Editing Lift (force) (section)

===Boundary layer and profile drag===
No matter how smooth the surface of an airfoil seems, any surface is rough on the scale of air molecules. Air molecules flying into the surface bounce off the rough surface in random directions relative to their original velocities. The result is that when the air is viewed as a continuous material, it is seen to be unable to slide along the surface, and the air's velocity relative to the airfoil decreases to nearly zero at the surface (i.e., the air molecules "stick" to the surface instead of sliding along it), something known as the [[no-slip condition]].<ref>White (1991), Section 1-4</ref> Because the air at the surface has near-zero velocity but the air away from the surface is moving, there is a thin boundary layer in which air close to the surface is subjected to a [[shear force|shearing]] motion.<ref>White (1991), Section 1-2</ref><ref name="Anderson 1991, Chapter 17">Anderson (1991), Chapter 17</ref> The air's [[viscosity]] resists the shearing, giving rise to a [[shear stress]] at the airfoil's surface called [[skin friction drag]]. Over most of the surface of most airfoils, the boundary layer is naturally turbulent, which increases skin friction drag.<ref name="Anderson 1991, Chapter 17"/><ref name="Doenhoff 1958">Abbott and von Doenhoff (1958), Chapter 5</ref>

Under usual flight conditions, the boundary layer remains attached to both the upper and lower surfaces all the way to the trailing edge, and its effect on the rest of the flow is modest. Compared to the predictions of [[inviscid flow]] theory, in which there is no boundary layer, the attached boundary layer reduces the lift by a modest amount and modifies the pressure distribution somewhat, which results in a viscosity-related pressure drag over and above the skin friction drag. The total of the skin friction drag and the viscosity-related pressure drag is usually called the [[profile drag]].<ref name="Doenhoff 1958"/><ref>Schlichting (1979), Chapter XXIV</ref>