What happens to total drag of an aircraft if the airspeed decreases in level flight below the speed for maximum L/D?

When the airspeed of an aircraft decreases in level flight below the speed for maximum lift-to-drag ratio (L/D), the total drag experienced by the aircraft increases primarily due to increased induced drag. Induced drag is a byproduct of lift generation, and it increases as the angle of attack (AoA) increases, which is necessary to maintain altitude at lower speeds.

At airspeeds slower than the optimal point for maximum L/D, the aircraft must increase the AoA to generate sufficient lift to counteract its weight. As AoA increases, the induced drag also rises because induced drag is directly related to the square of the amount of lift being produced. Consequently, even though parasitic drag, which depends mainly on speed and surface area, may decrease due to lower airspeed, the increase in induced drag more than offsets that decrease. Therefore, the overall total drag increases when flying below the optimum airspeed for L/D.

This insight helps in understanding aircraft performance and efficiency, particularly in scenarios where maintaining altitude is critical while managing drag and lift dynamics.