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.


you are probably correct there.

I'm slowly restoring an Auster built from T45. that is an alloy which
preceeded 4130 and has a tensile strength of 45 tons to the square
inch.
you get to appreciate lots of things beadblasting a 50 year old
fuselage. the thing I appreciate most is that the alloying components
have protected the steel from corrosion so much that a simple bead
blast and a repaint is all that 99% of the entire fuselage requires.

you get the same corrosion resistance with 4130.
moisture that only sees light surface corrosion in 4130 will probably
deeply pit a mild steel component.

my tailwind fuselage is 4130 and has no discernable rust in 18 years.

for corrosion resistance alone I'd go the extra mile for the
recognised alloy.
Stealth Pilot
Australia