On a lift coefficient polar, which condition corresponds to the best glide?

• A. The point of maximum lift.
• B. The point of maximum L/D on the CL-CD curve.
• C. The minimum CD.
• D. The zero-lift condition.

Answer: (B)

Best glide means traveling the farthest horizontally for each unit of altitude lost. That forward-to-downward trade-off is the lift-to-drag ratio, L/D. So the glide performance is optimized where L/D is as large as possible. On a lift coefficient polar, L/D equals CL/CD, so the best glide occurs at the point where CL/CD is maximized. This point represents a balance: lift is sufficient to support the descent, but drag isn’t excessive.

Choosing maximum lift would push drag up, reducing L/D and making the glide steeper. Choosing minimum drag places you at a condition with low lift, which also tends to limit L/D. Zero lift cannot support the aircraft at all, so glide is not possible. Hence the condition with the highest CL/CD on the polar is the one that yields the best glide.