Interference Drag is drag that is generated by the mixing of airflow streamlines between airframe components such as the wing and the fuselage, the engine pylon and the wing or, in the case of a military or other special purpose aircraft, between the airframe and attached external stores such as fuel tanks, weapons or sensor pods.
Interference drag is generated when the airflow across one component of an aircraft is forced to mix with the airflow across an adjacent or proximal component. If one considers two parts of the aircraft that intersect at a particular point, such as the vertical and horizontal components of the empennage, it is obvious where the point of intersection occurs. Each of these two components generate high velocity (potentially transonic or even supersonic) airflow across their respective surfaces. At the intersection of the two surfaces, there is less physical space for the airflow to occupy resulting in the turbulent mixing of the two airflows and the production of a localized shock wave. Due to this shock wave, the resulting total drag from the empennage is greater than the sum of the drag produced individually by the vertical tail and the horizontal tail surfaces. Other significant locations which generate interference drag include the wing/fuselage junction and the wing/engine pylon or fuselage/engine pylon convergence.
Interference drag can be minimized by the appropriate use of fairings and fillets to ease the transition between components. Fairings and fillets use curved surfaces to soften the transition at the junction of two aircraft components. This, in turn, allows the airflow streamlines to meet gradually rather than abruptly and reduces the amount of interference drag that is generated.