What is the primary cause of form drag on an aircraft?

The primary cause of form drag on an aircraft is the disruption of airflow over the aircraft shape. Form drag, also known as pressure drag, occurs due to the pressure differential between the front and rear of the aircraft as it moves through the air. When the streamlined shape of an aircraft meets the air, some of that airflow is disturbed or separated, creating a region of low pressure at the rear. This difference in pressure generates drag, as the high-pressure air at the front pushes back against the aircraft to fill the space left by the disrupted airflow.

While other factors like friction over surfaces and surface imperfections contribute to overall drag, they do not primarily cause form drag. Airfoil lift generation is also unrelated to form drag, as it pertains more to the lifting forces created by the wings rather than resistance due to shape.