. . . Say you have a design which calls for 1/4" square balsa spars, and you wanted to use spruce instead, what size spruce would you select for comparable strength of the 1/4" balsa. And would there be any savings in weight?
It is fairly accurate to say that across all woods commonly used for building things, strength is roughly proportional to density. That is, in general, you can build a structure to a specified weight and strength spec with anything from balsa to spruce to oak to hickory and make it work – not too dissimilar weights and strengths. But back in the day of wood shafted golf clubs, you didn’t see 3” diameter balsa shafted clubs, and you don’t see microfilm covered indoor models made of spruce. More on that below. In lay terms, heavier woods are about as much heavier as they are stronger, and vice versa.
A specific example is the ¼” balsa spar question. Assuming 9 lb/ft^3 balsa and 23 lb/ft^3 spruce, we get a 5/32” spruce equivalent. However, the bending strength of anything depends a lot on its size (too complex to go into all that), so the balsa will be stiffer – probably. It is more accurate to say for two ¼” spars, the spruce would be 23/9 heavier and the balsa one would be 9/23 as strong. Balsa is more often the “go to” wood for our size of models because we can gain “geometric strength” with thicker pieces than the thinner pieces of heavier, stronger wood.
Tables of strengths and densities for various woods are available on the internet for those interested enough to download and do some arithmetic. I did that a few years ago, and found significant variation for the same woods in different tables. The various strength tests mean different things, so you need understand how those work.
Off topic notes:1. An interesting wood not used much for models is cedar. The early U-Keys had cedar fuselages. Cedar is a little lighter and a little less strong than spruce.
2. The AN-W-2 spec for spruce is 360 kg/m^3 (~22.47 lb/ft^3), and Aircraft Spruce guarantees all their spruce meets that spec. That means you won’t get any super light spruce from them.
Larry FulwiderPhil C. Posted while I was writing this. His post explains why the "purely mathematical" approach doesn't give all the answers.