The drag curve or drag polar is the relationship between the drag on an aircraft and other variables, such as lift, the coefficient of lift, angle-of-attack or speed. It may be described by an equation or displayed as a graph (sometimes called a "polar plot"). Drag may be expressed as actual drag or the coefficient of drag. Drag curves are closely related to other curves which do not show drag, such as the power required/speed curve, or the sink rate/speed curve. The drag curve 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 can be plotted against , or can be plotted against each other. The lift and the drag forces, and , are scaled by the same factor to get and , so = . and are at right angles, with parallel to the free stream velocity (the relative velocity of the surrounding distant air), so the resultant force lies at the same angle to as the line from the origin of the graph to the corresponding , point does to the axis. If an aerodynamic surface is held at a fixed angle of attack in a wind tunnel, and the magnitude and direction of the resulting force are measured, they can be plotted using polar coordinates. When this measurement is repeated at different angles of attack the drag curve 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", however drag curves are rarely plotted today using polar coordinates. Depending on the aircraft type, it may be necessary to plot drag curves at different Reynolds and Mach numbers. The design of a fighter will require drag curves for different Mach numbers, whereas gliders, which spend their time either flying slowly in thermals or rapidly between them, may require curves at different Reynolds numbers but are unaffected by compressibility effects. During the evolution of the design the drag curve will be refined. A particular aircraft may have different curves even at the same and values, depending for example on whether undercarriage