Lift is the force that holds an aircraft
in the air. Lift can be generated by any part of the airplane, but
most of the lift on an airplane is generated by the wings.
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 for many years.
One of the most popular
incorrect theories
states that the upper surface of an airfoil is shaped longer than the lower
surface so that the flow over the top travels faster to reach the trailing edge
at the same time. The higher velocity produces lower pressure according to
Bernoulli's equation
and the difference in pressure produces the lift.
However, if we look at the
airfoil shapes
used by the Wright brothers, we see that they are thin curved plates.
Here's a picture of a model of the Wright 1903 wing:
You are looking out towards the right wing tip and
the cloth skin has been removed from the wing so that you can view the
construction.
The distance
on the upper and lower surface is exactly the same, yet these airfoils create plenty
of lift. So the "equal transit" theory must be incorrect; it is not necessary for the
upper surface to be longer than the lower surface, or for the
lower surface to be flat to create lift.
Lift occurs when a 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 airfoil, both the
upper and lower surfaces contribute to the flow turning.
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. For a
kite,
the lift force is generated by the wind blowing over a surface that
is fixed in space.
Lift acts perpendicular to the fluid motion while
drag
acts in the direction opposed to the motion.
There are many factors
that affect the amount of lift generated by an object.
We can express the relation between these factors and the generated lift
by a mathematical
lift equation.
At the time of the Wright brothers a slightly different version of the
lift equation
was used, but both old and new version of the equation express the dependence
of lift on the
square of the velocity,
the surface area, the
shape of the object,
inclination of the object to the flow,
and the properties of the air.
You can view a short
movie
of "Orville and Wilbur Wright" discussing the lift force
and how it affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
Activities:
Navigation..
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page
- http://www.nasa.gov
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