Drag & the Minimum-Drag Speed

Parasite drag — skin friction, form and interference — grows with the square of speed, so it dominates when you fly fast. Induced drag is the price of making lift: at low speed the wing works at a high angle of attack, the wing-tip vortices strengthen, and induced drag climbs. Their sum is a U-shaped curve, and its lowest point is V_MD, where the two are equal — the speed for minimum drag, best lift-to-drag ratio, best glide range and best endurance (for a jet). Fly slower than V_MD, on the 'back of the drag curve', and drag actually rises as you slow. Add weight and the wing needs more lift, so induced drag and V_MD both increase.

• Try this: Slow toward 60 kt — watch the tip vortices swell as induced drag takes over.
• Try this: Speed up to 360 kt — the airflow streaks intensify as parasite drag dominates.
• Try this: Park the speed exactly on V_MD and see parasite = induced.
• Try this: Raise the weight to 130% — the whole curve lifts and V_MD moves to a higher speed.