This comes from the intersection of airstreams that creates eddy currents (like whirlpools), turbulence, or restricts smooth airflow. Which drag type is described?

• A. Induced drag
• B. Parasite drag
• C. Form drag
• D. Interference drag

Answer: (D)

The main concept here is how flows interact where different surfaces meet, causing disturbances in the air that create extra drag. Interference drag happens when airstreams from separate surfaces intersect and disturb each other, producing eddy currents and turbulence that disrupt smooth airflow. This is why you get whirlpool-like eddies and restricted flow at junctions such as where the wing meets the fuselage or near tail interfaces. Those flow interactions add drag beyond what each surface would generate alone, which is exactly what the description is describing. Induced drag is tied to lift and wingtip vortices, form drag comes from the overall shape not the interaction of streams, and parasite drag covers drag not due to lift but includes multiple contributors; the specific cause described—eddies and turbulence from intersecting airstreams—points to interference drag.