Square this sunshine:

1. Fly direct from Seattle to London. (I've done it multiple times) take note of how often the wings bank to the right. On your flat earth map, you would constantly have to course correct to the right, yet you don't. Once on course, you fly straight and only turn while ascending or descending because of other aircraft.

2. Signup for the IILS space station and get alerted each time they fly over your house. On a clear night, you can see it with your own eyes without a telescope. Since the orbit adjusts only a few degrees every 90 minutes, you will see it come around again on the new track. It travels overhead in a straight line. If the earth were flat, the path would be arcing to the right or left to follow your flat map, so how would it go over your head in a straight line and 90 min. later follow the same path?

https://ifttt.com/applets/YfkYtQB2-get-a-notification-when-the-international-space-station-passes-over-your-house