
The Drag Polar is the relationship between the lift on an aircraft and its drag, expressed in terms of the dependence of the lift coefficient on the drag coefficient. It may be described by an equation or displayed in a diagram called a polar plot.〔 ==The drag polar== The significant aerodynamic properties of aircraft wings are summarised by two dimensionless quantities, the lift and drag coefficients and . Like other such aerodynamic quantities, they are functions only of the angle of attack , the Reynolds number and the Mach number . and are often presented individually, plotted against , but an alternative graph plots as a function of , using parametrically.〔〔 Similar plots can be made for other components or for whole aircraft; in all cases they are referred to as drag polars. The drag polar of an aircraft contains almost all the information required to analyse its performance and hence to begin a design.〔 Since the lift and the drag forces, and are scaled by the same factor to get and , = . As and are at right angles, the latter parallel to the free stream velocity or relative velocity of the surrounding, distant, air, the resultant force lies at the same angle to that direction as the line from the origin of the polar plot to the corresponding , point does to the axis. If, in a wind tunnel or whirling arm system an aerodynamic surface is held at a fixed angle of attack and both the magnitude and direction of the resulting force measured, they can be plotted using polar coordinates. When this measurement is repeated at different angles of attack the drag polar is obtained. Lift and drag data was gathered in this way in the 1880s by Otto Lilienthal and around 1910 by Gustav Eiffel, though not presented in terms of the more recent coefficients. Eiffel was the first to use the name drag polar.〔 Because of the Reynolds and Mach number dependence of the coefficients, families of drag polars may be plotted together. The design of a fighter will involve a set at different Mach numbers, whereas gliders, which spend their time either flying slowly in thermals or rapidly between them may require polars at different Reynolds numbers but are unaffected by compressibility effects. During the evolution of the design the drag polar will be refined. A particular aircraft may have different polar plots even at the same and values, depending for example on whether undercarriage and flaps are deployed.〔 The accompanying diagram shows a drag polar for a typical light aircraft. Such diagrams identify a minimum point at the leftmost point on the plot, where the drag is locally independent of lift; to the right, the drag is lift related. One component here is the induced drag of the wing, an unavoidable companion of the wing's lift, though one that can be reduced by increasing the aspect ratio. Prandtl's lifting line theoretical work shows that this increases as . The other drag mechanisms, parasitic and wave drag, have both constant components, totalling say, and dependent contributions that are often assumed to increase as . If so, then :::::::::: The effect of is to lift the polar curve upwards; physically this is caused by some vertical asymmetry, such as a cambered wing or a finite angle of incidence, which ensures the minimum drag attitude produces lift and increases the maximum lift to drag ratio.〔〔 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「Drag Polar」の詳細全文を読む スポンサード リンク
