Eli5: When a super fast plane like blackbird is going in a straight line why isn’t it constantly gaining altitude as the earth slopes away from it?

Question

In a debate with someone who thinks the earth could be flat, not smart enough to despute a point they are making plz help.

In: 1372

Answer 1

It’s would appear to be gaining altitude, so the pilot would adjust the plane so that it stays at the same altitude.