Sam Marcus, age 10, of Baton Rouge, La., for his question:
HOW DOES AN AIRPLANE FLY?
Four basic forces govern the flight of an airplane: gravity, lift, drag and thrust.
Gravity is the natural force that pulls a plane toward the ground. Lift is the force that pushes a plane upward against the force of gravity. Drag is the natural force of air opposing an airplane's forward movement. Thrust is the force that opposes drag and moves a plane forward.
For a plane to become airborne and to stay in the air, its wing must create a lifting force greater than the downward force of gravity. Lift is created by a change of air pressure around an airplane's wing as the plane pushes along the ground or through the air.
As a plane starts to move forward on the ground, air begins to flow over and under the wing. The air moving over the curved upper surface flows in a curve. As it does so, its speed increases and its pressure drops.
The air moving under the flat bottom of the wing moves in a straight line. Its speed and pressure are not changed by the wing. A high pressure area always moves toward a low pressure area, and so the air under the wing tries to move upward to the air over the wing. But the wing is in the way. Instead of meeting the low pressure area, the high pressure area lifts the wing into the air.
The faster the airplane moves, the greater the lift its wing produces. As an airplane increases its speed down the runway before take off, its wing builds up more and more lift. The air pressure beneath the wing finally becomes greater than the weight of the plane, and so lift becomes greater than the force of gravity. The plane then takes off.
A wing can produce lift only if it is moving forward through the air. It needs engine thrust to create the forward movement. As thrust increases, it moves a plane forward faster.
In a jet airplane, the rapid movement of gases through the jet engine produces thrust.
Propeller blades are shaped much like airplane wings. As the propeller spins, the air pressure on the front surface of the blades is reduced. The higher pressure on the back of the blades moves toward the lower pressure on the front. As it does so, it pushes against the propeller blades and moves the plane forward.
The faster the jet engine works or the propeller spins, the greater the force of thrust.
An airplane cruising in level flight has lift balanced against gravity and thrust balanced against drag.
To make a plane descend, the pilot decreases the engine power. The propeller or jet engines slow down, reducing the plane's thrust. Reducing the plane's thrust also reduces lift and the plane begins to move downward. At the same time, drag increases its effect.
If the pilot wants to climb, he increases engine power. The propeller or jet engines work faster, creating more thrust. The increased thrust also increases lift and the plane starts to climb.