The secret of streamlining lies in an understanding of the dangerous situation which arises when a boundary layer is required to flow from region of low pressure to a. region of high pressure. With a thin body having a long tapered tail, like a streamlined airfoil, the rate of pressure rise from shoulder to tail is so moderate that the boundary layer gets nearly all the way to the tail without becoming stalled. This produces no pressure drag. A shape is said to be streamlined if there is no boundary layer separation. The object of smoothing components is to avoid sharp edges, corners rapid changes in contour, or shapes that might cause turbulence or unnecessary air resistance.
Rough surfaces can also cause excess air friction.