wood species question

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 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.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

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

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

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

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

"Gordon Arnaut" wrote

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.
???????????????????????????????????/

You HAVE to be totally kidding. Unless that wood was sitting under a 10,000
lbs box on the UPS truck, it WILL NOT get compressive failure like that.

Most compressive fractures take place when the tree is felled, and lands
across a swag, or on another log.

You had a pretty good writing going, but you lost all credibility, with that
last line of crap.

Also, rupture is not the only mode of failure that is important. You have
to know if the part you are replacing is in tension, compression, bending,
or what. There are different values for each mode.
--
Jim in NC