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Parasitic drag is drag that results when an object is moved through a fluid medium (in the case of aerodynamic drag, a gaseous medium, more specifically, the atmosphere). Parasitic drag is a combination of form drag, skin friction drag and interference drag. (The other components, induced drag and wave drag, are separate components of total drag, and are NOT components of parasitic drag.) In flight, induced drag results from the need to maintain lift. It is greater at lower speeds where a high angle of attack is required. As speed increases, the induced drag decreases, but parasitic drag increases because the fluid is striking the object with greater force, and is moving across the object's surfaces at higher speed. As speed continues to increase into the transonic and supersonic regimes, wave drag enters the picture. Each of these drag components changes in proportion to the others based on speed. The combined overall drag curve therefore shows a minimum at some airspeed; an aircraft flying at this speed will be close to its optimal efficiency. Pilots will use this speed to maximize the gliding range in case of an engine failure. However, to maximize the gliding endurance, the aircraft's speed would have to be at the point of minimum power, which occurs at lower speeds than minimum drag. At the point of minimum drag, CD,o (drag coefficient of aircraft when lift equals zero) is equal to CD,i (induced drag coefficient, or coefficient of drag created by lift). At the point of minimum power, CD,o is equal to one third times CD,i. This can be proven by deriving the following equations: : and : where : ==Form drag== Form drag or pressure drag arises because of the shape of the object. The general size and shape of the body are the most important factors in form drag; bodies with a larger presented cross-section will have a higher drag than thinner bodies; sleek ("streamlined") objects have lower form drag. Form drag follows the drag equation, meaning that it increases with the square of velocity, and thus becomes more important for high-speed aircraft. Form drag depends on the longitudinal section of the body. A prudent choice of body profile is essential for a low drag coefficient. Streamlines should be continuous, and separation of the boundary layer with its attendant vortices should be avoided. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「parasitic drag」の詳細全文を読む スポンサード リンク
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