Which of the following is true about drag?

Drag is a force that always acts in opposition to the direction of the aircraft's motion through the air. When an aircraft is moving forward, drag works against it, thereby contributing to the overall aerodynamic forces experienced by the aircraft. This opposing force is critical to understand because it affects how much thrust is needed to maintain or increase speed.

In terms of its presence, drag is not limited to specific phases of flight, such as takeoff. Instead, it is constantly acting on the aircraft whenever it is in motion through the air, regardless of altitude. The characteristics of drag, such as increased resistance experienced with higher speeds, can influence performance metrics across different scenarios, including climb, cruise, and descent.

Additionally, while drag tends to vary with altitude due to changes in air density, the relationship is not as direct as simply stating it increases as altitude decreases. At lower altitudes, there is indeed denser air which can contribute to increased drag compared to higher altitudes, but this is a differential relationship based on various factors like speed and aircraft design. Understanding that drag opposes motion is fundamental to aviation principles and the operation of aircraft.