What does minimum drag refer to in aerodynamics?

• A. The speed at which the total drag is minimized and lift-to-drag ratio is greatest.
• B. The speed for maximum endurance.
• C. The speed for maximum range.
• D. The speed at which lift equals weight.

Answer: (A)

Minimum drag is the airspeed where the airplane’s total drag is at its lowest for the current weight. Drag comes from two main sources: induced drag, which is large at low speeds because you must produce the required lift with a higher angle of attack, and parasite drag, which grows with speed as the aircraft pushes through more air. As you speed up, induced drag falls while parasite drag rises, so the sum reaches a minimum at a certain mid-range speed.

At that same speed, in level flight lift equals weight, so lift-to-drag ratio (L/D) is at its maximum. Since L/D = Weight / Drag, a minimum Drag with constant weight means the L/D is greatest. Practically, this is the most aerodynamically efficient speed, giving the best glide performance—the farthest distance you can cover for a given altitude.

That’s why the described speed—where total drag is minimized and the lift-to-drag ratio is greatest—best captures the concept of minimum drag.