I think this is a good intuition about it, and Xyla Foxlin goes into this a bit in her video explaining the build for Veritasium's video. https://youtu.be/VUga

I think this is a good intuition about it, and Xyla Foxlin goes into this a bit in her video explaining the build for Veritasium's video. https://youtu.be/VUgajGv4Aok?t=432 https://youtu.be/VUgajGv4Aok?t=432 The essential 'gear' ratio is between forward motion of the vehicle and the propeller pitch (essentially how far the prop would screw through a tub of jello for a given amount of rotation). The vehicle has to move *faster* on the ground than the prop does through the air for this to work at all, and it sounds like it should be roughly twice as fast for it to work on something that is reasonably simple to engineer. So if your prop pitch is 5", your wheel size and gearing should allow the vehicle to move forward approximately 10" for each rotation of the prop. (Note that her video does not explain 'why' this is the case, just that it is a phenomenon that the Blackbird inventors had determined/discovered and is an essential design criteria).