The strength to weight issue is interesting. "aircraft" spruce does very well because the FAA has strict specifications for the density, number of growth rings per inch, lack of knots or deformities, etc. Generally, a larger, lower density piece of balsa will have better strength to weight and better stiffness than other woods in model applications. Go to too low a density, grain and firmness of the wood become very important. Mush soft 4 lb. wood will not be very strong. But if it is firm, has a definite grain, and isn't abused otherwise, it has the best strength/weight of any wood.
Balsa, especially low density wood, is prone to wind checking, which is cracks across the grain in the wood(usually nearly invisible) which make it useless for anything but carving blocks. Other wood can also get wind checked, so take a close look at any spars and try bending and pulling on the wood. A wind-checked piece will break very easily with no strings or sign of anything going across the break. Just a more or less flat across break that looks like it was almost cut(which it almost has since the wood fibers are mostly already broken).
Overall, the strength of a piece of wood is primarily proportional to its density. Heavy dense wood is stronger than lighter, less dense wood, across the board for a given cross section. But carefully selected wood with straight grain and no defects can easily be twice as strong as a poorly selected piece. So a spruce spar will be pretty much the same strength as a balsa spar of the same weight(not the same dimensions).
Topic: Spruce (Read 1778 times)