Something to measure physical pressure.

Weight will be an issue. But that will also be part of the
comparisons.
Lou



My only question is how much weight is added replacing fabric.

Richard

"cavelamb himself" wrote

He was asking about test samples, not finished product.

For a replacement for fabric skin?

So if it punches good, it's ok, right?

My only question is how much weight is added replacing fabric.

My take was that he wanted to test alternative materials, like how strong
would fiberglass have to be for putting on a wing, instead of fabric. Or,
how strong would plywood have to be to replace fabric. Add to that many
different materials, and many different applications-all over the plane.

If that is not the jist of it, so sorry, I guess.

If I am right, then I don't see how you can get around testing the
alternative material in a "similar to the real application" type of test
like I described. Creativity would need to be applied to figure out other
tests for other areas.
--
Jim in NC

On Wed, 31 Oct 2007 10:28:53 -0700, Bob Kuykendall
wrote:

On Oct 31, 10:02 am, Orval Fairbairn
wrote:

You should be able to find that information in the materials handbooks.

I would start with a Google search for "Forest Products Laboratory",
which is the central source of information on wood products. They test
materials and publish the results.

That's true as far as it goes. However, the design and development of
sport aircraft occasionally demands that one innovate beyond the
boundaries of what established laboratories and institutions have
deigned to test and approve.

Even fiberglass strength varies over quite a range depending on the
ratio of fiber to resin, fiber orienttion between layers, and type of
resin.

Plywood layups also vary some what.

Roger (K8RI)

Consider the practice of using cellulose fiber composite panels for
the reinforcement of truss junctions in chordwise wing members. First
developed in the 1930s, this practice was validated in a successful
glider design and is currently under evaluation for at least one light
sport aircraft.

However, it would never have seen the light of day of some poor guy
hadn't snipped a bunch of wing rib gussets out of cereal box cardboard
and tried them out. Necessity is the mother of invention. Theory often
follows practice.

Thanks, Bob K.

On Wed, 31 Oct 2007 05:19:17 -0700, Lou wrote:

Well Roger, I was really just looking for a couple of simple
answers. Seeing how this is an experimental hobby, I thought
that I would compare the strengths of the plywood that the designer
put on the plans (old design) against some newer materials and idea's.
It may cost me a few bucks but I want to measure strength and breaking
points against each other.

Wellll... for an unscientific and relative comparison that might be
considered a bit crude but it is based on the way it's done in the
labs: We just have better equipment and measurement techniques. it
should even work with doped fabric.

You can get relative numbers this way to compare and these should work
with fiberglass, plywood, and even spruce scarf joints.

You would need to decided on the spacing between supports but take a
pair of pipes or even 2 X 4s and lay the sample over them. Place a
pipe or 2 X 4 over the sample and then start placing weight on the
pipe or 2 X 4. You can measure the deflection per pound of Kg and
that required for failure. This would also work in two axies on a
scarf joint. Of course with a two layer lay-up or thin plywood the
supports would need to be relatively close together. The important
thing is to use the same spacing and equipment for the different types
of material. You could probably come up with some methods a bit more
specific with a web search.

For pull I'd just take two pair of flat metal plates. create some
coupons or wood sections and clamp them in place on one end with a
pair of plates. On the other end use a pair of ViseGrips to clamp the
metal plates on the sample. Then hang a weight from the ViseGrips via
a scale.Keep adding weight. (be careful not to crush the sample when
using ViseGrips) You can measure stretch/elongation, creep, and
failure this way. It is crude, but should give a pretty good
comparison between materials.

For those not familiar with creep it is simply elongation or stretch
measured over time. This one can be surprising when using fiberglass
and resin at elevated temperatures like you'd get out in the sun on a
hot summer day. You can also find why it's not a good idea to clean
the wings with a solvent like acetone. :-))

I saw a beautiful plane that had this done. The wings went from works
of art to having the outline of every rib visible.

Roger (K8RI)
Lou

On Nov 2, 12:27 am, "Morgans" wrote:
"cavelamb himself" wrote

He was asking about test samples, not finished product.

For a replacement for fabric skin?

So if it punches good, it's ok, right?

My only question is how much weight is added replacing fabric.

My take was that he wanted to test alternative materials, like how strong
would fiberglass have to be for putting on a wing, instead of fabric. Or,
how strong would plywood have to be to replace fabric. Add to that many
different materials, and many different applications-all over the plane.

If that is not the jist of it, so sorry, I guess.

If I am right, then I don't see how you can get around testing the
alternative material in a "similar to the real application" type of test
like I described. Creativity would need to be applied to figure out other
tests for other areas.


Me too. Do ragwings derive any of their strength from the skin?

If cloth is an option then the wing skins are so thin that they
will not carry a compressive load--they'll just buckle instead.

I also don't see how you could load cloth in shear, other than right
where it is glued, or when you're cutting it. Testing the strength of
the skin-to-underlying-structure bond is very important. But since
that wasn't the questiont, I'll continue on.

That leaves tension.

You can take strips of each material to be tested, rig up a clamp
for each end, and use that arrangement to hang a bucket. Fill
the bucket with weights until the sample fails. Weigh the bucket.
Keep you sore toes out of the way and wear eye protection.

That gives you a fair comparison of the ultimate tensile strengths
of the materials with the following caveats: Tensile strength of
a material is defined as the stress at rupture in pure tension.
Stress is force per unit area, you would need to calculate the
cross sectional areas of your samples and divide the force (weight)
at rupture by that number to arrive at the correct answer.

BUT, unless I am mistaken, OP is not interested in the intrinsic
properties of the materials so much as how much load a finished
wing skin would carry. So long as the samples are the same width
as each other and the same thicknesses as the proposed wing
skins what OP would want to compare is the actual force, not
the stress, at rupture.

Obviously you do not want to make the mistake of supposing this
comparison is all you need to know. A part may fail to perform it's
desired function due to deformation long before it actually breaks.
E.g. if you wing skins balloon out enough it may not matter if the
fabric
tears or not. A part may also deform so as to shift the load onto
another part, precipitating its failure whereas a weaker but stiffer
material might not.

I encourage OP to read up a bit on "Strength of Materials", perhaps
some introductory literature is available on the web. Commonly
used words like stiffness, strength, stress, force, and pressure
need to be carefully defined and used in their defined context in
order to understand and engage in a meaningful discussion of the
design issues.

--

FF