Just one more quick point about stiffness and elasticity, since this was
another issue that nutjob was screaming about.

As I explained earlier, stiffness of all the species mentioned in AC43.13b
is quite acceptable and does not really vary by a significant degree. Also
stiffness is not a structural concern -- although it can be a factor in
flying qualities.

For example, an airplane built of fir will be stiffer than than the same
airframe built of spruce, and could provide a little crisper control
response. However, the more flexible spruce or pine airplane will give a
little more comfortable ride in bumpy air.

I think that puts to rest all of the dust that nutjob has kicked up.

Regards,

Gordon.




"Gordon Arnaut" wrote in message
...
Just to wrap up this thread for those who were seriously interested in
wood substitution issues, before the discussion was rudely disrupted by a
know-nothing peon who has since departed with tail between legs -- as soon
as he realized the jig was up.

As was discussed before the rude interruption, the FAA has approved a
number of wood species for substitution for sitka spruce on certified
aircraft, including certain varieties of pine, fir, cedar, and poplar.
There is specific guidance on this is Chapter 1 of AC43.13b.

Since we are primarily interested in experimental aircraft, I think it is
quite prudent to follow what guidance exists for certified craft -- it is
a good standard to adhere to. For example, AC43.13b specifically states
that white pine can be substituted for sitka if the member is
appropriately resized to account for the slightly lower strength (and
weight) of pine.

I did not set out to provide a lecture or tutorial on wood substitution as
this is not the appropriate forum for such an exercise. Instead, I urge
each airplane builder to educate himself using good reference material,
such ANC-18 and the many NACA documents that can be easily downloaded.

However, I thought it would be useful to show how easy it really is to
accomplish the calculations necessary for resizing even the most critical
structural member of the airplane, the wing spar.

Subsequently I also worked through a simple calculation for compression,
which may be relevant in parts like wing compression members and drag
braces. (Stress in tension is calculated the same way as compression:
force per area).

The main reason for sbustituting wood is that it allows the builder to buy
lumber locally, where he can inspect it closely and subject it to proper
selection techniques -- assuming he has learned how to do that.

I think personally that shipping aircraft lumber cross country is not a
good idea. Others may have different views and indeed many airplanes are
flying successfully that have been built with mail-order lumber. However,
it is quite conceivable for such small pieces of wood to be placed in
awkward bending situations in transit. And as we have seen, it does not
take much bending at all to cause compression failure that is nearly
invisible to the naked eye.

It is a fact that there have been catastrophic structural failures on
homebuilt wooden airframes. That's not to say that those accidents were
caused by wood damaged in transit. In some cases, accident investigators
have been able to pinpoint the cause of the failure -- and compression
failure in wood members has been one such cause, as have other things,
like improper techniques, materials, etc.

In some of those cases were compression failure was a factor, it may never
be known when or how the wood was damaged. Still it is enough of an issue
for me that I prefer not to buy mail order lumber. It is also quite a bit
cheaper to buy wood locally, which is a nice ancillary benefit.

Another benefit is that I prefer to mill my own boards to size, which lets
me make best use of the grain direction, slope, etc. When you get a
cardboard tube of wood from a mail-order house, it is a fait accompli --
everything is already cut to size and you must use it regardless of how
the grain pleases you.

The fact of the matter is that in every part of this continent -- and
indeed many parts of the world -- there is suitable lumber available
locally. In the Great Lakes region where I live, northern white pine grows
tall and straight, yielding nice long runs of knot-free planks. There is
also spruce locally, but it is difficult to get clear wood because eastern
spruce does not tend to grow very tall, so branches start much lower to
the ground than pine.

In Australia, there is an abundance of hoop pine, which is actually
stronger than sitka spruce, also grows very tall and makes fine wood for
airplanes.

So yes, I think it is important to know these things when one is starting
out and considering what to do and how to get started.

That's why I have gone to considerable effort here to silence the noise
from a completely worthless clown who thinks it is okay to completely
disrupt an intelligent and friendly discussion despite the fact that he
has zero knowledge of the subject.

In any case, I think everyone can see that this annoying moron has been
completely discredited, hence his silence.

I just want to address a couple of things and then I will call it a wrap.
As I have shown, making calculations for resizing lumber is not that
difficult or time-consuming, even for complex stresses like bending
moments.

And I have also shown that making calculations for compression and tension
is even simpler still. (I gave an example for compression, but tnesion is
calculated in exactly the same way -- stress equals area per force.

Column buckling is not a consideration on any wooden part of an airframe,
there is simply no such structure that has a length to thickness ratio
high enough and that is completely unsupported along its length.

I'm adding this just to clear up the confusion that the irritating moron
has raised -- throwing verbiage out willy-nilly without even knowing the
first thing about what those things mean.

All in all, substituting local wood is a very attractive option for a
number of reasons. It should not be dismissed out of hand because of some
erroneous impressions we may have, or some hangar tales we may have
heard -- and above all, not because of some nut screaming "fire" in a
crowded theatre.

Regards,

Gordon.