Model Forum / General / Rockets / March 2004

For the same size... spruce is marginally stronger than basswood. Weight is
about the same. Its easier to get a good clue joint with the basswood than
spruce with some glues.  also Basswood is a hardwood where Spruce is a
softwood..Basswood density is listed as 26 pounds per cubic foot, while both
Sitka and White spruce are slightly heavier, at 28 pounds per cubic foot,
all at 12% moisture content by weight.
Basswood has a modulus of rupture of 8700 psi (pounds per square inch),
while Sitka and White spruce chime in at 10,200 and 9800 psi, respectively.
Thus, basswood appears to have a strength to weight ratio about 92% as high
as Sitka spruce's.

Basswood's tensile strength across the grain is nearly the same as spruce's,
at 350 psi versus 370 psi, so it should have about the same resistance to
splitting as spruce. Basswood's shear strength parallel to the grain is a
bit lower than spruce's, at 990 psi versus 1150 psi. Basswood's modulus of
elasticity is given as 1460 psi, versus Sitka spruce's 1570 psi, so it is
the same as spruce in relation to its weight

Sitka spruce has been very popular among full-size aircraft builders for
many decades. The reason may be its truly outstanding attribute, impact
strength, at 56% higher than basswood's. Basswood finishes in a distant
second place in crash damage resistance.

Basswood also takes a back seat to spruce in crushing strength across the
grain, at 4730 versus 5610 psi. Sitka spruce appears to have the highest
strength to weight ratio of any wood, higher even than that of hickory, the
strongest wood listed.

Alot of  model aircraft people have used spruce in replacement of basswood
for their spars with excellent results.....and the spar of course is a major
load bearing member....

shockie B)