Skin Friction & Force Drag

Skin friction

The accurate measurement of wall shear stress provides key input to understanding the basic fluid physics involved. For example, the local skin friction (or wall shear), plays a central role in the correlation of all turbulent boundary layer flows through the friction velocity. Skin friction data is also critical in assessing the performance of a device by determining drag, transition, and any local regions of flow separation. Knowledge of wall property variations provides feedback to the design engineer leading to improvements in the design of the device itself for increased performance and efficiency.

http://www.lunainnovations.com/luna/products/sensors/skin_friction.htm

Form drag

In aerodynamics, form drag, or profile drag, is a drag force created by the wind hitting the aircraft. Form drag follows the drag equation, meaning that it rises with the square of speed, and thus becomes more important for high speed aircraft. This form of drag is reduced through the use of streamlining to create a shape with as little overall drag as possible.

The amount of form drag is generally related to two components. One is the general size and shape of the aircraft, those with many protrusions will have a higher drag than "clean" designs. This source of drag is sometimes referred to as parasitic drag. Another is the friction between the wind and the surfaces themselves. This is typically measured as wetted area, the area of the surface of the plane that would become wet if sprayed with water flowing in the wind.

At lower speeds induced drag tends to dominate, whereas at higher speed form drag becomes dominant, at at even higher speeds in the transonic, wave drag enters the picture. Each of these forms of drag changes in proportion to the others based on speed. The combined overall drag curve therefore shows a minimum at some airspeed - an aircraft flying at this speed will be at or close to its optimal efficiency. Pilots will use this speed to maximize endurance (minimum fuel consumption), or maximize gliding range in the event of an engine failure.