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