Drag (physics)
Additional information for aerodynamics

The pressure distribution acting on a body's surface exerts normal forces on the body. Those forces can be added together and the component of that force that acts downstream represents the drag force, D p r {\displaystyle D_{pr}}. The nature of these normal forces combines shock wave effects, vortex system generation effects, and wake viscous mechanisms.

Viscosity of the fluid has a major effect on drag. In the absence of viscosity, the pressure forces acting to hinder the vehicle are canceled by a pressure force further aft that acts to push the vehicle forward; this is called pressure recovery and the result is that the drag is zero. That is to say, the work the body does on the airflow is reversible and is recovered as there are no frictional effects to convert the flow energy into heat. Pressure recovery acts even in the case of viscous flow. Viscosity, however results in pressure drag and it is the dominant component of drag in the case of vehicles with regions of separated flow, in which the pressure recovery is infective.

The friction drag force, which is a tangential force on the aircraft surface, depends substantially on boundary layer configuration and viscosity. The net friction drag, D f {\displaystyle D_{f}}, is calculated as the downstream projection of the viscous forces evaluated over the body's surface. The sum of friction drag and pressure (form) drag is called viscous drag. This drag component is due to viscosity.
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