Which method can reduce skin friction drag?

Skin friction drag is a type of aerodynamic drag that occurs when air flows over a surface, such as an aircraft's skin. This drag is influenced by the surface texture, and using flush riveting, smooth paint, and waxing can significantly reduce it.

Flush riveting minimizes protrusions on the aircraft’s surface that could disrupt airflow, resulting in a smoother surface. Smooth paint further enhances this by preventing turbulence at the surface level. Waxing creates an even smoother finish and reduces the friction that air encounters as it moves over the aircraft. By employing these techniques, pilots and aircraft manufacturers can enhance aerodynamic efficiency, leading to improved performance and fuel efficiency.

In contrast, options that suggest adding more power, increasing weight, or reducing angle of attack do not target the reduction of skin friction drag effectively. Instead, they may either increase drag or not impact the skin's surface characteristics at all. Therefore, focusing on the surface's smoothness and finish through the correct methods is the key to reducing skin friction drag effectively.