Over the weekend I made my first model in #freecad. The design is of a contra-rotating gearbox, based off this video https://www.youtube.com/watch?v=xda1WgL9fd0&list=PLJH7IqpOoNLpc9NVpY-d0J0OlvtPcMqAi&index=12
It allows two propellers to rotate in opposite directions with one motor. This reduces the torqueing that occurs with just one.
Oops, that's an older image. This is the latest version: