Is there any (current, or planned, or hypothesized) practical application for engines of low thrust - like, below 40N - in the atmosphere?
I'm asking this in relation to the Non-vacuum ion propulsion question: it can be used as an educational tool or technology demonstrator, or a hobby model, but I can't imagine a scenario where someone would find a ion engine or similar actually useful.
Am I missing something or are they simply a thing destined for space and will never find actual use down on Earth?