What primarily affects the aircraft's drag during flight?

The primary factor affecting an aircraft's drag during flight is its speed and aerodynamic shape. Drag is the resistance an aircraft encounters as it moves through the air, and it is significantly influenced by how streamlined the aircraft is—essentially its aerodynamic design. A well-designed aircraft minimizes drag by allowing air to flow smoothly over its surface.

As the speed of the aircraft increases, the drag also increases due to the dynamic pressure exerted by the air. This happens because the amount of air the aircraft has to push through increases with speed, creating greater resistance. Conversely, at lower speeds, the drag is less, allowing for more efficient flight.

Changes in altitude have some effect on drag as well, particularly due to changes in air density; however, the most direct influences are the aircraft's speed and its shape. Thrust adjustments and engine power primarily affect how the aircraft overcomes drag but do not directly alter the drag itself. Thus, speed and aerodynamic shape are the key factors that primarily dictate the amount of drag an aircraft experiences during flight.