Spars: wood density and growth rings

I did find a brief reference to wood density in the following article, but short of cutting up a board and taking out the usable wood and weighing it, the article doesn't help much.

http://www.oshkosh365.org/saarchive/eaa_articles/1961_06_20.pdf
"The strength of spruce varies also within large limits.
For stress analysis purposes it is usually taken at 8300
PSI tensiles strength parallel to the grain. The number of
annual rings varies widely between specimens - anywhere
from two to 30 rings to the inch. The best specimens average
from 10 to 20 rings to the inch and a minimum of 6
rings to the inch is acceptable. The number of rings to the
inch is related to the rate of growth of the tree. A large
number of rings mean slow growth, higher density and
the timber may be brashy and brittle. (See Fig. 6).
Wood can be classified into three grades with the
following characteristics:
Grade A — Used for stressed parts. Minimum density
24 Ibs./cu. ft. Moisture content: 10 to 17%
Grain slope not less than 1 in. in 15 in., No knots
Grade B — Used for secondary parts. Density between
22 and 24 Ibs./cu. ft. Moisture Content:
10 to 17%, Grain slope not less than 1 in. in 12
in., Pin knots may be present
Grade C — Not used for aircraft construction."

  See if this will help:

http://www.recreationalflying.net/tutorials/scratchbuilder/beams.html

From the Recreational flying website in Austrailia

Thanks David.  That's a really good article.  The part on density reads:

 "The rate of growth is shown by the width of the annual rings, or the number of growth rings per 25 mm. Generally, for those softwoods typically used in aircraft construction, the number of radial growth rings appearing in the end or on the face of sawn boards, should be at least six, possibly eight, per 25 mm but fewer than 15 to 18 — and with a high percentage (50%?) of the stronger late wood. (If the tree is grown too slowly, the strong late wood bands are too narrow; if grown too fast, the weaker early wood bands are too wide [in softwoods], or the late wood cell walls are too thin [in hardwoods].)"

That's about as good an explanation as I've seen.

The same article continues with:

"Measuring density: the only way to really determine the density of a particular seasoned board is to cut a piece from the board, carefully measure both volume and weight, and convert to kg/cubic metre. If you dry it in the microwave for 20 or 30 minutes before measuring and weighing you will have the density at 0% MC and the density at 12% MC will be about 6% greater. When drying perhaps it may be better if you check the weight after 15 minutes in the microwave then every five minutes or so until the weight stops reducing. To check MC see below.

Density classification: the density of seasoned timber is usually measured for classification purposes at 12% air-dry MC. The density classification is typically:

      • exceptionally light — under 300 kg/m³
      • light — 300 to 450 kg/m³
      • medium — 450 to 650 kg/m³
      • heavy — 650 to 800 kg/m³
      • very heavy — 800 to 1000+ kg/m³

Aircraft weight restraints mandate the use of wood having low weight but ample strength. Such timber is most likely contained in the lower band of the medium density classification; except perhaps timber for propellers. For 90 years top-quality North American Sitka spruce (average density perhaps 440 kg/m³ has been the aircraft designer's timber of choice for most of the airframe, and regarded as the benchmark for comparison purposes. "

Publications helpful regarding wood for aircraft are:

ANC-19  Wood Aircraft Inspection and Fabrication

ANC-18  Design of Wood Aircraft Structures

Both these publications are available from the EAA Library in photocopy form for a small charge.  You can contact the library by calling EAA Headquarters