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Induced drag on a finite wing arises physically because

  • A.Skin friction increases towards the wing tips
  • B.Shock waves form at the leading edge in subsonic flow
  • C.The local lift vector is tilted rearward by the downwash
  • D.The boundary layer separates over the whole upper surface

Answer: C

The local lift acts perpendicular to the local relative wind, and the downwash rotates that wind downward. The lift vector rotates with it, so it acquires a component along the free-stream direction. That streamwise component is induced drag. It is a pressure drag, not a viscous one, and it exists even in an inviscid calculation.

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