axial loading, then a straight member is better, because it wont be as likely to buckle.

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lr27

Well-Known Member
In this case, unless there is something stabilizing that curved tube under compression, it's going to buckle a lot easier. OTOH, I suspect that if the tube is pretty thin, fabric might pull it in if it was straight, but maybe not if it was arched. Once it's pulled in, it would probably buckle just as easily.

Tom Kay

Well-Known Member
Well, kinda what I expected, a few pro's and cons for each approach.

Thanks for the replies. Tom.

Fluffy

Hey

I've been browsing this forum for a while, and as I'm also trying to figure out how to design my own steel tube fuselage I noticed this topic. Not sure if this helps anyone, but the book The Race Car Chassis by Forbes Aird (published by HP Books) goes through the basics of steel tube chassis race cars. While the book isn't about aircraft (though 1/2 of the sources in the bibliography are aviation-related publications) most of the content (from what I can tell) on space frame theory and design is applicable to aircraft. It's a fairly non-technical book with only a few basic equations. I bought a copy at Chapters a few months ago for about $25. If you need formulas for stress analysis or want to know some more advanced theory, NASA's got a structures manual available online at Astronautic Structures Manual Volume I. I've got a bunch of textbooks from school so I haven't had to use the NASA manual yet, but it appears to be very comprehensive Tom Kay Well-Known Member Fluffy; Thanks for the lead. I guess the 2 applications of space frames would be quite similar, wouldn't they? There's a couple copies of the book on Ebay, too. Tom. GESchwarz Well-Known Member Any suggestion that fabric is going to keep structural steel tube in line is crazy. BBerson Well-Known Member HBA Supporter EAA had a builders tip in an old EAA publication: "Strike the tube with a rubber mallet to set a slight outward bow, then the fabric will pull the tube straight" PTAirco Well-Known Member Hey I've been browsing this forum for a while, and as I'm also trying to figure out how to design my own steel tube fuselage I noticed this topic. Not sure if this helps anyone, but the book The Race Car Chassis by Forbes Aird (published by HP Books) goes through the basics of steel tube chassis race cars. While the book isn't about aircraft (though 1/2 of the sources in the bibliography are aviation-related publications) most of the content (from what I can tell) on space frame theory and design is applicable to aircraft. It's a fairly non-technical book with only a few basic equations. I bought a copy at Chapters a few months ago for about$25.
Almost all the race car chassis I have seen make use of tubing that is also stressed in bending, as beams, rather than typical aircraft structures that avoid bending loads like the plague and try to keep everything tension or compression through proper triangulation. (This is why it is damned to awkward to get into some steel tube airplanes.)

Anytime you use steel tubes as beams the weight goes up; and you have to pay a lot of attention to avoiding stress concentrations at the ends of the beams or your welds won't last long. (Remember the Liberty ship breaking in half in WW2?)

Fluffy

Member
Almost all the race car chassis I have seen make use of tubing that is also stressed in bending, as beams, rather than typical aircraft structures that avoid bending loads like the plague and try to keep everything tension or compression through proper triangulation.
The book I mentioned says it is the same for a race car chassis: you don't want to have bending, just tension and compression. Race cars just do this a bit differently, using a flat sheet of a stiff material (such as carbon fibre sandwich panels) bonded or fastened to the steel tubes instead of a diagonal cross member. Both the plates and the diagonals work to prevent/reduce deformation of the frame. Most of the steel-tube fuselages that I can think of were WW1 - pre 1960 when composites weren't really available so stiff and lightweight plates weren't an option. In addition, welding a steel tube to act as a diagonal is a lot easier than making and installing a panel, it's easier to visualize the load-path, and you don't need to worry about fastening it your frame.

If I remember correctly, I think I saw in my Aviation Spruce and Specialty catalog a book on designing steel tube fuselages. However, as I haven't read or seen it I can't vouch for it's content.