As
an airplane moves through the air, its wings cause changes in the speed and
pressure of the air moving past them. These changes result in the upward force
called lift. To understand lift, you first have to understand how air (a gas)
behaves under certain conditions.
Let's
start with the Bernoulli principle. The Bernoulli principle states that an
increase in the speed of a fluid occurs simultaneously with a decrease in the
pressure exerted by the fluid.
When
moving air encounters an obstacle—a person, a tree, a wing—its path narrows as
it flows around the object. Even so, the amount of air moving past any point at
any given moment within the airflow is the same. For this to happen, the air
must either compress or speed up where its flow narrows. While air can be
compressed more easily than water, freely flowing air acts much like water—at
least at relatively low speeds. So when you "squeeze" a stream of
air, two things happen. The air speeds up, and as it speeds up, its
pressure—the force of the air pressing against the side of the object—goes
down. When the air slows back down, its pressure goes back up.
Why
does the air speed up? Because of conservation of mass, which states that mass
is neither created nor destroyed, no matter what physical changes may take
place. This means that if the area in which the air is moving narrows or
widens, then the air has to speed up or slow down to maintain a constant amount
of air moving through the area.
For
a stream of air to speed up, some of the energy from the random motion of the
air molecules must be converted into the energy of forward stream flow. The
random motion of air molecules is what causes air pressure; so transferring
energy from the random motion to the stream flow results in lower air pressure.
You
can see the Bernoulli principle at work in rivers. The water speeds up (and the
pressure goes down) where the river narrows. The water slows down (and pressure
goes up) where the river widens.
A
wing is shaped and tilted so the air moving over it moves faster than the air
moving under it. As air speeds up, its pressure goes down. So the faster-moving
air above exerts less pressure on the wing than the slower-moving air below.
The result is an upward push on the wing—lift!