Wingtip vortices are best described as

• A. The boundary layer separation at the wing root
• B. The natural straightening of airflow over the wing at cruise
• C. The rapidly rotating air that spills over an airplane's wings during flight
• D. A method of increasing lift by wing curvature

Answer: (C)

Wingtip vortices arise from lift. When a wing generates lift, air from beneath the wing rushes to the upper surface, and some of this high-pressure air spills around the wingtip to the lower-pressure region above. That tunneling around the tip creates two swirling, rapidly rotating streams that trail behind the aircraft. This is essence of wingtip vortices—the rotating air mass behind each wing. The description “rapidly rotating air that spills over an airplane's wings during flight” captures this swirling wake. The other ideas describe different air-flow phenomena not related to the wingtip vortices: boundary layer separation at the wing root is a stall-related issue on the surface, not a trailing vortex; straightening of airflow over the wing doesn’t involve swirling; and increasing lift by wing curvature refers to the airfoil shape that produces lift, not the vortical wake behind it.