FAA Commercial Pilot Airplane (CAX)Aerodynamics and PerformanceEasy
A pilot studies a total drag curve (parasite drag plus induced drag) plotted against airspeed. At one specific airspeed, the curve reaches its lowest point, meaning total drag is at a minimum. What does this airspeed represent?
- ANever-exceed speed (Vne)
- BThe speed for maximum lift-to-drag ratio (L/D max)
- CManeuvering speed (Va)
- DBest angle-of-climb speed only during a full-power climb
Show answer & explanationAnswer & explanation
Correct answer: B. The speed for maximum lift-to-drag ratio (L/D max)
The point where total drag is lowest is also where the lift-to-drag ratio is highest, since drag is minimized relative to the lift being produced. This speed is called L/D max and is the reference speed for best glide performance.
Why the other options are wrong
- A. Vne is a structural limit speed unrelated to the drag curve shape.
- C. Va relates to structural load limits, not the drag curve minimum.
- D. Vx is related to climb performance and may coincide with L/D max in some cases, but the concept described is drag minimization, not climb angle.
L/D Max (Minimum Drag Speed)
The airspeed at which total drag (parasite + induced) is lowest, giving the highest lift-to-drag ratio.
- Parasite drag increases with speed squared; induced drag decreases with speed squared.
- Their sum is minimized at one specific speed — L/D max.
- This speed is the basis for best glide speed.
Memory trick: Where the two drag curves cross, drag is least — and glide is best.