Which drag component is caused by friction between air layers and the surface?

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Study for the Pilot's Handbook of Aeronautical Knowledge Exam. Study with flashcards and multiple choice questions, each question has hints and explanations. Get ready for your exam!

Multiple Choice

Which drag component is caused by friction between air layers and the surface?

Explanation:
Friction between air layers and the surface is the shear in the boundary layer caused by viscosity. The air in contact with the aircraft surface sticks and has to accelerate from zero at the surface to the free-stream velocity, creating a velocity gradient. The resulting shear stress acts parallel to the surface and, integrated over the aircraft, produces a drag force known as skin friction drag. This is a viscous drag component distinct from pressure-difference drag, lift-related induced drag, or wave drag from shock waves. Smooth surfaces and laminar flow reduce skin friction, while roughness and turbulence increase it.

Friction between air layers and the surface is the shear in the boundary layer caused by viscosity. The air in contact with the aircraft surface sticks and has to accelerate from zero at the surface to the free-stream velocity, creating a velocity gradient. The resulting shear stress acts parallel to the surface and, integrated over the aircraft, produces a drag force known as skin friction drag. This is a viscous drag component distinct from pressure-difference drag, lift-related induced drag, or wave drag from shock waves. Smooth surfaces and laminar flow reduce skin friction, while roughness and turbulence increase it.

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