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

Planes are trimmed to maintain a constant altitude. Gravity is trying to pull the plane down, engines and lift are trying to hold it up. When these forces are balanced in a properly trimmed aircraft altitude is maintained. Gravity is always pulling the plane straight down. Assuming the atmosphere at a given altitude is of a consistent density these forces remain in balance as the Earth “drops away” under the nose of the aircraft. As a result no adjustments in the attitude of the aircraft are necessary to maintain a constant altitude relative to the center of the Earth.