Which statement best describes the drag generated between air molecules and the solid surface of the aircraft?

• A. Interference drag from wing-fuselage junctions
• B. Pressure drag due to form drag
• C. Induced drag from lift
• D. Drag generated between air molecules and the solid surface of the aircraft

Answer: (D)

Drag arising from the viscous interaction between air and the aircraft’s surface is the frictional or skin friction drag. It comes from air molecules sticking to and sliding along the solid surface, creating shear stresses in the boundary layer. The statement describing drag generated between air molecules and the solid surface matches this exactly, because it’s specifically about the surface shear between air and the aircraft.

Other named forms refer to different phenomena: interference drag results from how flow fields around different parts of the aircraft interact at junctions, forming additional pressure effects; pressure (form) drag comes from the pressure differences around the body due to its shape; induced drag is tied to generating lift and the wingtip vortices that accompany it. Skin friction drag is the surface-based, viscous component, separate from those pressure- or lift-related effects.