How does induced drag change with increasing speed?

• A. Increases
• B. Decreases
• C. Remains constant
• D. Becomes negative

Answer: (B)

Induced drag comes from the wingtip vortices created to generate lift, and it grows with how much lift the wing must produce. In level flight at a constant weight, you can maintain the same lift with a smaller angle of attack as you speed up, so the required lift coefficient drops. Since the induced-drag tendency is tied to the square of the lift coefficient, and lift coefficient falls roughly as 1 over speed squared, induced drag ends up decreasing roughly as 1 over speed squared. In short, faster in level flight means less induced drag.