mamba
Legend
I agree, you do not need to be able to see the curvature to detect that it exists, clearly we figured that out way before we could see it from ‘space’But honestly, all that about flight is moot. Eratosthenes measured the circumference of the Earth back in third century BC, a couple thousand years before the airplane was invented. Being able to fly really isn't relevant to the discovery.
The most easily detectable sign should be a sunset / sunrise already, you do not even need ships sailing past the horizon on an ocean
The other thing is that on a flat plane the horizon should always rise to your eye level, even on mountains (given a large enough flat plane), and that clearly does not happen either.
And yet a few thousand years ago the ‘common sense’ said that the Earth is flat. What we consider common sense is frequently not common or not sensible, so I would not see this as a foundation for worldbuilding that everyone should adhere to.
The point was not that it is undetectable, but that you have to fly very high to see it and is not what common sense would / did indicate. It took a little more than that for us to figure out the right answer, otherwise we would have started out with believing that the Earth is a sphere.