A flying body's (3D) lift coefficient CL is the dimensionless value defined as follows:
CL=L/(1/2)*ρ*V2*S
A related value is the section, or 2D, lift coefficient, cl. This is defined by the similar formula (where l represents lift per unit wingspan):
cl=l/(1/2)*ρ*V2*c
In the study of complete flying bodies, the 3D lift coefficient is most useful. However, early in the design process, knowing the section lift coefficient of an airfoil can lead to a rough estimate of the performance of an aircraft. The conversion CL=cl/b is helpful in this regard. However, the use of CFD in modern aircraft design makes this much less useful because, even without testing in a wind tunnel, the 3D lift coefficient can be accurately estimated by the computer.
Flying things without wings get much less use of these values as such, mainly because there is no good reference area to use for these objects (though if needed, the surface area of a blimp or balloon envelope is recommended). When trying to optimize performance, it is suggested to work backward from the well-defined drag coefficient to a value for drag, and work with lift and drag directly.
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