The force acting on an aircraft in opposition to gravity which keeps the aircraft in the air.
Lift is the upwards force exerted on a body (such as an aircraft) by the surrounding fluid (which, for an aircraft, is generally air). There are three main types of lift, but of the most interest for heavier-than-air aircraft is dynamic lift, which occurs when an object moves through a homogeneous fluid which surrounds it on all sides. The other two types of lift are aerostatic lift, also called buoyancy (which occurs when the object in question has a lower average density than that of the surrounding fluid, and is mainly of interest for balloons and dirigibles), and planing lift (which occurs when only one side of the object is in contact with the fluid, and is mainly of interest for some watercraft).
Dynamic lift is produced by a combination of several processes:
- If the aircraft has a positive angle-of-attack, it deflects the air it hits downwards, which, in turn, pushes the aircraft upwards.
- Most aircraft have wings with a cambered (curved) upper surface, with the greatest curvature being at or just behind the wing's leading edge, which limits the space available for the air passing over the top of the wing and forces it to move faster than the air passing underneath the wing; this decreases the air pressure above the wing relative to that underneath, causing the wing to be sucked upwards.
- The air moving over the aft portion of the wing moves downwards as it does so in order to follow the curve of the wing's upper surface; when this air reaches the wing's trailing edge, it continues to move downwards, pushing the wing upwards.
- Some supersonic aircraft are shaped so that the shock-waves generated by supersonic flight are stronger under the aircraft than above it and are partially trapped under the aircraft, forcing it upwards.