Originally Posted by BMatthews
Hashem, looking at your drawing I'd suggest a couple of things.
First off I assume that your carbon tube will be used as a wing joiner to let you transport your model more easily. That's fine. But the tube is best put in between your spars and boxed in with filler so it connects directly to the spars instead of using the ribs as support.
While old growth Sitka spruce is the standard for aircraft spars over here in North America you could consider some of the European pines or other softwoods as an alternate that would be easier to obtain. What you want is some wood that is flexible and spring like but does not break too easily. Some soft woods are flexible but not springy. If a piece is bent it tends to stay bent. You want one of the woods that springs back to straight to use for spars. Generally this means it will be a slow growth wood with probably 6 or more growth rings per centimeter when you look at the end of the wood. And obviously you want a wood where the grain is fairly straight. To be a good spar stock the grain ring runout where the grain twists and runs side to side instead of along the wood must be less than a full runout in 30 cm. Anything much less and if you test break a sample you will find that it snaps with short pointed ends. That's no good for a spar.
Something you might want to consider since you are building a quarter scale version. If you make the wing struts functional and load bearing you can then switch to a two spar setup just like the original Cub used and use the struts to carry the flight loads. This means making the strut connections VERY strong. But the advantage is that you don't even need a joiner tube at the center. Instead the wing just needs a carry through box through the fuselage. The two spars would end up being located on the ribs at the same spots where the struts connect on the real Cub.
Originally Posted by Lnewqban
The size should be enough to avoid buckling of the spar under compression loads, whatever type of wood you can get.
This technique helps reducing the size of the needed spars:
Now, the location of the spars along the airfoil is favored to be at the highest point for the simple reasons of keeping a good shape of the airfoil (formed by the cover) between the ribs where is more critical and more efficient structurally (more distance between top and bottom).
oh thanks guys for well informative posts. I learned new things and will absoloutley consider them.
BMatthews would you add some images about wood grain runout becaue of bad english understaning! (sorry).
I added one: which one do you mean?