What is the primary mechanism by which induced drag arises in finite wings?

• A. Downwash generated by wingtip vortices reduces the effective angle of attack and creates induced drag.
• B. Downwash reduces drag but not lift.
• C. Downwash arises from surface roughness only.
• D. Induced drag is independent of downwash.

Answer: (B)

The main idea is that finite wings shed wingtip vortices, which create a downward deflection of the flow behind the wing (downwash). This downwash makes the lift vector tilt rearward relative to the flight direction. That rearward tilt adds a forward-backward component of force opposite the motion, which we call induced drag. In other words, the energy needed to produce lift on a finite wing comes with a byproduct: a drag force tied to the wingtip vortices and the resulting downwash. As lift increases, downwash grows and so does induced drag; longer, high-aspect-ratio wings reduce this effect for a given lift.

The idea that downwash reduces drag is not correct—downwash creates a backward (drag) component of the lift force. Induced drag isn’t due to surface roughness, and it doesn’t vanish or become independent of downwash.