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Total Drag Curve
As indicated in Figure 2, adding both induced drag and parasite drag together forms a "U" shaped total drag curve. At the airspeed where induced drag equals parasite drag, the total drag is at a minimum, and the ratio of lift to drag produced is at a maximum. This is referred to as "L/DMAX".
Ignoring propeller effects, L/DMAX generally has great significance for high performance aircraft. If drag is at a minimum and thrust equals drag, then the fuel flow will also be at a minimum; therefore, the aircraft has maximum endurance at this airspeed. Maximum climb angle (Vx) is achieved at L/DMAX because thrust available in excess of drag is at a maximum. This is not the case with maximum rate of climb (Vy). In case of a flameout or engine failure, maximum glide range is also obtained. If L/DMAX equals 15:1, then the glide ratio is also 15:1.
Ignoring propeller effects:
• Maximum Endurance occurs at L/D max.
• Maximum Climb Angle occurs at L/D max.
• Maximum Glide Range occurs at L/D max.