Which term describes the aerodynamic resistance due to the contact of moving air with the surface of an aircraft, including microscopic roughness?

• A. The drag due to engine cowlings
• B. The aerodynamic resistance due to the contact of moving air with the surface, including microscopic roughness
• C. Drag due to lift
• D. Drag due to interference

Answer: (B)

Skin friction drag is the aerodynamic resistance caused by viscous shear between the moving air and the aircraft’s surface, including the effects of microscopic roughness. As air flows over the fuselage and wings, viscous forces act along the surface, dragging it slightly and producing drag. The roughness of the surface—even on a microscopic scale—disturbs the boundary layer, increasing turbulence and the shear stresses that contribute to this drag. This type of drag is a part of parasite drag, separate from drag caused by lift (induced drag) or from interference effects between components. It also isn’t limited to any single feature like engine cowlings; it represents the general frictional resistance of the air against the aircraft surface.