The skin friction coefficient (cf) is a dimensionless quantity used in aerodynamics to characterize the frictional drag that a fluid, such as air, exerts on the surface of an object as it moves through the fluid. It quantifies the drag caused by the shear stress between the fluid and the object’s surface. For the upper surface of an airfoil (wing or other aerodynamic shape), the skin friction coefficient is often denoted as cf,u to specifically refer to the frictional drag on the upper surface.Â
- cf,u is the skin friction coefficient on the upper surface of the airfoil
- Ï„w is the wall shear stress
- q∞ is the dynamic pressureÂ