RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
From what I can see in the pictures it looks like the belt and the exposed gears don't provide any gear reduction. They just take the output from the motor gearbox and split it into a counterclockwise, and a clockwise rotation for the front and rear props.
This means that the weight of the belt and exposed gears is in addition to the weight of a planetary gearbox, which is looks like it's located directly behind the exposed gears, and in front of the motor, so you are probably looking at a fairly significant increase in weight over a single prop setup, maybe up to 300g or so.
However, this might be a good solution for a glow engine like the DZ170. Maybe you could use it to get the DZ to spin a pair of 22X20 props, and make a DZ powered plane fly similar to an electric. I wonder if the guys from YS are looking at this.