Drag polar
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 significant aerodynamic properties of aircraft wings are summarised by two dimensionless quantities, the lift and drag coefficients CL and CD. Like other such aerodynamic quantities, they are functions only of the angle of attack α, the Reynolds number Re and the Mach number M. CL and CD are often presented individually, plotted against α, but an alternative graph plots CL as a function of CD, using α 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, L and D are scaled by the same factor to get CL and CD, L/D = CL/CD. 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 CL, CD point does to the CD 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.
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