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