Profile Drag

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Profile drag develops from the frictional resistance of the blades passing through the air. It does not change significantly with the airfoil’s angle of attack, but increases moderately when airspeed increases. Profile drag is composed of form drag and skin friction.

Form drag results from the turbulent wake caused by the separation of airflow from the surface of a struc-ture. The amount of drag is related to both the size and shape of the structure that protrudes into the relative wind. [Figure 2-12]

Figure 2-12. It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreases form drag by reducing the airflow separation.

Skin friction is caused by surface roughness. Even though the surface appears smooth, it may be quite rough when viewed under a microscope. Athin layer of air clings to the rough surface and creates small eddies that contribute to drag.