Where is the compression on a compression strut in a tube and fabric airplane coming from?
You want a nice, taut fabric cover on this wing, don't you?

The main function of the compression struts is to keep both spar tubes from being pulled closer by the fabric cover. Of course, Niels is right, aerodynamic forces come on top of this and will also pull on the tubes in the wing's plane.

In order to calculate those forces, integrate the tension in the fabric over wingspan when loaded by maximum lift forces. Below I have sketched those lift forces – they want to pull the fabric up, increasing tension within it and so the compression forces on the struts.



I hope I got the basic structure about right. Please correct me where needed.