Lift depends on the density of the air, the square of the velocity, the air's viscosity and compressibility, the surface area over which the air flows, the shape of the body, and the body's inclination to the flow. In general, the dependence on body shape, inclination, air viscosity, and compressibility is very complex.

One way to deal with complex dependencies is to characterize the
dependence by a single variable. For lift, this variable is called
the lift coefficient, designated "Cl." This
allows us to collect all the effects, simple and complex, into a
single equation. The lift equation states that lift **L** is equal to the
lift coefficient **Cl** times the density **r** times half of the
velocity **V** squared times the wing area **A**.

L = Cl * A * .5 * r * V^2

For given air conditions, shape, and inclination of the object, we have to determine a value for Cl to determine the lift. For some simple flow conditions and geometries and low inclinations, aerodynamicists can determine the value of Cl mathematically. But, in general, this parameter is determined experimentally.

In the equation given above, the density is designated by the
letter "r." We do not use "d" for density, since "d" is often used to
specify distance. In many textbooks on aerodynamics, the density is
given by the Greek symbol "rho" (Greek for "r"). The combination of
terms "density times the square of the velocity divided by two" is
called the **dynamic pressure** and appears in Bernoulli's
pressure equation.

You can investigate the various factors that affect lift by using the FoilSim II Java Applet. (Have fun!) Use the browser "Back" button to return to this page. If you want your own copy of FoilSim to play with, you can download it at no charge.

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*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *