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.