During the evolution of the design the drag polar will be refined. 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. īecause of the Reynolds and Mach number dependence of the coefficients, families of drag polars may be plotted together. Eiffel was the first to use the name drag polar. 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. When this measurement is repeated at different angles of attack the drag polar is obtained. 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. As L and D are at right angles, the latter parallel to the free stream velocity or relative velocity of the surrounding, distant, air, the resultant force R lies at the same angle to that direction as the line from the origin of the polar plot to the corresponding C L, C D point does to the C D axis. Since the lift and the drag forces, L and D are scaled by the same factor to get C L and C D, L/ D = C L/ C D. The drag polar of an aircraft contains almost all the information required to analyse its performance and hence to begin a design. Similar plots can be made for other components or for whole aircraft in all cases they are referred to as drag polars. C L and C D are often presented individually, plotted against α, but an alternative graph plots C L as a function of C D, using α parametrically. Like other such aerodynamic quantities, they are functions only of the angle of attack α, the Reynolds number R e and the Mach number M. The significant aerodynamic properties of aircraft wings are summarised by two dimensionless quantities, the lift and drag coefficients C L and C D. Drag polar for the Nairfoil, colour-coded as opposite plot.
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