At what condition does total drag decrease to L/Dmax?

Total drag decreases to L/Dmax at the condition where velocity increases and reaches a certain point because this is the point at which the lift-to-drag ratio (L/D) is maximized.

As an aircraft accelerates, it initially experiences an increase in lift, and while drag also increases, the rise in lift occurs at a slower rate compared to drag at lower speeds. At a certain velocity, the ratio of lift to drag reaches its optimal value, which is L/Dmax. This indicates that for that particular speed, the aircraft is able to generate the most lift for the least amount of drag, resulting in a more efficient flight condition.

Beyond this point, if velocity continues to increase, the drag will start to increase more significantly than the lift, thus decreasing the L/D ratio. Therefore, identifying this specific velocity where the highest lift-to-drag ratio occurs is essential for efficient flight maneuvers and can greatly affect performance characteristics, especially during cruise phases and while gliding.