What causes parasitic skin friction drag?

Parasitic skin friction drag is primarily caused by the roughness of the aircraft's skin surface. When air flows over the surface of an aircraft, the roughness disrupts the smooth flow of air, creating turbulence. This turbulence increases the friction between the air and the surface, resulting in drag. The smoother the surface of the aircraft, the less turbulent airflow will be created, thereby reducing skin friction drag.

Other factors, such as wind speed variations and air pressure differences, can influence overall drag but do not specifically lead to parasitic skin friction drag. Excessive weight of the aircraft primarily affects lift and overall performance rather than directly impacting skin friction drag. Thus, the roughness of the surface plays a crucial role in determining the amount of skin friction drag experienced during flight.