Spruce or basswood.

The qualities of each are different. Exactly where do you plan to use one or the other?

In my experience bass is more brittle and where spruce may give a little, just a little bending will cause catastrophic failure under the same stress. I would not use bass for spars. Ever.

Test bend one of each of the same size. You will see for yourself.

Robert

Yesterday when I went to build a leadout slider, I was out of 1/8" square spruce which I normally use, but I did have some basswood so I used that instead.  Then I got to wondering about how the two materials compare, weight and strength wise.
Later I did get to wondering if bass would be OK for wing spars.  When I compare the samples I have they seem about the same in stiffness and weight, althought I admit I did not actually put them on the scale.  In a spar application I would think there is very little bending, and the bulk of the load / stress is in tension.   So here is another question.  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?

Aircraft quality spruce(they actually specify the number of growth rings per inch) is measurably stronger.  For run of the mill wood that we can easily get virtually all the woods are as strong as they are dense.  Just like balsa, a heavier piece will be stonger, and it has all the grain problems you find in balsa- A grain, B grain, C grain,  you even get cross grain, where the wood fails on a slant because the grain is not straight down the piece.

The biggest difference between bass and spruce is that spruce tends to shatter when it breaks or gets hit, bass tends to snap at one particular spot.  So for spars, bass is generally easier to repair after a crash.  Bass sands and carves more easily and doesn't tend to split.  I've used bass for spars for years because it is easier to work with, and plenty strong.

For the best strength to weight no wood beats carefully selected balsa.  To a point, the less dense balsa is, the stronger it is per lb.  at least until you get down to the mushy, soft 4 lb stuff.

. . . 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 Fulwider

Phil C. Posted while I was writing this. His post explains why the "purely mathematical" approach doesn't give all the answers.