hi - just about to fix the leading edges to my ugly stick wings - but am wondering if i should use hard balsa stock or will medium be ok which is what i have bought - where should i use hard balsa - any response welcome - thanks

Don't think it will matter at all which you use since weight of some balsa is not significant. Perhaps you can determine what present ld. edge was made from and go by that.

In my opinion, hard is good for perimeters of built up flying surfaces ( rudders, etc) and shear webs, ribs spars (if balsa is used) in the wing, where strenght but lighter weight are needed.

If you wish to know the density of a 3 x 36 in piece of balsa, just divide the thickness in inches into the weight in oz and it will give you the density in pounds per cubic foot. For example, a 1/8 in sheet weighing 3/4 oz is 6 lbs, 1/2 sheet weighing 5 oz is 10 lbs, etc.

There's always a story ...

While waiting to pay for something at my LHS I struck up a conversation with another modeler. We were inspecting a nicely built 3-D ARF checkin' all of the good stuff. He commented that he uses very light balsa. If the piece is likely to take a hit (leading edges) he cuts a slot in the piece and glues a piece of CF in the slot. Light and strong.