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Drag coefficients in fluids with Reynolds number approximately 10 4 [1] [2] Shapes are depicted with the same projected frontal area. In fluid dynamics, the drag coefficient (commonly denoted as: , or ) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water.
Lift is always accompanied by a drag force, which is the component of the surface force parallel to the flow direction. Lift is mostly associated with the wings of fixed-wing aircraft , although it is more widely generated by many other streamlined bodies such as propellers , kites , helicopter rotors , racing car wings , maritime sails , wind ...
It is also useful to show the relationship between section lift coefficient and drag coefficient. The section lift coefficient is based on two-dimensional flow over a wing of infinite span and non-varying cross-section so the lift is independent of spanwise effects and is defined in terms of ′, the lift force per unit span of the wing. The ...
Sometimes a body is a composite of different parts, each with a different reference area (drag coefficient corresponding to each of those different areas must be determined). In the case of a wing, the reference areas are the same, and the drag force is in the same ratio as the lift force. [14]
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"). [1] Drag may be expressed as actual drag or the coefficient of drag.
drag force F d. Using the algorithm of the Buckingham π theorem, these five variables can be reduced to two dimensionless groups: drag coefficient c d and; Reynolds number Re. That this is so becomes apparent when the drag force F d is expressed as part of a function of the other variables in the problem:
The rates of change of lift and drag with angle of attack (AoA) are called respectively the lift and drag coefficients C L and C D. The varying ratio of lift to drag with AoA is often plotted in terms of these coefficients. For any given value of lift, the AoA varies with speed. Graphs of C L and C D vs. speed are referred to as drag curves ...
In these cases, lift and drag are the same, but the decomposition of total aerodynamic force (F T) into forward driving force (F R) and lateral force (F LAT) vary with point of sail. Forward driving force ( F R ) increases, as the direction of travel is more aligned with the wind, and lateral force ( F LAT ) decreases.