Drop a stone into the ocean and it will sink into the deep. Chuck a
stone off the side of a mountain and it will plummet as well. Sure,
steel ships can float and even very heavy airplanes can fly, but to
achieve flight, you have to exploit the four basic aerodynamic forces:
lift, weight, thrust and drag. You can think of them as four arms
holding the plane in the air, each pushing from a different direction.
First, let's examine thrust and drag. Thrust,
whether caused by a propeller or a jet engine, is the aerodynamic force
that pushes or pulls the airplane forward through space. The opposing
aerodynamic force is drag, or the friction that resists
the motion of an object moving through a fluid (or immobile in a moving
fluid, as occurs when you fly a kite).
If you stick your hand out
of a car window while moving, you'll experience a very simple
demonstration of drag at work. The amount of drag that your hand creates
depends on a few factors, such as the size of your hand, the speed of
the car and the density of the air. If you were to slow down, you would
notice that the drag on your hand would decrease.
We see another example of drag reduction when we watch downhill skiers in the Olympics.
Whenever they get the chance, they'll squeeze down into a tight crouch.
By making themselves "smaller," they decrease the drag they create,
which allows them to zip faster down the hill.
A passenger jet
always retracts its landing gear after takeoff for a similar reason: to
reduce drag. Just like the downhill skier, the pilot
wants to make the aircraft as small as possible. The amount of drag
produced by the landing gear of a jet is so great that, at cruising
speeds, the gear would be ripped right off the plane.
For flight
to take place, thrust must be equal to or greater than the drag. If, for
any reason, the amount of drag becomes larger than the amount of
thrust, the plane will slow down. If the thrust is increased so that
it's greater than the drag, the plane will speed up.
On the next page, we'll discuss weight and lift.
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