system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If you got in a plane and started flying flat along with Earth then maintained that direction, would you eventually begin flying out of the atmosphere?

Or would the plane follow the curvature of the Earth?
gpt
tl;dr: The plane would need to follow the curvature of the earth. 

As an airplane gets higher in the atmosphere a few things happen. First, as the plane encounters less air resistance, it can move faster, generating more lift. Counteracting this, however, is the fact that the air gets thinner, so less of it is displaced by the airfoil, so overall less lift is created. 

At a certain point, even if you have a plane that can somehow generate enough lift to get to the very top of the atmosphere, the oxygen content is not high enough to fuel the combustion of the fuel inside the jet engines, so the engines will flame out. 

There are certain ways to get around this problem by using different air compression designs. Ramjets and scramjets are for ultra high altitude planes that travel many times the speed of sound. These engines do not have traditional fans to compress the air, relying instead on the shockwave of the air hitting the engine intake to compress the air for it. Because of this, you have to already be flying at very high speeds to use these types of engines, making them a logistical challenge on commercial aircraft (along with many other reasons this is completely infeasible.) 

However, even these engines fail at high enough altitudes, so you would need to carry your own oxygen supply for combustion, like traditional rockets. You may notice now that we are no longer talking about the original subject, so the short answer is that the plane would follow the curvature of the Earth unless you had very special engines to get it to the upper atmosphere. Even then, it would eventually need to follow the earth's curvature unless you are flying a rocket ship.