Why does drag decrease with increasing altitude?

Drag decreases with increasing altitude primarily due to the decrease in air density. At higher altitudes, the atmosphere becomes less dense, which means that there are fewer air molecules for the aircraft to interact with. Since drag is essentially the resistance that the aircraft faces from the air as it moves through it, lower air density results in less aerodynamic drag.

As the altitude increases, the reduced air density leads to a decrease in both lift and drag forces experienced by the aircraft. This is crucial for pilots to understand because it affects aircraft performance, fuel efficiency, and the overall flying characteristics.

In contrast, true airspeed, aircraft weight, and thrust available are less directly related to the relationship between altitude and drag. While true airspeed does play a role in the drag equation, it is not the primary factor that accounts for the decrease in drag with altitude; instead, true airspeed typically increases at higher altitudes to maintain lift, which can complicate the drag scenario rather than simplifying it. Additionally, aircraft weight does not inherently decrease with altitude unless fuel is burned, and thrust available is related to engine performance, which means that their effects are more nuanced and not as directly correlated with drag reduction at altitude.