Definition
A form of parasite drag caused by air leaking through gaps, seams, or openings in an aircraft's structure, such as around doors, control surfaces, cowlings, or inspection panels. The leaking air creates pressure differences and turbulence that disturb the smooth flow over the airframe, adding to total drag without contributing to lift.
Plain English
Drag created when air sneaks in or out through small gaps and seams on the aircraft, disturbing the smooth airflow and slowing the airplane down.
Context Anchor
Seen in aircraft performance, maintenance, and aerodynamic discussions about doors, cowling seams, control-surface gaps, and other surface openings.
Derivation
From 'leak' (Old English / Old Norse, meaning to let fluid escape through a hole) plus 'drag.' The term describes drag produced by air that has 'leaked' through the airframe rather than flowing smoothly over it.
Why Pilots Care
It lowers cruise speed, raises fuel consumption, and reduces range; proper sealing improves efficiency and handling.
Analogy
It is like driving with a window not quite closed: the small opening lets air rush through and creates extra resistance and noise.
Intuition Check
Leakage drag does not mean fuel, oil, or hydraulic fluid is leaking. Here, “leakage” means air is slipping through gaps and creating extra drag.
Example Sentence 1
A loose cowling fastener can increase leakage drag and noticeably reduce cruise speed.
Example Sentence 2
Mechanics taped gaps on the wing fairings to minimize leakage drag before the long cross-country flight.