I have a small size 25" flying wing with 280 motor. It does not come with a spar, but I think one would be a good idea. Where would you add one, if you are allowed only one?
The way this plane is designed, if a spar is put at the CoG, it will extend outward to about the mid point of the leading edge. If I want the longest spar extending from tip to tip, it will be quite a way behind the CoG which could be a balancing problem.