Which factor primarily increases skin friction drag on an aircraft?

Surface roughness is the primary factor that increases skin friction drag on an aircraft. Skin friction drag occurs as airflows over the surface of the aircraft, generating resistance due to the friction between the airflow and the surface of the aircraft. When the surface is rough, it disrupts the smooth flow of air, leading to an increase in turbulent flow around the surface. This turbulence enhances drag, as the air has a harder time flowing smoothly along the surface, thereby increasing the overall resistance to motion.

In contrast, while speed, wing design, and the weight of the aircraft can influence overall drag, they do not specifically pertain to skin friction drag in the same direct manner as surface roughness. Speed affects the dynamic pressure and can increase total drag but does not change the inherent characteristics of skin friction. Wing design and aircraft weight can influence aerodynamic efficiency and the distribution of lift and drag, but surface roughness directly impacts the friction between the air and the aircraft's skin.