How does Induced drag vary with airspeed?

• A. Drag varies inversely with airspeed; as speed decreases, induced drag increases.
• B. Drag increases with higher airspeed.
• C. Drag is independent of airspeed.
• D. Drag only occurs at stall.

Answer: (A)

Induced drag is generated by the wingtip vortices that form when the wing produces lift. In level flight with a constant weight, the lift the wing must provide is fixed, so as speed increases the wing can reach that lift with a smaller angle of attack, which reduces the strength of the wingtip vortices and the associated energy lost to swirl. This means induced drag decreases as speed rises.

A straightforward way to see it is that lift in level flight is proportional to dynamic pressure (L ≈ qS CL), so the required CL falls roughly as 1/V^2 when weight is constant. Since induced drag is tied to CL (roughly proportional to CL^2) and to dynamic pressure (D_i ∝ q), the overall relationship becomes D_i ∝ 1/V^2. Therefore, as speed decreases, induced drag increases; at higher speeds, it diminishes.

The other statements aren’t accurate for induced drag: it does not increase with speed, it’s not independent of speed, and induced drag is present whenever the wing is producing lift, not only at stall.