Drag generated between air molecules and the solid surface of the aircraft.

• A. Skin friction drag
• B. Induced drag
• C. Form drag
• D. Wave drag

Answer: (A)

Drag from the air’s viscous interaction with the aircraft’s surface comes from the boundary layer where air slides along the skin and experiences shear. This frictional drag is known as skin friction drag. It depends on how much skin is in contact with the flow (surface area), the air’s viscosity, the surface roughness, and the speed of the aircraft (Reynolds number). It’s a component of parasite drag, and it’s distinct from other drag types: induced drag is related to lift and the trailing vortices it creates; form drag arises from pressure differences around the shape and flow separation; wave drag appears at higher speeds when shock waves form. So the frictional interaction between air molecules and the surface is skin friction drag.