Contra-rotating Propeller Model

A contra-rotating propeller system consists of two propellers rotating in opposite directions co-axially. Not to be confused with counter-rotating propellers, which refers to two propellers rotating in opposite directions mounted on different locations (as in either side of the wing on an aircraft). For more information, refer to https://en.wikipedia.org/wiki/Contra-rotating_propellers/ .

This is a demonstration toy to show how bevel gears can be used to make coaxial shafts rotate in opposite directions. In a practical propulsion system, the inner shaft would be driven directly by the engine, and the outer shaft made to rotate in the opposite direction via the gear chain. As a demonstration toy, I made the handle to come out from the "intermediate" gear.

I was going to design this system with bearings for smooth rotation, but I scrapped the idea as it over-complicated the system, and I also wanted to design something that many people can build without the need to purchase additi