Parasite Drag
Parasite drag (or zero lift drag) is caused by any aircraft surface which is deflected or has an interferring effect on the smooth airflow around the airplane. Normally, parasite drag is divided into three types, namely: form drag, interference drag and skin friction drag. We'll discuss these items into detail below.

Form drag is related to turbulent wake which is caused by airflow separation from the surface of a structure. Its strength is greatly related to both size and shape of the structure protruding into the relative wind. To keep form drag as low as possible it is of greatest importance to have laminar airflow over the surface with the separation point located as far afterward as possible. This is because a laminar airflow has only one boundary airflow exposed to the aircrafts surfaces while a turbulent airflow exposes more air to the surface due to mixing with high speed layers of air further away from it. Figure 1.1 gives us a detailed representation of this situation.

Constant efforts are made in order to place the separation point as much afterward as possible. In case the airflow separates from the surface, a turbulent airflow is favoured when trying to retain attachement of the boundary layer. This is due to the higher kinetic energy contained in a turbulent boundary layer.
Figure 1.1 Boundary layer and separation point

Interference drag occurs when various currents over an airplane meet at some point and interact with eachother. A common place where this kind of drag occurs is at the junction of the wings, fuselage, nacelles etc. Testing both structures separately may have less effect on the interference drag than when these structures are place adjacent to eachother.

Skin friction drag has everything to do with the roughness of the airplane's surfaces and is largely determined by the total area of the aircraft which is exposed to the air flowing past it. Since these surfaces are exposed to high speed airflow it is necessary to keep these surfaces clean and smooth. Although a surface may look smooth it may appear to be quite rough when examined under a microscope. What happens is that a small layer of air may cling to these rough surfaces and create small eddies which contribute to drag. The only way to reduce skin friction drag is by controlling the boundary layer. To keep friction drag to the minimum, the transition from laminar to turbulent airflow has to be delayed as long as possible.

The graph below (figure 1.2) shows us that parasite drag increases with speed. As speed doubles, parasite drag increases fourfold. therefore, an airplane at a constant altitude has four times a much parasite drag at 180 knots as it does when flying at 90 knots.

Because of its rapid increase with increasing airspeed parasite drag is predominant at high speeds. At lower speeds, near a stall for example, induced drag predominates due to high lift production. As mentioned earlier, when the airspeed doubles the parasite drag fourfolds. This is proved by the same formula that applies to lift, namely:

Drag formula

Where C(d) stands for the drag coefficient and depends on the shape of the aircraft and the angle-of-attack. Rho stands for the air density while S stands for the total area.
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