Which statement is correct at the speed for minimum drag (subsonic)?

• A. The lift is maximum.
• B. The gliding angle is minimum (assume zero thrust).
• C. Parasite drag is zero.
• D. Induced drag is maximum.

Answer: (B)

At subsonic speeds, total drag D consists of parasite drag, which grows with speed, and induced drag, which falls as speed increases. The total drag is minimized where these two components are equal. That speed also coincides with the best glide condition when flying with zero thrust: the descent path is decided by the balance L = W cos gamma and D = W sin gamma, so the glide angle gamma satisfies tan gamma = D/L. Since L is roughly equal to the weight W in a shallow glide, minimizing D minimizes tan gamma, giving the smallest possible glide angle. So the speed for minimum drag gives the minimum descent angle in a zero-thrust glide. Lift isn’t at its maximum (that occurs near stall), parasite drag isn’t zero, and induced drag isn’t maximum at that speed.