Skin friction on the airfoil is what type of drag?

• A. Form
• B. Parasite
• C. Induced
• D. Profile

Answer: (D)

The main idea is how drag on a wing is categorized. Drag not tied to lift is parasite drag, which for a lifting surface is described as profile drag. Skin friction comes from the viscous shear of air sliding along the airfoil’s surface; this frictional drag is part of the airfoil’s overall drag that arises from its shape and surface, not from pressure differences alone. Profile drag specifically refers to the drag related to the airfoil’s surface geometry and surface interactions at a given lift, encompassing skin friction in combination with any associated form drag on the profile. Induced drag is caused by lift and wingtip vortices, and form drag is the pressure-based portion; skin friction is the viscous portion that, together with pressure effects, makes up profile drag. So the skin friction on the airfoil is profile drag.