"Eric R Snow" wrote in message
...
| From what I've read the stiffness, by weight, of steel and aluminum is
| almost equal. So when building a tubular space frame there's really no
| advantage, from a weight standpoint, to using aluminum over steel.

An aircraft skin consists of several layers of aluminum. The outside
layer is soft aluminum, a sacrificial layer than can corrode, be polished,
and tolerate blending of scratches. Inner layers are different alloys, and
in different thicknesses at key points, to provide the strength needed.
It's really cool to look at a body panel and see the small marks made by the
mill to create different thicknesses.
The other reason is weight. Aluminum, carefully arranged, can be as
strong as necessary. Steel can be stronger, obviously. I seem to recall
that the Russians made their shuttle carrier (Anton AN-225?) out of
stainless for the strength needed. Massive bird, by any standard. It's
also a major gas hog. If the plane were steel, in a monocoque design (vs. a
frame) then more gas is needed. More gas, more weight, stronger and bigger.
Continuous circle, so aluminum is by far the better choice. Go stand on a
pair of soda pop cans. Providing you keep your feet flat, the cans will
hold you up. Beer cans are a lot thinner, so that trick is a lot harder to
do with them, but it can be done. Having a board over both helps.

But
| modern aircraft are built using aluminum almost exclusively for the
| frame and skin. Is the reason this makes sense due to the skin? By how
| much does making the skin a structural component favor aluminum?

The strength of an airframe is based on the fact that a cylinder is one
of the strongest shapes there is. If you pressurize the cylinder, it wants
to be even stiffer, as the stretching makes it less flexible. Aluminum is
easier to shape and work with than steel, and can tolerate all kinds of
fatigue that steel won't. On older small planes like the 727, the skin is
thin enough that trimming it took shears. Skin panels on the bigger birds
are almost 3/8" in certain places like around the wings and high stress
areas. Not something trimmed by hand, that's for sure! The stress of
pressurization that the aluminum can stand in the radial direction is
excellent, but in order to attach to it, there needs to be an underlying
frame to spread the load and make it still enough on the ground to be able
to hold seats, cargo, wings, and so forth.


And
| what are the other factors that favor aluminum over steel for the
| frame? Is this because of the frame shape?

Aluminum is the best balance of weight, price, fatigue life, and many
other criteria. Titanium is good stuff and has very specific applications
on modern airplanes, but is very expensive, hard to shape, and costs a lot.
Stainless gets used on planes too, but it's almost as hard to use as
titanium, in addition to the weight. Magnesium has some issues, one of them
being fireworthiness. Steel is way to heavy, slightly harder to shape than
aluminum, and transferring loads is a whole different ball game. Don't know
those details much more than that, as I just build the planes, I don't get
to spend much time designing them them unless I'm trying to help engineering
fix a boo-boo.

| Thanks,
| Eric