On 17 Mar 2004 18:19:11 -0800, (Leon McAtee)
wrote:

Why do we homebuilders use 4130 tube? My old Aeronca does just fine
being made of mild steel. A bit of napkin calcs says that going up
just one tube diameter for the size tube we normally use, the area,
and the strength/weight goes up between 15% and 20%. This pretty
much offsets the difference in tensile strength between 4130 N and
1026, and more than offsets it for something like 1040. The "mild
steels" can be welded using MIG or TIG with little worries about HAZ
and since we are not heat treating the 4130 to obtain its strength
advantage it seems to me to actually be a poorer choice for amateur
aircraft construction.

For a typical rag and tube plane, properly choosing the tube sizes
should result in a weight gain of less than 15% for the same strength
which is, what, around 20 pounds for something like a Tailwind or
Aeronca. This to me seems like a good trade off to eliminate the
possibility of cracked welds due to poor technique. Not to mention
maybe saving a few bucks and being able to get the steel locally.

Could the availability of cheap WWII surplus steel have created a
tradition that has persisted in spite of other possibly superior
options?

There's really nothing wrong with using mild steel tubing to construct
a fuselage with the following caveat's: 1. That you properly size the
tubing and wall thickness to give you the proper strength. 2. That
you can find it.

I've read several times over the last few years that mild steel tubing
is almost as expensive as 4130, but I don't know that from personal
experience.

The last thing I'd mention is that because 4130 is stronger and
stiffer, you can probably use tubing that is lighter than mild steel
to construct your fuselage and still have a strong fuselage. In
airplanes, saving weight is almost a formal religion. The more weight
you save, the more weight you can carry, or the better the performance
or both.

Corky Scott