Which drag is caused by the friction of air moving over the aircraft surface and increases with airspeed?

• A. Induced drag
• B. Profile drag
• C. Parasite drag
• D. Skin drag

Answer: (C)

The drag produced by air sliding along the aircraft’s surface is skin friction drag, often called skin drag. It arises from viscous forces in the boundary layer that shear the air against the surface. As the aircraft speeds up, these shear forces and the friction losses increase, so skin drag grows with airspeed. This type of drag is a component of parasite drag—the portion of drag not tied to lift—along with form and interference drag, which come from pressure differences around the shape and at surface junctions. Induced drag, on the other hand, comes from lift production and actually decreases as speed increases. So the description points to skin drag as the specific drag caused by friction on the surface; it’s the frictional portion of parasite drag.