Posted: 3/16/2011 09:02:24
I'm looking for some guidance on selection of wood spar material. I'll be using White Spruce or Western Hemlock since they are available locally.
AC 43.13 recommends at least 6 rings per inch for these 2 species. But I find there is enormous variation in the density of wood that meets these guidelines. For example, it's common to find pieces with not only 6 but even 15 or more rings per inch. And then there is the ratio of Summer wood (dark rings) vs Spring wood (light rings). Some of these are very dense and heavy. Even with 6 rings per inch there is a lot of variation in density, again depending on the ratio dark vs light wood.
I even find some pieces with a high ring count but the dark rings are very thin and almost imperceptible. This wood is very very light but seems to be strong just the same, i.e., based on bending capstrips.
So, with so much variation what should I look for in selecting wood for spars (laminated)?
Posted: 3/16/2011 09:41:58
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.
"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
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."
Posted: 3/16/2011 17:18:27
See if this will help:
From the Recreational flying website in Austrailia
Posted: 3/16/2011 21:07:00
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
That's about as good an explanation as I've seen.
Posted: 3/16/2011 21:10:01
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.
Posted: 3/18/2011 08:54:34
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