Matt,

You are right that resizing creates complications with brackets and other
hardware. But this will take only a little time and effort to address.

The calculations for the structural sizing are not that time consuming
either.

Let's take a Baby Ace spar for example which is made of sitka spruce, and
for which we want to substitute white pine.

The first thing is to calculate the moment of inertia (I) of the spar: I =
width x height (cubed) divided by 12.

So for the 3/4" wide by 5-1/8" high Baby Ace spar, the moment of inertia
calculates to 0.75 x 5.125(cubed) / 12 = 8.41.

Now we simply look up the modulus of rupture (Fbu), which is the strength in
bending, for spruce and pine: 10,100psi for sitka and 8,800psi for pine
(according to Forest Products Laboratory data).

By plugging in the moment of inertia into the bending stress formula, we
arrive at the maximum load this spar is capable of carrying: My = Fbu x I

Where M = bending moment in inch pounds
y = distance of neutral axis of spar to outer surface on
compression side
I = 8.41 (as we just calculated)

So to arrive at the ultimate strength of the Baby ace spar we simply
multiply I (8.41) x modulus of rupture of sitka (10,100). The answer is
84,941 inch pounds. This figure is the amount of load the spar was designed
to carry, using sitka spruce.

Now to subsitute pine all we have to do is rearrange the bending stress
formula using the slightly lower modulus of rupture (Fbu) of pine.

So first we want to solve for bending moment (I) using the substitute wood:
I = My / Fbu = 84,941 / 8,800 = 9.65

Now that we know the moment of inertia we can solve for the increased width
of the spar using pine:
w = I x 12 / h(cubed) = 9.65 x 12 / 134.61 = 0.860

So the new width (thickness) of the pine spar is 0.860 inch, a little less
than 7/8" (0.875).

So we would only need to increase the thickness of the spar by a mere 1/8".
Remember the stock Baby Ace sitka spar is 3/4", so our 7/8" pine spar would
actually be a little stronger.

That's all the calculation you would need to do for the whole airplane if
most of the structure is made of 3/4" stock. If the longerons were specified
as 3/4" sitka, you would again simply substitute 7/8" pine.

I don't think this is a lot of work, because now I can go down to Home Depot
and pick out some nice clear pine, bring it right home and start building an
airplane.

I think this is so much better than sending hundreds of dollars to some
mail-order outfit and wondering what kind of beating the boards took in
transit.

One of the biggest dangers in using wood as a structural material is
compression failures that are almost invisible to the naked eye. A piece of
wood that has been severely stressed (such as sitting under some big heavy
boxes on the UPS truck) may look perfectly good, but its fibers may be have
completely lost their strength. A small amount of load and it will now snap
like a twig.

That's one of the reasons I don't like mail-order wood. That's also why you
need to test a sample from each board you buy and look very carefully for
compression failures or wood "crush." The EAA book I mentioned previously
has a good article on this, written by Sam Evans, the designer of the
Volksplane.

Regards,

Gordon.



"Matt Whiting" wrote in message
...
Gordon Arnaut wrote:
Matt,

You are correct that resizing structural members is not as simple as
simply increasing size by the same percentage amount that the substitute
wood varies in strength.

Yes, you do have to recalculate the structural stresses, but this is not
that difficult. You can do this by applying the bending stress formula.
This will give you the exact dimensions that you will need of the
substitute material, in order to carry the same loads.

There is an old Sport Aviation article that works through this, called
"Selection and Evaluation of Wood," by Noel J. Becar. It is included in
the EAA book, "Wood: Aircraft Building Tecniques."

Yes, not that difficult, but definitely tedious and time consuming. I'd
rather spend a little more time locating quality wood of the species
specified by the designer than recalculating the sizes of all of the
stressed members of the structure - which is a lot of calculation even on
simple airframes.

And then you may have to adjust a lot of other items (brackets, etc.) to
accomodate the different dimensions. All in all, a lot of work and the
increased chance of a miscalculation that could cause problems later.

If someone was planning to make many airplanes using the new wood, then it
would be worthwhile, but for a single airplane, it seems to me that the
work would greatly outweight any benefit of using a different specie.


Matt