A drag polar relates drag coefficient to lift coefficient for specified geometry, configuration and flow conditions. It is commonly displayed with horizontally and vertically. The slope of a ray from the origin to a measured point is then , the lift-to-drag ratio when common references are used.
A wind tunnel can obtain a polar by varying angle of attack while recording both forces and the reference dynamic pressure. Reynolds number, Mach number, surface condition and deployed devices must also be recorded; changing one of those can change the curve. A polar is not a geometry-only fingerprint.
For an original illustrative dataset, the pairs lie on . Their positive-lift ratios at and are about and . The larger ratio among those samples does not establish the continuous maximum lift-to-drag ratio; the unsampled neighborhood near performs better in this assumed model.
A parabolic drag polar summarizes such data with a small number of parameters. A shifted parabolic drag polar allows minimum drag to occur at nonzero lift coefficient. Flow separation and changes of flow state can also produce branches or hysteresis that one parabola does not describe. A fit supplies a model whose residuals need inspection, rather than evidence that every unmeasured state lies exactly on the fitted curve.
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