Lift coefficient

The lift coefficient is a dimensionless coefficient for the dynamic lift of a body around which a fluid flows. It is an important parameter in the characterization of airfoils in fluid mechanics. In the case of passenger cars, the lift coefficient is one of six coefficients that are determined in the wind tunnel, for example. In formulas, the abbreviation c_{{\mathrm a}}chosen. In English texts it is often c_{{\mathrm l}}(l for lift) or c_{{\mathrm z}}.

The lift coefficient is a special form of the lateral lift coefficient {\displaystyle c_{\mathrm {F} }}or {\displaystyle c_{\mathrm {q} }}. A lift coefficient can be determined experimentally for all elongated bodies with all cross-sections flowed by fluids.

Lift coefficients depending on the angle of attack β are given graphically,\beta galloping) or bridge travel paths (English: galloping; example: "Galloping Gertie").

The lift coefficient results from the lift force F_{{\mathrm a}}normalized to the dynamic pressure qand the area Athe reference surface; the reference surface is the wing area for airfoils and the frontal area for vehicles:

c_{{\mathrm a}}={\frac {F_{{\mathrm a}}}{q\,A}}.

The lift coefficient, like other aerodynamic coefficients such as the drag coefficient, depends on the orientation of the body in the flow, expressed by the angle of attack. The relationship between lift and drag coefficients as a function of the angle of attack is given by the polar diagram, which differs significantly for different airfoil shapes.

Reduction at finite long wing

The data in a profile polar can be directly transferred to an infinitely long wing with this profile. For a finite-length wing, on the other hand, the influence of the wing tip must also be taken into account. This is because at the outermost end of a wing, crossflows reduce the pressure difference between the upper and lower surfaces further inside the wing, which means a smaller dynamic lift. The crossflow also causes the edge vortex.

Thus, the lift coefficient of a real wing is smaller than specified in the polar. The longer the wing in relation to its depth (i.e. the greater its aspect ratio), the closer the wing comes to the coefficient of an infinitely long wing. The lift coefficient c_{{{\mathrm a},{\text{endlich}}}}of a finitely long wing with aspect ratio Λ \Lambda can be approximated as follows from the lift coefficient c_{{{\mathrm a},{\text{unendlich}}}}an infinitely long wing:

c_{{{\mathrm a},{\text{endlich}}}}={\frac {\Lambda }{\Lambda +2}}\,c_{{{\mathrm a},{\text{unendlich}}}}


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