Definition
The resistance produced when air flows across the surface of an aircraft and rubs against the skin, creating drag along the boundary between the air and the airframe. Surface roughness — including ice, frost, or contamination — increases this resistance and reduces aerodynamic efficiency.
Plain English
The slowing effect created by air sliding across the surface of the aircraft. The rougher the surface, the more it slows the airplane down.
Context Anchor
Seen in icing discussions, where ice roughens the wings and other surfaces and makes the airplane harder to move through the air.
Derivation
From Latin frictio, meaning 'a rubbing.' Frictional drag is literally the drag caused by air rubbing against the aircraft's surface.
Why Pilots Care
Ice roughens surfaces and increases frictional drag, reducing airspeed and climb capability.
Grounding Statement
A clean, smooth wing lets air pass over it more easily; a rough, icy wing makes the air resist the airplane more.
Intuition Check
Frictional drag is not wheel braking or runway friction. Here, friction means air rubbing along the aircraft’s surface, and drag means the rearward resistance that slows the airplane.
Example Sentence 1
A coating of frost on the wings increases frictional drag and can prevent the aircraft from achieving a safe climb after takeoff.
Example Sentence 2
A clean, polished wing reduces frictional drag during normal cruise.