FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightHard
A pilot experiences an engine failure and wants to establish best glide airspeed to maximize gliding distance. This airspeed corresponds to which point on the total drag curve?
- AThe point of minimum induced drag only, regardless of parasite drag
- BThe point of minimum parasite drag only, near stall speed
- CThe point where induced drag equals parasite drag, producing minimum total drag (L/D max)
- DThe point of maximum total drag, where both drag components are highest
Show answer & explanationAnswer & explanation
Correct answer: C. The point where induced drag equals parasite drag, producing minimum total drag (L/D max)
Best glide speed occurs at L/D max, the point where the total drag curve is at its lowest value. This occurs where the induced drag curve (which decreases with speed) and the parasite drag curve (which increases with speed) intersect, meaning induced drag equals parasite drag, minimizing total drag and maximizing glide ratio.
Why the other options are wrong
- A. Minimum induced drag alone occurs at a much lower, near-stall speed where parasite drag is very low but total drag is not minimized.
- B. Minimum parasite drag occurs near stall speed but induced drag is very high there, so total drag is not minimized.
- D. Maximum total drag occurs at very high or very low speeds, the opposite of what is needed for best glide.
Total Drag Curve / L/D Max
The total drag curve is the sum of parasite drag (increasing with speed²) and induced drag (decreasing with speed); minimum total drag occurs at L/D max, which is the best glide/best range speed.
- Induced drag dominates at low speed; parasite drag dominates at high speed
- L/D max = point where the two curves intersect
- Above L/D max is the region of normal command; below is region of reversed command
Memory trick: Where the curves cross, glide the boss (best glide)