Friction Drag, also known as Skin Friction Drag, is drag caused by the friction of a fluid against the surface of an object that is moving through it. It is directly proportional to the area of the surface in contact with the fluid and increases with the square of the velocity. In aerodynamics, the fluid concerned is the atmosphere.
Skin Friction Drag
Friction Drag is created in the boundary layer due to the viscosity of the air and the resulting friction against the surface of the aircraft. The air molecules in direct contact with the aircraft surface are most affected. As the molecules flow past the surface and past each other, the viscous resistance to that flow becomes a force which retards forward motion. The amount of friction drag that is created per square metre of surface area is relatively small. However, as the boundary layer covers much of the surface of the aircraft, friction drag can become quite significant in larger aeroplanes.
Turbulent flow creates more friction drag than laminar flow due to its greater interaction with the surface of the airplane. Rough surfaces accelerate the transition of boundary layer airflow from laminar to turbulent which, in turn, increases the thickness of and the airflow disruption within the boundary layer. These increases result in more air molecules being affected by the movement of the aircraft and a corresponding increase in friction drag. Friction drag can be reduced by delaying the point at which laminar flow becomes turbulent. This can be accomplished by smoothing the exposed surfaces of the aeroplane by using flush rivets on the leading edges and through painting, cleaning, waxing, polishing or the application of surface coatings.