Lift coefficient
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The lift coefficient (CL or CZ) is a number associated with a particular shape of an airfoil, and is incorporated in the lift equation to predict the lift force generated by a wing using this particular cross section.
The lift coefficient CL is equal to:
where L is the lift force, ρ is density, v is speed and A is area.
Airfoil theory also incorporates the concept of an infinte wing, in which case it is not practical to define the lift coefficient in this manner. Rather, the lift is defined per unit span of the wing. In such a situation, the above formula becomes:
where c is the chord length of the airfoil.
Note that the lift equation does not include terms for angle of attack — that is all wrapped up within the description of airfoil geometry, and the coefficient of lift incorporates this term. The graph for lift coefficient vs. angle of attack follows the same general shape for all airfoils, but the particular numbers will vary. The graph shows an almost linear increase in lift coefficient with increasing angle of attack, up to a maximum point, after which the lift coefficient falls away rapidly. This is known as the stall angle of the airfoil.
The coefficient of lift is a dimensionless number.
Note that in the graph here, there is still a small but positive lift coefficient with angles of attack less than zero. This is true of any airfoil with camber (asymmetrical airfoils). The pressures on the upper surface of the airfoil are lower than on the bottom surface, even at zero angle of attack.
