David Robach, age 9, of Rice Lake, Wisconsin, for his question:

How can a plane stay up in the air?

The birds, of course, have been flying for millions of years. Nobody has to teach them    but maybe they can show us how it is done. This is not so silly as it sounds. As a matter of fact, the first people to design airplanes did study the flight of birds. What’s more, students who plan to become airplane designers still learn all they can from the birds.

A bird seems to fly through the air with the greatest of ease. But if a person tried to do likewise, he would plonk down to the ground like a stone. The earth’s gravity pulls down people and stones. It pulls them down through the air very fast because they are heavy and the air is so thin. The obstacles to flying are the pull of gravity and the thin air. But there are tricks to outsmart them both.

The basic trick is to make things move. When you toss a ball, it flies through the air instead of dropping straight down, at least for a while. Another way to make something fly is to make the air move. Wind, as you know, is really fast moving air. Notice how it makes the leaves fly through the air. So, for something to fly, it must be moving or the air must be moving.

An airplane, of course, is a huge and heavy object. In order to make it fly, we need to use both of these tricks. We must make it move, pushing or pulling it forward. For this it needs an engine, much more powerful than the engine that drives an automobile. Let’s be on the safe side and have two engines, or maybe a pair on each side. That should be enough to provide the pushing power for a big plane.

Now let’s think about moving some air to help it along. The filmy air tends to flow around like water. It is easy to shove it out of the way    but it tends to stack up in thick piles. Also, when we push it out of the way to pass through it, the piles flow around somewhat like streams of water. This is very useful to know. But to make a plane fly as it should, we must make those moving currents push where we need them. This is where the designer does his best work. He makes the plane streamlined so that the thin edges at the front slice easily through the air. The thicker parts follow on through the first thin slices. This starts strong currents of air flowing around the rest of the airplane. If the plane is well designed, the strength of these air currents help to lift it up and keep it up.

The engine provides the force that thrusts the plane forward. It helps to outsmart the downward pull of gravity. The shape of the moving plane starts the air currents that help to lift it and keep it going. These things must be in balance to overcome the drag of the plane’s weight. Then the thrust and the lift can make it fly.