What effect does cleanliness have on skin friction drag?

Cleanliness plays a significant role in influencing skin friction drag on a surface, particularly in aerodynamics. When a surface such as an aircraft's wing or fuselage is clean, it tends to have a smoother texture which allows air to flow more smoothly over it. This smooth airflow reduces the turbulence that can occur, which is a key factor in skin friction drag.

In contrast, when a surface is dirty, it often exhibits increased roughness due to the presence of contaminants like dirt, oil, or other residues. This roughness disrupts the smooth airflow, leading to increased turbulence and greater skin friction drag. Therefore, maintaining cleanliness on aerodynamic surfaces is crucial for minimizing drag and enhancing overall performance.

Considering this context, the correct answer indicates that cleanliness increases skin friction drag, which is contrary to the principles of fluid dynamics where smoother surfaces decrease drag. This highlights the importance of regular maintenance and cleanliness in aerodynamics to optimize efficiency and performance.