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Parasite DragParasite drag is caused by any aircraft surface which deflects or interferes with the smooth airflow around the airplane. Parasite drag normally is divided into three types: form drag, interference drag, and skin friction drag. Form drag results from the turbulent wake caused by the seperation of airflow from the surface of a structure. The amount of drag is related to both the size and shape of the structure which protrudes into the relative wind. Interference drag occurs when varied currents of air over an airplane meet and interact. Placing two objects adjacent to one another may produce turbulence 50% to 200% greater than the parts tested separately. An example of interference drag is the mixing of the air over structures such as wing and tail surfaces brace struts and landing gear struts. Skin friction drag is caused by the roughness of the airplane's surfaces. Even though these surfaces may appear smooth, under a microscope, they may be quite rough. A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag. Each type of parasite drag varies with the speed of the airplane. The combined effect of all parasite drag varies proportionately to the square of the airspeed. For example, a particular airplane at a constant altitude has four times as much parasite frag at 160 knots as it does at 80 knots. |
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This page was last modified June 11, 2000 |
