What is skin friction drag primarily affected by?

Skin friction drag is primarily influenced by the cleanliness of the aircraft's surface, as it affects the smoothness and overall texture of the area in contact with the airflow. A cleaner surface allows for a more streamlined flow of air, reducing turbulence and the friction between the air and the surface of the aircraft. When dirt, oil, or other contaminants are present on the surface, they can create irregularities that increase drag, making it harder for the air to flow smoothly. This phenomenon is directly linked to the principles of aerodynamics, where smoother surfaces contribute to lower drag and improved fuel efficiency.

While the aircraft's weight, altitude, and speed can influence other forms of drag or overall performance, they do not directly alter the skin friction drag as significantly as the condition of the surface does. A clean aircraft surface is vital for maintaining optimal performance and efficiency during flight.