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."

Regards,

Gordon Arnaut.



"Matt Whiting" wrote in message
...
Ernest Christley wrote:
Matt Whiting wrote:

Gordon Arnaut wrote:

There are many types of wood that can be used in constructing a proper
airframe, as per AC-43.13b.

I happen to like northern white pine, which has nearly the same
strength-to-weight ratio as spruce -- and better than that of Douglas
fir. NWP is about 15 percent lighter and about 15 percent less strong
than spruce, so if your plans specify spruce you will want to increase
the dimensions by about 15 percent.



Trouble is it isn't that simple. Strength of many load bearing members
(those loaded in bending or torsion, for example), is a linear function
of size. It would take virtually a re-engineering of the structure to
change species in most cases.


Matt


Matt, did you mean to say that it is NOT a linear function of size.

Take a cantilevered beam. Regardless of the thickness, it's bending
strength is the square of the thickness times the tensile strength. Say
the beam as designed is 1" thick and can hold 1000lbs. You substitute a
material twice as strong. Make it 1" thick and it can hold 2000lbs. Cut
it in half (because it's twice as strong) and it can only hold

(.5")^2 * 2000lbs = 500lbs.

I'm not a mechanical engineer, and I've learned just enough to know that
I don't know enough, so I may be wrong on the particulars; but I know
for a fact that twice as strong but half as thick doesn't get you to
where you started.


Yes, that is what I meant to say. Too bad my fingers aren't always
connected to my brain. Hopefully, the context of the rest of what I wrote
made the typo obvious.


Matt