Gentlemen,
I have no real experience and very little formal training. The
absolute reliance on 4130 is a wives tale. Very serviceable aircraft
were and are built regularly with mild steel tube. For amateur
construction mild steel makes more sense than 4130. On the Yahoo
Piper Cub site in the files section is a factory drawing of a Super
Cub fuselage. It is predominently mild steel in the sizes of 3/8-7/8
..035 wall. These planes did not rust away or disappear from tubing
failure. The US military materials book from WW2 notes that after
welding 4130 is reduced substantially in tension, 74 thousand #/ as
compared to mild steels 54 thousand #/, and the compression of tubes
is a minor difference in the lengths we deal with. Please reference
Bob Whittier's book on tubing for this discussion.

The problem of weight moving backward is real but not a reason for not
using mild steel. The Circa Nieuports with thier aluminum tubes work
fine, it is a matter of design. A second coat of paint, a big swivel
tail wheel, or balancing the elevator will be as much of a change and
many homebuilts have these changes made over the plans regularly. If
a guy can't keep control of his C of G then he needs help. I have
done a design study using .049 mild steel to replace .035 4130 in a
standard low wing Warren truss fuselage. The effect on the C of G was
neglidgeable due to the increased weight in the forward fuselage were
the largest and more cocentrated collections of tubing existed. The
added weight in the tail feathers was compensated for with a movement
of the speced A65 forward 2-3 inches. I've decided to use wooden tail
feathers and actually save a lot of weight.

Many homebuilts do not come close to the designers empty weight and
operate over stated design grosses all the time, even those built from
4130. Or the guy who puts an extra hundred #'s of Lyc. and electrics
in an A65 design is never questioned so severly as a guy who wants to
use mild steel tube accepted and certified for aircraft construction
by civil and military specs for 70 years.

Many fine planes were built and designed using common sense, alternate
materials and the engineering from established designs. Pete Bowers
discussed this in his book on Homebuilts he wrote in the seventies
prior to the litigation era we now live in. In fact I have a
collection of fuselage plans from the past and they would appear to be
designed in 1930 out of light gauge mild steel and copied since.

Recently I saw some sitka from an established aircraft supplier. I
wouldn't have used it for ladder rails. I have hand picked perfect
quarter sawn Western Hemlock for a fraction of the cost at a local
building store that was far superior to the aircraft grade stuff that
cost more in brokerage fees than my wood. Get educated about
inspection and alternatives using established experts and build around
the limitation. A can of line oil or linseed oil used according to
Tony Bingelis in a properly ventilated fuselage will take care of
rust. Probably good idea with 4130 as well.