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Coefficient of lift

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Coefficient of lift

The lift coefficient (CL, Ca or Cz) is a dimensionless coefficient that relates the lift generated by a lifting body to the density of the fluid around the body, its velocity and an associated reference area. A lifting body is a foil or a complete foil-bearing body such as a fixed-wing aircraft. CL is a function of the angle of the body to the flow, its Reynold number and its Mach number. The lift coefficient cl is refers to the dynamic lift characteristics of a two-dimensional foil section, with the reference area replaced by the foil chord.[1][2]

Definition of the lift coefficient CL

The lift coefficient CL is defined by[2][3]

C_\mathrm L = {\frac{L}{\frac{1}{2}\rho v^2S}} = {\frac{2 L}{\rho v^2S}} = \frac{L}{q S} ,

where L\, is the lift force, \rho\, is fluid density, v\, is true airspeed, S\, is planform area and q\, is the fluid dynamic pressure.

The lift coefficient can be approximated using the lifting-line theory,[4] numerically calculated or measured in a wind tunnel test of a complete aircraft configuration.

Section lift coefficient

Lift coefficient may also be used as a characteristic of a particular shape (or cross-section) of an airfoil. In this application it is called the section lift coefficient c_\mathrm l\, . It is common to show, for a particular airfoil section, the relationship between section lift coefficient and angle of attack.[5] 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 l\,, the lift force per unit span of the wing. The definition becomes

c_l={l \over \frac{1}{2}\rho v^2c} ,

where c\, is the chord of the airfoil.

For a given angle of attack cl can be calculated approximately using the thin airfoil theory,[6] calculated numerically or determined from wind tunnel tests on a finite-length test piece, with end-plates designed to ameliorate the three-dimensional effects. Plots of cl versus angle of attack show the same general shape for all airfoils, but the particular numbers will vary. They show an almost linear increase in lift coefficient with increasing angle of attack with a gradient known as the lift slope. For a thin airfoil of any shape the lift slope is π2/90 ≃ 0.11 per degree. At higher angles a maximum point is rteached, after which the lift coefficient reduces. The angle at which maximum lift coefficient occurs is the stall angle of the airfoil.

Symmetric airfoils necessarily have plots of cl versus angle of attack symmetric about the cl axis but for any airfoil with positive camber, i.e. asymmetrical, convex from above, there is still a small but positive lift coefficient with angles of attack less than zero. That is, the angle at which cl=0 is negative. On such airfoils at zero angle of attack the pressures on the upper surface are lower than on the lower surface.

See also

Notes

References

  • Clancy, L. J. (1975): Aerodynamics. Pitman Publishing Limited, London, ISBN 0-273-01120-0
  • Abbott, Ira H., and Von Doenhoff, Albert E. (1959): Theory of Wing Sections. Dover Publications Inc., New York, Standard Book Number 486-60586-8
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