The lift force, lifting force or simply lift consists of the sum of all the fluid dynamic forces on a body perpendicular to the direction of the external flow approaching that body.
Sometimes the term dynamic lift (dynamic lifting force) is used in reference to the vertical force resulting from the relative motion of the body and the fluid, as opposed to the static lifting force resulting from the buoyancy.
The most straightforward and frequently-mentioned application of lift is the wing of an aircraft. However there are many other common, if less obvious, uses such as propellers on both aircraft and boats, rotors on helicopters, fan blades, sails on sailboats and even some kinds of wind turbines.
While the common meaning of the term "lift" suggests an "upwards" action, in fact, the direction of lift (and its definition) does not actually depend on the notions of "up" and "down", e.g., as defined with respect to the direction of the gravity. Specifically, the term negative lift refers to the lift force directed "down".
There are a number of ways of explaining the production of lift, all of which are equivalent. That is, they are different expressions of the same underlying physical principles....
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