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. 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? And
what are the other factors that favor aluminum over steel for the
frame? Is this because of the frame shape?
Thanks,
Eric

Eric R Snow wrote:
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. 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? And
what are the other factors that favor aluminum over steel for the
frame? Is this because of the frame shape?
Thanks,
Eric

I'm just guessing, but my guess is that there are several reasons. The
first that comes to mind is galvanic corrosion where a steel frame meets an
aluminum skin due to contact of dissimilar metals. The second is thing that
comes to me is that an airplane has to heat and cool just like any ship and
they may want the metals to have similar temperature properties so they don't
overstress any rivets. Planes go into very hot and very cold places. If the
skin stretched a lot but the airframe only a little maybe it would shear
rivets. It also may be something to do with the way their shops are tooled,
maybe they only want to work with aluminum ..

GWE

Aluminum is stiffer in many cases because for the same weight the
thickness is greater. IIRC aluminum is about 1/3 the weight and 1/3
the stiffness. So if a sheet of steel would need to be .030 inch
thick to provide the strength, the aluminum would be .090 thick and
because it is so much thicker, the aluminum would be stiffer. Since
the aluminum is stiffer, ribs can be further apart.

Dan



Eric R Snow wrote:
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.
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? And
what are the other factors that favor aluminum over steel for the
frame? Is this because of the frame shape?
Thanks,
Eric

I'm not an metallurgical expert by any means, but my under standing is that
because aluminum is lighter than steel, one can make aluminum tubing that
weighs the same as steel, but is a good deal larger in diameter and in wall
thickness. This results in a stiffer/stronger tube while weighing the same
(or less, depending what trade offs get made).

When aluminum frame bicycles first appeared using tubing of the same size as
steel tubing (and bonded lugs IIRC), they were horribly flexible and while
comfortable, were pretty scary in a pack sprint or while going down hill
quickly. When Klien came out with the first big tube aluminum bike in the
early eighties (?) it was a revelation. The bikes were light and really
stiff - too stiff for many. anyhow, the market (mostly Cannondale for a
while) took off and now the world is lousy with aluminum frames.

Peter

"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. 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? And
what are the other factors that favor aluminum over steel for the
frame? Is this because of the frame shape?
Thanks,
Eric

"Eric R Snow" wrote in message
...
From what I've read the stiffness, by weight, of steel and aluminum is
almost equal.

Correct.

So when building a tubular space frame there's really no
advantage, from a weight standpoint, to using aluminum over steel.

Slight. There are three examples you can read about, if you can find the
info. Mid-'60s Bobsy sports-racer; Porsche 917 (evolved from steel to
aluminum to magnesium); and a recent hillclimber built by the Locost group,
owned by a guy named Champion.

You're dealing with complex statics. Aluminum's advantage is in bending --
"plate stiffness," based on the third-power phenomena involved when you bend
a plate, sheet, or rod. The implications of that for a tubular space frame
have some practical interest but are confusing from the theoretical side; in
theory, an aluminum space frame has no advantage over steel. In practice,
you get into columnar stiffness and bending issues that can, if everything
else is perfect, give aluminum a very slight advantage.

We discussed this here once before. Somebody who was sharp on the structural
engineering side had some interesting things to say about it. But it was
pretty arcane for most of us. The point is, aluminum ain't worth it unless
you have a very, very special reason to use it.

But
modern aircraft are built using aluminum almost exclusively for the
frame and skin. Is the reason this makes sense due to the skin?

Plate stiffness.

By how
much does making the skin a structural component favor aluminum?

I think it's something like 3:1, but I'd have to look it up.

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

Shape is everything. If you gain an advantage from plate stiffness, aluminum
will make a lighter frame. Aluminum tubs are lighter than steel tubs. But
space frames are not necessarily lighter.

In theory, a space frame is loaded ONLY in tension and compression. Thus,
there is no theoretical advantage for aluminum. In practice, tubes bend a
little and tubing walls are columns when loaded in compression. Depending on
the specifics of the frame design, aluminum can give a very small advantage.

Don't worry about it. You'll drive yourself up a tree. Stick to steel,
either 1020 or 4130. When you start to read about it, you'll see why those
two are used.

The Brits used some manganese-steel alloys in the '50s and early '60s.
Again, don't worry about it. It's old news.

--
Ed Huntress

wrote in message
ups.com...
Aluminum is stiffer in many cases because for the same weight the
thickness is greater. IIRC aluminum is about 1/3 the weight and 1/3
the stiffness. So if a sheet of steel would need to be .030 inch
thick to provide the strength, the aluminum would be .090 thick and
because it is so much thicker, the aluminum would be stiffer. Since
the aluminum is stiffer, ribs can be further apart.

Dan



Eric R Snow wrote:
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.
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? And
what are the other factors that favor aluminum over steel for the
frame? Is this because of the frame shape?
Thanks,
Eric

Also because its thicker it is easier to connect together.
Pat

"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