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

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.

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.