What is this strut in biplane rigging, attached only to wires, not bearing any load?
It's just a stick set between the flying and landing wires (the &quot;wires&quot; are actually solid stainless steel rods with an almond cross section for streamlining - they are VERY expensive).  Normally they are made from wooden dowel with notches for the wires to fit, retained in the notches by wraps of lockwire, or on fancier biplanes they may be made from aluminum tube with a bullet nose and pointed tail.
They are usually called &quot;wire stays&quot;. Their main function is to stop the wires from vibrating in the airflow (they're more or less metal vocal cords), which can create a lot of noise (that's the traditional diving airplane sound effect from the movies, as you can hear hear here, at 1:50:30, in Hell's Angels, one of the earliest sound movies with airplanes). The noise is from the &quot;Wind in the Wires&quot; (the title of a book by WWI pilot Duncan Grinnell-Milne). More importantly, the vibration can also induce fatigue cracking of the streamlined rods over the long term. Linking them together changes the natural frequency of the wires to something much higher than an individual wire, and stops them from vibrating in the airplane's speed range.