Lift is the force that directly
opposes the weight of an airplane and holds the airplane in the air. Lift is
generated by every part of the airplane, but most of the lift on a normal
airliner is generated by the wings. Lift is a mechanical aerodynamic force
produced by the motion of the airplane through the air. Because lift is a force,
it is a vector quantity, having both a magnitude and a direction associated with
it. Lift acts through the center of pressure of the object and is directed
perpendicular to the flow direction. There are several factors which affect the
magnitude of lift.
HOW IS LIFT GENERATED?
There are many explanations for the generation of lift found in encyclopedias,
in basic physics textbooks, and on Web sites. Unfortunately, many of the
explanations are misleading and incorrect. Theories on the generation of lift
have become a source of great controversy and a topic for heated arguments. To
help you understand lift and its origins, a series of pages will describe the
various theories and how some of the popular theories fail.
Lift occurs when a moving flow of gas is turned by a solid object. The flow is
turned in one direction, and the lift is generated in the opposite direction,
according to Newton's Third Law of action and reaction. Because air is a gas and
the molecules are free to move about, any solid surface can deflect a flow. For
an aircraft wing, both the upper and lower surfaces contribute to the flow
turning. Neglecting the upper surface's part in turning the flow leads to an
incorrect theory of lift.
NO FLUID, NO LIFT
Lift is a mechanical force. It is generated by the interaction and contact of a
solid body with a fluid (liquid or gas). It is not generated by a force field,
in the sense of a gravitational field,or an electromagnetic field, where one
object can affect another object without being in physical contact. For lift to
be generated, the solid body must be in contact with the fluid: no fluid, no
lift. The Space Shuttle does not stay in space because of lift from its wings
but because of orbital mechanics related to its speed. Space is nearly a vacuum.
Without air, there is no lift generated by the wings.
NO MOTION, NO LIFT
Lift is generated by the difference in velocity between the solid object and the
fluid. There must be motion between the object and the fluid: no motion, no
lift. It makes no difference whether the object moves through a static fluid, or
the fluid moves past a static solid object. Lift acts perpendicular to the
motion. Drag acts in the direction opposed to the motion.
You can learn more about the factors that affect lift at this web site. There
are many small interactive programs here to let you explore the generation of
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